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Ceres

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

TAGS: DAWN, CERES, VESTA, ASTEROID BELT

  • Marc Rayman
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Mariner Jupiter Saturn 1977 (Voyager) Logo

In August and September 1977, two Voyager spacecraft were launched on a Grand Tour of the solar system. In 1973, the mission had been named Mariner Jupiter-Saturn 1977 (MJS ‘77) and was intended to go only as far as Jupiter and Saturn.

In March 1977 the mission name was changed to Voyager. In October 1978, a Voyager Fact Sheet mentioned the possibility of sending Voyager 2 to Uranus and Neptune. It would happen only if the primary science objectives were met at Saturn first. Even though the extended mission was not certain before launch, Voyager engineers (unofficially) designed and built the spacecraft to be capable of navigating to Uranus and Neptune, and surviving the longer trip. On-board computers were reprogrammed during the voyage, giving the spacecraft the ability to successfully return many more images and much more information than were expected. It’s unlikely the Voyager team imagined that both spacecraft would still be operating 40 years after launch.

For more information about the history of JPL, contact the JPL Archives for assistance. [Archival and other sources: Various Voyager and JPL History web pages; Voyager Fact Sheet, 10/6/1978; Section 260 photo album and index.

TAGS: VOYAGER, MARINER, URANUS, NEPTUNE, GRAND TOUR, JUPITER

  • Julie Cooper
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Lower Manhattan

Projecting regional changes

“Sea level scientists have a pretty good grasp on global mean sea level,” said Steve Nerem, a professor in the Aerospace Engineering Sciences Department at the University of Colorado and the team leader for NASA’s Sea Level Change Team (N-SLCT). “It’s the regional sea level change that’s the next big question, the next big step for sea level science,” he added.

Nerem and much of the rest of the N-SLCT were in New York City this July where more than 300 scientists from 42 countries gathered at Columbia University for a weeklong Regional Sea Level Changes and Coastal Impacts Conference. The international conference was organized by the World Climate Research Programme (WCRP), Climate and Ocean – Variability, Predictability, and Change (CLIVAR), and the UNESCO Intergovernmental Oceanographic Commission and was co-sponsored by NASA.

Regional sea level change is more variable, over both space and time, than global sea level change and can diverge by up to 7 inches (20 centimeters) or more from the global mean. Additionally, making regional projections about future sea level differs from making global mean sea level projections. This is due to the fact that different processes contribute to sea level change in coastal regions.

Global sea level rise is caused by thermal expansion of warmer water plus contributions from ice sheets and glaciers. Regional sea level change, especially along coastlines, is influenced by additional factors, including vertical land movements, waves and tides, and winds and storms. So in order to estimate sea level inundation and flood risk, scientists have to understand all the factors that contribute to extreme water levels such as local sea level rise, land subsidence, tides, waves and storm surge.

Members of the N-SLCT understand the importance of studying coastal sea level change and improving the accuracy of regional projections. Ben Hamlington, assistant professor in the Ocean, Earth and Atmospheric Sciences Department at Old Dominion University in Norfolk, Virginia, and upcoming team leader for N-SLCT is serious about understanding sea level.

“The overarching theme of my scientific research,” he said, is “to consistently improve regional sea level projections.” Manhattan, where the conference was held, for example, lies within a few feet of sea level, and furthermore, the U.S. East Coast has some of the highest amounts of projected sea level increase.

“Global means aren’t very useful for someone who’s on the coast of Virginia where I live,” Hamlington said. A main part of the challenge of predicting regional sea level is that what causes the sea level changes and the flooding varies dramatically from place to place. Hamlington described a term called “nuisance flooding,” which is a type of persistent tidal flooding that leads to public inconveniences like road closures and backed-up storm water systems.

“Basically it means your path to work has to change because a certain road is blocked or impassable. You can still get to work, but it might take longer,” he explained. Right now, these nuisance-flooding events occur multiple times a year. But as sea level continues to rise, the nuisance flooding will get more and more frequent and will become even more of a problem. “Where I live, it’s hard to separate the pure science from the applications. With all this flooding, the broader significance of your work is very clear,” he said.

In Norfolk, Virginia, glacial isostatic adjustment (GIA) is around 0.04 inches (1 millimeter) per year, another millimeter per year of subsidence is due to slow subsidence into the Chesapeake Bay Meteor Impact Crater plus ground water pumping. Finally add 0.08 inches (2 millimeters) per year from the ocean rising and “You get the long-term tide gauge rate of relative sea level rise of just lower than 0.20 inches (5 millimeters) per year over the last 100 years. That’s a pretty high rate of sea level rise over a long period of time,” Hamlington explained. “Beyond nuisance flooding, there are also extreme events,” he continued. “During a storm event, you can get several feet of water in some parts of Norfolk.”

Actionable science

Stakeholders and decision makers are the ones driving the demand for improved regional sea level projections, Hamlington continued. “They’re the ones driving the discussion toward regional projections and that’s what’s needed for planning efforts.” These stakeholders include state and local public works officers responsible for infrastructure such as stadiums, roads, seawalls, and dykes plus pumps, water utilities, other utilities, businesses, and coastal inhabitants.

Scientists are responsible for helping society. This is why decision makers and scientists have come together to co-produce actionable science, to discuss how to communicate and collaborate, and to ensure that sea level science is being understood by the adaptation community.

“This is one of the biggest sea level conferences that we’ve had, when everybody who is working in different areas of the field comes together,” said Nerem. There were presentations on a variety of techniques to measure sea level change: tide gauges, measurements in marshes, paleo-sea level, corals, but from the perspective of the N-SLCT, “ We’re really focused on how to use remote sensing, satellite altimetry from Jason-1, 2 and 3 and Gravity Recovery and Climate Experiment (GRACE) combined with GPS measurements to improve regional sea level measurements and projections.”

Nerem’s project targets regions around the globe that are susceptible to inundation but don’t have much measurement infrastructure, such as Bangladesh. Many of these regions do not have detailed digital elevation models or 50 years of tide gauge measurements like we do in the United States.  “If we use our satellite techniques and test them in a place we understand, then we can go out where we don’t have that infrastructure and assess future sea level change in those regions.”

The N-SLCT hopes to leverage the satellite observations as much as possible to try to better understand future regional sea level change. This will help decision makers, coastal managers and stakeholders better adapt and prepare for the impacts of sea level rise.

According to Nerem, “We would like to produce a new assessment of future regional sea level change that benefits from the extensive record of satellite measurements collected by NASA.”

Thank you for reading,
Laura

TAGS: SEA LEVEL, REGIONAL SEA LEVEL, GLOBAL SEA LEVEL

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Mars Pathfinder Rover Team, 1994

In December 1994, a group of Mars Pathfinder team members gathered for a photo with the Sojourner Rover model.

In December 1994, a group of Mars Pathfinder team members gathered for a photo with the Sojourner Rover model.  They were working on rover technology development efforts about two years before the anticipated launch date.

On February 1, 1995, Mars Day was held on the JPL mall – an event for JPLers, schoolchildren, and visitors.  The Office of Mars Exploration sponsored presentations, booths, and demonstrations of technology from Mars Pathfinder and Mars Global Surveyor.  Mars Exploration Program Manager Donna Shirley said, “We wanted people from other projects and those who aren’t involved in our office to see what we’re up to, what kind of technologies we’ve developed.  We’re excited about what we’re doing and we wanted to share that excitement.”

If you would like to help the Archives staff identify people in this photo, please see the partial list at https://pub-lib.jpl.nasa.gov/docushare/dsweb/Services/Document-2749 (click on title to open PDF document).

For more information about the history of JPL, contact the JPL Archives for assistance. [Archival and other sources: Collection JPL508, various issues of Universe, photo index, Allen Sirotta, Brian Wilcox, and David Braun.]

TAGS: MARS, PATHFINDER, ROVER, TEAM, 1994

  • Julie Cooper
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Dawn observed this scene on Sept. 28, 2015

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

TAGS: DAWN, NEPTUNE, CERES

  • Marc Rayman
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Greenland’s ice sheet

Reflections at the top of the world

It was 11:30 in the morning and GLISTIN-A instrument engineer Ron Muellerschoen and I were in northern Greenland at the Thule Air Base pier looking over the frozen Wolstenholme Bay. We’d been talking about the time Ron was wearing shorts here during the summer, but today it was the typical -22 Fahrenheit (-30 Celsius.) And even though over the past week we’d somehow gotten used to the cold and I was wearing a big parka, my legs were starting to get cold after walking for an hour. So we decided to head back.

As we turned around to go, I was struck in the face by the sun’s rays reflecting off the ice-covered ground. The brightness was astounding. And in that instant the meaning of “albedo” was seared into my brain in a way that went beyond reading about the science or looking at illustrations and animations.

There was something special about the experience of having the rays of the sun, which was sitting low in the high latitude sky, hit the ice surface at that extremely low angle and reflect off into my eyes.

Albedo is a measure of how reflective a surface is, how much light energy bounces off and reflects away and how much light energy gets absorbed. (Think hot asphalt on a sunny day. Black asphalt has a low albedo and absorbs light energy, while the brightest white has the highest albedo and reflects the light.)

Walking in Greenland
Team Oceans Melting Greenland (OMG) stands on the Greenland Ice Sheet. Notice the long shadows made by small rocks on the ground.
Albedo is part of what has stabilized Earth’s climate for millennia, because under normal conditions, the white of the polar ice reflects light energy back to space, keeping average global temperature stable. The more area covered by ice, the more heat reflected back to space. The more ice that melts, the more heat absorbed. Increasing temperatures are melting more ice, which exposes darker brown and green land and dark blue ocean. Those darker surfaces have a lower albedo, so they absorb more of the heat from the sun’s rays.

I stood for a moment, looking at the ground — a hard, dry, crusty mixture of ice and snow that made an exceptionally satisfying crunch crunch noise as our boots marched through it — and tried to figure out the color: 50 shades of white. I settled on white/light blue/silvery sparkle. Due to the low angle of the sun, the tiniest rough edge the size and shape of a pebble on the ground’s textured surface left a long, dark shadow.

No matter where we were or how we stood or what time of day, all day, every day, there were always long shadows — crazy long shadows. At 78 degrees north latitude, a full 12 degrees above the Arctic Circle, the sun will never be overhead. Never. I know that seems unbelievable, but even during the summer solstice, when Earth’s North Pole is tilted toward the sun, or during the four summer months of 24-hour daylight, the sun is always low, low, low at this latitude.

Low on the horizon

In that moment, I also understood another science question that had been bothering me. I’d been wondering why the meter-thick sea ice hadn’t yet begun to melt. Even though it was the end of March, even though the equinox had passed, the sun was out and the days were getting longer. In fact, up here the days were getting much longer, very quickly. On March 23, just three days after the equinox, we were already having 14-hour days with sunsets lasting past 9 p.m. That’s because in these high latitudes, the day length can increase by as much as 40 minutes per day. And by mid-April, just a few weeks after spring equinox, there will be 24 hours of daylight and the sun won’t set again until September. 

By mid-April the meter-thick layer of frozen seawater that covers the sea surface and fills the fjords will completely melt and expose the dark blue ocean underneath. But today, even in this brilliant sunshine, even on this day of 14-hour sunlight, the ocean was still completely frozen over. 

Greenland ice
The sun sits low in the sky even during midday. This iceberg calved off one of Greenland's coastal glaciers, floated into Wolstenholme Fjord and then became trapped in sea ice.
Just the other day in fact, a group of us walked about 3 miles (5 kilometers) across the frozen ocean into the middle of Wolstenholme Fjord to visit icebergs trapped in the frozen seawater. We hadn't worried at all about breaking through. 

But “Why?” I’d been wondering. Why, with all this extra sunshine, was the sea surface still so frozen? And why did that hard, dry, crusty mixture of ice and snow still remain on the ground?

In that instant, as the glint of the sunlight reflecting off the icy ground hit my face, I knew exactly why. It was the extraordinarily low angle of the sunlight that bounced right off the stunning bright whiteness of the ice. The sunlight was not absorbed by the ice and snow and instead was reflected away. It wouldn’t be until another month or so that the sun would get a little higher in the sky. And although the sun would never be directly overhead up here, it would be high enough to begin melting the ice.

No matter how much a person studies Greenland, or the northern latitudes, or albedo, or Earth in general, going into the field to experience those things can change your entire understanding of the world and how it works. I stood there for a moment, just allowing the high-latitude sun’s cold rays to glance off the snowy ice and shine straight into my face.

NASA’s Oceans Melting Greenland (OMG) team is here in Greenland; here to find out specifically how much ice the island is losing due to warmer ocean waters around the coastline. There is almost no ocean data in remote places like this, but OMG is busy working to change that, studying the complex ocean processes that affect Greenland's coastline because gathering data is critical to understanding Earth’s complex climate. This information will help us understand the amount of sea level rise we're going to have around the world.

Thank you for reading,

Laura

TAGS: EARTH, OCEANS, MELTING, GREENLAND

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Exhibit  explained the flow of data from a spacecraft to the Deep Space Network stations

In October 1967 Mariner 5 had just reached Venus, JPL was looking forward to the 10th anniversary of Explorer 1 and the launches of Surveyor 6 and 7 to the Moon, and Mariner 6 and 7 were in development.

When visitors were escorted into the lobby of the Space Flight Operations Facility (SFOF), they saw the reception/security desk, a waiting area, and this new exhibit. It explained the flow of data from a spacecraft to the Deep Space Network stations (or Deep Space Instrumentation Facilities) to the SFOF. A series of photos showed various work stations in the SFOF, as well as the technology being used in the facility (in the main operations area and behind the scenes). During 1967 and 1968, JPL hosted visits by NASA staff, members of Congress, foreign dignitaries, JPL contractors/partners, former employees, student groups, professional groups, celebrities, and the press.

For more information about the history of JPL, contact the JPL Archives for assistance. [Archival sources: 318 and P photo albums and index.]

TAGS: DEEP SPACE NETWORK, SPACE FLIGHT OPERATIONS EXHIBIT, MARINER, EXPLORER 1, SURVEYOR

  • Julie Cooper
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Illustration of the Dawn spacecraft flying towards Ceres.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The sun!

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

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

TAGS: CERES, DAWN

  • Marc Rayman
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NASA's modified G-III aircraft, with the GLISTIN-A radar instrument visible below, on the runway at Thule Air Base, Greenland.

Dive into a sea of Oceans Melting Greenland data

"Get to work." The phrase stuck in my head.

I had just walked out of a two-and-a-half-hour debriefing with NASA’s Oceans Melting Greenland (OMG) Principal Investigator Josh Willis, but the whole meeting could be summed up in those three little words of his: Get to work.

It was as though he’d been ringing one of those big ol’ dinner gongs. Data! Hot off the press! Come and get your data! Calling all oceanographers, geologists, paleo-climate scientists: come and get a big ol’ helping of free data.

He made me hungry for data, too.

OMG has just returned from its second spring season. Every April for five years, just before the ice starts to melt, OMG flies a radar instrument over almost every glacier in Greenland that reaches the ocean and collects elevation measurements within a 6.2-mile (10-kilometer)-wide swath for each glacier individually so we can measure how quickly each one is thinning. That’s literally hundreds of glaciers.

“We have more than 70 of these swaths that cover a couple hundred glaciers to create new elevation maps that are high accuracy, high resolution and high quality,” Willis said.

Greenland probe map
The blue squares on this Greenland map show 250 planned locations for probes dropped by plane into ocean waters near the coast. Called Aircraft Expendable Conductivity Temperature and Depth probes, they measure ocean temperature and salinity.

OMG also has bathymetry data from sonar and gravimetry. And we have a year’s worth of Airborne Expendable Conductivity Temperature Depth Probes AXCTD data collected last September plus hundreds of vertical profiles of temperature and salinity taken from ship surveys. “We have temperature measurements in many glacial fjords that have never had a historical temperature profile before. And none of that data is being used to its fullest extent yet.” OMG will set the baseline so we know what the water temperatures are today, and as we look to the future, we can watch them warm. That’s huge. 

I recounted all the times I’ve told someone that many parts of the ocean are still so unknown. I thought about all the times I’ve written about the OMG aircraft flying into remote, uncontrolled airspace, or researching the ocean water-ice interface around Greenland: So many of these places still nameless, still anonymous, still unidentified, still unknown. It’s mind blowing.

And somewhere in all this new data is information about the correlation between the ocean water and the ice as well as the answer to the question of how each glacier may or may not be affected by the waters offshore. “We know that warm water reaches a lot of glaciers. And there have been surveys in few places, but we’ve never had a comprehensive survey of the shelf water before,” Willis said.

OMG is mapping out the edges of glaciers and watching them change year on year on year. The mission measures glacial elevation in the last few kilometers before the glacier hits the water to see exactly how much the glacier shrank or retreated or both. In a few cases, the opposite might happen. Over a single year, a glacier might not have had as much calving or it might have slowed down, which would cause it to thicken and advance.

Aerial shot of a Greenland fjord
Aerial shot of a Greenland fjord shows, at the top, the glacier's origin in the ice sheet, and, at the bottom, its termination point, where it enters a frozen ocean.

There are literally hundreds of glaciers to research and dozens of papers buried in that data. And anybody who wants to can sift through it and publish. “You could get a Ph.D. done really fast,” Willis added enticingly. Here are some recommendations for interesting scientific research:

  • OMG’s temperature data could be used to write oceanography papers about where the warm water is on the shelf and to map out and catalogue which glaciers terminate in deep Atlantic water and which ones sit in shallow water. OMG has enough data to catalogue the depth of the faces of two-thirds of the glaciers around Greenland.
  • Paleo-climatologists and geologists can use new clearly mapped-out OMG bathymetry data to study how ancient glaciers carved troughs in the sea floor. Looking at maps of the seafloor will help us understand the implications for Greenland’s ancient ice sheet. Some flat-bottomed troughs, for example, show evidence of where little ancient rivers must have carved their way through to erode the paleo-glaciers. And sea floor sediments could be analyzed to find out how far the ancient glaciers advanced.
  • Overview papers that compare and contrast the east, west, north and south coasts of Greenland would be incredibly useful to have.
  • Some elevation maps made from historical datasets as well as a few decades’ worth of temperature measurements already exist for some isolated regions across Greenland. Using these historical maps, it’s now possible to compare them with current measurements of temperature and elevation in these locations to observe the changes.
  • OMG is also gathering oceanography data around Greenland. Since the Atlantic Ocean water is very warm and salty and the Arctic Ocean water is cold and fresh, the ratio of those two could be analyzed. Warm Atlantic Ocean water has been in the coastal area around Greenland forever, but how much Atlantic water makes it onto the shelf and reaches the glaciers? This is affected by the bathymetry and the winds, which affect the local currents. And according to Willis, “There’s really still a lot to learn.”

Already there are four downloadable datasets right here! So, come and get it, all you hungry Ph.D. oceanographers.

Get to work.

I can't wait to read your papers,

Laura

TAGS: EARTH, OCEANS, MELTING, GREENLAND

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

Dawn has accomplished an extraordinary orbital dance.

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

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

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

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

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

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

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

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

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

Flow on Ceres

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

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

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

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

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

Landslide photo

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

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

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

Navigation photo

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

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

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

Navigation photo

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

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

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

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

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

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

TAGS: CERES, DAWN

  • Marc Rayman
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