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A veteran explorer is leisurely orbiting the only dwarf planet in the inner solar system. Measuring space radiation high over Ceres, Dawn revolves once every 30 days in its gravitational master's firm grip. Dawn is well-known for its patience, and the pace of its activities has been decidedly relaxed in this orbit. That is about to change. There is now only one revolution to go before the spacecraft begins the final campaign of its long and rewarding deep-space adventure.

For eight months in 2015-2016, Dawn circled Ceres once every 5.4 hours at only 240 miles (385 kilometers). (The orbit has been variously designated as LAMO, then XMO1, and often as "the lowest orbit.") It then flew higher to pursue new objectives. The probe's orbit now takes it from slightly under 2,800 miles (4,400 kilometers) up to 24,300 miles (39,100 kilometers) and then back down again. (These values are a little different from what we presented in December, principally because the Sun's gravity gradually alters the orbit.) The orbit is known to people who call it extended mission orbit 5, or XMO5, as "extended mission orbit 5" or "XMO5" (following the nomenclature described here). XMO5 is illustrated in a figure below.

In contrast to the distant, serene probe, the operations team has been working quite intensively to prepare for a bold new phase of the mission. They have been assiduously working through all the tasks necessary to prepare for piloting this unique spaceship, late in its life and low on supplies, through maneuvers it was never designed for and to conduct observations never conceived of prior to late last year. Since the previous Dawn Journal, the team has generated more than 45,000 trajectories to study how to fly Dawn to two new orbits. Often there are more than 100 computers operating simultaneously to perform the necessary calculations. Many thousands more trajectories are yet to be computed and analyzed. If all goes well, by June, the probe will have followed an intricate flight plan that will allow it to glide a mere 22 miles (35 kilometers) above the alien landscapes almost every day in an orbit dramatically and poetically designated XMO7 (but occasionally summarized as "Whoa, that's low!").

Dawn took this picture of Juling Crater in LAMO from an altitude of 240 miles (385 kilometers) on April 30, 2016. When we presented a different view of Juling, taken four months later, we described the surprising discovery of ice there. In October 2016, in XMO2, Dawn successfully accomplished the challenging assignment of acquiring infrared spectra of Juling's north wall, where the ice had been spotted, at three different times of the Cerean day. Comparing these five observations, scientists have determined that the area of ice increased from 1.4 square miles (3.6 square kilometers) to 2.1 square miles (5.5 square kilometers). In other words, the ice grew by 470 acres (190 hectares) over those six months. This is the first detection of a change on Ceres during Dawn's exploration. Scientists attribute the change to a seasonal cycle of solar heating of the crater floor. During that period, late in southern hemisphere winter, the Sun was moving south (toward Juling, which is at 36°S). As the ground warmed, it released water vapor. The vapor then condensed on the colder north wall of the crater, which faces away from the Sun. The crater wall acts as a "cold trap," collecting ice. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/ASI/INAF

Let's take a look at some of the plans the flight team is developing. As always, we will provide more details when Dawn is executing its complex assignments. In addition, as some parts of the plan are still being refined, there may be a few changes, and we will keep you updated on those as well. But plans are firm enough now that a preview is warranted.

On April 17, the spacecraft will fire up ion engine #2 and begin a downward spiral, gradually shrinking its elliptical orbit. Along the way to its final space destination, XMO7, the ship will moor at an intermediate orbit. On May 14, when it is in an orbit that ranges from about 235 miles (375 kilometers) to almost 3,000 miles (4,800 kilometers), it will shut down the engine. (This orbit is illustrated in the next two figures below.)

It is only coincidental that the lowest altitude of this intermediate orbit, XMO6, is so close to height of the lowest orbit so far. Indeed, the lowest point is not the most important point. The motivation for stopping in XMO6 is to collect infrared spectra and take pictures in the southern hemisphere in a range of about 900 miles (1,500 kilometers) to 1,600 miles (2,500 kilometers). It just so happens that when flying from XMO5 to XMO7, an orbit that provides that viewing opportunity dips down to the height of LAMO/XMO1 elsewhere in the orbit.

The XMO6 altitude in the south was chosen to be comparable to the altitude from which Dawn observed Ceres so extensively in its third and fifth mapping orbits (known as HAMO and XMO2, respectively). XMO6 will afford the probe views of the terrain with the illumination of southern summer that will make for the best comparison with what it has already observed farther north on the dwarf planet. Dawn photographed all of Ceres in full color in those earlier orbits, but it was not possible then to cover the vast surface with the infrared mapping spectrometer, which has a much smaller field of view than the camera. Therefore, scientists had focused their spectral mapping in the northern hemisphere, taking advantage of the lighting then. While some of the southern hemisphere was studied in infrared as well, the opportunity now to observe more of it will allow a more complete understanding of the distribution of minerals.

In XMO6 the spacecraft will fly over the south pole and then head north over the hemisphere of Ceres facing the Sun. It will go lower and lower as it does so. The lowest point in the orbit will occur between 50° and 60°N. Dawn already mapped that territory from LAMO/XMO1, but now it will take advantage of being low again to acquire some new color photography in the northern hemisphere.

As the spacecraft continues farther north, the altitude will increase again. It will sail higher as it travels over the night side before beginning its fall back down. It will take about 37 hours to complete one elliptical revolution.

Some readers may recall that for all of the mapping orbits at Vesta and Ceres, Dawn traveled south over the sunlit side and north over the hemisphere shrouded in the dark of night. (Readers who don't recall that are invited to trust that it's true.) Experts readily recognize that it is very, very difficult to reverse the orbital direction. Dawn did so, however, with the extensive maneuvering in February-April 2017 that allowed it to make the unique observation of opposition. Those who are interested can review the skilled piloting that reversed the direction.

The explorer will observe Ceres on 10 consecutive orbits in XMO6. To conserve precious hydrazine, Dawn will turn to point its main antenna to Earth and radio its findings after every other transit over the sunlit landscapes. In the other orbits, it will wait patiently, saving both data and hydrazine onboard for later.

On May 31, the spaceship will resume maneuvering. It will take about a week of ion thrusting to push down to the final orbit of the mission.

In XMO7 (shown in the two figures below), Dawn will range from as high as 2,500 miles (4,000 kilometers) to as low as about 22 miles (35 kilometers). (The minimum altitude will vary by a few miles, or kilometers, from revolution to revolution, for reasons we will explain in a future Dawn Journal.) It will take a little more than a day to complete one loop.

We have described before that photography will be very challenging, both because of the difficulty pointing the camera accurately enough to capture specific targets and the high speed so close to the ground. We will return to this problem in an upcoming Dawn Journal.

At the high point of XMO7, Dawn will move at only about 120 mph (190 kph). Then as gravity pulls it back down, the spacecraft will accelerate until it streaks northward at 1,050 mph (1,690 kph) above a relatively narrow strip of ground before starting to soar up again. Dawn was designed for mapping uncharted worlds, not making specialized observations under such conditions, and traveling so fast and so low means it cannot take pictures as sharp as you might expect. Nevertheless, even with a little bit of motion-induced blur at low altitude, any sights we photograph certainly will reveal finer details than we have seen before. This is going to be exciting!

The highest priority measurements will be the nuclear spectra, giving scientists the opportunity to take a sharper picture of the elemental composition of the faraway world, making a more accurate map of the concentration of atomic species that are important for Ceres' geology and chemistry. Dawn's gamma ray and neutron detector (GRaND) is not subject to the limitations of pointing accuracy and blur that can affect the photography. You can think of GRaND's gamma ray vision and its neutron vision as being broader but less acute than the camera's visible-light vision. Getting closer to the ground will help ensure the instrument sees a stronger nuclear signal than ever before and takes a clearer picture.

As the spacecraft races over the ground, GRaND will measure gamma rays and neutrons escaping into space from the atoms down to about a yard (meter) underground. It collected a large volume of such data from LAMO/XMO1, but being so much lower in XMO7 will allow scientists to identify and locate elements more accurately.

There are several GRaND (if not grand) objectives for XMO7. One is to see how the elemental composition differs at different latitudes. The instrument has already revealed that water is more plentiful near the surface at higher latitudes than near the equator, and now it may be able to refine this finding. One of the properties of XMO7 is that the low point will shift almost 2° of latitude south on each revolution. That is, each time Dawn swoops down to its lowest point, it will be south of the low point on the previous orbit. That will provide GRaND the opportunity to survey the concentration and distribution of underground ice at different latitudes. GRaND also may tell us more about other constituents, providing clues about the geological processes that shaped this exotic world.

Of course, as Dawn orbits Ceres, Ceres turns on its axis, pirouetting beneath her admiring companion. So each time Dawn zooms down for a close look, it will not only be farther south than the time before but it will also be at a different longitude. The next Dawn Journal will focus on this and what it means for GRaND and for photography.

Controlling Dawn's orientation in the zero-gravity of spaceflight is harder at low altitude, where Ceres' gravitational pull is stronger. Dawn will use hydrazine much more quickly in XMO7 than at any other part of the mission, and the last of the propellant will be expended before the end of this year.

Dawn just celebrated the third anniversary of arriving at its permanent residence in the solar system. In the natural perspective of its current home, Dawn arrived about two-thirds of a Cerean year ago, or nearly 3,000 Cerean days ago. The explorer has now completed 1,600 orbits. Although hydrazine is dwindling, and the adventure is nearing its end, there is still plenty to look forward to. Stay onboard as Dawn prepares to delve further into the unknown. It's going to be a great ride!

Dawn is 10,800 miles (17,400 kilometers) from Ceres. It is also 1.87 AU (174 million miles, or 280 million kilometers) from Earth, or 740 times as far as the Moon and 1.88 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 31 minutes to make the round trip.

Dr. Marc D. Rayman
9:15 am PDT March 20, 2018

TAGS: DAWN, CERES

ABOUT THE AUTHOR

Marc Rayman, Dawn Mission Director and Chief Engineer

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

ABOUT THE AUTHOR

Marc Rayman, Dawn Mission Director and Chief Engineer

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.

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 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).

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.

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.

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 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.

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 am PDT September 27, 2017

TAGS: DAWN, CERES, VESTA, ASTEROID BELT, ION PROPULSION

ABOUT THE AUTHOR

Marc Rayman, Dawn Mission Director and Chief Engineer

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.

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.

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.

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 pm PDT May 24, 2017

TAGS: CERES, DAWN

ABOUT THE AUTHOR

Marc Rayman, Dawn Mission Director and Chief Engineer

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 is concluding a remarkable year of exploring dwarf planet Ceres. At the beginning of 2016, the spacecraft was still a newcomer to its lowest altitude orbit (the fourth since arriving at Ceres in March 2015), and the flight team was looking forward to about three months of exciting work there to uncover more of the alien world’s mysteries.

This animation shows many views of Occator Crater and its distinctive, captivating bright features. Dawn team members at the German Aerospace Center (DLR) combined photographs and other data collected by Dawn to make this video. (Unlike the visuals, the sounds are entirely speculative.) We have discussed the Occator findings shown here before. For details, see our last description, and follow the links from there to earlier Dawn Journals. Original video and caption. Video/image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

As it turned out, Dawn spent more than eight months conducting an exceptionally rewarding campaign of photography and other investigations, providing a richly detailed, comprehensive look at the extraterrestrial landscapes and garnering an extraordinary bounty of data. In September, the craft took advantage of its advanced ion propulsion system to fly to a new orbit from which it performed still more unique observations in October. Last month, the ship took flight again, and now it is concluding 2016 in its sixth science orbit.

Dawn is in an elliptical orbit, sailing from about 4,670 miles (7,520 kilometers) up to up to almost 5,810 miles (9,350 kilometers) and back down. It takes nearly eight days to complete each orbital loop. Flying this high above Ceres allows Dawn to record cosmic rays to enhance the nuclear spectra it acquired at low altitude, improving the measurements of atomic constituents down to about a yard (meter) underground.

This animation shows Vesta (Dawn's first destination) and Ceres. Based on measurements of hydrogen, the colors encode the water content of the material within about a yard (meter) of the surface. We have seen before how the spacecraft’s neutron spectrometer can make such a measurement. Here, as before, scientists have good reason to assume the hydrogen is in water molecules. Some of the water is in the form of ice and some is bound up in hydrated minerals. Even if it not exactly soggy, Ceres is much, much wetter than Vesta. In some regions on Vesta, there is no evidence of water at all (represented by red), and even the greatest concentration (the deepest blue) is only 0.04 percent. On Ceres, water is abundant, varying from 1.8 to 3.2 percent, or 45 to 80 times more prevalent than the highest concentration on Vesta. (The interior of Ceres harbors even more water than that.) Note that on Ceres, there is very little difference at different longitudes. The variability is much stronger with latitude: at greater distances from the equator, water is more plentiful. This fits with the temperatures being lower near the poles, allowing ice to be closer to the surface for very, very long times without sublimating away. (Below, we will discuss the presence of ice on the ground.) Vesta and Ceres are shown to scale in this animation. They are the two largest objects in the main asteroid belt. Vesta’s equatorial diameter is 351 miles (565 kilometers). Ceres is 599 miles (963 kilometers) across at the equator. (Their rotation rates are not shown to scale. Vesta turns once in 5.3 hours, whereas Ceres takes 9.1 hours.) Video/image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The spacecraft has been collecting cosmic ray data continuously since reaching this orbit (known to the Dawn team, imaginative readers of last month’s Dawn Journal and now you as extended mission orbit 3, or XMO3). These measurements will continue until the end of the extended mission in June. But there is more in store for the indefatigable adventurer than monitoring space radiation.

Based on studies of Dawn’s extensive inspections of Ceres so far, scientists want to see certain sites at new angles and under different illumination conditions. Next month, Dawn will begin a new campaign of photography and visible spectroscopy. All of Dawn’s five previous science orbits had different orientations from the sun. And now XMO3 will provide another unique perspective on the dwarf planet's terrain. The figure below shows what the orientation will be when the explorer turns its gaze once again on Ceres for the first set of new observations on Jan. 27, 2017.

Dawn XMO2 Image 10 — 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*

We mentioned in the figure caption that the alignments are simplified. One of the simplifications is that some of the orbits covered a range of angles. There is a well-understood and fully predictable natural tendency for the angle to increase. In some phases of the mission, the flight team allows that, and in others they do not, depending on what is needed for the best scientific return. At the lowest altitude (orbit 4 in the diagram, and sometimes known as LAMO, XMO1 or "the lowest orbit"), navigators held the orbit at a fixed orientation. Had they not done so, it would have changed quite dramatically over the course of the eight months Dawn was there. For XMO3, the team has decided not to keep the angle constant. Therefore, later observations will provide still different views. We will return to this topic in a few months.

We have described before how places that remain shadowed throughout the Cerean year can trap water molecules. Dawn’s pictures have revealed well over 600 craters high in the northern hemisphere that are permanently in darkness, covering more than 800 square miles (more than 2,000 square kilometers). (It has not been possible to make as thorough a census of the southern hemisphere, because it has been fall and winter there during most of Dawn’s studies, so some areas were not lit well enough. Now that spring has come, new photography will tell us more.)

Ceres Persistent Shadow — This animation shows the lighting during a full Cerean day at high northern latitude. The 11-mile-diameter (18-kilometer-diameter) unnamed crater is at 82°N and 78°E, only 40 miles (65 kilometers) from the north pole. Because the sun is overhead near the equator, it never rises much above the horizon as seen from this location, so shadows are long, and deep sites never receive direct sunshine. More than half of this crater, about 53 square miles (137 square kilometers), is never illuminated. This is the largest permanently shadowed area identified on Ceres. Below, we can glimpse the interior of a nearby crater. Full image and caption. Image credit: *NASA/JPL-Caltech/UCLA/MPS/DLR/IDA*

Dawn peered into craters to see what was hidden on the dark floors. Long exposures could reveal hints of the scenery using the faint light reflected from crater walls. In 10 of the craters, scientists found bright deposits. In one of those craters, the reflective material extends beyond the permanent shadow and so is occasionally illuminated, albeit still with the sun very low on the horizon. And sure enough, right there, Dawn’s infrared mapping spectrometer found the characteristic fingerprint of ice. These shadowed crater floors accumulate water that happens to land there, preserving it in a deep freeze that may be colder than -260°F (-163°C). Readers are invited to formulate their own business plans for how best to utilize that precious resource.

Jan. 1 is the anniversary of the discovery of Ceres. When Giuseppe Piazzi spotted the faint smudge of light in his telescope that night in 1801, he did not know that it would be known as a planet for almost two generations. (After all, he was an astronomer and mathematician, not a clairvoyant.) And he could never have imagined that more than two centuries later (by which time Ceres was known as a dwarf planet, reflecting progress in scientific knowledge), humankind would undertake an ambitious expedition to explore it, dispatching a sophisticated ship to take up residence at that distant and mysterious place. What Piazzi discovered was a lovely jewel set against the deep blackness of space and surrounded by myriad other gleaming stellar jewels. What Dawn has discovered is a unique and fascinating world of complex geology, composed of rock and ice and salt, with exotic and beautiful scenery. And as Dawn continues to build upon Piazzi’s legacy, unveiling Ceres’ secrets, everyone who has ever looked in wonder at the night sky, everyone who has ever hungered for new understanding, everyone who has ever felt the lure of a thrilling adventure far from home and everyone who has ever yearned to know the cosmos will share in the rewards.

Dawn is 5,640 miles (9,070 kilometers) from Ceres. It is also 2.43 AU (226 million miles, or 364 million kilometers) from Earth, or 915 times as far as the moon and 2.48 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
4:00 p.m. PST December 29, 2016

TAGS: DAWN

ABOUT THE AUTHOR

Marc Rayman, Dawn Mission Director and Chief Engineer

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.

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.

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.

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.)

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, ION PROPULSION, DWARF PLANET

ABOUT THE AUTHOR

Marc Rayman, Dawn Mission Director and Chief Engineer

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 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.

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.

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.

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.

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

ABOUT THE AUTHOR

Marc Rayman, Dawn Mission Director and Chief Engineer

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.

Nine years ago today, Dawn set sail on an epic journey of discovery and adventure.

The intrepid explorer has sailed the cosmic seas and collected treasures that far exceeded anything anticipated or even hoped for. It began its voyage at Earth with a fiery ascent atop a Delta rocket. After escaping from its home planet’s gravitational grasp, it flew through the solar system perched on a pillar of blue-green xenon ions that enabled the probe to accomplish a mission that would have been impossible with conventional propulsion. In 2009, with its sights set on more distant lands, Dawn swept past Mars, taking some of the planet’s orbital energy for its own. By its fourth anniversary, Dawn was conducting an extensive orbital investigation of protoplanet Vesta, the second most massive resident of the main asteroid belt. Dawn found it to be quite unlike typical asteroids. Rather than a big chunk of rock, Vesta is like a small planet, and scientists recognize it as being more closely related to the rocky planets of the inner solar system (including Earth) than to the much smaller asteroids. Vesta’s nearer brethren are the blue and white planet where Dawn began its mission nine years ago and the red one it flew by 17 months later. By its fifth anniversary of leaving Earth, the interplanetary spaceship was on its way to yet another distant, alien world. Under the careful guidance of its human colleagues, Dawn completed its 2.5-year journey from Vesta to Ceres last year. Now a perpetual companion of the first dwarf discovered, the veteran space traveler will spend all future anniversaries in orbit around Ceres, even after its operational lifetime has concluded.

By February of this year, the spacecraft had exceeded all of its original objectives established by NASA. Doing so involved orbiting Vesta for 14 months and, at that time, Ceres for almost a year. On June 30, Dawn’s prime mission concluded, and on July 1, its "extended mission" began.

One year ago today, the ship was in its third Ceres mapping orbit, scrutinizing the exotic landscapes 915 miles (1,470 kilometers) beneath it. Less than four weeks later, it started powering its way down through the uncharted depths of Ceres gravitational field to undertake the final planned observations of its long mission.

When ion thrusting concluded on Dec. 13, 2015, Dawn was orbiting closer to Ceres than the International Space Station is to Earth. From its vantage point only 240 miles (385 kilometers) high, the probe used its suite of sophisticated sensors to develop a richly detailed portrait of the only dwarf planet in the inner solar system. Dawn’s reason for venturing to its fourth mapping orbit was to collect about 35 days of neutron spectra, 35 days of gamma-ray spectra and 20 days of gravity measurements. Given the complexity of operating in the low, tight orbit, mission planners expected it could take about three months to acquire these precious data and transmit them to Earth. Operations turned out to be essentially flawless, and by the time Dawn left that orbit on Sept. 2, it had accumulated 183 days of neutron spectra, 183 days of gamma-ray spectra and 165 days of gravity measurements. In addition, the spacecraft amassed a sensational bonus of 38,000 high resolution photos (including stereo and color) as well as more than 11 million infrared spectra and 12 million spectra in visible wavelengths. The original plan was not to take any pictures or visible or infrared spectra at the lowest altitude.

For such an overachiever, it’s fitting that now, on its ninth anniversary, the spacecraft is engaged in activities entirely unimagined on its eighth. With the critical loss of two of the four reaction wheels used to orient and stabilize the ship in space, the flight team (and your correspondent) considered it unlikely Dawn would survive long enough to celebrate a ninth anniversary. And everyone was confident that whether it was operating or not, it would still be in the fourth mapping orbit. There was a clear intent never to go anywhere else. But as we explained last month, with the extraordinary wealth of information Dawn gleaned, the team has been developing plans for new and previously unforeseen work at higher altitudes. Next month, we will detail the first set of new observations from an orbital perch of about 920 miles (1,480 kilometers).

For now, Dawn is using its ion engine #2 to gradually raise its orbit. We have seen how the spacecraft’s uniquely capable propulsion system leads to intriguing spiral trajectories. Right now, on the ninth anniversary of the last moment Dawn’s rocket stood motionless at Cape Canaveral’s Space Launch Complex 17B, Dawn is 660 miles (1,060 kilometers) above Ceres. With its signature combination of exceptional gentleness and exceptional efficiency, the ion engine will propel Dawn to an altitude 20 miles (35 kilometers) higher by the end of the day today. (In contrast, by the end of the day it launched nine years ago, Dawn had gained about 175,000 miles, or 280,000 kilometers, in altitude. The Delta rocket provided a much stronger thrust at much lower efficiency. We will discuss this further below.)

You can follow Dawn’s ascent to its new orbit by flying right behind it as it loops around Ceres or by checking the frequent mission status reports.

Nine years after launch, as Dawn maneuvers in orbit around a distant dwarf planet in order to conduct new observations, it is convenient to look back over its long trek through deep space. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the ninth annual summary, reusing text from previous years with updates where appropriate. Readers who wish to reflect upon Dawn’s ambitious journey may find it helpful to compare this material with the Dawn Journals from its first, second, third, fourth, fifth, sixth, seventh and eighth anniversaries.

In its nine years of interplanetary travels, the spacecraft has thrust for a total of 2,044 days (5.6 years), or 62 percent of the time (and 0.000000041 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 890 pounds (404 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sept. 27, 2007. The spacecraft has used 68 of the 71 gallons (256 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space.

The thrusting since then has achieved the equivalent of accelerating the probe by 24,800 mph (39,900 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 97 percent of the change in speed that the entire Delta rocket provided.

Since launch, our readers who have remained on or near Earth have completed nine revolutions around the sun, covering 56.6 AU (5.3 billion miles, or 8.5 billion kilometers). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 38.6 AU (3.6 billion miles, or 5.8 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 nine years since Dawn began its voyage, Vesta has traveled only 36.6 AU (3.4 billion miles, or 5.5 billion kilometers), and the even more sedate Ceres has gone 34.0 AU (3.2 billion miles, or 5.1 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 nine 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 a few 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, Vesta, Ceres and Dawn) follow their paths around the sun, they sometimes move closer and sometimes move farther from it.

Trajectory diagram — 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 July. (And, to answer an important question raised last month, this image, along with others, also will be seen for a short time this afternoon on a yummy chocolate cake at the Dawn flight team’s novennial celebration.) 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°
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 five 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. But now, Dawn has gone even beyond that. Having discovered so many of Vesta’s secrets, the stalwart adventurer left it behind in 2012. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. Dawn devoted another 2.5 years to reshaping and tilting its orbit even more so that now it is identical to Ceres’. Once again, that was essential to the intricate celestial choreography in March 2015, when the behemoth tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.

Oxo crater — This shows where Dawn’s infrared mapping spectrometer detected water ice in Oxo Crater. The crater is 6 miles (10 kilometers) in diameter. This view was constructed from bonus photographs Dawn took from an altitude of 240 miles (385 kilometers). Blue, green and infrared pictures were combined with stereo pictures to provide this perspective. (Colors are enhanced to bring out subtle differences your eye would not otherwise detect, and the vertical scale has been exaggerated by a factor of two.) Compare this with the Oxo Crater photograph shown in the April Dawn Journal. Here, we are looking from the upper left of that photo toward the lower right. Full image and caption. Image credit: *NASA/JPL-Caltech/UCLA/MPS/DLR/IDA*

Its ion propulsion system has allowed Dawn to do even more than orbit two distant and fascinating bodies. At each one, the spacecraft has changed its orbits extensively, optimizing its views to conduct detailed studies, something it would not have been able to do with conventional propulsion.

Dawn passed a coincidental pair of milestones in its orbital mission at Ceres last week. The dwarf planet reached out to take Earth’s emissary into a gentle but permanent gravitational embrace on March 6, 2015. Sept. 23, 2016, was 1,500 Cerean days later. (Ceres turns on its axis in 9 hours, 4 minutes, considerably faster than Earth, although not all that different from the giant planet Jupiter, which takes 9 hours, 56 minutes). Interestingly, on Sept. 22, Dawn completed its 1,500th orbital revolution around Ceres.

Given the equality between the number of orbits and the number of Cerean days, you may be tempted to conclude that Dawn orbits at the same rate that Ceres rotates. Please resist this temptation! Dawn’s early orbits took weeks to complete, and as the spacecraft maneuvered to lower altitudes, eventually they took days and then hours. In its lowest altitude, the spacecraft circled Ceres in only 5.4 hours. (For a reminder of the details of the orbits, see this table and this diagram depicting preliminary orbit sizes.) So, it truly is a coincidence that the average has worked out so that Dawn has revolved as many times as Ceres has rotated. And now that Dawn is raising its altitude and thus increasing the time required to complete an orbit, such a coincidence will not occur again. Ceres is very stubborn and will keep rotating at the same rate. Dawn, much nimbler and more flexible, is currently in a 13-hour orbit. By the time it completes ion thrusting next week, the orbit period will be almost 19 hours.

Topographical map of Ceres — This topographical map of Ceres was made from Dawn’s stereo photos taken in the third mapping orbit. (For experts, the topography is referenced to an ellipsoid of 299.5 by 299.5 by 277.1 miles, or 482.0 by 482.0 by 446.0 kilometers.) 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 map shows all the 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. (To avoid confusion, note that the topographical map here has the prime meridian on the left, but the IAU map has it in the middle.) The scales for horizontal distance in this figure apply at the equator. Rectangular maps like this distort distances at other latitudes. A similar version of this map is here. Image credit: *NASA/JPL-Caltech/UCLA/MPS/DLR/IDA*

Now in the 10th year of its deep-space expedition, Dawn is not satisfied simply to rest on its laurels. The explorer (along with its support team on distant Earth) is committed to remaining as prolific and profitable at Ceres as it was during earlier years of its extraordinary and innovative mission of discovery. The largest body between Mars and Jupiter is a relict from the dawn of the solar system, a strange and fascinating world of rock, ice and salt that likely has been geologically active for more than 4.5 billion years. Ceres was first glimpsed from Earth more than 200 years ago but held her secrets close until Earth finally answered her cosmic invitation. Now, after so very long, Ceres is whispering those wondrous secrets to her permanent companion. Dawn is listening carefully!

Dawn is 660 miles (1,060 kilometers) from Ceres. It is also 1.99 AU (185 million miles, or 297 million kilometers) from Earth, or 760 times as far as the moon and 1.98 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 33 minutes to make the round trip.

Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2016

TAGS:

ABOUT THE AUTHOR

Marc Rayman, Dawn Mission Director and Chief Engineer

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.

Humankind dispatched Dawn on an extraordinary extraterrestrial expedition in 2007.

It visited Mars briefly in 2009 and spent 14 months orbiting protoplanet Vesta in 2011-2012, revealing fascinating details of that uncharted, alien world. After traveling for another two and a half years through the interplanetary void, the spacecraft arrived at Ceres in March 2015. It has now conducted an outstandingly successful exploration of the only dwarf planet in the inner solar system. Dawn greatly surpassed its objectives at both Vesta and Ceres, accomplishing well more than was envisioned when NASA decided to undertake this ambitious mission. Having realized its raison d'être, the official end of Dawn's prime mission was June 30.

Following the conclusion of the prime mission, the adventurer began its "extended mission" of performing more Ceres observations without missing a beat. We described in April some of what Dawn can do as it continues investigating many of the mysteries there. Dawn's extension allows for even better measurements with the gamma ray and neutron detector of the nuclear radiation emanating from Ceres. This is like taking a longer exposure of the very faint nuclear glow, yielding a brighter, sharper picture that reveals more about the atomic constituents down to about a yard (meter) underground. The spacecraft is taking more stereo photos, continuing to improve the topographical map it created from four times higher. Scientists also are taking advantage of this opportunity to study more geological features with the visible and infrared mapping spectrometers, providing important insight into Ceres' mineralogical inventory.

Dawn has already made extraordinary discoveries at Ceres, some of which we have described in recent months. But on a dwarf planet of 1.1 million square miles (2.8 million square kilometers), there is a great deal to see. That, after all, is the benefit of being in orbit, lingering long enough to make a richly detailed portrait of the exotic expanse. Indeed, Ceres has 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. In such a vast territory, there are innumerable mysteries to unravel. And that is only the surface.

Dawn also is continuing its studies of the gravitational field to discover more about the interior structure of the largest body between Mars and Jupiter.

Dawn captured this view inside Occator Crater on March 26 from an altitude of 240 miles (385 kilometers). We have explained that the bright areas are salts, which reflect much more sunlight than typical materials on Ceres. Recent analysis of Dawn's infrared spectra shows the salt is mostly sodium carbonate. (This is the brightest region on Ceres, but you can see another of the many reflective deposits in one of the pictures below of two adjoining craters.) Occator Crater formed 80 million years ago. Another part of this geologically young crater is shown immediately below. Full image and caption. Image credit: *NASA/JPL-Caltech/UCLA/MPS/DLR/IDA*

In the coming months we will discuss other intriguing activities and how Dawn will make measurements never even considered before. But for now, let's look at how this extension came about.

As readers of the Dawn Journals know (and as you will be reminded below), there has been very good reason in recent years to believe the spacecraft would not operate beyond the end of its prime mission. However, the veteran explorer is in very good health. It is one of Earth's most experienced and capable ambassadors to the cosmos, we want to squeeze as much out of this mission as we can. Ever resourceful, the Dawn team recognized in March 2016 that the probe had the capability to do yet more and decided to give NASA Headquarters a unique choice: remain at Ceres (as always expected) or go elsewhere.

It is worth pondering how extraordinary this is. Most spacecraft can only make minor adjustments to their trajectories, so at the end of their prime missions, they generally go wherever they were already headed. If a spacecraft is in orbit around some planetary body, it remains in orbit. If a spacecraft is not in orbit, having previously flown past one or more bodies that orbit the sun, its course is largely determined by the targeting for the last encounter. A planet's gravity may have redirected it, but otherwise its propulsion system has to do the work, and that usually can produce only a tiny change in direction. If a spacecraft is not already in orbit around a planetary body, it won't be able to enter orbit.

Dawn is different. With its uniquely capable ion propulsion system, Dawn is the only spacecraft ever to travel to a distant destination, orbit it, later break out of orbit, then travel to another faraway destination, and orbit it. And even while in orbit around Vesta and Ceres, Dawn maneuvered extensively, optimizing its orbits for its scientific investigations. And yet this remarkable ship can do still more. It has the capability to leave its second destination and continue its travels.

Dawn's brilliant and creative navigators analyzed possible missions to more than 68,000 known objects. That alone is a nice illustration of the powerful potential.

The project team very quickly narrowed the list to the most interesting body Dawn could reach after leaving Ceres, a large asteroid named Adeona. That mission offered the best alternative to further studies of the dwarf planet.

But how to decide between these two attractive possibilities? Some members of the Dawn team preferred continuing the exploration of Ceres and others preferred going to Adeona. Similarly, some people prefer cake and some prefer pie. (That's not a perfect example, because it's obvious cake is better, but you get the idea.)

NASA thoroughly evaluated the scientific potential and other aspects of the options. Part of this was an assessment by an independent group of esteemed scientists. The conclusion was that either would be valuable but that studying Ceres further was preferable.

From the perspective of your correspondent -- passionate about space exploration since the age of four, a professional scientist (as well as a scientist at heart), an engineer and a taxpayer -- this is a wonderful outcome. How could one want anything other than such a well-considered decision?

But how is it even possible that the team could have offered to NASA the option of visiting Adeona for the extended mission? We have emphasized for several years that Ceres would be Dawn's final home. If you had asked even as recently as early this year whether the spacecraft could leave Ceres (and many of you did), we would have responded that such a prospect was unrealistic and inconceivable (and we did). We have described in great detail how the failure of two of Dawn's four reaction wheels was so serious that it was only with heroic effort that the distant robot was able to complete its original assignments. We have explained repeatedly that the spacecraft will soon expend the last of its hydrazine propellant, then immediately lose the ability to point its solar arrays at the sun, its antenna at Earth, its scientific sensors at Ceres or its ion engine in the direction needed to fly elsewhere. Why the change now, and how could Dawn operate for a multiyear journey?

We have discussed in recent months how remarkably well the flight team has done in conserving hydrazine, significantly exceeding any reasonable expectations and thereby extending Dawn's functional lifetime. Moreover, mission controllers know that the probe consumes less hydrazine at higher altitudes. Contrary to many people's notions, the dwarf planet's gravity is appreciable, and operating so close to it requires a very high rate of hydrazine consumption. Dawn is circling only 240 miles (385 kilometers) above Ceres, closer than the International Space Station is to Earth. But during the long deep-space journey to Adeona, Dawn would use the precious propellant much more sparingly. So despite the loss of the two reaction wheels, under the expert guidance of its terrestrial colleagues, the ship could set sail once again for a new and distant land beyond the horizon.

Isn't it incredibly cool that humankind has the capability to fire up the ion engine on a distant interplanetary spaceship and pilot it out of orbit around a dwarf planet to fly more than halfway around the sun on a bold expedition of 900 million miles (1.5 billion kilometers) to investigate a huge asteroid? (Hint: the answer is yes.)

Exciting as such a voyage might seem, it is gratifying that a thoughtful, rationale decision was made that yields an even better outcome. Rather than terminate the present mission after it has exceeded all of its original objectives, and rather than embark on that new mission, the best possible use of Dawn is to do what it is doing right now: extracting secrets from dwarf planet Ceres. And now we can look forward to more, as Dawn pursues new objectives. As the extended mission progresses, we will describe marvelous new findings from the rich trove of data Dawn is returning, and we will see how the team plans to take advantage of this unique opportunity to learn more about the nature of the solar system.

If you share in the passion for exploration, if you thrill to new discoveries and new knowledge or even if you just want to see how many more silly Dawn Journal greetings your correspondent can concoct, stay onboard as Dawn's adventure at Ceres continues.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 2.69 AU (250 million miles, or 403 million kilometers) from Earth, or 1,090 times as far as the moon and 2.65 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 45 minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT July 27, 2016

TAGS: CERES, DAWN

ABOUT THE AUTHOR

Marc Rayman, Dawn Mission Director and Chief Engineer

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 is continuing to record the extraordinary sights on dwarf planet Ceres. The experienced explorer is closer to the alien world than the International Space Station is to Earth.

Dawn has completed more than 1,000 orbital revolutions since entering into Ceres' gentle but firm gravitational grip in March 2015. The probe is healthy and performing its ambitious assignments impeccably. In the last few months, we have described how Dawn has greatly exceeded all of its original objectives at Ceres and the excellent progress it has been making in collecting bonus data. On schedule on May 25, the spacecraft completed the mapping campaign it began on April 11, in which it took photographs with the camera pointed to the left and forward as it circled Ceres. Now it is looking to the right and forward to get another stereo view.

In January we mentioned that, having already acquired far more measurements with the visible and infrared mapping spectrometer than anticipated, scientists were devoting further observations to infrared rather than visible. Now Dawn is operating both spectrometers again. Having seen much more of Ceres in the infrared from this low altitude than planned, mission controllers now can afford to allocate some of the spacecraft's data storage and interplanetary radio transmissions to visible spectra in exchange for limiting the infrared to a few select targets. In addition, a device in the infrared spectrometer that lowers the sensor's temperature to -307 degrees Fahrenheit (-188 degrees Celsius) is showing signs of age. (We saw here that the sensor can detect heat. So to avoid interference from its own heat, it needs to be cooled.) Its symptoms are not a surprise, given that the instrument has acquired far, far more data at Vesta and Ceres than it was designed for. It is continuing to function quite productively, but now its use is being curtailed.

One of the mission's objectives was to photograph 80 percent of Ceres' vast landscape with a resolution of 660 feet (200 meters) per pixel. Dawn has now photographed nearly the entirety (99.9 percent) with a resolution of 120 feet (35 meters) per pixel. The adventurer has shown us 25 percent more terrain than planned with 5.7 times the clarity. We can see detail 830 times sharper than the Hubble Space Telescope revealed.

What is the value of that much detail? The more detailed the portrait, the better understanding geologists can obtain. Imagine the difference (not only visually but also emotionally and socially) between seeing a person at the opposite end of a soccer field and seeing them from five inches (12 centimeters) away.

The pictures speak quite eloquently (and succinctly) for themselves, but let's take a look at one of the many uses of these sharp photographs: determining the age of geological features.

In December, we gave an approximate age of 80 million years for Occator Crater, site of the famous "bright spots" (or famously bright spots). It takes more than an experienced geological eye to estimate such an age.

Now don't forget that we are trying to ascertain the age, but we are going to get there on a long and winding path, mostly because it's an opportunity to touch on some fun and interesting topics.

To begin, we go back in time, not quite 80 million years, to the Apollo program. Astronauts returned from the moon with many treasures, including 842 pounds (382 kilograms) of lunar material collected on six missions. In addition, three Soviet robotic Luna spacecraft came back with a total of 11 ounces (0.3 kilograms).

Earth's total inventory of lunar samples is larger. By comparing the chemical composition of that material with a great many meteorites, scientists have identified nearly 120 pounds (54 kilograms) of meteorites that were blasted from the moon by asteroid impacts and then landed on our planet.

Other meteorites are known to have originated on Mars. The principal method by which that connection was made was comparison of gasses trapped in the meteorites with the known constituents of the Martian atmosphere as measured by the two Viking spacecraft that landed there 40 years ago. Scientists thus have 276 pounds (125 kilograms) of Martian material.

Of course, unlike the Apollo and Luna samples, the lunar and Martian meteorites were selected for us by nature's randomness from arbitrary locations that are not easy to determine.

The moon and Mars are two of only three (extant) extraterrestrial bodies that are clearly established as the source of specific meteorites. The third is Vesta, the fascinating protoplanet Dawn explored in 2011-2012. That world is farther away even than Mars, and yet we have 3,090 pounds (1,402 kilograms) from Vesta, or more than 11 times as much as from the red planet and more than three times as much as from the moon. We reflected on these meteorites during our travel from Vesta to Ceres.

It is thanks to Dawn's detailed measurements of the composition of Vesta that scientists were able to clinch the connection with the meteorites that were under study in terrestrial laboratories. The impact of an asteroid perhaps 20 to 30 miles (30 to 50 kilometers) in diameter more than one billion years ago excavated Vesta's Rheasilvia Crater. It left behind a yawning basin more than 300 miles (500 kilometers) across, a mountain more than twice the height of Mt. Everest, and a network of about 90 canyons with dimensions rivaling those of the Grand Canyon. And it launched a tremendous amount of material into space. Some of it settled back onto Vesta, resurfacing much of the southern hemisphere, but some of it departed with so much energy that it escaped Vesta's gravitational hold. Some of the biggest pieces liberated by that tremendous impact are now visible as small asteroids known as vestoids. And some of the small pieces eventually made their way to the part of the solar system where many of our readers (perhaps including you) reside. After Earth's gravity took hold of any of those wandering interplanetary rocks and pulled them in, they became meteors upon entering the atmosphere, meteorites upon hitting the ground, and keys to studying the second largest object in the main asteroid belt upon entering laboratories. One esteemed scientist on the Dawn team opined that with Dawn's detailed data and our Vestan samples, Vesta joined the ranks of the moon and Mars as the only extraterrestrial bodies that have been geologically explored in a rigorous way.

With so many meteorites from Vesta, why have we not linked any to Ceres? Is it because the rocks didn't get blasted away in the first place, or they didn't make it to the vicinity of Earth or to the ground, or we have not recognized that they are in our collections? While there are some ideas, the answer is not clear. For that matter, although Vesta and Ceres are the two largest residents of the main asteroid belt, why have we not tied meteorites to any of the smaller but still sizable bodies there? We will return to this question in a future Dawn Journal, but for now, let's get back to the question of how Dawn's pictures help with measuring the ages of features on Ceres.

Crater on Ceres — Dawn took this picture on March 22 from an altitude of 240 miles (385 kilometers). The impact that formed the crater in the upper left deposited material outside the crater, partially covering the smaller craters that were already there. The area on the lower right of the picture, including the other large crater in this scene, has many more small craters and so must be older. Sunlight in this photograph is coming from the right, so all the craters are dark on the right side where their walls descend into shadow. The crater walls on the left face the sun and so are illuminated. Look closely around the young crater and on its floor to see many very small features with the opposite lighting: they are bright on the right and dark on the left. Unlike all the craters, they are not depressions but rather are very large boulders, catching sunlight on the right side. (Each pixel in this picture is 120 feet, or 35 meters.) The tremendous punch that excavated the young crater must have produced these boulders. The Dawn project does not recommend doing the same thing at home. Full image and caption. Image credit: *NASA/JPL-Caltech/UCLA/MPS/DLR/IDA*

Scientists have measured the relative abundance of different atomic species in the Apollo and Luna samples from different locations. Elements with known radioactive decay rates serve as clocks, providing a record of how old a sample is. This process enabled scientists to pin down the ages of many craters on the moon, and from that, they developed a history of the rate at which craters of different sizes formed.

During some periods in the moon's history, it was pelted with more interplanetary debris, forming more craters, than at other times. This uneven history is a reflection of solar-system-wide events. For example, it seems that the giant planets of the outer solar system jockeyed for their orbital positions around the sun about four billion years ago. Their gravitational jostling over the course of about 300 million years may have sent a flurry of material into the inner solar system, where the moon recorded the bombardments.

The moon lives at one astronomical unit (1 AU, which is 93 million miles or 150 million kilometers) from the sun (because that's where Earth is). Scientists can extrapolate the cratering history the moon experienced to other locations in the solar system, so they can calculate what other bodies should have been subjected to. Ceres lives between 2.6 and 3.0 AU from the sun.

Dawn observed this scene on March 28 from an altitude of 240 miles (385 kilometers). The prominent crater on the left lies on the western rim of Azacca Crater, which goes vertically through the center of the picture. (Azacca is a Haitian god of agriculture.) With a diameter of 31 miles (50 kilometers) Azacca, is too large to fit in a single picture from this low altitude. Note the many deposits of bright material, which is likely some kind of salt. Full image and caption. Image credit: *NASA/JPL-Caltech/UCLA/MPS/DLR/IDA*

Scientists count the number and size of craters in an area of interest, like inside Occator Crater and on the blanket of ejected material surrounding it. (See the picture above.) With their mathematical description of how many impacts should have occurred over time, they can estimate how long the surface has been exposed and accumulating craters. Although the ages have not been computed yet, compare the third and fourth pictures presented in April for a clear illustration of areas that are of very different ages.

The method of determining the age involves many subtleties we did not touch on here, and there are many complicating factors that limit the accuracy. But the dating results are improved substantially by including smaller craters in the count.

It is readily apparent in pictures of Ceres, Vesta, the moon, and elsewhere that small craters are more prevalent than large ones. There has simply been more small stuff than large stuff flying around in the solar system and crashing into surfaces to make craters. There are more bits like sand grains than pebbles, more pebbles than boulders, more small boulders than big boulders, etc.

Extending Dawn's photographic documentation of the Cerean landscapes to finer resolution provides the means to develop a better census of the population of craters, yielding a better measure of the age.

Dawn's bonus observations thus give us not only a sharper view of the dwarf planet beneath it today but also a more accurate view of the mysterious world's past. As this extraordinary journey through space and time continues, next month, we will look to the future.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.42 AU (318 million miles, or 512 million kilometers) from Earth, or 1,400 times as far as the moon and 3.38 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 57 minutes to make the round trip.

Dr. Marc D. Rayman
3:30 p.m. PDT May 31, 2016

TAGS: CERES, DAWN, EZINU CRATER

ABOUT THE AUTHOR

Marc Rayman, Dawn Mission Director and Chief Engineer

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.

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