Blogs by Marc Rayman

Blogs by Marc Rayman

Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.


Dawn's long and productive expedition in deep space is about to enter a new phase.

Building on the successes of its primary mission and its first extended mission, NASA has approved the veteran explorer for a second extended mission. Dawn will undertake ambitious new investigations of dwarf planet Ceres, its permanent residence far from Earth.

It was not a foregone conclusion that Dawn would conduct further operations. In part, that's because it is only one of many exciting and important missions NASA has underway, and more are being designed and built. But the universe is a big place, as you may have noticed if you've ever gazed in awestruck reflection at the night sky (or had to search for a parking space in Los Angeles). It simply isn't possible to do everything we want. Entrusted with precious taxpayers' dollars, NASA has to make well-considered choices about what to do and what not to do.

In addition, as we have discussed in detail, Earth's ambassador to two giants in the main asteroid belt has had to contend with severe life-limiting problems. Dawn's reaction wheels have failed, and now it has consumed most of its original small supply of hydrazine that it uses in compensation. It has also expended most of the xenon propellant for its uniquely capable ion propulsion system. It was not clear that a truly productive future would be possible for this aged, damaged ship with some supplies that are so limited. Fortunately, Dawn has endless supplies of creativity, ingenuity, dedication and enthusiasm.

For several months, the flight team has been studying the feasibility of flying the spaceship closer to Ceres than had ever been seriously considered. Dawn spent more than eight months in 2015-2016 circling about 240 miles (385 kilometers) above the dwarf planet. It had spectacular views of mysterious landscapes and acquired a wealth of data far beyond what the team had anticipated. Then Dawn flew to a higher altitude during its first extended mission for new observations. Now engineers are making progress on ways to operate the spacecraft in an elliptical orbit that would allow it to swoop down to below 125 miles (200 kilometers) for a few minutes on each revolution. Their results so far are very encouraging. There are still many complex technical problems to solve, and months of additional work remain. Dawn can wait relatively patiently in its current orbit, where it expends hydrazine quite parsimoniously as it measures cosmic rays.

Axomama Crater on Ceres
Dawn had this view from an altitude of about 240 miles (385 kilometers) on July 24, 2016, during its first extended mission. A segment of the western wall of Dantu Crater is visible at lower left. Pressure from underground liquid water is one of the possible explanations for the origin of the fractures visible here in Dantu's floor. (We have seen other views of Dantu, most recently in June. The scene above is in the lower left part of Dantu in that previous photo.) The crater below and right of center is Axomama. (Axomama, literally "potato mother," was an Incan goddess of potatoes.) At three miles (five kilometers) in diameter, Axomama's sharp rim indicates the crater was excavated in the recent geological past. This scene is visible at 23°N, 131°E on the map presented last month. Axomama is one of the newly named features on that map. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The promising potential for observing Ceres in elliptical orbits from closer than ever before makes a second extended mission there extremely attractive. NASA and the panel of scientists and engineers convened to provide an independent, objective assessment concluded that further exploration of Ceres would be the most valuable assignment for the spacecraft. It is noteworthy that Dawn is the only spacecraft ever to orbit two extraterrestrial destinations and even now, having significantly exceeded its original objectives, has the capability to leave Ceres and pay a brief visit to a third (although it does not have enough xenon left to orbit a third), but the prospects for new discoveries at Ceres are too great to pass up.

Ceres is not only the largest object between Mars and Jupiter but also certainly one of the most intriguing. In fact, motivated by what Dawn has found, there is now great interest in the possibility of sending a lander there someday. Anything more Dawn can do to learn about Ceres or to help pave the way for a subsequent mission will be of great importance.

Ceres is just too fascinating to abandon! Dawn has already revealed the dwarf planet to be an exotic world of ice, rock and salt, with organic materials and other chemical constituents, and now we can look forward to more discoveries. After all, the benefit of having the capability to orbit a distant destination, rather than being limited to a quick glimpse during a fleeting flyby, is that we can linger to scrutinize it and uncover even more of the secrets it holds. (Some readers may also draw inspiration from Ceres' ingredients to concoct recipes for treats to give out to Halloween visitors.)

In addition to the possibility of observing Ceres from unprecedentedly close, there are other benefits to keeping our sophisticated probe at work there. For now, let's consider two of them, both related to how long it takes Ceres to complete its stately orbit around the sun. One Cerean year is 4.6 terrestrial years.

The dwarf planet carries its robotic moon with it as it follows its elliptical path around the sun. In fact, all orbits, including Earth’s, are ellipses. Ceres’ orbit is more elliptical than Earth’s but not as much as some of the other planets. The shape of Ceres’ orbit is between that of Saturn (which is more circular) and Mars (which is more elliptical). (Of course, Ceres’ orbit is larger than Mars’ and smaller than Saturn’s, but here we are considering how much each orbit deviates from a perfect circle, regardless of the size.)

When Dawn arrived at Ceres in March 2015, they were 2.87 AU from the sun. That was well before the dwarf planet's orbit carried them to the maximum solar distance of 2.98 AU in January 2016. Now, with the second extended mission, the spacecraft will still be operating when Ceres reaches its minimum solar distance of 2.56 AU in April 2018. Dawn will keep a sharp eye out for any changes caused by being somewhat closer to the sun.

Xevioso Crater on Ceres
Dawn photographed this scene from an altitude of 915 miles (1,470 kilometers) on Oct. 15, 2015, from its third mapping orbit. Near the lower left is the cryovolcano Ahuna Mons, the highest mountain on the dwarf planet. (We have seen many view of Ahuna Mons before, most recently here.) Near the center top is Xevioso Crater, with bright material that was blasted when the crater formed. (Xevioso was a god of thunder who, among his other talents, nourished the land for the Fon people in the Dahomey Kingdom in what is now Benin.) The presence of the ejected material on Xevioso's left must be because the object that hit the ground came from the right. At 5.3 miles (8.5 kilometers) in diameter, Xevioso is relatively small, suggesting that this highly reflective material was relatively shallow. This scene is centered at 4°S, 314°E on the map presented last month. Like Axomama above, Xevioso is one of the recently named features on that map. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The extension also will give scientists the opportunity to examine Ceres with the different lighting caused by the change of seasons. Ceres' slower heliocentric orbit than Earth's means seasons last longer on that distant world. It was near the end of autumn in the southern hemisphere when Dawn took up residence at Ceres. Winter came to that hemisphere on July 24, 2015, when the sun reached its greatest northern latitude. The sun crossed the equator, bringing spring to the southern hemisphere, on Nov. 13, 2016, and summer begins on Dec. 22 of this year. Autumn, when the sun will leave the southern hemisphere, is more than one (terrestrial) year later. Most of Dawn's observations so far were made with the sun in the northern hemisphere. Now Dawn will have new opportunities to see the southern hemisphere with similar illumination.

In the coming months, as the team develops and refines its plans, we will describe how they will pilot the ship down to very low altitudes and what new measurements they will make. Before the new phase gets underway, however, you can explore Ceres (and other planets) yourself with Google maps (some functions don't work in some web browsers). Even though it does not use Dawn's sharpest photos, it should be more than adequate for most of your navigational needs. (It isn't quite adequate for Dawn's needs, but that's no cause for worry, because JPL navigators employ somewhat more sophisticated and accurate methods.)

What will Dawn find when it ventures closer to the ground than ever before? What will the new perspectives reveal about a strange world from the dawn of the solar system? What new challenges will the adventurer confront as it pushes further into uncharted territory? We don't know, but stay onboard as we find out together, for that is an essential element both of the tremendously successful process of science and the powerful thrill of exploration.

Dawn is 21,600 miles (34,700 kilometers) from Ceres. It is also 2.47 AU (229 million miles, or 369 million kilometers) from Earth, or 970 times as far as the moon and 2.49 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.

Dr. Marc D. Rayman
2:30 p.m. PDT October 31, 2017

TAGS: DAWN, CERES, EXTENDED MISSION

  • Marc Rayman
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Ceres

A decade after leaving its first home in the solar system, Dawn is healthy and successful at its current residence.

Even as the veteran explorer orbits high over dwarf planet Ceres and looks forward to continuing its mission, today it can reflect upon 10 exciting and productive years (or equivalently, with its present perspective, 2.17 exciting and productive Cerean years).

The ambitious adventurer embarked on an extraordinary extraterrestrial expedition on Sept. 27, 2007. With its advanced ion propulsion system, Dawn soared past Mars in 2009. The spacecraft took some of the Red Planet’s orbital energy around the sun to boost itself on its journey. (Nevertheless, this extra energy amounts to less than a quarter of what the ion engines have provided.) Ever a responsible citizen of the cosmos, Dawn fully adheres to the principle of the conservation of energy. So to compensate for speeding up, it slowed Mars down.

Dawn launch
Dawn launched at dawn (7:34 a.m. EDT) from Cape Canaveral Air Force Station, Sept. 27, 2007. Note the sun rising on the right edge of the picture. The intricate sequence of activities between the time this photo was taken and Dawn separated from the rocket to fly on its own is described here. Image credit: KSC/NASA

In 2011, the spacecraft arrived at Vesta, the second largest object in the main asteroid belt between Mars and Jupiter. Dawn gracefully entered into Vesta’s firm but gentle gravitational embrace. The probe maneuvered extensively in orbit, optimizing its views to get the best return possible from its photography and other observations. During 14 months in orbit, Dawn completed 1,298 revolutions around Vesta, taking nearly 31,000 pictures and collecting a wealth of other scientific measurements. From the perspective it had then, Dawn was in residence for nearly a third of a Vestan year (or almost 1,900 Vestan days). The explorer revealed a strange, ancient protoplanet, now recognized to be more closely related to the terrestrial planets (including the one Dawn left 10 years ago) than to the typical and smaller asteroids.

Unlike all other deep-space missions, Dawn had the capability to leave its first orbital destination and voyage to and enter orbit around another. After smoothly disengaging from Vesta, the interplanetary spaceship flew more than 900 million miles (1.5 billion kilometers) in 2.5 years to Ceres, the largest object in the asteroid belt. Indeed, prior to Dawn’s arrival, that dwarf planet was the largest body between the sun and dwarf planet Pluto that a spacecraft had not yet visited. And just as at Vesta, thanks to the maneuverability of ion propulsion, Dawn did not have to be content with a one-time flyby, gathering only as much data as possible during a brief encounter. By going into orbit around Ceres, the spacecraft could linger to scrutinize the exotic, alien world. And that is exactly what it has done.

Both Vesta and Ceres have held secrets since the dawn of the solar system, and both have beckoned since they were first spotted in telescopes at the dawn of the 19th century. For the next two centuries, they appeared as little more than faint smudges of light amidst myriad glittering stellar jewels, waiting for an inquisitive and admiring visitor from Earth. Finally, Dawn answered their cosmic invitations and eventually developed richly detailed, intimate portraits of each.

As the last stop on a unique interplanetary journey of discovery, Ceres has proven well worth the wait. Since arriving in March 2015 (more than half a Cerean year ago, or nearly 2,500 Cerean days ago), Dawn has completed 1,595 revolutions. It has beheld mysterious and fascinating landscapes and unveiled a complex world of rock, ice and salt, along with organic compounds and other intriguing constituents. The dwarf planet may have been covered by an ocean long ago, and there might even be liquid water underground now. The 57,000 pictures and numerous other measurements with the sophisticated sensors will keep scientists busy for many years (both terrestrial and Cerean).

By early 2016, during its ninth year in space, Dawn had accomplished so much that it exceeded all of the original objectives established for it by NASA before the ship set sail. Along the way, Dawn encountered and ultimately overcame many obstacles, including equipment failures that could well have sunk the mission. Against all odds and expectations, however, when its prime mission concluded in June 2016, the spacecraft was still healthy enough that NASA decided to extend the mission to learn still more about Ceres. Since then, Dawn has conducted many investigations that had never even been considered prior to last year. Now it has successfully achieved all of the extended mission objectives. And, once again defying predictions thanks to expert piloting by the flight team (and a small dose of good luck), Dawn still has some life left in it. Before the end of the year, NASA will formulate another new set of objectives that will take it to the end of its operational life.

Dawn has flown to many different orbital altitudes and orientations to examine Ceres. Now the probe is in an elliptical orbit, ranging from less than 3,200 miles (5,100 kilometers) up to 23,800 miles (38,300 kilometers). At these heights, it is measuring cosmic rays. Scientists mathematically remove the cosmic ray noise from Dawn’s 2015-2016 recordings of atomic elements from a low, tight orbit at only 240 miles (385 kilometers).

Juling Crater
Dawn took this picture of Juling Crater on Aug. 25, 2016, during its extended mission at an altitude of 240 miles (385 kilometers). (Juling is a crop spirit of the Orang Asli in the Malay Peninsula. The word also can mean strabismus or squint in the local language, and the spirit has been called the Squinting Demon. We leave it to you to make the connection with this particular crater apart from the general Ceres naming convention.) The 12-mile (20-kilometer) diameter crater is young, as seen by its sharp features and the absence of many smaller craters inside and nearby. Dawn’s infrared mapping spectrometer spotted the clear signature of ice on the ground in Juling. Ice is not stable for long at this location, so although the crater formed in the recent geological past, the ice must have been exposed even more recently. Scientists have found ice elsewhere as well, and other measurements show that there is a vast amount underground. One of the objectives of the second extended mission orbit was to follow up on the detection of ice in Juling by observing it under different lighting conditions and at different times of the Cerean day. Juling is at 36°S, 169°E on the map below. The next picture partially overlaps with this one, displaying more of the scenery in this area. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In its present orbit, Dawn can make these measurements to clarify Ceres’ nuclear signals while being very frugal with its precious hydrazine, which is so crucial because of the loss of three reaction wheels. (The small supply was not loaded onboard with the intention of compensating for failed reaction wheels.) When the hydrazine is expended, the mission will end. So this high elliptical orbit is a very good place to be while NASA and the Dawn project are determining how best to use the spacecraft in the future.

Meanwhile, this anniversary presents a convenient opportunity to look back on a remarkable spaceflight. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the tenth annual summary, reusing text from previous years with updates where appropriate. Readers who wish to investigate Dawn’s ambitious journey in detail may find it helpful to compare this material with the Dawn Journals from its first, second, third, fourth, fifth, sixth, seventh, eighth and ninth anniversaries.

In its 10 years of interplanetary travels, the spacecraft has thrust with its ion engines for a total of 2,109 days (5.8 years), or 58 percent of the time (and 0.000000042 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 908 pounds (412 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sept. 27, 2007. The spacecraft has used 69 of the 71 gallons (262 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space.

Dawn launch
Dawn observed this rugged terrain on Aug. 24, 2016, during its extended mission at an altitude of 240 miles (385 kilometers). The upper crater is Juling (which we saw above), and the one at lower right is Kupalo, which is 16 miles (26 kilometers) in diameter. Although this and the picture above overlap, they were taken more than 27 hours apart, during which Dawn completed five orbital revolutions of Ceres. This scene is at 38°S, 169°E on the map below. We have seen other views of Kupalo and the area around it, most recently on the ninth anniversary of Dawn’s launch. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The thrusting since then has achieved the equivalent of accelerating the probe by 25,400 mph (40,900 kilometers per hour). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.) It is remarkable that Dawn’s ion propulsion system has provided nearly the same change in speed as the entire Delta rocket.

Since launch, our readers who have remained on or near Earth have completed 10 revolutions around the sun, covering 62.8 AU (5.8 billion miles, or 9.4 billion kilometers). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 42.4 AU (3.9 billion miles, or 6.3 billion kilometers). As it climbed away from the sun, up the solar system hill to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It had to go even slower to perform its graceful rendezvous with Ceres. In the 10 years since Dawn began its voyage, Vesta has traveled only 40.5 AU (3.8 billion miles, or 6.1 billion kilometers), and the even more sedate Ceres has gone 37.8 AU (3.5 billion miles, or 5.7 billion kilometers). (To develop a feeling for the relative speeds, you might reread this paragraph while paying attention to only one set of units, whether you choose AU, miles, or kilometers. Ignore the other two scales so you can focus on the differences in distance among Earth, Dawn, Vesta and Ceres over the 10 years. You will see that as the strength of the sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)

Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with even more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. (If you prefer not to skip it, click here.) In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.

Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family (including Earth, Dawn, Vesta and Ceres) follow their individual paths around the sun, they sometimes move closer and sometimes move farther from it.

Dawn launch
Dawn’s interplanetary trajectory (in blue). The dates in white show Dawn’s location every Sept. 27, starting on Earth in 2007. Note that Earth returns to the same location, taking one year to complete each revolution around the sun. When Dawn is farther from the sun, it orbits more slowly, so the distance from one Sept. 27 to the next is shorter. In addition to seeing Dawn’s progress on this figure on previous anniversaries of launch, we have seen it other times as well, most recently in May. (This graphic also will be at a Dawn flight team celebration this afternoon, but it will be in a form that is much more transitory and delectable, although perhaps not much more nutritious, than the way it is displayed here.) Image credit: NASA/JPL

In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth’s orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft may travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body’s orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and part of the arduousness of Dawn’s journey has been changing the inclination of its orbit, an energetically expensive task.)

Now we can see how Dawn has done by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts’ websites when their readership extends to one more elliptical galaxy than ours does.)

The table below shows what the orbit would have been if the spacecraft had terminated ion thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on Sept. 27, 2007, its orbit around the sun was exactly Earth’s orbit. After launch, it was in its own solar orbit.

Minimum distance
from the Sun (AU)
Maximum distance
from the Sun (AU)
Inclination
Earth’s orbit 0.98 1.02 0.0°
Dawn’s orbit on Sept. 27, 2007 (before launch) 0.98 1.02 0.0°
Dawn’s orbit on Sept. 27, 2007 (after launch) 1.00 1.62 0.6°
Dawn’s orbit on Sept. 27, 2008 1.21 1.68 1.4°
Dawn’s orbit on Sept. 27, 2009 1.42 1.87 6.2°
Dawn’s orbit on Sept. 27, 2010 1.89 2.13 6.8°
Dawn’s orbit on Sept. 27, 2011 2.15 2.57 7.1°
Vesta’s orbit 2.15 2.57 7.1°
Dawn’s orbit on Sept. 27, 2012 2.17 2.57 7.3°
Dawn’s orbit on Sept. 27, 2013 2.44 2.98 8.7°
Dawn’s orbit on Sept. 27, 2014 2.46 3.02 9.8°
Dawn’s orbit on Sept. 27, 2015 2.56 2.98 10.6°
Dawn’s orbit on Sept. 27, 2016 2.56 2.98 10.6°
Dawn’s orbit on Sept. 27, 2017 2.56 2.98 10.6°
Ceres’ orbit 2.56 2.98 10.6°

For readers who are not overwhelmed by the number of numbers, investing the effort to study the table may help to demonstrate how Dawn patiently transformed its orbit during the course of its mission. Note that six years ago, the spacecraft’s path around the sun was exactly the same as Vesta’s. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to slip into orbit around Vesta. While simply flying by it would have been far easier, matching orbits with Vesta required the exceptional capability of the ion propulsion system. Without that technology, NASA’s Discovery Program would not have been able to afford a mission to explore the massive protoplanet in such detail. Dawn has long since gone well beyond that. Having discovered so many of Vesta’s secrets, the stalwart adventurer left it behind. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. From 2012 to 2015, the stalwart craft reshaped and tilted its orbit even more so that now it is identical to Ceres’. Once again, that was essential to accomplishing the intricate celestial choreography in which the behemoth reached out with its gravity and tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.

Dawn launch
This map of Ceres has all 138 feature names approved so far by the International Astronomical Union (IAU), including 25 approved last month. (We described the naming convention here.) As more features are named, this official list and map are kept up to date. The dwarf planet is 1.1 million square miles (2.8 million square kilometers). That’s about 36 percent of the land area of the contiguous United States, or the combined land areas of France, Germany, Italy, Norway, Spain, Sweden and the United Kingdom. The scales for horizontal distance in this figure apply at the equator. Rectangular maps like this distort distances at other latitudes. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Even after a decade of daring space travel, flying in deep space atop a blue-green pillar of xenon ions, exploring two of the last uncharted worlds in the inner solar system, overcoming the loss of three reaction wheels, working hard to stretch its shrinking supply of hydrazine, Dawn is ready for more. And so is everyone who yearns for new knowledge, everyone who is curious about the cosmos, and everyone who is exhilarated by bold adventures into the unknown. More is to come. Dawn -- and all those who find the lure of space irresistible -- can look forward to whatever lies ahead for this unique mission.

Dawn is 16,600 miles (26,700 kilometers) from Ceres. It is also 2.92 AU (271 million miles, or 437 million kilometers) from Earth, or 1,080 times as far as the moon and 2.91 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 49 minutes to make the round trip.

Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2017

TAGS: DAWN, CERES, VESTA, ASTEROID BELT

  • Marc Rayman
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Dawn observed this scene on Sept. 28, 2015

Orbiting the only dwarf planet inside the orbit of Neptune, Dawn is healthy and continuing to carry out its assignments at Ceres with the masterful skill to be expected for such an experienced space explorer.

As Earth and Ceres took up positions on opposite sides of the sun for the first part of this month, the probe operated for almost two weeks without being able to count on assistance from its human handlers, even if it encountered a serious problem. The powerful interference of the sun could have prevented radio communications. But Dawn had no need. When the changing geometry allowed the radio silence to break, the ship confirmed that all was well.

Dawn’s primary responsibility in this phase of its mission continues to be monitoring cosmic rays. For eight months in 2015-2016, circling closer to Ceres than the International Space Station is to Earth, the probe measured nuclear radiation that contains the signatures of geologically important elements down to about a yard (meter) underground. Since December, when it reached a much greater altitude, it has been listening to the faint hiss of cosmic rays. Scientists will mathematically remove that from the earlier recordings of Ceres. This procedure will allow them to squeeze even more information out of the low-altitude census of atomic species.

Ceres
Ceres’ largest crater is Kerwan, which fills most of this picture. The crater is 174 miles (280 kilometers) in diameter. That may seem large, and indeed it is large compared to all other Cerean craters and probably also compared to the block you live on and the landscape you can see out your window (unless you happen to have a view of the moon). But as we have discussed before, scientists recognize that Ceres should have even larger craters. Those ancient craters probably were erased by the gradual movement of the ice and rock in the ground as it "relaxes" after a disfiguring impact, just as your skin restores its shape after pressure has been removed. That process is stronger for larger craters and likely contributed to making Kerwan’s features appear softened. Kerwan is noticeably polygonal because the crater walls formed along preexisting underground fractures when the impactor struck, and we will see another example of that below. Dawn took this picture on June 12, 2015, from an altitude of 2,700 miles (4,400 kilometers) in its second mapping orbit. We have seen Kerwan from a different perspective as well as a close-up of one area in it photographed from Dawn’s lowest altitude orbit. (The crater is so large that it took about 50 pictures from low altitude to cover it.) Below is a photo of the center of Kerwan from an intermediate altitude. The crater at the center is Insitor, which is 16 miles (26 kilometers) in diameter. (Insitor was a minor Roman god concerned with sowing crops. Perhaps his being minor is appropriate, as the crater is less than 1 percent of Kerwan’s area.) You can locate Kerwan at 11°S, 124°E on this map. The dark material at the upper right of this picture was blasted out by the impact that formed Dantu Crater, which we will see below. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn had to fly far enough above Ceres that it could measure the cosmic rays alone, rather than the combination of Ceres radiation and cosmic radiation it detected at low altitude. The mission continued to go so well after they had sent the spacecraft to a high altitude, that the team devised more new objectives. To start, they had Dawn photograph some very nice scenes of a gibbous Ceres. Then they guided it through two months of intricate orbital maneuvers, allowing the spacecraft to fly across the line from the sun to Ceres, providing a view of the fully illuminated dwarf planet (like a full moon). In addition to yielding lovely new movies and color pictures, these opposition measurements may help scientists discover details of the material on the ground that would otherwise be impossible to descry from orbit.

That orbit extended so high that it took two months to complete one long elliptical loop around Ceres. The opposition observations worked extremely well, but it’s not a convenient orbit for most other investigations (except the cosmic ray measurements). Therefore, earlier this month, mission controllers instructed the spacecraft to use its ion engine to adjust the orbit again, this time reducing the period for one revolution to 30 days and improving the opportunities for future scientific measurements.

Insitor Crater
Near the center of this picture is Insitor Crater, which is at the center of Kerwan Crater, as we saw above. Insitor is 16 miles (26 kilometers) in diameter and is at 11°S, 125°E on this map. Dawn captured this view from an altitude of 915 miles (1,470 kilometers) in its third mapping orbit on Sept. 23, 2015. Full image (rotated from the one here) and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In coming months, we will look ahead to new observations the team is just beginning to consider. It has not been assured that further activities would be possible. For half of the time since it embarked on its extraordinary extraterrestrial expedition, Dawn has managed to complete its work without the use of the full complement of equipment it was supposed to have at its disposal. Even with the failures of three reaction wheels, however, the mission has far exceeded its original objectives and well outlasted its expected lifetime. Nevertheless, the spacecraft’s lifetime certainly is limited, most likely by the dwindling supply of hydrazine, although possibly instead by one of the many risks that are part of the very nature of conducting complex operations in the unforgiving far reaches of space. For now, however, it appears that Dawn has enough life left in it to warrant pursuing even more new goals.

On July 16, as the sophisticated ship from distant Earth continues to carry out its mission, it will celebrate the 271st birthday of Giuseppe Piazzi, the first person to spot Ceres. It was a faint point of light amid the stars, one tiny jewel among too many to count. When the 54-year-old made his serendipitous discovery, which gave him an honored place in the history of science, he certainly could not have foreseen what Dawn has now seen. (And there's no reason he should have. He was an astronomer and mathematician, not a clairvoyant.)

Dawn had this view on June 24, 2015
Dawn had this view on June 24, 2015, from its second mapping orbit at an altitude of 2,700 miles (4,400 kilometers). The scene displays quite a variety of geological features. Most salient is Occator Crater, with its deposits of sodium carbonate and other salts shining in reflected sunlight, at upper left (although you probably didn’t need that helpful guidance in order to find it.) Occator is 57 miles (92 kilometers) in diameter. It is the youngest major feature here. The largest crater, to the right and slightly below Occator, is Kirnis. (Kirnis was a Lithuanian god of cherries. It isn’t known whether he collaborated with deities of chocolate or of sundaes.) This 71-mile (115-kilometer) crater is old, as shown by its degraded appearance, gradually worn down by the particles large and small that fall from space. (Kirnis is too small to have been affected much by the movement of the crust that erases craters.) Notice how the crater rim seems to consist of straight segments. That is most evident for the lower rim, where there is a clear overlap with one of the long linear structures in the right half of the picture. When an impact occurs in an area with fractures, the resulting crater may be shaped by them, yielding a similar polygonal structure, even if there is no other evidence of those fractures visible on the ground. We saw that with Kerwan as well. The fractures in this picture are collectively known as Samhain Catenae. (Samhain, meaning "summer’s end," is a Celtic agricultural festival marking the end of summer and beginning of winter. Halloween can trace its origins to Samhain.) A catena is usually a chain of craters (and is a Latin word for chain), but the term also is applied more generally to large grooves that can be formed by a variety of geological processes. Scientists have not yet determined the mechanism responsible for Samhain Catenae. We will see another catena below. Lociyo Crater, well below Occator, is 21 miles (34 kilometers) across. (Although he was the god of lightning, Lociyo is fortunate to qualify for the naming convention for Cerean craters, because he was associated with agriculture. When the Zapotecs, in what is now Oaxaca, Mexico, cut the first chili of the harvest, they would sacrifice it to Lociyo.) The impact that excavated Lociyo obliterated half of an older crater of about the same size. This scene is centered on this map at 2°N, 249°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In addition to revealing Ceres’ overall appearance, Dawn has acquired a wealth of pictures and other information that scientists are now actively studying. The mission has shown us mesmerizing bright regions and an extensive network of ground fractures in Occator Crater. The shapes and sizes of many craters provide intriguing clues about the strength and other properties of the interior, and the measurements of the gravity field yield still more insight into the inside. The towering cryovolcano Ahuna Mons rises up as a compelling monument to internal geological forces (which we will discuss below). Organic chemicals spotted in and near Ernutet Crater and elsewhere are of special interest for astrobiology. We see ice on the ground and have determined there is a tremendous amount underground (and there may be liquid underground as well). Piazzi discovered -- and Dawn uncovered -- a truly alien world, and its vastness and diversity are part of what make it so fascinating.

Among the minerals Dawn has found is a group known as carbonates, and they are abundant on Ceres. We see two types there. One, which is omnipresent, is known as dolomite and contains calcium and magnesium. It is mixed with another Cerean mineral, serpentine. A different type of carbonate is prominent in Occator Crater. The sodium carbonate there reflects so much sunlight that it seems almost to be luminous, like a giant spotlight casting its brilliance far out into space, perhaps to show off that it contains the highest concentration of any kind of carbonates known anywhere in the solar system except Earth. Occator’s specific kind, sodium carbonate, has been observed only on Earth and in the plumes of Saturn’s watery moon Enceladus. Interestingly, the carbonates and serpentine are formed by chemical reactions between rocks and water under high pressure. How could these minerals be both widespread and exposed?

One possibility is that they formed deep underground and were later pushed to the surface by internal geological processes. Just as on Earth, those internal forces are mostly powered by heat from the decay of radioactive elements. The heat is carried away by the motion of the material, just as heating water at the bottom of a pot causes it to rise and then make complex convection patterns. The strength of the forces depends on the rate at which the heat leaks from the deep interior to the ground. That is, heat is a form of energy, and a faster flow of heat energy (and thus of material) would provide a more powerful internal engine to drive minerals to the surface.

Heat flows from hot (far underground) to cold (the surface, which is exposed to space). It is at least 80 degrees Fahrenheit (50 degrees Celsius) colder near Ceres’ north and south poles than near the equator. That means the strength of the geological pressure pushing minerals to the surface should depend on the latitude, which would translate into different compositions at different latitudes. But that is not what Dawn sees. The minerals show up everywhere we look. Their prevalence is a fact that is inconsistent with a deep underground origin followed by a heat-driven movement to the surface. Science tells us we need to formulate a different explanation for why minerals produced in water under high pressure now can be found on the ground.

Scientists recognize a more likely explanation. The minerals may have formed in an ocean early in Ceres’ history, when radioactive elements were so abundant that it would have been warm enough to keep a large volume of water as a liquid. But as Ceres aged, it would have cooled (perhaps some readers have experienced this as well), because the supply of radioactive elements would have gradually been depleted as they decayed. Almost the entire ocean would have frozen, encasing Ceres in a shell of ice. But that wouldn’t be the end of the story.

Ice cannot last long on Ceres (except in special places). Cold though it is on that world, there is enough warmth from the distant sun that ice sublimates, turning from a solid into a gas as the water molecules escape into space. Even as that gradual phenomenon occurred at the microscopic level, ice was lost through a much more dramatic and abrupt process. It was blasted away by asteroids that slammed into it. The rain of rocks that fall onto Ceres over millions of years is a familiar hazard to anyone who has lived in the main asteroid belt for millions of years. In fact, scientists estimate that a frozen ocean three miles (five kilometers) thick could have been lost in only a few tens of millions of years, a blink in geological time. (And even if that ice shell had been much thicker, it would still have been lost on a geologically short timescale.)

Yalode crater
Dawn took this picture of a part of Yalode Crater from an altitude of 915 miles (1,470 kilometers) in its third mapping orbit on Sept. 27, 2015. At 162 miles (260 kilometers) across, Yalode is the second largest crater on Ceres (and too large to be captured with a single picture even from this high). Note the distinctly polygonal craters, including the largest one on the right, Lono, which is 12 miles (20 kilometers) wide. Below Lono is Besua Crater, with a diameter of 11 miles (17 kilometers). Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Before it froze and dispersed, chemical reactions between the water and rocks would have produced a rich inventory of minerals. As Dawn peers down from its orbital perch, it sees their testimony to that long-lost ocean. And even now there may still be reservoirs of liquid within Ceres, as it is warm enough inside.

None of this could have been imagined by Piazzi on the night he first glimpsed Ceres from his observatory in Sicily. Because he wasn’t prescient, he also did not expect that what he discovered would be known at times as a planet, an asteroid, a dwarf planet and eventually as "home" by Dawn. Nor would he have anticipated the Tunisian-Sicilian War, the extraordinary intellectual achievements in the scientific discoveries of evolution, relativity and quantum mechanics, or the inventions of the safety pin, granola, integrated circuits and remotely controlled interplanetary spacecraft. If Piazzi thought seriously about the unique successes of science or about the nature of exploration, he did not leave much of a record.

The largest crater here is Dantu
The largest crater here is Dantu, 78 miles (126 kilometers) in diameter. We have seen other views of this impressive landscape, most recently here. Dawn took this picture on Sept. 24, 2015, from an altitude of 915 miles (1,470 kilometers) in its third mapping orbit. Some of the material ejected by the violent excavation of Dantu is visible in the photo of Kerwan Crater above. This scene is centered on this map at 22°N, 133°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

For the perspective of someone who did, let’s go back to a time before Piazzi’s 1801 sighting of Ceres but after the dwarf planet’s formation nearly 4.6 billion years ago. Sometime between 1607 and 1620, the polymath and early champion of modern science Francis Bacon wrote this in Cogitata et Visa (Thoughts and Conclusions):

    It would disgrace us, now that the wide spaces of the material globe, the lands and seas, have been broached and explored, if the limits of the intellectual globe should be set by the narrow discoveries of the ancients. Nor are those two enterprises, the opening up of the earth and the opening up of the sciences, linked and yoked together in any trivial way. Distant voyages and travels have brought to light many things in nature, which may throw fresh light on human philosophy and science and correct by experience the opinions and conjectures of the ancients.

Bacon realized that archaic ideas had such a tight grip that they prevented the expansion of Europe’s intellectual horizons. The startling and exciting discoveries of the explorers who pushed the physical horizons during the century or so that preceded his writings broke that suffocating squeeze. New realizations about the reality of the natural world, and how dramatically it differed from the untested notions of old, inspired an ardor for intellectual exploration as daring and vigorous as what had been undertaken in traversing those distant lands and seas.

The reward has been discoveries by Piazzi and uncounted other scientists who have revealed the staggering richness of nature in all its forms, a universe of such majesty, such beauty, such complexity that it would seem to defy explanation. And yet science not only uncovers myriad mysteries but also lifts the veil, revealing inner workings and showing us why things are the way they are. The ultimate rewards of science are knowledge and understanding.

Dawn is both a beneficiary of and a contributor to the extraordinary successes of science since Bacon’s time. The mission’s "distant voyages and travels have brought to light many things in nature." And its exploration of alien lands and its journeys on interplanetary seas continue to "throw fresh light on human philosophy and science." The real beneficiaries are we ourselves. How fortunate we all are to behold what that light has illuminated!

Dawn is 20,000 miles (32,200 kilometers) from Ceres. It is also 3.67 AU (341 million miles, or 549 million kilometers) from Earth, or 1,400 times as far as the moon and 3.61 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and one minute to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT June 30, 2017

TAGS: DAWN, NEPTUNE, CERES

  • Marc Rayman
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Ceres

On the other side of the solar system, invisible by virtue both of the blinding glare of the sun and by the vastness of the distance, Dawn is continuing its remarkable cosmic adventure.

Orbiting high above dwarf planet Ceres, the spacecraft is healthy and performing all of its assignments successfully even when confronted with what appears to be adversity.

In the last four Dawn Journals, we described the ambitious plans to maneuver the craft so it would cross the line from the sun to Ceres on April 29 and take pictures plus infrared and visible spectra from that special perspective. With Dawn between the sun and Ceres, the alignment is known as opposition, because from the spacecraft’s point of view, Ceres is opposite the sun.

As explained in March, those opposition measurements may provide clues to the nature of the material on the ground with much greater detail than the camera or other sensors could ever discern from orbit. The veteran explorer carried out its complex tasks admirably, and scientists are overjoyed with the quality of the data.

Ceres at opposition from Sun
On April 29, Dawn watched a fully illuminated Ceres rotating on its axis for a little more than three hours. (One Cerean day, the time to complete one full rotation, is nine hours. Because Ceres turns faster than Earth, this movie spans what would be the equivalent of nearly nine hours of Earth rotation.) The spacecraft was about 12,000 miles (20,000 kilometers) high when it witnessed this scenery at opposition. Cerealia Facula and Vinalia Faculae in Occator Crater look like a pair of bright beacons casting their reflected sunlight back into the cosmos. Occator is on this map at 20°N, 239°E, and you can use it as a reference to identify other features. It is worth noting that Ceres appears somewhat washed out here compared to all the pictures we have seen of it, despite a slight enhancement of the contrast. The reason is that we are looking along the same direction as the incoming light, so shadows have mostly disappeared. This phenomenon is known as shadow hiding. With nearly uniform illumination and no shadows visible, the principal variations in how bright or dark Ceres appears are a result of intrinsic differences in the material on the ground, such as composition or texture. (Differences are more evident in the color picture below.) Full movie and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The flight team had worked out a plan to provide a backup opportunity to study Ceres at opposition on June 28. The results of the April 29 observations are so good, however, that the backup was deemed unnecessary and so has been canceled. In this phase of Dawn’s mission, the highest priority continues to be recording cosmic rays so scientists can improve their measurements of the atomic constituents down to about a yard (meter) underground.

Dawn’s latest success followed less than a week after what might have seemed to some people to be a very serious problem. Indeed, in other circumstances, it could have been devastating to the mission. Fortunately, the expert team piloting this spaceship was well prepared to steer clear of any dire scenarios.

On April 23, reaction wheel #1 failed. This was Dawn’s third incident of losing a reaction wheel. (In full disclosure, the units aren’t actually lost. We know precisely where they are. But given that they stopped functioning, they might as well be elsewhere in the universe; they don’t do Dawn any good.) Reaction wheels are disks that spin to help control the orientation of the spacecraft, somewhat like gyroscopes. By electrically changing a wheel’s speed (as high as 75 revolutions per second), the spacecraft can turn or hold steady.

We have discussed Dawn’s reaction wheels many times, and reaction wheel enthusiasts are encouraged to review the detailed history by rereading the last 275,000 words posted. But because this is the last time we will ever need to discuss them, we will summarize the entire story to its conclusion here.

Ceres at opposition from Sun
This view of Ceres at opposition is made from pictures Dawn took on April 29 from an altitude of about 12,000 miles (20,000 kilometers) with the color filters in its primary camera. (The color pictures from the backup camera are essentially the same.) The colors are enhanced to bring out subtle differences in the composition or texture our eyes would not detect. Bluish material tends to be younger. (We saw that here as well.) As in the rotation movie above, Occator Crater is the most salient feature, and you can use its location at 20°N, 239°E as a reference on this map to find other sites. Notice that the bright crater is adjacent to an unusually dark area. The dark material was excavated and ejected when Occator formed by the powerful impact of an asteroid. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The wheels do not help propel Dawn through space. The ion propulsion system does that (and, by the way, does it amazingly well). The wheels are used to rotate the spacecraft around its three axes, which can be called pitch, roll and yaw; x, y and z; left-right, front-back and up-down; Kirk, Spock and McCoy; animal, vegetable and mineral; or many other names. Regardless of the designations, three wheels are needed because there are three dimensions of space. Always conservative, designers equipped Dawn with four wheels. On a nearly decade-long interplanetary odyssey to well over one million times farther from Earth than astronauts can travel, the probe was designed with enough spare hardware to tolerate the loss of almost any component, including a reaction wheel. (The spacecraft is also outfitted with a backup radio receiver, radio transmitter, central computer, ion engine, camera, heaters, valves and on and on.)

One reaction wheel failed in June 2010, about a year before Dawn arrived at its first destination, Vesta, the second largest body orbiting the sun between Mars and Jupiter. A second one failed in August 2012 as Dawn was escaping from Vesta, having far surpassed its objectives in exploring the protoplanet. (That second failure is so long ago, that now, for half of its time in space, Dawn has not had three operable wheels, despite the intent of its cautious designers.)

The flight team was able to overcome the loss of the two reaction wheels, even though that had never been planned for (nor even considered) when the spacecraft was being designed and built. It required not only a great deal of work but also exceptional ingenuity and diligence. That heroic effort paid off very handsomely in allowing the spacecraft to continue its ambitious deep-space expedition, trekking for 2.5 years from Vesta to Ceres and then conducting a comprehensive study of that dwarf planet, the first one humankind had ever seen. Dawn exceeded all of its goals and successfully concluded its prime mission in June 2016. And even with the malfunctions of two reaction wheels, the team kept the spacecraft so healthy and productive that it is now conducting an extended mission, gathering even more riches at Ceres.

There was no basis for predicting when another wheel would fail, but it was widely considered to be only a matter of time. Because the four wheels are of the same design, and some had failed on other spacecraft as well, confidence that the two remaining wheels would function for long was low. Indeed, your faithful correspondent, in his technical role on Dawn, occasionally referred to the "two failed wheels and two doomed wheels."

When the spacecraft reported on April 24 that another wheel had failed, no one on the team was very surprised. In fact, the biggest surprise was that the two doomed wheels had continued to operate as long as they did after the other two stopped.

Navigation picture 1
Dawn had this view on May 16 from an altitude of 26,400 miles (42,500 kilometers). Most of the terrain beneath the orbiting spacecraft was on the night side of the dwarf planet, leaving only a narrow crescent illuminated. To get an idea of where Dawn was relative to Ceres and the sun, look at this figure. The large green ellipse is the current orbit, which Dawn flew to in order to observe Ceres at opposition on April 29. Orbiting clockwise, the spacecraft was at about the 4:00 position from Ceres (remember, the sun is on the left in that figure) when it captured this scene. Dawn took this and similar pictures to help navigators refine their measurements of its orbital position, as explained here and below. Visible at the left is Zadeni Crater. Zadeni is 80 miles (128 kilometers) in diameter and is on this map at 70°S, 39°E. (Zadeni is thought to have been a god of fruitfulness for the ancient Georgians, but the details are murky because that information is based on medieval records.) The larger crater on the right is Urvara, which we have seen a number of times from different altitudes, most recently last month. (If you try to compare the craters’ positions on the map with this scene, the perspective here deep in the southern hemisphere may prove a bit confusing.) An earlier photo of Zadeni from a lower altitude is below, and another May 16 navigation photo is below that. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The strategy for recovering from each of the two earlier failures and preparing for another was complex and multifaceted. Let’s recall just a few aspects.

Dawn carries a small supply of conventional rocket propellant called hydrazine, expelled from small jets of the reaction control system. (Yes, Dawn has a full set of backup jets.) The reaction wheels occasionally need a little bit of hydrazine help, and that is why the reaction control system is onboard. (For propulsion, it is far less efficient than the ion propulsion system, and Dawn has never used hydrazine for that purpose.) In principle, the reaction control system could do the job of the reaction wheels, but that would require a great deal more hydrazine than Dawn carried when it left Earth. Indeed, the reason for reaction wheels is that they control the orientation for much less mass. Well, to be more precise, they control the orientation when they work. When they fail, they don’t do as well. The flight team invested a tremendous effort in stretching the hydrazine so it could be used in place of the wheels, and that has proven to be extremely successful. In fact, Dawn arrived at Ceres ready to complete its mission here with zero wheels in case a third wheel was on the verge of failing.

The amount of hydrazine Dawn uses depends on its activities. Whenever it fires an ion engine, the engine controls two of the three axes, significantly reducing the consumption of hydrazine. In orbit around Vesta and Ceres, the probe often trains its sensors on the alien landscapes beneath it. The lower the orbital altitude, the faster the orbital velocity, so Dawn needs to turn faster to keep the ground in its sights. Also, the gravitational attraction of these massive worlds tends to tug on the unusually large solar arrays in a way that would turn the ship in an unwanted direction. (For more on this, see here.) That force is stronger at lower altitude, so Dawn needs to work harder to counter it. The consequence is that Dawn uses more hydrazine in orbit around Vesta and Ceres than when it is journeying between worlds, orbiting the sun and maneuvering with its ion engine. And it uses more hydrazine in lower orbits than in higher ones. Following the first reaction wheel problem, mission controllers decided to hold the wheels in reserve for the times that they would be most valuable in offsetting hydrazine use.

Zadeni Crater
Dawn snapped this picture of Zadeni Crater at an altitude of 920 miles (1,480 kilometers) on Oct. 18, 2016. Dawn was in its second extended mission orbit then. We saw Zadeni higher up (both in altitude and in this Dawn Journal), but here it fills the frame. As we discussed here, the many craters on and in Zadeni indicate it is relatively old. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

From August 2010 to May 2011, the spacecraft flew with the one failed wheel and the three healthy (but doomed) wheels all turned off. As it approached Vesta, controllers reactivated the three wheels, and they served well for almost all of Dawn’s work there. The second malfunction occurred in August 2012 as Dawn was ascending on its departure spiral, and the spacecraft correctly deactivated all of them and reverted to hydrazine control even before radioing the news to distant Earth. The wheels had been scheduled to be turned off again shortly after Dawn pulled free of Vesta, so the team decided to leave them off then and complete the escape without reaction wheels. They were not used again (except for four brief periods) until 1.2 billion miles (1.9 billion kilometers) later, in December 2015, when Dawn reached its lowest altitude orbit around Ceres.

At Ceres, of course, only two reaction wheels were operable, and Dawn was not designed to use fewer than three. But the day after the first reaction wheel problem occurred in 2010, engineers at JPL and Orbital ATK (back then, it was Orbital Sciences Corporation) began preparing for another failure. They started working on a method to control the orientation with two wheels plus hydrazine, a combination known as hybrid control. That would consume less hydrazine than using no wheels, although more than if three wheels were available. Following an unusually rapid development of such complex software for a probe in deep space, the team installed the new capability in Dawn’s central computer in April 2011, shortly before Vesta operations began. That software performed flawlessly from December 2015 until the third reaction wheel failed last month.

The team determined in 2010 that the benefits of operating the spacecraft with only one wheel would not justify the investment of effort required. So now that three have failed, the last operable wheel is turned off, and it will never be used again. But as we saw above, the team has a great deal of experience flying Dawn with no wheels at all. They had piloted the ship in that configuration through the solar system and around Ceres for a total of four years, so they were well prepared to continue.

Navigation picture 2
Dawn took this navigational photograph on May 16 from an altitude of 26,400 miles (42,600 kilometers). We’ll get to the real importance in a moment, but let’s cover the technical details first. This picture was taken 20 minutes after the one above. The perspective is nearly identical, but Ceres has rotated so scenery has shifted slightly. (As we discussed with the movie above, 20 minutes on Ceres would be the equivalent of 53 minutes of Earth rotation.) In the time between these two pictures, Dawn progressed 24 miles (39 kilometers) in its slow, high orbit. (Some readers may have noted that the altitude at the beginning of this caption differs by 100 kilometers from the altitude given for the previous navigation image. This writer rounds the values to the nearest multiple of 100.) With their accurate maps constructed from Dawn’s earlier observations, navigators analyzed the precise location of landmarks in each picture to help establish where Dawn was at the moment the photo was taken. They then plotted Dawn’s successive positions to refine their knowledge of its orbit. For technical reasons, the orbit is more difficult to measure at this high altitude than closer to Ceres. Without these pictures, navigators would know the ship’s position to an accuracy of about three miles (five kilometers). The pictures allowed them to reduce that uncertainty to about 700 feet (200 meters). Perhaps more important than the navigational application is that these May 16 pictures show Dawn’s final view of Ceres in its one-year extended mission. This image serves as a reminder that the nature of a distant, alien world can be elusive, like a small, thin crescent, with most of the secrets veiled by an impenetrable cloak of darkness. But since early 2015, Dawn has scrutinized this dwarf planet and produced an exquisitely detailed, intimate portrait of what was for two centuries little more than an indistinct dab of light on the inky black canvas of space. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

With the third wheel failure, we can be grateful that each wheel provided as much benefit as it did. The wheels allowed Dawn to conduct extremely valuable work while using the hydrazine very sparingly. Now that we are finished with the wheels, the members of the flight team are not despondent, dear reader, and you shouldn’t be either. Dawn can continue to operate until the hydrazine is depleted or some unforeseen problem arises. But risks are the nature of venturing into the forbidding depths of space. For now, Dawn has life left in it. Next month we will describe the plans for using the remaining hydrazine.

Less than a week after the third reaction wheel failed, Dawn performed perfectly in collecting all of the planned pictures (using both the primary camera and the backup camera) as well as visible spectra and infrared spectra at opposition. Reaching that special position on the line from the sun to Ceres required two months of intricate maneuvers. By coincidence, another special alignment occurs very soon. This one is called conjunction.

Earth and Ceres follow independent orbits around the sun. Earth carries with it the moon and thousands of artificial satellites. The dwarf planet has one companion, a native of Earth, a temporary resident of Vesta and a resident of Ceres since March 2015.

Because Earth is closer to the sun than Ceres, it is bound by a stronger gravitational leash and so circles faster. Early next month, their separate orbital paths will bring them to opposite sides of the sun. From the terrestrial perspective (shared by some readers, perhaps even including you), the sun and Ceres will appear to be at the same location in the sky. This is conjunction.

schematic of orbit conjunction
Dawn’s location in the solar system is shown on June 5, 2017, when the spacecraft and Ceres will be on the opposite side of the sun from Earth. We have charted Dawn’s progress on this figure before, most recently in November. Image credit: NASA/JPL-Caltech

Communicating with distant interplanetary spacecraft is not easy. (Surprise!) It is even more difficult near conjunction, when the radio signals between Earth and the spacecraft travel close to the sun on their way. The solar environment is fierce indeed, and the stormy plasma that surrounds the star interferes with the radio waves, like hot, turbulent air making light shimmer. Communications will be unreliable from May 31 to June 12. Even though some signals may get through, mission controllers can’t count on hearing from the spacecraft or contacting it. But they are confident the stalwart ship will manage on its own, executing the instructions transmitted to it beforehand and handling any problems until Earth and Ceres are better positioned for engineers to provide any help. Occasionally Deep Space Network antennas, pointing near the sun, will listen amid the roaring solar noise for Dawn’s faint whisper, but receiving any crackling messages will simply be a bonus. In essence, conjunction means radio silence.

Dawn’s proximity to the sun presents a convenient opportunity for terrestrial observers to locate Dawn in the sky. On June 5-6, it will be less than one solar diameter from the sun. Ceres does not orbit the sun in the same plane as Earth, so it does not always go directly behind the disk of the sun. The spacecraft and dwarf planet will be a little bit south of the sun.

If you hold three fingers (preferably your own) together at arm’s length and block the sun any time from June 1 to 10 (and you are encouraged to do so), you will also cover Dawn. From June 3 to June 8, you can cover the dazzling celestial signpost and Dawn at the same time with your thumb.

Dawn is very big for an interplanetary spacecraft (or for an otherworldly dragonfly, for that matter), with a wingspan of nearly 65 feet (19.7 meters). However, it will be 346 million miles (557 million kilometers) away during conjunction, more than 3.7 times as far as the sun.

Dawn Spacecraft
This is an artist’s concept of Dawn. The two wings of solar cells make the spacecraft very large. Nevertheless, when at conjunction, it will be so far away that it will appear comparable to the width of a human hair at a distance of more than 1,000 miles (2,000 kilometers). In other words, the ship is much too far for your eyes to see. It would be better to use your mind’s eye. Even the most powerful telescopes could not detect the spacecraft. For that matter, observing Ceres with a telescope would be difficult at this range. Sunlight makes it impossible, but even if we ignore the overwhelming glare, the dwarf planet would appear about as large as a soccer ball seen from 81 miles (130 kilometers.) It’s a good thing we have a spacecraft there to examine it in such great detail. Image credit: NASA/JPL-Caltech

Those who lack the requisite superhuman (or even supertelescopic) vision to discern the fantastically remote spacecraft through the blinding light of the sun needn’t worry. We can overcome the limitation of our visual acuity with our passion for exploring the cosmos and our burning desire for bold adventures far from home. For this alignment is a fitting occasion to reflect once again upon missions deep into space.

There, in that direction, is Earth’s faraway emissary to alien worlds. You can point right to where it is. Dawn has traveled more than 3.8 billion miles (6.1 billion kilometers) on a remarkable odyssey. It is the product of creatures fortunate enough to be able to combine their powerful curiosity about the workings of the cosmos with their impressive abilities to wonder, investigate, and ultimately understand. While its builders remain in the vicinity of the planet upon which they evolved, their robotic ambassador now is passing on the far side of the extraordinarily distant sun.

The sun!

This is the same sun that is more than 100 times the diameter of Earth and a third of a million times its mass. This is the same sun that has been the unchallenged master of our solar system for more than 4.5 billion years. This is the same sun that has shone down on Earth all that time and has been the ultimate source of much of the heat, light and other energy upon which residents of the planet have depended. This is the same sun that has so influenced human expression in art, literature, mythology and religion for uncounted millennia. This is the same sun that has motivated impressive scientific studies for centuries. This is the same sun that is our signpost in the Milky Way galaxy. Daring and noble missions like Dawn transport all of us well beyond it.

Dawn is 31,600 miles (50,800 kilometers) from Ceres. It is also 3.72 AU (346 million miles, or 557 million kilometers) from Earth, or 1,555 times as far as the moon and 3.68 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and two minutes to make the round trip.

Dr. Marc D. Rayman
5:00 p.m. PDT May 24, 2017

TAGS: CERES, DAWN

  • Marc Rayman
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Orbiting Ceres, a section of the wall of the crater at the top of the picture collapsed, allowing material to flow downhill into the larger Ghanan Crater, only a portion of which is shown.

Dawn has accomplished an extraordinary orbital dance.

Dawn has accomplished an extraordinary orbital dance. It completed the cosmic choreography with the finesse and skill that have impressed fans since its debut in space nearly a decade ago. Dawn’s latest stellar performance with Ceres took two months and four acts. (Although Ceres played an essential role in the performance, it was much easier than Dawn’s. Ceres’ part was to exert a gravitational pull, which, thanks to all the mass within the dwarf planet, is pretty much inevitable.)

In February, we presented a detailed preview of the spacecraft’s extensive orbital maneuvering with its ion engine. Now, like so many of Dawn’s cool plans, that complex flight is more than an ambitious goal. It is real. (And the Dawn project will negotiate with any theme park that would like to turn that or any of our other deep-space feats into rides. Another good candidate is here.)

But there is more to do. The reason for such dramatic changes in the orbit is not to show off the flight team’s prowess in piloting an interplanetary spaceship. Rather, it is so Dawn’s new orbital path will cross the line from the sun to the gleaming center of Occator Crater on April 29. From the explorer’s point of view at that special position, Occator will be opposite the sun, which astronomers (and readers of the last three Dawn Journals) call opposition. Last month we explained the opposition surge, in which photographing the crater’s strikingly bright region, known as Cerealia Facula, may help scientists discover details of the reflective material covering the ground there, even at the microscopic level.

Dawn is multitasking. Even as it was executing its space acrobatics, and when it measures the opposition surge later this week, its most important duty is to continue monitoring cosmic rays. Scientists use the spacecraft’s recordings of the noise from this space radiation to improve the measurements it made at low altitude of radiation emitted by Ceres.

Now that Dawn is on course for opposition, let’s take a look at the observations that are planned. Measuring the opposition surge requires more than photographing Cerealia Facula right at opposition. The real information that scientists seek is how the brightness changes over a small range of angles very near opposition. They will compare what Dawn finds for Cerealia Facula with what they measure in carefully designed and conducted laboratory experiments.

To think about Dawn’s plan, let’s consider a clock. Ceres is at the center of the face with its north pole pointing toward the 12. As in this figure, the sun is far, far to the left, well outside the 9 and off the clock. This arrangement matches the alignment in this figure.

Now let’s put the spacecraft on the tip of the second hand, so it takes only one minute to orbit around Ceres. (In reality, it will take Dawn 59 days to complete one revolution in this new orbit, but we’ll speed things up here. We can also ignore for now that Dawn’s orbit is not circular. That would correspond, for example, to the length of the second hand changing as it goes around. This clock doesn’t have that feature.) If the clock were one foot (30 centimeters) across, Ceres would be a little more than a quarter of an inch (seven millimeters) in diameter, or smaller than a pea. Dawn is at a high altitude now, which is why Ceres is so small on the clock.

With this arrangement, opposition is when the second hand is on the 9 and Occator is pointed in that direction as well, so the sun, spacecraft and crater are all on the same line. All of the opposition surge measurements need to occur within about one second of the 9, and most of them have to be within a quarter of a second of that position. This precision has created quite a challenge to the flight team for navigating to and performing the observations.

Readers have long clamored for more information on clocks in the Dawn gift shops, which we have not addressed in more than three years. (Most, of course, clamor for refunds. For that, please take your clock in person to the refund center nearest you, which usually is near the largest black hole in your galaxy.) We hope the discussion this month has filled that horological void.

Flow on Ceres

Dawn had this view in its third mapping orbit at an altitude of 915 miles (1,470 kilometers). It shows another example of material that flowed on the ground. A powerful impact occurred on the northwest rim of Datan Crater, creating the unnamed 12-mile (20-kilometer) near the top of the picture. The impact melted or even vaporized some material and unleashed a flow that extends south as much as 20 miles (32 kilometers). With a thickness of a few tens of yards (meters), it is not nearly as deep as the flow in the photo above. This scene is at 60°N, 247°E on this map. Dawn obtained more detailed photos of this region from a lower altitude, but this terrain covers such a large area that it’s easier to take it all in with this picture. (We presented an even broader view of this region here.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The problem would be difficult enough if Ceres presented Occator to Dawn as a bright bullseye for the camera, but the dwarf planet is not that cooperative. Rather, like all planetary bodies, Ceres turns on its axis, so even if Dawn managed to hover on the line from the sun to Ceres, Occator would be visible only half the time. The rest of the time, the crater would be on the other side of Ceres, cloaked in the darkness of night (which would compromise a measurement of how much sunlight it reflects) and blocked from Dawn’s view by an opaque dwarf planet 584 miles (940 kilometers) in diameter.

Of course, Dawn can’t hover, and Occator is a moving target that’s not visible half the time. That introduces further complications. As Ceres’ rotation brings Occator from night into day (that is, it is sunrise -- dawn! -- at Occator), the crater will be on the limb from Dawn’s perspective. (Remember, Dawn is aligned with the sun.) The foreshortening would make a poor view for measuring the opposition surge. We need to have the crater closer to the center of the disc of Ceres, displaying its bright terrain for Dawn to see, not near the edge, where Cerealia Facula would appear compressed. (In November we saw a photo of Occator near the limb. When Dawn measures the opposition surge, it will be more than 13 times higher.)

Dawn’s orbit has been carefully designed so the spacecraft will cross the line from the sun to Occator when the crater is along the centerline of Ceres. That will give Dawn the best possible view. At that time, the sun will be as high as it can be that day from Occator’s perspective. Because the crater is at 20°N latitude, and Ceres’ axis is tilted only 4 degrees, the sun does not get directly overhead, but it reaches its highest point at noon.

If that is confusing, think about your own location on your planet. For most terrestrial readers, the sun never gets directly overhead (and for all, there are long stretches of the year in which it does not). But as the sun arcs across the sky from morning until evening, its highest point, closest to the zenith, is at noon. Now think about the same thing from the perspective of being far out in space, along the line from the sun to Earth, looking down on Earth as it rotates. That location will come over the limb at sunrise. (That sunrise is for someone still there on the ground. From your vantage point in space, the sun is behind you and Earth is in front of you.) Then the turning Earth will carry it to the other limb, where it will disappear over the horizon at sunset. The best view from space will be in the middle, at noon. If you have a globe, you can confirm this. Just remember that because of the tilt of Earth’s axis, the sun always stays between 23.5°N and 23.5°S. If it’s still confusing, don’t worry! You don’t need to understand this detail to follow the description of the observation plan, and you may rest assured that the Dawn team has a reasonably good grasp of the geometry.

Landslide photo

Dawn observed this pair of overlapping craters near 50°N, 126°E from an altitude of 915 miles (1,470 kilometers) in its third mapping orbit. A broad landslide reaches as much as nine miles (15 kilometers) northeast from both craters. Flows with characteristics like this are found in many locations on Ceres, taking long paths on shallow slopes outside crater walls rather than inside. In general, they did not form at the time the associated craters did but are the result of subsequent processes. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn’s orbital path is timed to make opposition occur as close as possible to 12:00:00 in the Occator Standard Time zone, and that happens to be 2:46:20 a.m. PDT on April 29. (We are glossing over many complications, but one fortunate simplification in the problem is that Cereans do not use daylight saving time. The Cerean day is only nine hours and four minutes long, but they’re so far from the sun that they don’t even bother trying to save daylight.)

Dawn will photograph Ceres extensively during the brief period around opposition. The spacecraft will be around 12,400 miles (20,000 kilometers) above Ceres, a view that would be equivalent to seeing a soccer ball 15 feet (4.7 meters) away. Occator Crater will be like a scar on the ball less than seven-eighths of an inch (2.2 centimeters) wide. The principal target, Cerealia Facula, would be a glowing pinhead, not even a tenth of an inch (about two millimeters) across, at the center of the crater.

Navigation photo

Dawn took this photo of Ceres on March 28 from an altitude of 30,100 miles (48,400 kilometers) during its long coast to even greater heights. (The trajectory is described here.) Navigators used this and other pictures taken then to help pin down the spacecraft’s position in orbit in preparation for the third period of ion thrusting on April 4-12. (When we described the plan in February, the thrusting was scheduled for April 3-14. Dawn’s orbital trajectory following the two previous thrust segments was so good that not as much thrusting was needed.) Another navigation image taken after that maneuver is below. When Dawn photographs Occator Crater at opposition on April 29, they will be closer together, so Ceres will show up with 2.4 times more detail than here. More significant will be that the sun will be directly behind Dawn, so Ceres will appear as a fully illuminated disc (like a full moon rather than a half moon, or, to be more appropriate for this mission, like a full dwarf planet). This scene is centered at 33°S, 228°E, and most of what’s illuminated here is east of that location on this map. Near the top is Occator Crater, with its famously bright Cerealia Facula appearing as a bright spot. The crater is 57 miles (92 kilometers) across. Just below and to the right of center is the prominent Urvara Crater. At 106 miles (170 kilometers) in diameter, Urvara is the third largest crater on Ceres. We have seen Urvara in much finer detail several times before, most recently in October. To its right is Yalode, the second largest crater, 162 miles (260 kilometers) in diameter. We saw some intriguing details of its geology last month. The picture below includes the largest crater on Ceres. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn has spent a great deal of time scrutinizing Ceres from more than 50 times closer (see this table for a summary, including comparisons with a soccer ball for other altitudes). To accomplish this new goal, however, we don’t need high resolution. There are other technical considerations that require the greater altitude. We have already seen Cerealia Facula in as much detail as Dawn will ever reveal. But thanks to the team’s creativity, we have the possibility of learning about it on a far finer scale than had ever been considered.

As we have discussed before, scientists will study the handful of pixels in each image that contain Cerealia Facula to determine how the brightness changes as the viewing angle changes. Throughout its observations, Dawn will take pictures covering a range of exposures. After all, we don’t know how large or small the surge in brightness will be. The objective is to find out. The plan also includes taking pictures through the camera’s color filters to help determine whether the strength of the opposition surge depends on the wavelength of light. (Coherent backscatter may be more sensitive to the wavelength than shadow hiding.) In addition, the probe will collect visible and infrared spectra. (Dawn’s photos and spectra will capture a great many more locations on Ceres than Cerealia Facula. Indeed, well over half of the dwarf planet will be observed near opposition. The data for all these other locations will provide opportunities for still more valuable insights.)

Navigation photo

Dawn took this photo of Ceres on April 17 from an altitude of 27,800 miles (44,800 kilometers). Like the one above, this was taken to help navigate the spacecraft to opposition. Based on the navigation pictures and other data, the operations team developed a pair of trajectory correction maneuvers to fine tune the orbit. (This maneuvering was depicted in the figures in February as the fourth and final thrusting segment. The spacecraft executed the first with five hours of ion thrusting on April 22. It was scheduled to perform the second with a little less than 4.5 hours on April 23-24, but, as the last update to this Dawn Journal before it was posted, that did not occur. See the postscript.) This scene is centered at 52°S, 110°E, and the landscape in sunlight is to the east on this map. In the upper right is Kerwan, the largest Cerean crater at 174 miles (280 kilometers) in diameter. (We saw a close-up of part of this crater in October.) Kerwan is noticeably polygonal because the crater walls formed along preexisting underground fractures when the impactor struck. The largest crater in the grouping just below and right of center is Chaminuka Crater, which is 76 miles (122 kilometers) across. (Chaminuka was a spirit and prophet among the Shona people in what is now Zimbabwe. He could cause a barren tree to bear food and rain to come during a drought. Chaminuka also could turn into a child, a woman, an old man or even a ball. Despite these talents, there’s no evidence the prophet foretold anything about the geology of Ceres nor ever turned into a crater.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Although observing the opposition surge is a bonus in the extended mission, and not as high a priority as many of Dawn’s other scientific assignments, the operations team has taken extra measures to improve the likelihood of it working. Occasionally the camera experiences a glitch, perhaps from cosmic rays, that temporarily prevents the instrument from taking pictures. Therefore, for the opposition surge, the spacecraft will use both the primary camera and the backup camera. Even with well over 85,000 photos during Dawn’s exploration of Vesta and Ceres, the two cameras have been operated simultaneously only once. That was in February, and the purpose then was to verify that the cameras and all other systems (including spacecraft thermal control, data management and even extensive mission control software on distant Earth) would perform as engineers predicted. That test was successful and helped prepare for this upcoming observation.

The plan to measure the opposition surge on Ceres is complex and challenging, and the outcome is by no means assured. But that’s the nature of most efforts to uncover the universe’s secrets. After all, an expedition to orbit and explore two uncharted worlds that had appeared as little more than pinpoints of light among the stars for two centuries, the two largest bodies between Mars and Jupiter, is complex and challenging, and yet it has accomplished a great deal more than anticipated. The reward for such a bold undertaking is the thrill of new knowledge. But there are also rewards in engaging in the endeavor itself, as the spacecraft transports us far from the confines of our humble planetary residence. Such a journey fuels the fires of our passion for adventure far from home and our yearning for new sights and new perspectives on the cosmos.

Dawn is 17,800 miles (28,700 kilometers) from Ceres. It is also 3.64 AU (339 million miles, or 545 million kilometers) from Earth, or 1,505 times as far as the moon and 3.62 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and one minute to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT April 25, 2017

P.S. Just before this Dawn Journal was to be posted on April 24, when a scheduled telecommunications session began, the flight team discovered that the third of the spacecraft’s four reaction wheels had failed. We have written a great deal about these devices and the team’s extraordinary creativity in conducting an extremely successful mission without a full complement. The unit failed before the final, short period of ion thrusting, and the spacecraft correctly responded by entering one of its safe modes and assigning control of its orientation to the hydrazine thrusters. That meant it could not execute the brief maneuver, which would have changed the speed in orbit by 1.4 mph (2.3 kilometers per hour). The team quickly diagnosed the condition and returned the spacecraft to normal operation (still using hydrazine control) on April 25. They also determined that Dawn’s trajectory is close enough to the original plan that the opposition surge measurements can still be conducted. This experienced group of space explorers knows how to do it without the reaction wheels. (For most of the time since Dawn left Vesta in 2012, including the first year of Ceres operations, all four wheels were turned off. This will be no different.) See this mission status update for additional information. Next month’s Dawn Journal will include this new chapter in the reaction wheel story, the outcome of the attempt to observe the opposition surge and more.

TAGS: CERES, DAWN

  • Marc Rayman
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Yalode -  ts the second largest crater on Ceres.

Now in its third year of orbiting a distant dwarf planet, a spacecraft from Earth is as active as ever. Like a master artist, Dawn is working hard to add fine details to its stunning portrait of Ceres.

In this phase of its extended mission, the spacecraft’s top priority is to record space radiation (known as cosmic rays) in order to refine its earlier measurements of the atomic species down to about a yard (meter) underground. The data Dawn has been collecting are excellent.

As we explained in January, the ambitious mission has added a complex bonus to its plans. The team is piloting the ship through an intricate set of space maneuvers to dramatically shift its orbit around Ceres. They are now about halfway through, and it has been smooth sailing. Dawn is on course and on schedule. (If you happen to be one of the few readers for whom it isn’t second nature to plan how to change a spacecraft’s orbit around a dwarf planet by 90 degrees and then fly it under control of ion engine, last month’s Dawn Journal presents a few of the details that may not be obvious. And you can follow the adventurer’s orbital progress with the regular mission status updates.)

If all goes well, on April 29 the new orbit will take Dawn exactly between the sun and the famous bright region at the center of Occator Crater. Named Cerealia Facula, the area is composed largely of salts. (Based on infrared spectra, the strongest candidate for the primary constituent is sodium carbonate). The probe will be at an altitude of about 12,400 miles (20,000 kilometers), or more than 50 times higher than it was in 2016 when it captured its sharpest photos of Occator (as well as the rest of Ceres’ 1.1 million square miles, or 2.8 million square kilometers). But the objective of reaching a position at which the sun and Ceres are in opposite directions, a special alignment known as opposition, is not to take pictures that display more details to our eyes. In fact, however, the pictures will contain intriguing new details that are not readily discerned by visual inspection. Dawn will take pictures as it gets closer and closer to opposition, covering a range of angles. In each image, scientists will scrutinize the handful of pixels on Cerealia Facula to track how the brightness changes as Dawn’s vantage point changes.

Occator Crater

Dawn took this photo of Occator Crater on Oct. 18, 2016, at an altitude of 920 miles (1,480 kilometers) in extended mission orbit 2. We have seen other views of Occator, from farther, from closer, with exposures optimized for the brightest areas, in color, with the crater on the limb of Ceres and more, but you can never have too many pictures of such a captivating scene. The central bright region is Cerealia Facula, and the collection of others is Vinalia Faculae. (A bright region on a planet is a facula. Here is more on these names.) These are the brightest areas on Ceres. One scenario for how they formed is that underground briny water made its way to the surface through fractures. When the water was on the ground, exposed to the cold vacuum of space, it froze and sublimated (that is, it transformed from a solid to a gas). The dissolved salt was left behind, with sodium carbonate being the likely principal constituent, and that reflective material is what we see here. We will see below that opposition surge measurements may provide evidence to support or modify this scenario. (A recent estimate is that Cerealia Facula may be some tens of millions of years younger than the crater itself. We discussed last year how ages are determined.) Since we can’t have too many views of this exotic scenery, another is below (and it shows the fractures that may have served as conduits for the water). Occator is on this map at 20°N, 239°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

We described the opposition surge, in which the reflected sunlight at opposition may be significantly brighter than it is in any other geometrical arrangement. A few degrees or even a fraction of a degree can make a large difference. But why is that? What is the underlying reason for the opposition surge? What can we learn by measuring it? And is the best cake better than the best candy?

Those are all interesting and important questions. We will address some of them here and leave the rest for your own thorough investigation.

There are at least three separate physical effects that may contribute to the opposition surge. One of them is known as shadow hiding. When the sun shines on the ground, tiny irregularities in the surface, even at the microscopic level, will cast shadows. When you look at the ground, those shadows collectively detract from its overall brightness, even if each individual shadow is too small for you to see. The total amount of light reflected off the ground and into your eyes (or your camera) is less than it would be if every point, no matter how small, were well lit. However, if you look along the same direction as the incoming light, then all the shadows will be hidden. They will all be on the opposite side of those tiny irregularities, out of reach of both the incident light and your sight. In that case, anything you can see will be illuminated, and the scene will be brighter. The figure below is intended to illustrate this phenomenon of shadow hiding (and excluding the caption, the picture is probably worth almost 480 words).

Illustration of Shadow hiding

Illustration of shadow hiding. At the bottom is the ground on Ceres with greatly exaggerated crystals of salt pointing in random directions. (Shadow hiding occurs even with very small grains.) The white dashed lines show light from the sun, and each ray traces the light to the tip of a crystal and then to the point beyond. The solid black lines along the ground and the crystals are in shadow. That is, the incoming light cannot reach those places. Therefore, when Dawn is in the position on the right, looking along the same direction as the incoming light, it cannot see those shadows, because there is no line of sight to those hidden locations. In that special position, where Ceres is at opposition, every point on the ground Dawn sees is lit. When Dawn is in the position on the left, it does have a direct line of sight to some (although not all) shadows, as shown by the black dotted lines. Some of the ground it sees is lit and some is not. The difference between these two perspectives is the shadow-hiding component of the opposition surge. (Remember that these crystals are too tiny for Dawn to discern. One pixel in the explorer’s camera would take in this entire scene, so what matters is the total lit surface here, not the fine details.) Now at location 1, there are crystals that happen to point directly at Dawn when it is on the left, and at location 2, there are crystals that point directly at Dawn when it is on the right. You can see that at opposition, the shadows are hidden for both crystal orientations. But when Dawn is on the left, crystals pointing directly at it don’t provide a fully lit scene. Shadows are still visible. So, shadow hiding does not depend on any special alignment of crystals on the ground. It is the special observing location that matters. In summary, the ground appears brighter to Dawn when it is at opposition than when it is elsewhere. Although all crystals here are the same size, different crystal sizes may yield different shadowing and hence different opposition surge signatures. So, with a good measurement of the opposition surge, the crystal sizes may be determined. The self-portrait at right (biceps not to scale) is provided to illustrate your correspondent’s artistic skills. It should help you calibrate the fine details of the rest of the image. There are many simplifications here. In other words, take this diagram with a grain of sodium carbonate. Image credit: NASA/JPL-Caltech

The opposition surge was first described scientifically in 1887 by Hugo von Seeliger, an accomplished astronomer and highly esteemed teacher of astronomers. He analyzed data collected by Gustav Müller when Earth’s and Saturn’s orbits around the sun brought Saturn into opposition, and the brightness of the rings increased unexpectedly. Seeliger realized that shadow hiding among the myriad particles in the rings could explain Müller’s observations. The opposition surge is occasionally known as the Seeliger effect. (Although astronomers had been observing the rings for more than two centuries by then, a careful scientific analysis to show that the rings were not solid but rather composed of many small particles had only been completed about 30 years before Seeliger’s advance.)

Now astronomers recognize the opposition surge on many solar system bodies, including Earth’s moon and the moons of other planets, as well as Mars and asteroids. In fact, it also occurs on many materials on Earth, including vegetation. Scientists exploit the phenomenon to determine the character of materials at a distance when they can make careful measurements at opposition.

For many solar system objects, however, it is difficult or impossible to position the observer along the line between the sun and the target. But thanks to the extraordinary maneuverability provided by Dawn’s ion engine, we may be able to perform the desired measurement in Occator Crater.

3-D Anaglyph of Cerealia Facula

This 3-D image of part of Occator Crater, the brightest area on Ceres, was created with photos from Dawn’s lowest altitude orbit at 240 miles (385 kilometers). The spacecraft took pictures of this scenery from different angles, forming stereo views. To perceive the 3-D, you need colored filters, with red for your left eye and blue for your right. (You can get a 4-D view by looking at it for a while. However, apart from the daily and annual changes in the angle of the incoming sunlight, no changes are expected to be discernible even over a few years.) If you don’t have access to stereo glasses, you can see a more conventional photo here. The bright region on the left, Cerealia Facula, is about nine miles (14 kilometers) across, and the stereo reveals a dome that rises to about 1,300 feet (400 meters). The other bright areas are collectively called Vinalia Faculae. Occator is on this map at 20°N, 239°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

It was nearly a century after Seeliger’s description of shadow hiding before scientists realized that there is another contributor to the opposition surge, which we mention only briefly here. It depends on the principle of constructive interference, which applies more in physics than in politics. Waves (in this case, light waves) that have their crests at the same places can add up to be especially strong (which makes the light bright). (Destructive interference, which may be more evident outside of the physics realm, occurs when troughs of one wave cancel crests of another.) We will not delve into why constructive interference tends to occur at opposition, but anyone with a thorough understanding of classical electromagnetic theory can work it out, as physicists did in the 1960s to 1980s. (More properly, it should be formulated not classically but quantum mechanically, but we recognize that some readers will prefer the former methodology because it is, as one physicist described it in 1968, "much simpler and more satisfying to the physical intuition." So, why make it hard?) For convenient use to ruin parties, the most common term for constructive interference in the opposition surge is coherent backscatter, but it sometimes goes by the other comparably self-explanatory terms weak photon localization and time reversal symmetry. Regardless of the name, as the light waves interact with the material they are illuminating at opposition, constructive interference can produce a surge in brightness.

The intensity of the opposition surge depends on the details of the material reflecting the light. Even the relative contributions of shadow hiding and coherent backscatter depend on the properties of the materials. (While both cause the reflected light to grow stronger as the angle to opposition shrinks, coherent backscatter tends to dominate at the very smallest angles.)

Especially sensitive laboratory measurements show that sometimes shadow hiding and coherent backscatter together are not sufficient to explain the result, so there must be even more to the opposition surge. The unique capability of science to explain the natural world, shown over and over and over again during the last half millennium, provides confidence that a detailed theoretical understanding eventually will be attained.

Part of science’s success derives from its combination of experiment and theory. For now, however, the opposition surge is more in the domain of the former than the latter. In other words, translating any opposition surge observation into a useful description of the properties of the reflecting material requires controlled laboratory measurements of well characterized materials. They provide the basis for interpreting the observation.

Occator’s Bright Spots in 3-D

This short animation shows how the illumination of the northern hemisphere changes as Ceres’ axial tilt changes from 2 to 12 to 20 degrees. (In each frame, the lighting is shown on the summer solstice, when the sun reaches its greatest northern latitude.) We have discussed the orientation of the dwarf planet’s axis before. As we saw, it is tipped only 4 degrees, causing much more modest changes in lighting throughout each Cerean year (which is 4.6 terrestrial years) than Earth (and perhaps your planet) experiences. However, the gravitational tugs of Jupiter and Saturn, despite their distance, tip the axis. The angle can change from as little as 2 degrees to as much as 20 degrees in only about 12,000 years, which astronomers consider to be very fast. (Earth’s axis is tilted 23.5 degrees and is stabilized by the moon. Mars, which lacks a sizable moon, also goes through dramatic changes in axial tilt, although much more slowly than Ceres.) The angle of the sun near the poles is an important factor for where ice might accumulate. The animation shows the regions that would stay in shadow throughout every Cerean day of a full Cerean year, with blue for 2 and 12 degrees and red for 20 degrees. (The blue at 12 degrees is difficult to see.) When the sun goes farther north, it shines deeper into craters, illuminating and warming locations that would remain in shadow if the sun could not rise as high in the sky. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

If Dawn accomplishes the tricky measurements (which we will describe next month), scientists will compare the Cerealia Facula opposition surge with lab measurements of the opposition surge. As always in good science, to establish the details of the experiments, they will start by integrating the knowledge already available, including the tremendous trove of data Dawn has already collected -- spectra of neutrons, gamma rays, visible light and infrared light plus extensive color and stereo photography and gravity measurements. In the context of their understanding of physics, chemistry and geology throughout the solar system, scientists will determine not only the mixtures of chemicals to test but also the properties such as grain sizes and how densely packed the particles are. They will perform experiments then on many combinations of credible facular composition and properties. Comparing those results with Dawn’s findings, they will be able to elucidate more about what really is on the ground in that mesmerizing crater. For example, if they determine the salt crystals are small, that may mean that salty water had been on the ground and sublimated quickly in the vacuum of space. But if the salt came out of solution more slowly underground and was later pushed to the surface by other geological processes, the crystals would be larger.

It is an impressive demonstration of the power of science that we can navigate an interplanetary spaceship to a particular location high above the mysterious, lustrous landscape of a distant alien world and gain insight into some details that would be too fine for you to see even if you were standing on the ground. Using the best of science, Dawn is teasing every secret it can from a relict from the dawn of the solar system. On behalf of everyone who appreciates the majesty of the cosmos, our dedicated, virtuoso artist is adding exquisite touches to what is already a masterpiece.

Dawn is 31,400 miles (50,500 kilometers) from Ceres. It is also 3.48 AU (324 million miles, or 521 million kilometers) from Earth, or 1,430 times as far as the moon and 3.48 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 58 minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT March 30, 2017

TAGS: DAWN, BLOG, JOURNAL, CERES, VESTA

  • Marc Rayman
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Ceres

Dear Pedawntic Readers,

A sophisticated spaceship in orbit around an alien world has been firing its advanced ion engine to execute complex and elegant orbital acrobatics.

On assignment from Earth at dwarf planet Ceres, Dawn is performing like the ace flier that it is.

The spacecraft’s activities are part of an ambitious bonus goal the team has recently devised for the extended mission. Dawn will maneuver to a location exactly on the line connecting Ceres and the sun and take pictures and spectra there. Measuring the opposition surge we explained last month will help scientists gain insight into the microscopic nature of the famous bright material in Occator Crater. Flying to that special position and acquiring the pictures and spectra will consume most of the rest of the extended mission, which concludes on June 30.

This month, we will look at the probe’s intricate maneuvers. Next month, we will delve more into the opposition surge itself, and in April we will describe Dawn’s detailed plans for photography and spectroscopy. In May we will discuss further maneuvers that could provide a backup opportunity for observing the opposition surge in June.

Dawn launch
This image combines several photographs of Ernutet Crater taken through different color filters in Dawn's science camera. (Ernutet was an Egyptian goddess, often depicted with the head of a cobra, who provided food and protected grains by eating pests such as rodents.) The colors have been enhanced to bring out subtle differences in the chemical composition of the material covering the ground that would not be visible to your unaided eye (even assuming your unaided eye were in the vicinity of Ceres). Using data acquired by the spacecraft's infrared mapping spectrometer, scientists have determined that the red regions are rich in organic compounds. The organic molecules are based on chains of carbon atoms and represent a class of chemicals important in biochemistry. Such a finding, along with Dawn's earlier discoveries of ice and other chemicals that likely were formed through interactions with water, makes Ceres very interesting for studies of astrobiology. Nevertheless, future colonists on Ceres would be expected to have little need for protection from native pestilential threats. The 32-mile (52-kilometer) Ernutet Crater is on this map at 53°N, 46°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

First, however, it is worth recalling that this is not Dawn’s primary responsibility, which is to continue to measure cosmic rays in order to improve scientists’ ability to establish the atomic species down to about a yard (meter) underground. Sensing the space radiation requires the spacecraft to stay more than 4,500 miles (7,200 kilometers) above the dwarf planet that is its gravitational master. The gamma ray and neutron detector will be operated continuously as Dawn changes its orbit and then performs the new observations. The ongoing high-priority radiation measurements will not be affected by the new plans.

The principal objective of the orbital maneuvers is to swivel Dawn’s orbit around Ceres. Imagine looking down on Ceres’ north pole, with the sun far to the left. (To help your imagination, you might refer to this figure from last month. As we will explain in May, Dawn’s orbital plane is slowly rotating clockwise, according to plan, and it is now even closer to vertical than depicted in January. That does not affect the following discussion.) From your perspective, looking edge-on at Dawn’s orbit, its elliptical path looks like a line, just as does a coin seen from the edge. In its current orbit (labeled 6 in that figure), Dawn moves from the bottom to the top over the north pole. When it is over the south pole, on the other side of the orbit, it flies from the top of the figure back to the bottom. The purpose of the current maneuvering is to make Dawn travel instead from the left to the right over the north pole (and from the right to the left over the south pole). This is equivalent to rotating the plane of the orbit around the axis that extends through Ceres’ poles and up to Dawn’s altitude. From the sun’s perspective, Dawn starts by revolving counterclockwise and the orbit is face-on. We want to turn it so it is edge-on to the sun.

That may not sound very difficult. After all, it amounts mostly to turning right at the north pole or left at the south pole. Spaceships in science fiction do that all the time (although sometimes they turn right at the south pole). However, it turns out to be extremely difficult in reality, not to mention lacking the cool sounds. When going over the south pole, from the top of the figure to the bottom, the spacecraft has momentum in that direction. To turn, it needs to cancel that out and then develop momentum to the left. That requires a great deal of work. It is energetically expensive. Fortunately, the ever-resourceful flight team has an affordable way.

As we discuss this more, we will present three diagrams of the trajectory. It may be challenging to follow Dawn’s three-dimensional motion on two-dimensional figures, especially if you are not accustomed to reading such depictions. Don’t worry! The team has it all under control, and it works. But consider that however complicated the figures seem, designing and flying the maneuvers is somewhat more complicated. Nevertheless, if you want to try, it might help to try to reproduce Dawn’s movements with your finger as you read the text and study the illustrations. (And if the figures are not helpful for understanding the trajectory, they may at least serve as fun optical illusions, as they did for one member of the test audience.)

Diagram
This shows how Dawn is changing its orbit in order to accomplish the opposition surge measurements. The perspective here is close to that of the figure from last month but shifted a little away from the north pole so you don’t see the orbit exactly along the edge. (As noted above, Dawn’s orbit has rotated slightly and is now more vertical than shown in January.) With the sun far to the left, the spacecraft starts in the vertical green orbit (known as extended mission orbit 3, or XMO3). When it is just to the left of Ceres, it is over the south pole, farther from you than the plane of the figure and traveling toward the bottom. Then the orbit takes it through your monitor, and it is closer to you as it skirts to the right of Ceres, over the north pole. The blue (which we’ll get to in a moment) obscures the right half of that green ellipse. The horizontal green orbit is the destination, and the plus sign shows where Dawn will be when it conducts the new observations. At that point, it will be on the line from Occator Crater to the sun. To maneuver to that new orbit, Dawn will follow the blue trajectory, thrusting with its ion engine where the trajectory is solid and coasting where it is dashed. As explained in the text in more detail, the spacecraft uses the first two thrusting segments (the solid vertical sections) to raise its orbital altitude. After the second one, Dawn’s orbit carries it to greater and greater heights. As it flies the arc at the top of the picture, it is receding from you, on the other side of the plane of this diagram, beyond your computer screen. It is not yet at its highest altitude, although it appears that way here because of the foreshortening of a two-dimensional figure. It is still ascending. When it does reach its highest altitude, it executes the third thrusting segment to accomplish the turn. Then with one more short thrust period (on the left of the figure), it reaches the desired new orbit. Dawn is flying north (and approaching you) when it reaches the plus sign. The two figures below show the same trajectory from different perspectives. Image credit: NASA/JPL-Caltech

Suppose you are driving from north to south and want to turn east at an intersection. You have to decrease your southward (forward) velocity somehow; otherwise, you will continue moving in that direction. You also have to increase your eastward (left) velocity, which initially is zero. That means putting on the brakes and then turning the wheel and reaccelerating, which takes work. (If you’re a stunt driver in the movies, it also may mean making smoke come out near the tires.) With your car, there are two major forces at work: the engine and the friction between the wheels and the road. For a spacecraft, the forces available are the propulsion system and the gravity of other bodies (like moons). Ceres’ only moon is Dawn itself, and there are no other helpful gravitational forces, so it’s all up to the probe’s ion engine.

Dawn was not built to perform these new maneuvers. The main tank and the xenon propellant loaded in it shortly before the spacecraft launched from Cape Canaveral did not account for such an addition to the interplanetary itinerary. The plan was to travel from Earth past Mars to Vesta, enter orbit and maneuver around the protoplanet, then break out of orbit and travel to Ceres, slip into orbit, and maneuver there. Dawn has now done all that with great distinction and already moved around more while orbiting Ceres than originally planned. Indeed, the mission has accomplished far, far more propulsive flight than any other, but now its xenon supply is very low. Navigators needed an efficient way to swivel the spacecraft’s orbit, and that meant finding an efficient way to change the direction of its orbital motion.

An orbit is the perfect balance between the inward tug of gravity and the fundamental tendency of free objects to travel in a straight line. Orbital velocity thus depends on the strength of the gravitational pull. At low altitude, orbiting objects travel faster than at higher altitude. (We have considered this topic in some detail, including with examples, several times before.) Dawn is flying to a very high altitude, where Ceres’ grip will not be as strong so the orbital velocity will naturally be much lower and therefore easier to change. Then it will turn left and swoop back down for the photo op. Any hotshot spaceship pilot would be proud to fly the same profile.

Ceres
Dawn took this photo of Ceres on Feb. 11 from XMO3 at an altitude of about 4,700 miles (7,500 kilometers). Most striking are the reflections from Cerealia Facula (the brightest region, at the center of the crater) and Vinalia Faculae (the grouping to the right), sodium carbonates concentrated in Occator Crater. The salt was left behind when the water it had been dissolved in sublimated. Sodium carbonates have been found at only three solar system bodies: Ceres, Earth, and Saturn’s moon Enceladus. Visible in profile on the limb at the right, only slightly higher in the picture than Occator, is the cryovolcano Ahuna Mons. From this distance, it is not very prominent, but the towering mountain is the tallest structure on the dwarf planet. You can locate this scene on this map using these two features. Occator is at 20°N, 239°E, and Ahuna Mons is at 11°S, 316°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In December 2016, Dawn reached extended mission orbit 3 (XMO3), which ranged in altitude between 4,670 miles (7,520 kilometers) and 5,810 miles (9,350 kilometers). Now the spacecraft is climbing, and it will peak at more than 32,000 miles (52,000 kilometers) in early April when it will pivot the orbit almost 90 degrees. It will then glide down to about 12,400 miles (20,000 kilometers) for the targeted observations.

The maneuvering will be conducted in four stages. The first part of the ion powered ascent was Feb. 22-26, and the next will be March 8-12 when the orbital position is optimal. Although the spacecraft will stop thrusting then at an altitude of 8,000 miles (13,000 kilometers), it will have built up so much momentum that it will continue soaring upward for almost a month as Ceres’ gravitational attraction slows its down. (Dawn uses that pull as a means of putting on the brakes to reduce the forward momentum.) A third period of thrusting on April 3-14 at the apex of its arc will accomplish the turn. Dawn will then be in an orbit that will intersect the line between Occator Crater and the sun on April 29. (After turning, Dawn allows Ceres to do the work of accelerating it, as gravity brings the ship back down.)

Trajectory XMO4-eq_1
This figure (and the one below) shows Dawn’s trajectory from high above Ceres’ equator to provide a different view from the figure above. North is at the top. The sun is now far behind you and off to your left a little. (Congratulations on moving so far in the short time since viewing the previous figure.) The smaller green orbit is XMO3, in which Dawn orbited counterclockwise in the plane of this diagram. Ion thrusting is shown in solid blue, and the dashed segments of the trajectory are coasting. The maneuvers began on the right side as the spacecraft was heading north. The first two thrust periods propel the ship to higher orbital altitudes. Far above the south pole, the third thrust segment, at the bottom of the figure, swivels the orbit so Dawn flies out of the screen toward you. Following the fourth thrust period to fine tune its path as it travels to the north, the spacecraft settles into the new green orbit, and when it gets to the plus sign, it is exactly on the line from Occator Crater to the sun. Image credit: NASA/JPL-Caltech
Figure depicts a trajectory above the equator
Like the figure immediately above, this depicts Dawn’s trajectory from a vantage point far above the equator, again with north at the top. Having shifted your position again, now, as in the first trajectory figure above, the sun is far to the left, not behind you, so you see XMO3 (the inner green orbit) almost edge-on. Just to the right of Ceres, Dawn is closer to you than the plane of the figure and is traveling toward the top (north). As in the first figure, part of the green ellipse is blocked by the blue. As described in the text and the other figures, Dawn uses its ion engine initially to raise its altitude above Ceres, then it turns when it crests far over the south pole (bottom). In the long vertical dashed section, the last arc before the turn, Dawn is flying south on the other side of the plane of the diagram. Its new orbit is the large green ellipse, and as the spacecraft flies north on its clockwise progression, it will measure the opposition surge at the plus sign. Image credit: NASA/JPL-Caltech

This complex flight plan is different from all the prior powered flight, both at Vesta and at Ceres. Most of the orbit changes have been lovely spirals, and the ship rode the gravitational currents at Vesta to shift the orbital plane by a much smaller angle than it is working on now. Some of the graceful steps in this new choreography are especially delicate and require exquisite accuracy to reach just the right final trajectory. For the first time in almost two years, the spacecraft will need to take pictures of Ceres for the express purpose of helping navigators plot its progress. (In the intervening time, Dawn has taken more than 55,000 photos specifically to study the dwarf planet. Many of them also have been used for navigation.) Combining these "optical navigation" pictures with their other navigational techniques, the team will design a final, fourth stage of ion thrusting for April 22-24 to fine tune the orbit. We have described such trajectory correction maneuvers before. (It’s easier for you to chart the spacecraft’s progress than it is for the Dawn team. All you have to do is read the mission status reports.)

By the time it began ion thrusting last week, Dawn had successfully completed all of its assignments in XMO3. That included three photography sessions. In the last, the spacecraft used the primary and backup cameras simultaneously for the first time in the entire mission. In its extensive investigations of Vesta and Ceres, Dawn has taken more than 85,000 pictures, but all of them had been with only one camera powered on at a time, the other being held in reserve. In April we will discuss the reason for operating differently before leaving XMO3.

Dawn’s adventure has been long and its experiences manifold. In just a few days, the bold explorer will mark its second anniversary of arriving at Ceres. (That’s the second anniversary as reckoned by inhabitants of Earth. In contrast, for locals, the immigrant from distant Earth has been in residence for less than half a Cerean year, although more than 1,900 Cerean days.) In 2011-2012, the probe spent almost 14 months in orbit around the giant protoplanet Vesta, the second largest object in the main asteroid belt. The only craft ever to orbit two alien destinations, it is a denizen of deep space. In its nearly 9.5-year solar system journey, Dawn has traveled 3.7 billion miles (6.0 billion kilometers). For most of this time, the spaceship has been in orbit around the sun, just as its erstwhile home Earth is. Now it has been in orbit around remote worlds for a third of its total time in space. And for you numerologists, March 5 will mark Dawn’s being in orbit around its targets for pi years. (Happy pi-th anniversary.)

Readers on or near Earth who appreciate following such an extraordinary extraterrestrial expedition can take advantage of an opportunity this week to do a little celestial navigation of their own. On March 2, the moon will serve as a helpful signpost to locate the faraway ship on the interplanetary seas. From our terrestrial viewpoint, the moon will move very close to Dawn’s location in the sky. The specifics, of course, depend on your exact location. For many afternoon sky watchers in North America, the moon will come to within about a degree, or two lunar diameters, of Dawn. As viewed by some observers in South America, the moon will pass directly in front of Dawn. For most Earthlings, when the moon rises on the morning of March 2, it will be north and east of Dawn. During the day, the moon will gradually drift closer and, from many locations, pass the spacecraft and the dwarf planet it orbits. The angle separating them will be less than the width of your palm at arm’s length, providing a handy way to find our planet’s emissary. Although Dawn and Ceres will appear to be near the moon, they will not be close to it at all. The distant spacecraft will be more than 1,300 times farther away than the moon by then (and well over one million times farther than the International Space Station) and quite invisible. But your correspondent invites you to gaze in that direction as you raise a saluting hand to humankind’s insatiable appetite for knowledge, irresistible drive for exploration, passion for adventure, and longing to know the cosmos.

Dawn is 7,300 miles (11,800 kilometers) from Ceres. It is also 3.19 AU (296 million miles, or 477 million kilometers) from Earth, or 1,280 times as far as the moon and 3.22 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 53 minutes to make the round trip.

Dr. Marc D. Rayman
4:30 p.m. PST February 27, 2017

TAGS: CERES, DAWN, GAMMA RAY AND NETRON DETECTOR

  • Marc Rayman
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Ikapati Crater

A deep-space robotic emissary from Earth is continuing to carry out its extraordinary mission at a distant dwarf planet.

Orbiting high above Ceres, the sophisticated Dawn spacecraft is hard at work unveiling the secrets of the exotic alien world that has been its home for almost two years.

Dawn’s primary objective in this sixth orbital phase at Ceres (known as extended mission orbit 3, XMO3 or "this sixth orbital phase at Ceres") is to record cosmic rays. Doing so will allow scientists to remove that "noise" from the nuclear radiation measurements performed during the eight months Dawn operated in a low, tight orbit around Ceres. The result will be a cleaner signal, revealing even more about the atomic constituents down to about a yard (meter) underground. As we will see below, in addition to this ongoing investigation, soon the adventurer will begin pursuing a new objective in its exploration of Ceres.

With its uniquely capable ion propulsion system, Dawn has flown to orbits with widely varying characteristics. In contrast to the previous five observation orbits (and all the observation orbits at Vesta), XMO3 is elliptical. Over the course of almost eight days, the spacecraft sails from a height of about 4,670 miles (7,520 kilometers) up to almost 5,810 miles (9,350 kilometers) and back down. Dutifully following principles discovered by Johannes Kepler at the beginning of the 17th century and explained by Isaac Newton at the end of that century, Dawn’s speed over this range of altitudes varies from 210 mph (330 kilometers per hour) when it is closest to Ceres to 170 mph (270 kilometers per hour) when it is farthest. Yesterday afternoon, the craft was at its highest for the current orbit. During the day today, the ship will descend from 5,790 miles (9,310 kilometers) to 5,550 miles (8,930 kilometers). As it does so, Ceres’ gravity will gradually accelerate it from 170 mph (273 kilometers per hour) to 177 mph (285 kilometers per hour). (Usually we round the orbital velocity to the nearest multiple of 10. In this case, however, to show the change during one day, the values presented are more precise.)

As we saw last month, the angle of XMO3 to the sun presents an opportunity to gain a new perspective on Ceres, with sunlight coming from a different angle. (We include the same figure here, because we will refer to it more below.) Last week, Dawn took advantage of that opportunity, seeing the alien landscapes in a new light as it took pictures for the first time since October.

Figure illustrates orbits at Ceres
This illustrates (and simplifies) the relative size and alignment of Dawn’s six science orbits at Ceres. We are looking down on Ceres’ north pole. The spacecraft follows polar orbits, and seen edge-on here, each orbit looks like a line. (Orbits 1, 2 and 6 extend off the figure to the lower right, on the night side. Like 3, 4 and 5, they are centered on Ceres.) The orbits are numbered chronologically. The first five orbits were circular. Orbit 6, which is XMO3, is elliptical, and the dotted section represents the range from the minimum to the maximum altitude. With the sun far to the left, the left side of Ceres is in daylight. Each time the spacecraft travels over the illuminated hemisphere in the different orbital planes, the landscape beneath it is lit from a different angle. Ceres rotates counterclockwise from this perspective (just as Earth does when viewed from the north). So higher numbers correspond to orbits that pass over ground closer to sunrise, earlier in the Cerean day. (Compare this diagram with this figure, which shows only the relative sizes of the first four orbits, with each one viewed face-on rather than edge-on.) Click on this image for a larger view. Image credit: NASA/JPL

Dawn takes more than a week to revolve around Ceres, but Ceres turns on its axis in just nine hours. Because Dawn moves through only a small segment of its orbit in one Cerean day, it is almost as if the spacecraft hovers in place as the dwarf planet pirouettes beneath it. During one such period on Jan. 27, Dawn’s high perch moved only from 11°N to 12°S latitude as Ceres presented her full range of longitudes to the explorer’s watchful eye. This made it very convenient to take pictures and visible spectra as the scenery helpfully paraded by. (The spacecraft was high enough to see much farther north and south than the latitudes immediately beneath it.) Dawn will make similar observations again twice in February.

As Dawn was expertly executing the elegant, complex spiral ascent from XMO2 to XMO3 in November, the flight team considered it to be the final choreography in the venerable probe’s multi-act grand interplanetary performance. By then, Dawn had already far exceeded all of its original objectives at Vesta and Ceres, and the last of the new scientific goals could be met in XMO3, the end of the encore. The primary consideration was to keep Dawn high enough to measure cosmic rays, meaning it needed to stay above about 4,500 miles (7,200 kilometers). There was no justification or motivation to go anywhere else. Well, that’s the way it was in November anyway. This is January. And now it’s (almost) time for a previously unanticipated new act, XMO4.

Always looking for ways to squeeze as much out of the mission as possible, the team has now devised a new and challenging investigation. It will consume the next five months (and much of the next five Dawn Journals). We begin this month with an overview, but follow along each month as we present the full story, including a detailed explanation of the underlying science, the observations themselves and the remarkable orbital maneuvering entirely unlike anything Dawn has done before. (You can also follow along with your correspondent’s uncharacteristically brief and more frequent mission status updates.)

Map of Ceres
This map of Ceres has all 114 feature names approved so far by the International Astronomical Union (IAU). (We described the naming convention here.) As more features are named, this official list and map are kept up to date. We saw an earlier version of this map before Dawn had flown to its lowest orbit and obtained its sharpest pictures. The dwarf planet is 1.1 million square miles (2.8 million square kilometers). That’s about 36 percent of the land area of the contiguous United States, or the combined land areas of France, Germany, Italy, Norway, Spain, Sweden and the United Kingdom. The scales for horizontal distance in this figure apply at the equator. Rectangular maps like this distort distances at other latitudes. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

From the XMO3 vantage point, with sunlight coming from the side, Ceres is gibbous and looks closer to a half moon than full. The new objective is to peer at Ceres when the sun is directly behind Dawn. This would be the same as looking at a full moon. (In the figure above, it would be like photographing Ceres from somewhere on the dashed line that points to the distant sun.)

While Dawn obtained pictures from near the line to the sun in its first Ceres orbit, there is a special importance to being even closer to that line. Let’s see why that alignment is valuable.

Most materials reflect light differently at different angles. You can investigate this yourself (and it’s probably easier to do at home than it is in orbit around a remote dwarf planet). To make it simpler, take some object that is relatively uniform (but with a matte finish, not a mirror-like finish) and vary the angles at which light hits it and from which you look at it. You may see that it appears dimmer or brighter as the angles change. It turns out that this effect may be used to help infer the nature of the reflecting material. (For the purposes of this exercise, if you can hold the angle of the object relative to your gaze fixed, and vary only the angle of the illumination, that’s best. But don’t worry about the details. Conducting this experiment represents only a small part of your final grade.)

Now when scientists carefully measure the reflected light under controlled conditions, they find that the intensity changes quite gradually over a wide range of angles. In other words, the apparent brightness of an object does not vary dramatically as the geometry changes. However, when the source of the illumination gets very close to being directly behind the observer, the reflection may become quite a bit stronger. (If you test this, of course, you have to ensure your shadow doesn’t interfere with the observation. Vampires don’t worry about this, and we’ll explain below why Dawn needn’t either.)

If you (or a helpful scientist friend of yours) measure how bright a partial moon is and then use that information to calculate how bright the full moon will be, you will wind up with an answer that’s too small. The full moon is significantly brighter than would be expected based on how lunar soil reflects light at other angles. (Of course, you will have to account for the fact that there is more illuminated area on a full moon, but this curious optical behavior is different. Here we are describing how the brightness of any given patch of ground changes.)

A full moon occurs when the moon and sun are in opposite directions from Earth’s perspective. That alignment is known as opposition. That is, an astronomical body (like the moon or a planet) is in opposition when the observer (you) is right in between it and the source of illumination (the sun), so all three are on a straight line. And because the brightness takes such a steep and unexpected jump there, this phenomenon is known as the opposition surge.

Yalode Crater
Dawn observed this scene inside Yalode Crater on Oct. 13, 2015, from its third mapping orbit at an altitude of 915 miles (1,470 kilometers). At 162 miles (260 kilometers) in diameter, Yalode is the second largest crater on Ceres. (Scientists expected to see much larger craters than Ceres displays.) The two largest craters within Yalode are visible in this picture. Lono Crater, at top right, is 12 miles (20 kilometers) in diameter. (Lono is a Hawaiian god of agriculture, rain and other roles.) Below Lono is the 11-mile (17-kilometer) Besua Crater. (Besua is one of at least half a dozen Egyptian grain gods.) Note several chains of craters as well as fractures on the left and lower right. We saw a much more fractured area of Yalode, now named Nar Sulcus, here. (Nar is from a modern pomegranate feast in part of Azerbaijan. A sulcus is a set of parallel furrows or ridges.) You can locate this scene in the eastern part of Yalode on the map above near 45°S, 300°E. The photo below shows a more detailed view. You can see all of Yalode starting at 2:32 in the animation introduced here. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The observed magnitude of the opposition surge can reveal some of the nature of the illuminated object on much, much finer scales than are visible in photos. Knowing the degree to which the reflection strengthens at very small angles allows scientists to ascertain (or, at least, constrain) the texture of materials on planetary surfaces even at the microscopic level. If they are fortunate enough to have measurements of the reflectivity at different angles for a region on an airless solar system body (atmospheres complicate it too much), they compare them with laboratory measurements on candidate materials to determine which ones give the best match for the properties.

Dawn has already measured the light reflected over a wide range of angles, as is clear from the figure above showing the orbits. But the strongest discrimination among different textures relies on measuring the opposition surge. That is Dawn’s next objective, a bonus in the bonus extended mission.

You can see from the diagram that measuring the opposition surge will require a very large change in the plane of Dawn’s orbit. Shifting the plane of a spacecraft’s orbit can be energetically very, very expensive. (We will discuss this more next month.) Fortunately, the combination of the unique capabilities provided by the ion propulsion system and the ever-creative team makes it affordable.

Ceres
Dawn had this view on June 7, 2016, from its fourth mapping orbit. Taken at an altitude of 240 miles (385 kilometers), this picture shows greater detail in a smaller area than the picture above. Part of Lono Crater is at the bottom. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Powered by an insatiable appetite for new knowledge, Dawn will begin ion thrusting on Feb. 23. After very complex maneuvers, it will be rewarded at the end of April with a view of a full Ceres from an altitude of around 12,400 miles (20,000 kilometers), about the height of GPS satellites above Earth. (That will be about 50 percent higher than the first science orbit, which is labeled as line 1 in the figure.) There are many daunting challenges in reaching XMO4 and measuring the opposition surge. Even though it is a recently added bonus, and the success of the extended mission does not depend on it, mission planners have already designed a backup opportunity in case the first attempt does not yield the desired data. The second window is late in June, allowing the spacecraft time to transmit its findings to Earth before the extended mission concludes at the end of that month.

Occator Crater
Occator Crater is shown in this mosaic of photos Dawn took at its lowest altitude of 240 miles (385 kilometers). The central bright area, Cerealia Facula, is the prime target in the planned opposition surge measurements. Dawn’s infrared spectra show that this reflective material is principally sodium carbonate, a kind of salt. We described more about this mosaic here. For other views of Occator and its mesmerizing reflective regions, follow the links in the paragraph below. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

For technical reasons, the measurements need to be made from a high altitude, and throughout the complex maneuvering to get there, Dawn will remain high enough to monitor cosmic rays. Ceres will appear to be around five times the width of the full moon we see from Earth. It will be about 500 pixels in diameter in Dawn’s camera, and more than 180,000 pixels will show light reflected from the ground. Of greatest scientific interest in the photographs will be just a handful of pixels that show the famous bright material in Occator Crater, known as Cerealia Facula and clearly visible in the picture above. Scientists will observe how those pixels surge in brightness over a narrow range of angles as Dawn’s XMO4 orbital motion takes it into opposition, exactly between Occator and the sun. Of course, the pictures also will provide information on how the widespread dark material covering most of the ground everywhere else on Ceres changes in brightness (or, if you prefer, in dimness). But the big reward here would be insight into the details of Cerealia Facula. Comparing the opposition surges with laboratory measurements may reveal characteristics that cannot be discerned any other way save direct sampling, which is far beyond Dawn’s capability (and authority). For example, scientists may be able to estimate the size of the salt crystals that make up the bright material, and that would help establish their geological history, including whether they formed underground or on the surface. We will discuss this more in March.

Most of the data on opposition surges on solar system objects use terrestrial observations, with astronomers waiting until Earth and the target happen to move into the necessary alignment with the sun. In those cases, the surge is averaged over the entire body, because the target is usually too far away to discern any details. Therefore, it is very difficult to learn about specific features when observing from near Earth. Few spacecraft have actively maneuvered to acquire such data because, as we alluded to above and will see next month, it is too difficult, especially at a massive body like Ceres. The recognition that Dawn might be able to complete this challenging measurement for a region of particular interest represents an important possibility for the mission to discover more about this intriguing dwarf planet’s geology.

Meeting the scientific goal will require a careful and quantitative analysis of the pixels, but the images of a fully illuminated Ceres will be visually appealing as well. Nevertheless, you are cautioned to avoid developing a mistaken notion about the view. (For that matter, you are cautioned to avoid developing mistaken notions about anything.) You might think (and some readers wondered about this in a different phase of the mission) that with Dawn being between the sun and Ceres (and not being a vampire), the spacecraft’s shadow might be visible in the pictures. It would look really cool if it were (although it also would interfere with the measurement of the opposition surge by introducing another factor into how the brightness changes). There will be no shadow. The spacecraft will simply be too high. Imagine you’re standing in Occator Crater, either wearing your spacesuit while engaged in a thrilling exploration of a mysterious and captivating extraterrestrial site or perhaps instead while you’re indoors enjoying some of the colony’s specially salted Cerean savory snacks, famous throughout the solar system. In any case, the distant sun you see would be a little more than one-third the size that it looks from Earth, comparable to a soccer ball at 213 feet (65 meters). Dawn would be 12,400 miles (20,000 kilometers) overhead. Although it’s one of the largest interplanetary spacecraft ever to take flight, with a wingspan of 65 feet (20 meters), it would be much too small for you to see at all without a telescope and would block an undetectably small amount of sunlight. It would appear smaller than a soccer ball seen from 135 miles (220 kilometers). Therefore, no shadow will be cast, the measurement will not be compromised by the spacecraft blocking some of the light reaching the ground and the pictures will not display any evidence of the photographer.

Ceres
Dawn took this picture on Oct. 21, 2016, in its fifth observation orbit, at an altitude of 920 miles (1,480 kilometers). The two largest craters here display very different kinds of topography on their floors. The larger, Jarimba, is 43 miles (69 kilometers) across. (Jarimba is a god of fruit and flowers among the Aboriginal Aranda of central Australia.) Above Jarimba is part of Kondos Crater, which is 27 miles (44 kilometers) in diameter. (Kondos is a pre-Christian Finnish god of sowing and young wheat.) This scene is centered near 21°S, 27°E on the map above. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Even as the team was formulating plans for this ambitious new campaign, they successfully dealt with a glitch on the spacecraft this month. When a routine communications session with the Deep Space Network began on Jan. 17, controllers discovered that Dawn had previously entered its safe mode, a standard response the craft uses when it encounters conditions its programming and logic cannot accommodate. The main computer issues instructions to reconfigure systems, broadcasts a special radio signal through one of the antennas and then patiently awaits help from humans on a faraway planet (or anyone else who happens to lend assistance). The team soon determined what had occurred. Since it left Earth, Dawn has performed calculations five times per second about its location and speed in the solar system, whether in orbit around the sun, Vesta or Ceres. (Perhaps you do the same on your deep-space voyages.) However, it ran into difficulty in those calculations on Jan. 14 for the first time in more than nine years of interplanetary travel. To ensure the problematic calculations did not cause the ship to take any unsafe actions, it put itself into safe mode. Engineers have confirmed that the problem was in software, not hardware and not even a cosmic ray strike, which has occasionally triggered safe mode, most recently in September 2014.

Mission controllers guided the spacecraft out of safe mode within two days and finished returning all systems to their standard configurations shortly thereafter. Dawn was shipshape the subsequent week and resumed its scientific duties. When it activated safe mode, the computer correctly powered off the gamma ray and neutron detector, which had been measuring the cosmic rays, as we described above. The time that the instrument was off will be inconsequential, however, because there is more than enough time in the extended mission to acquire all the desired measurements.

The extended mission has already proven to be extremely productive, yielding a great deal of new data on this ancient world. But there is still more to look forward to as the veteran explorer prepares for a new and adventurous phase of its extraordinary extraterrestrial expedition.

Dawn is 5,650 miles (9,100 kilometers) from Ceres. It is also 2.87 AU (266 million miles, or 429 million kilometers) from Earth, or 1,135 times as far as the moon and 2.91 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 48 minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PST January 31, 2017

TAGS: CERES, DAWN, DWARF PLANET

  • Marc Rayman
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Occator Crater

Blue rope lights adorn Dawn mission control at JPL, but not because the flight team is in the holiday spirit (although they are in the holiday spirit).

The felicitous display is more than decorative. The illumination indicates that the interplanetary spacecraft is thrusting with one of its ion engines, which emit a lovely, soft bluish glow in the forbidding depths of space. Dawn is completing another elegant spiral around dwarf planet Ceres, maneuvering to its sixth science orbit.

Dawn’s ion propulsion system has allowed the probe to accomplish a mission unlike any other, orbiting two distant extraterrestrial destinations. Even more than that, Dawn has taken advantage of the exceptional efficiency of its ion engines to fly to orbits at different altitudes and orientations while at Vesta and at Ceres, gaining the best perspectives for its photography and other scientific investigations.

Dawn has thrust for a total of 5.7 years during its deep-space adventure. All that powered flight has imparted a change in the ship’s velocity of 25,000 mph (40,000 kilometers per hour). As we have seen, this is not the spacecraft’s actual speed, but it is a convenient measure of the effect of its propulsive work. Reaching Earth orbit requires only about 17,000 mph (less than 28,000 kilometers per hour). In fact, Dawn’s gentle ion engines have delivered almost 98 percent of the change in speed that its powerful Delta 7925H-9.5 rocket provided. With nine external rocket engines and a core consisting of a first stage, a second stage and a third stage, the Delta boosted Dawn by 25,640 mph (41,260 kilometers per hour) from Cape Canaveral out of Earth orbit and onto its interplanetary trajectory, after which the remarkable ion engines took over. No other spacecraft has accomplished such a large velocity change under its own power. (The previous record holder, Deep Space 1, achieved 9,600 mph, or 15,000 kilometers per hour.)

Early this year, we were highly confident Dawn would conclude its operational lifetime in its fourth orbit at Ceres (and remain there long after). But unexpectedly healthy and with an extension from NASA, Dawn is continuing its ambitious mission. After completing all of its tasks in its fifth scientific phase at Ceres, Dawn is pursuing new objectives by flying to another orbit for still more discoveries. Although we never anticipated adding a row to the table of Dawn’s orbits, last presented in December 2015, we now have an updated version.

Ceres
orbit
Dawn code
name
Dates
(mo.day.yr)
Altitude
in miles
(km)
Resolution
in ft (m)
per pixel
Orbit
period
Equivalent
distance of a soccer ball
1 RC3   04.23.15 – 05.09.15 8,400
(13,600)
4,200
(1,300)
15
days
10 ft
(3.2 m)
2 Survey   06.06.15 –06.30.15 2,700
(4,400)
1,400
(410)
3.1
days
3.4 ft
(1.0 m)
3 HAMO   08.17.15 – 10.23.15 915
(1,470)
450
(140)
19
hours
14 in
(34 cm)
4 LAMO/
XMO1
  12.16.15 – 09.02.16 240
(385)
120
(35)
5.4
hours
3.5 in
(9.0 cm)
5 XMO2   10.16.16 – 11.04.16 920
(1,480)
450
(140)
19
hours
14 in
(35 cm)

As with the obscure Dawn code names for other orbits, this fifth orbit’s name requires some explanation. The extended mission is devoted to undertaking activities not envisioned in the prime mission. That began with two extra months in the fourth mapping orbit performing many new observations, but because it was then the extended mission, that orbit was designated extended mission orbit 1, or XMO1. (It should have been EMO1, of course, but the team’s spellchecker was offline on July 1, the day the extended mission started.) Therefore, the next orbit was XMO2. Dawn left XMO2 on Nov. 4, and we leave it to readers’ imaginations to devise a name for the orbit the spacecraft is now maneuvering to.

Surprisingly, Dawn is flying higher to enhance part of the scientific investigation that motivated going to the lowest orbit. We have explained before that Dawn’s objective in powering its way down to the fourth mapping orbit was to make the most accurate measurements possible of gravity and of nuclear radiation emitted by the dwarf planet.

For more than eight months, the explorer orbited closer to the alien world than the International Space Station is to Earth, and the gamma ray spectra and neutron spectra it acquired are outstanding, significantly exceeding all expectations. But ever-creative scientists have recognized that even with that tremendous wealth of data, Dawn can do still better. Let’s look at this more carefully and consider an example to resolve the paradox of how going higher can yield an improvement.

Ceres
Dawn had this view of Ceres’ limb on Oct. 16 at an altitude of 920 miles (1,480 kilometers). The probe took this picture about 12 minutes after the picture above of Occator Crater. By this time, Dawn’s orbital motion had taken the center of Occator out of the view, but most of the shadowy eastern part is still visible at upper left. A Cerean day lasts about nine hours, so in the time between these two pictures, Ceres rotated as much as Earth would rotate in about 32 minutes. As a result, the change in the sun angle is quite noticeable. You can compare some craters in the two pictures to see how the lighting has changed. This is particularly evident not only in Occator but also in the crater near the center of the large crater visible here (on the lower right of the first picture) as well as the craters below and to the left of it. At the bottom right of this picture is part of the 45-mile (72-kilometer) Kaikara Crater. (Kaikara is a harvest goddess in the kingdom of Bunyoro in Uganda.) You can locate this scene on this map, with Kaikara at 43°N, 222°E and Occator at 20°N, 239°E. Full image (rotated differently and with different picture adjustments) and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The gamma ray and neutron detector (GRaND) reveals some of Ceres’ atomic constituents down to about a yard (meter) underground. The principal limitation in analyzing these spectra is "noise." In fact, noise limits the achievable accuracy of many scientific measurements. It isn’t necessarily the kind of noise that you hear from loud machinery (nor from the mouth of your unhelpful parent, inattentive progeny or boring and verbose coworker), but all natural systems have something similar. Physical processes other than the ones of interest make unwanted contributions to the measurements. The part of a measurement scientists want is called the "signal." The part of a measurement scientists don’t want is called the "noise." The quality of a measurement may be characterized by comparing the strength of the signal to the strength of the noise. (This metric is called the "signal to noise ratio" by people who like to use jargon like "signal to noise ratio.")

We have discussed that cosmic rays, radiation that pervades space, strike atomic nuclei on Ceres, creating the signals that GRaND measures. Remaining at low altitude would have allowed Dawn to enhance its measurement of the Cerean nuclear signal. But scientists determined that an even better way to improve the spectra than to increase the signal is to decrease the noise. GRaND’s noise is a result of cosmic rays impinging directly on the instrument itself and on nearby parts of the spacecraft. With a more thorough measurement of the noise from cosmic rays, scientists will be able to mathematically remove that component of the low altitude measurements, leaving a clearer signal.

For an illustration of all this, suppose you want to hear the words of a song. The words are the signal and the instruments are the noise. (This is a scientific discussion, not a musical one.) It could be that the instruments are so loud and distracting that you can’t make the words out easily.

You might try turning up the volume, because that increases the signal, but it increases the noise as well. If the performance is live, you might even try to position yourself closer to the singer, perhaps making the signal stronger without increasing the noise too much. (Other alternatives are simply to Google the song or ask the singer for a copy of the lyrics, but those methods would ruin this example.)

If you’re doing this in the 21st century (or later), there’s another trick you can employ, taking advantage of computer processing. Suppose you had a recording of the singing with the instruments and then obtained separate recordings of the instruments. You could subtract the musical sounds that constitute the noise, removing the contributions from both guitars, the drums, the harp, both ukuleles, the kazoo and all the theremins. And when you eliminate the noise of the instruments, what remains is the signal of the words, making them much more intelligible.

To obtain a better measure of the noise, Dawn needs to go to higher altitude, where GRaND will no longer detect Ceres. It will make detailed measurements of cosmic ray noise, which scientists then will subtract from their measurements at low altitude, where GRaND observed Ceres signal plus cosmic ray noise. The powerful capability to raise its orbit so much affords Dawn the valuable opportunity to gain greater insight into the atomic composition. Of course, it’s not quite that simple, but essentially this method will help Dawn hear Ceres’ nuclear song more clearly.

Ceres
Dawn took this photo on Oct. 17 at an altitude of 920 miles (1,480 kilometers). Above and to the right of center, part of the wall of a crater has collapsed, allowing material to flow into the larger crater. The area covered by the flow is less densely cratered than the surrounding terrain, because it is younger. We have seen how scientists use the number and size of craters to date geological features (no results are available yet in this area). The larger crater is Ghanan, one of the names of a Mayan maize god, although the devastating flow may not have been good for the maize harvest when the collapse occurred. Ghanan Crater, with an average diameter of 42 miles (68 kilometers), is on this map at 77°N, 31°E. Full image (with different picture adjustments) and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

To travel from one orbit to another, the sophisticated explorer has followed complex spiral routes. We have discussed the nature of these trajectories quite a bit, including how the operations team designs and flies them. But now they are using a slightly different method.

Those of you at Ceres who monitor the ship’s progress probably wouldn’t notice a difference in the type of trajectory. And the rest of you on Earth and elsewhere who keep track through our mission status updates also would not detect anything unusual in the ascent profile (to the extent that a spacecraft using ion propulsion to spiral around a dwarf planet is usual). But celestial navigators are now enjoying their use of a method they whimsically call local maximal energy spiral feedback control.

The details of the new technique are not as important for our discussion here as one of the consequences: Dawn’s next orbit will not be nearly as circular as any of its other orbits at Ceres (or at Vesta). Following the conclusion of this spiral ascent on Dec. 5, navigators will refine their computations of the orbit, and we will describe the details near the end of the month. We will see that as the spacecraft follows its elliptical loops around Ceres, each taking about a week, the altitude will vary smoothly, dipping below 4,700 miles (7,600 kilometers) and going above 5,700 miles (9,200 kilometers). Such a profile meets the mission’s needs, because as long as the craft stays higher than about 4,500 miles (7,200 kilometers), it can make the planned recordings of the cacophonous cosmic rays. We will present other plans for this next phase of the mission as well, including photography, in an upcoming Dawn Journal.

As Dawn continues its work at Ceres, the dwarf planet continues its stately 4.6-year-long orbit around the sun, carrying Earth’s robotic ambassador with it. Ceres follows an elliptical path around the sun (see, for example, this discussion, including the table). In fact, all orbits, including Earth’s, are ellipses. Ceres’ orbit is more elliptical than Earth’s but not as much as some of the other planets. The shape of Ceres’ orbit is between that of Saturn (which is more circular) and Mars (which is more elliptical). (Of course, Ceres’ orbit is larger than Mars’ and smaller than Saturn’s, but here we are describing how much each orbit deviates from a perfect circle.)

When Ceres tenderly took Dawn into its gravitational embrace in March 2015, they were 2.87 AU (267 million miles, or 429 million kilometers) from the sun. In January 2016, we mentioned that Ceres had reached its aphelion, or greatest distance from the sun, at 2.98 AU (277 million miles, or 445 million kilometers). Today at 2.85 AU (265 million miles, or 427 million kilometers), Ceres is closer to the sun than at any time since Dawn arrived, and the heliocentric distance will gradually decrease further throughout the extended mission. (If the number of numbers is overwhelming here, you might reread this paragraph while paying attention to only one set of units, whether you choose AU, miles or kilometers. Ignore the other two scales so you can focus on the relative distances.)

Ceres
Dawn’s location in the solar system is shown on Nov. 7, 2016. On that day, the spacecraft and Ceres were at the same distance from the sun as when Dawn arrived last year. Now as Ceres advances counterclockwise in its elliptical orbit, they will move somewhat closer to the sun. We have plotted Dawn’s progress on this figure before, most recently in September. Image credit: NASA/JPL

Another consequence of orbiting the sun is the progression of seasons. Right on schedule, as we boldly predicted in August 2015, Nov. 13 was the equinox on Ceres, marking the beginning of northern hemisphere autumn and southern hemisphere spring. Although it is celebrated on Ceres with less zeal than on Earth, it is fundamentally the same: the sun was directly over the equator that day, and now it is moving farther south. It takes Ceres so long to orbit the sun that this season will last until Dec. 22, 2017.

A celebration that might occur on Ceres (and which you, loyal Dawnophile, are welcome to attend) would honor Dawn itself. Although the spacecraft completed its ninth terrestrial year of spaceflight in September, on Dec. 12, it will have been two Cerean years since Dawn left Earth for its interplanetary journey. Be sure to attend in order to learn how a dawnniversary is commemorated in that part of the solar system.

Although a year on Ceres lasts much longer than on Earth, 2016 is an unusually long year on our home planet. Not only was a leap day included, but a leap second will be added at the very end of the year to keep celestial navigators’ clocks in sync with nature. The Dawn team already has accounted for the extra second in the intricate plans formulated for the spacecraft. And at that second, on Dec. 31 at 23:59:60, we will be able to look back on 366 days and one second, an especially full and gratifying year in this remarkable deep-space expedition. But we needn’t wait. Even now, as mission control is bathed in a lovely glow, the members of the team as well as space enthusiasts everywhere are aglow with the thrill of new knowledge, the excitement of a daring, noble adventure and the anticipation of more to come.

Dawn is 3,150 miles (5,070 kilometers) from Ceres. It is also 2.08 AU (194 million miles, or 312 million kilometers) from Earth, or 770 times as far as the moon and 2.11 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 35 minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PST November 28, 2016

TAGS: DAWN, CERES, ION ENGINE, DWARF PLANET

  • Marc Rayman
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Dawn photographed this scene in Yalode Crater on June 15, 2016

Dawn has just completed another outstandingly successful observation campaign at Ceres.

Far, far from Earth, the spacecraft has been making measurements at the alien world that were not even imagined until a few months ago. Once again, the experienced explorer has performed its complex assignments with distinction.

When Dawn arrived at Ceres in March 2015, becoming the first spacecraft to reach a dwarf planet, it was looking ahead to a very ambitious year of discovery from four different orbital altitudes. The great benefit of being able to enter orbit rather than fly by is that Dawn can scrutinize its subject over an extended period to develop a detailed, intimate portrait. Taking advantage of the ship’s ability to maneuver with its advanced ion propulsion system, mission planners had carefully selected the four orbits to enable a wide range of measurements.

By February of this year, Dawn had exceeded every one of its original mission objectives and was still going strong, accomplishing many new goals. Nevertheless, no one (at least, no one who was well informed) expected that the probe would complete its new assignments and yet still have the capability to maneuver to a fifth orbit and then undertake even more new observations. But that is exactly what occurred.

After more than eight months orbiting only 240 miles (385 kilometers) above the strange terrain of rock, ice and salt, Dawn ignited one of its ion engines on Sept. 2. By Oct. 6, when it had completed its graceful ascent, Dawn had made 93 spiral loops, reaching an orbit 920 miles (1,480 kilometers) high. From there, revolving once every 18.9 hours, the spacecraft has executed its new program of investigations.

With observations of Ceres from about the same altitude as a year ago in Dawn’s third mapping orbit, scientists will scour the expansive terrain, looking for changes. The most likely change is the presence of new, small craters. Everything in the solar system (including your planetary residence) is subject to strikes from rocks that orbit the sun. Ceres lives in the main asteroid belt between Mars and Jupiter, a particularly rough neighborhood, and being the largest resident there (by far) doesn’t give it any special protection or immunity. In fact, being the largest resident also makes Ceres the largest target.

Ceres
Dawn had this view on June 6, 2016, from an altitude of 240 miles (385 kilometers). The bright material at upper left is on the northwest rim of Kerwan Crater. Geologists have cataloged well over 130 locations on Ceres that are covered with reflective material. (The most famous deposits are in Occator Crater.) The brightness is because briny ice that had been on the surface sublimated, leaving behind salts, which reflect more sunlight than other minerals on the dwarf planet. Extending 174 miles (280 kilometers) across, Kerwan is the largest crater on Ceres. It is centered at 11°S, 124°E on the map shown last month. (Kerwan is a spirit of sprouting maize among the Hopi of Arizona in the US.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In addition to remapping Ceres with all of the camera’s color filters, the flight team has given Dawn other tasks. Controlling a sophisticated interplanetary spacecraft conducting complex operations so very far from Earth is never easy (but it’s always incredibly cool). There have been many challenges throughout this ambitious mission, quite unlike any ever undertaken. One of the significant ones was observing specific targets of interest from low altitude. We have explained that orbiting so close to the ground, the spacecraft’s motion was quite difficult to predict with sufficient accuracy far enough in advance to guide the craft so that the instruments’ narrow fields of view would hit specific features. Dawn was designed to map uncharted worlds, not to conduct targeted observations.

The difficulty was compounded by the loss in 2010 and 2012 of two of the four reaction wheels, used for controlling the probe’s orientation. An important side effect of the nudges from the small hydrazine-fueled jets of the reaction control system (even in combination with the two operable reaction wheels in hybrid control mode) was tiny distortions in the spacecraft’s orbital trajectory. The cumulative effect of many jet firings over days and weeks was enough to make it quite challenging to ensure the sensors could spot the targets as Dawn sped around the rapidly rotating orb beneath it.

This is not as difficult at higher altitude both because Dawn does not need to use its jets as often and because the instruments take in a wider area. As a result, the explorer has been better able to catch sight of preselected geological features, and it has acquired valuable new data.

Ceres
Dawn observed this area of craters, hills and canyons inside Urvara Crater on June 2, 2016, from an altitude of 240 miles (385 kilometers). The third largest crater on Ceres, it is 106 miles (170 kilometers) wide. We have seen Urvara several times before, and the crater on the right of this picture is visible in the northwestern part of Urvara shown here and here. Urvara is on the new map at 46°S, 249°E. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn also has studied selected sites at several times of the Cerean day. Mission planners may determine, for example, that if Dawn points not straight down on a particular orbit at a particular time but rather partially to the side, a certain crater could be spotted soon after Ceres’ nine-hour daily rotation has brought it into sunlight. In other words, it would be early in the morning at the crater when Dawn sees it, providing a nice dawn view. On another orbital revolution, Dawn might point in a different direction to see the same location longer after it has come into sunlight (that is, longer after sunrise), so from that same crater’s point of view, it is later in the day (albeit on a different day).

The spacecraft has done more than look at some special locations at different times of the Cerean day, corresponding to different lighting conditions. In taking pictures for a new map of Ceres this month, everywhere Dawn looked, the illumination was different from the photographs for the maps it compiled in its previous orbits. The orbit now is oriented at a different angle from the sun.

When the interplanetary adventurer was at Vesta, we described the orientation of the orbits in words. Thanks to changes in the Dawn Journal site since then, now we can present a picture showing that the scenery beneath Dawn has been illuminated from a different angle at each orbital altitude. And now in the fifth orbit, by seeing the sights from the same height as in the third mapping orbit but with different lighting, we gain a new perspective on the alien terrain.

Ceres
This illustrates (and simplifies) the relative size and alignment of Dawn’s five science orbits at Ceres. We are looking down on Ceres’ north pole. The spacecraft follows polar orbits, and seen edge-on here, each circular orbit looks like a line. (Orbits 1 and 2 extend off the figure to the right, on the night side. Like 3, 4 and 5, they are centered on Ceres.) The orbits are numbered chronologically. With the sun far to the left, the left side of Ceres is in daylight. Each time the spacecraft travels over the illuminated hemisphere in the different orbital planes, the landscape beneath it is lit from a different angle. Ceres rotates counterclockwise from this perspective (just as Earth does when viewed from the north). So higher numbers correspond to orbits that pass over ground closer to sunrise, earlier in the Cerean day. (Compare this diagram with this figure, which shows only the relative sizes of the orbits, with each one viewed face-on rather than edge-on.) Click on this image for a larger view. Image credit: NASA/JPL

In addition to all of its other work this month, the sophisticated robot has continued some specialized measurements it began at lower altitude. Being higher up does not cause as much of a reduction in the sharpness of some pictures as you might think. Held in a looser gravitational grip, Dawn’s orbital velocity is lower at higher altitude. As a result, observations that require a long exposure are not affected as much by the spacecraft’s movement. That’s helpful for some of the spectra and photographs. For example, Dawn has used its camera to peer into craters near the north and south poles that are in shadow continuously, every Cerean day of the Cerean year. These special locations might trap water molecules that escape from elsewhere on Ceres where it is too warm for them. With the benefits of a wider view from a higher altitude and a more predictable orbital path, Dawn’s coverage this month of these intriguing areas, faintly illuminated by sunlight reflected from crater walls, has been more complete than at lower altitude.

This fifth Ceres campaign was intricate and intensive, but it stayed right on the tight schedule. Dawn began collecting data as planned on Oct. 16 and finished transmitting its findings to Earth on Oct. 29. And it was exceedingly productive, yielding almost 3,000 photographs plus a great many infrared spectra and visible spectra containing a wealth of new information about Ceres.

This week controllers are going to check out the backup camera, as they do twice a year to confirm that it is still healthy and ready to take over should the primary camera develop a problem. Nevertheless, the primary camera remains fully functional. The team also is planning to switch to the backup set of reaction control system thrusters. Dawn has flown for so many years without a full complement of reaction wheels that these hydrazine thrusters have been used far more than anticipated when the ship was designed. They are healthy, but ever-cautious engineers do not want to overuse them.

Achita Crater
Dawn took this photo of Achita Crater on June 3, 2016, from an altitude of 240 miles (385 kilometers). Departing from what may seem to be the theme above of displaying interesting landscapes in the northwestern parts of the largest craters on Ceres, this scene includes most of the 25-mile (40-kilometer) Achita. Although many craters have a mountain peak in the center, this one has an extended ridge. (We have seen other craters on Ceres with central ridges, including Haulani and Urvara here and here.) Also note the bright material at the bottom of the southwest wall and a smaller deposit on the northeast rim. Achita Crater is at 26°N, 66°E on this map. (Achita is a god of agriculture in northern Nigeria.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn’s work in this fifth orbit is part of a comprehensive plan for exploring Ceres as thoroughly as possible. Surprising though it may be, we will see next month that scientists have determined that there is even more to learn about Ceres by flying to a higher altitude. So now that Dawn has accomplished all of its objectives for this phase of the mission, it is about to begin another month of maneuvering. On Nov. 4, the spaceship will once again power on ion engine #2 and start another spiral to a sixth orbital observing post.

As Earth and Ceres (accompanied by Dawn) follow their independent orbits around the sun, the distance between them is constantly changing. On Oct. 22, they were at their smallest separation in the 3.5 years from June 2014 to Dec. 2017. On that date, Dawn was a mere 1.900 AU (176.6 million miles, or 284.2 million kilometers) from its first solar system residence. Dawn never loses track of the rest of its team, still stationed on that faraway planet. But after many years of interplanetary travels and more than a year at Vesta, the denizen of deep space is now a devoted companion of Ceres, and that is where it focuses its attention. And it has more work to do as it seeks still greater insights into the nature of its mysterious and exotic home.

Dawn is 920 miles (1,480 kilometers) from Ceres. It is also 1.91 AU (178 million miles, or 286 million kilometers) from Earth, or 705 times as far as the moon and 1.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 32 minutes to make the round trip.

Dr. Marc D. Rayman
2:30 p.m. PDT October 31, 2016

P.S. Now that this Dawn Journal is complete, your correspondent can turn his attention to getting into costume for Halloween. This year, he will be disguised as someone who knew all along that Dawn would engage in a productive and innovative extended mission at Ceres. Just imagine what a great time the trick-or-treaters are going to have when they visit his home!

TAGS: DAWN, CERES, OBSERVATIONS

  • Marc Rayman
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