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31 Mar
2016
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | March 31

by Marc Rayman
 

Dear Resplendawnt Readers,

One year after taking up its new residence in the solar system, Dawn is continuing to witness extraordinary sights on dwarf planet Ceres. The indefatigable explorer is carrying out its intensive campaign of exploration from a tight orbit, circling its gravitational master at an altitude of only 240 miles (385 kilometers).

Even as we marvel at intriguing pictures and other discoveries, scientists are still in the early stages of putting together the pieces of the big puzzle of how (and where) Ceres formed, what its subsequent history has been, what geological processes are still occurring on this alien world and what all that reveals about the solar system.

For many readers who have not visited Ceres on their own, Occator Crater is the most mysterious and captivating feature. (To resolve the mystery of how to pronounce it, listen to the animation below.) As Dawn peered ahead at its destination in the beginning of 2015, the interplanetary traveler observed what appeared to be a bright spot, a shining beacon guiding the way for a ship sailing on the celestial seas. With its mesmerizing glow, the uncharted world beckoned, and Dawn answered the cosmic invitation by venturing in for a closer look, entering into Ceres’ gravitational embrace. The latest pictures are one thousand times sharper than those early views. What was not so long ago a single bright spot has now come into focus as a complex distribution of reflective material in a 57-mile (92-kilometer) crater.

Occator Crater and Ceres' Brightest Spots

Occator Crater is shown in this mosaic of photos Dawn took at its lowest altitude of 240 miles (385 kilometers). Go to the full image to see exquisite details of the crater walls and the many fractures in the floor. The exposure for these pictures was set for typical Ceres lighting to show the structure of the crater itself and the surrounding area. The pictures below use a shorter exposure to bring out more detail in the famously bright area. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Occator Crater and Ceres' Brightest Spots: Figure 1

Dawn took these pictures of Occator Crater on March 16. This is the most reflective area on Ceres. The exposure was optimized for the brightest part of the scene, revealing details that were indiscernible in longer exposures and in photos from higher altitudes. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Scientists are still working on refining their understanding of this striking region. As we described in December, it seems that following the powerful impact that excavated Occator Crater, underground briny water reached the surface. The detailed photographs show many fractures cutting across the bright areas, and perhaps they provided a conduit. Water, whether as liquid or ice, would not last long there in the cold vacuum, eventually subliming. When the water molecules disperse, either escaping from Ceres into space or falling back to settle elsewhere, the dissolved salts are left behind. This reflective residue covers the ground, making the spellbinding and beautiful display Dawn now reveals.

While the crater is estimated to be a geological youngster at 80 million years old, that is an extremely long time for the material to remain so reflective. Exposed for so long to cosmic radiation and pelting from the rain of debris from space, it should have darkened. Scientists don’t know (yet) what physical process are responsible, but perhaps it was replenished long after the crater itself formed, with more water, carrying dissolved salts, finding its way to the surface. As their analyses of the photos and spectra continue, scientists will gain a clearer picture and be able to answer this and other questions.

Center of Occator Crater (Enhanced Color)

The high resolution photo of the central feature of Occator Crater is combined here with color data from the third mapping orbit. With enhanced color to highlight subtle variations, this illustrates the red tinge that we described in December. (The scene would not look this colorful to your eye, even if you and your eye were fortunate enough to be in a position to see it.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI/LPI

These latest Occator pictures did not come easily. Orbiting so close to Ceres, the adventurer’s camera captures only a small scene at a time, and it is challenging to cover the entirety of the expansive terrain. (Perhaps it comes as a surprise to those who have not read at least a few of the 123 Dawn Journals that precede this one that operating a spacecraft closer to a faraway dwarf planet than the International Space Station is to Earth is not as easy as, say, thinking about it.) But the patience and persistence in photographing the exotic landscapes have paid off handsomely.

We now have high resolution pictures of essentially all of Ceres save the small area around the south pole cloaked in the deep dark of a long winter night. Seasons last longer on Ceres than on Earth, and Dawn may not operate there long enough for the sun to rise at the south pole. By the beginning of southern hemisphere spring in November 2016, Dawn’s mission to explore the first dwarf planet discovered may have come to its end.

This animation from NASA’s Dawn mission shows the spacecraft’s imaging coverage of dwarf planet Ceres during its low-altitude mapping orbit, 240 miles (385 kilometers) above the surface. The movie shows that the brightest area on Ceres, located in Occator Crater, was one of the last features to be imaged as Dawn progressively built its map.

This is an accelerated excerpt from this complete animation showing Dawn’s accumulated photographic coverage of Ceres during the lowest altitude mapping campaign from December 16 to March 11. To ensure that it can see all latitudes, Dawn travels in a polar orbit, flying from the north pole to the south pole over the illuminated hemisphere and back to the north over the nighttime hemisphere. Each orbital revolution takes 5.4 hours. Meanwhile, Ceres rotates from east to west, completing one Cerean day in just over nine hours. The combined motion causes the spacecraft’s path over the landscape to follow these graceful curves. Consecutive orbits pass over widely separated regions because Ceres continues to rotate beneath Dawn while the spaceship glides over the hidden terrain of the night side. The swaths that don’t fit the typical pattern are the extra pictures Dawn took as it turned away from the scenery below it, as described in January. The spacecraft does not take pictures on every orbit, because sometimes it performs other functions (such as pointing its main antenna to Earth), so that causes gaps that are filled in later. Note that the center of the popular Occator Crater (slightly above and to the right of center), just happened to be one of the last places to be imaged as Dawn progressively built its high-resolution map. Animation credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In addition to photographing Ceres, Dawn conducts many other scientific observations, as we described in December and January. Among the probe’s objectives at Ceres is to provide information for scientists to understand how much water is there, where it is, what form it is in and what role it plays in the geology.

We saw that extensive measurements of the faint nuclear radiation can help identify the atomic constituents. While the analysis of the data is complicated, and much more needs to be done, a picture is beginning to emerge from Dawn’s neutron spectrometer (part of the gamma ray and neutron detector, GRaND). These subatomic particles are emitted from the nuclei of atoms buried within about a yard (meter) of the surface. Some manage to penetrate the material above them and fly into space, and the helpful ones then meet their fate upon hitting GRaND in orbit above. (Most others, however, will continue to fly through interplanetary space, decaying into a trio of other subatomic particles in less than an hour.) Before it escapes from the ground, a neutron’s energy (and, equivalently, its speed) is strongly affected by any encounters with the nuclei of hydrogen atoms (although other atomic interactions can change the energy too). Therefore, the neutron energies can indicate to scientists the abundance of hydrogen. Among the most common forms in which hydrogen is found is water (composed of two hydrogen atoms and one oxygen atom), which can occur as ice or tied up in hydrated minerals.

GRaND shows Ceres is rich in hydrogen. Moreover, it detects more neutrons in an important energy range near the equator than near the poles, likely indicating there is more hydrogen, and hence more (frozen) water, in the ground at the high latitudes. Although Ceres is farther from the sun than Earth, and you would not consider it balmy there, it still receives some warmth. Just as at Earth, the sun’s heating is less effective closer to the poles than at low latitudes, so this distribution of ice in the ground may reflect the temperature differences. Where it is warmer, ice close to the surface would have sublimed more quickly, thus depleting the inventory compared to the cooler ground far to the north or south.

Ceres Neutron Counts Reflect Hydrogen Abundance

This map, centered over the northern hemisphere, uses color to depict the rate at which GRaND detected neutrons of a particular energy from an altitude of 240 miles (385 kilometers). (The underlying image of Ceres is based on pictures Dawn took with its camera at a higher altitude.) Red indicates more neutrons than blue. The relative deficiency of neutrons near the north pole (and near the south pole, although not shown here) is because hydrogen is more abundant there. The hydrogen atoms rob the neutrons of energy, so GRaND does not find as many at the special energy used for this study. (It does find them at other energies.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Dawn spends most of its time measuring neutrons (and gamma rays), so it is providing a great deal of new data. And as scientists conduct additional analyses, they will learn more about the ice and other materials beneath the surface.

Another spectrometer is providing more tantalizing clues about the composition of Ceres, which is seen to vary widely. As the dwarf planet is not simply a huge rock but is a geologically active world, it is no surprise that it is not homogeneous. We discussed in December that the infrared mapping spectrometer had shown that minerals known as phyllosilicates are common on Ceres. Further studies of the data show evidence for the presence of two types: ammoniated phyllosilicates (described in December) and magnesium phyllosilicates. Scientists also find evidence of compounds known as carbonates, minerals that contain carbon and oxygen. There is also a dark substance in the mix that has not been identified yet.

And in one place (so far) on Ceres, this spectrometer has directly observed water, not below the surface but on the ground. The infrared signature shows up in a small crater named Oxo. (For the pronunciation, listen to the animation below.) As with the neutron spectra, it is too soon to know whether the water is in the form of ice or is chemically bound up in minerals.

At six miles (10 kilometers) in diameter, Oxo is small in comparison to the largest craters on Ceres, which are more than 25 times wider. (While geologists consider it a small crater, you might not agree if it formed in your backyard. Also note that when we showed Oxo Crater before, the diameter was slightly different. The crater’s size has not changed since then, but as we receive sharper pictures, our measurements of feature sizes do change.) Dawn’s first orbital destination, the fascinating protoplanet Vesta, is smaller than Ceres and yet has two craters far broader than the largest on Ceres. Based on studies of craters observed throughout the solar system, scientists have established methods of calculating the number and sizes of craters that could be formed on planetary surfaces. Those techniques show that Ceres is deficient in large craters. That is, more should have formed than appear in Dawn’s pictures. Many other bodies (including Vesta and the moon) seem to preserve their craters for much longer, so this may be a clue about internal geological processes on Ceres that gradually erase the large craters.

Scientists are still in the initial stages of digesting and absorbing the tremendous wealth of data Dawn has been sending to Earth. The benefit of lingering in orbit (enabled by the remarkable ion propulsion system), rather than being limited to a brief glimpse during a fast flyby, is that the explorer can undertake much more thorough studies, and Dawn is continuing to make new measurements.

As recently as one year ago, controllers (and this writer) had great concern about the spacecraft’s longevity given the loss of two reaction wheels, which are used for controlling the ship’s orientation. And in 2014, when the flight team worked out the intricate instructions Dawn would follow in this fourth and final mapping orbit, they planned for three months of operation. That was deemed to be more than enough, because Dawn only needed half that time to accomplish the necessary measurements. Experienced spacecraft controllers recognize that there are myriad ways beautiful plans could go awry, so they planned for more time in order to ensure that the objectives would be met even if anomalies occurred. They also were keenly aware that the mission could very well conclude after three months of low altitude operations, with Dawn using up the last of its hydrazine. But their efforts since then to conserve hydrazine proved very effective. In addition, the two remaining wheels have been operating well since they were powered on in December, further reducing the consumption of the precious propellant.

As it turned out, operations have been virtually flawless in this orbit, and the first three months yielded a tremendous bounty, even including some new measurements that had not been part of the original plans. And because the entire mission at Ceres has gone so well, Dawn has not expended as much hydrazine as anticipated.

This is Ceres, the dwarf planet that Dawn’s been orbiting for more than a year now, providing us with fascinating views of an alien world. During its exploration, Dawn has moved closer and closer, allowing us to get a broad overview and then see exquisite detail.

This is an excerpt from an animation showing some of the highlights of Dawn’s exploration of Ceres so far, including Occator and Oxo craters, both of which are discussed above. You can also hear your correspondent’s pronunciation of the names of those and other features on Ceres. Full animation and transcript. Animation credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn is now performing measurements that were not envisioned long in advance but rather developed only in the past two months, when it was apparent that the expedition could continue. And since March 19, Dawn has been following a new strategy to use even less hydrazine. Instead of pointing its sensors straight down at the scenery passing beneath it as the spacecraft orbits and Ceres rotates, the probe looks a little to the left. The angle is only five degrees (equal to the angle the minute hand of a clock moves in only 50 seconds, or less than the interval between adjacent minute tick marks), but that is enough to decrease the use of hydrazine and thus extend the spacecraft’s lifetime. (We won’t delve into the reason here. But for fellow nerds, it has to do with the alignment of the axes of the operable reaction wheels with the plane in which Dawn rotates to keep its instruments pointed at Ceres and its solar arrays pointed at the sun. The hydrazine saving depends on the wheels’ ability to store angular momentum and applies only in hybrid control, not in pure hydrazine control. Have fun figuring out the details. We did!)

The angle is small enough now that the pictures will not look substantially different, but they will provide data that will help determine the topography. (Measurements of gravity and the neutron, gamma ray and infrared spectra are insensitive to this angle.) Dawn took pictures at a variety of angles during the third mapping orbit at Ceres (and in two of the mapping orbits at Vesta, HAMO1 and HAMO2) in order to get stereo views for topography. That worked exceedingly well, and photos from this lower altitude will allow an even finer determination of the three dimensional character of the landscape in selected regions. Beginning on April 11, Dawn will look at a new angle to gain still another perspective. That will actually increase the rate of hydrazine expenditure, but the savings now help make that more affordable. Besides, this is a mission of exploration and discovery, not a mission of hydrazine conservation. We save hydrazine when we can in order to spend it when we need it. Dawn’s charge is to use the hydrazine to accomplish important scientific objectives and to pursue bold, exciting goals that lift our spirits and fuel our passion for knowledge and adventure. And that is exactly what it is has done and what it will continue to do.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.90 AU (362 million miles, or 583 million kilometers) from Earth, or 1,505 times as far as the moon and 3.90 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and five minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT March 31, 2016

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29 Feb
2016
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | February 29

by Marc Rayman
 

Dear Indawnbitably Successful Readers,

A story of intense curiosity about the cosmos, passionate perseverance and bold ingenuity, a story more than two centuries in the making, has reached an extraordinary point. It begins with the discovery of dwarf planet Ceres in 1801 (129 years before its sibling Pluto; each was designated a planet for a time). Protoplanet Vesta was discovered in 1807. Following 200 years of telescopic observations, Dawn’s daring mission was to explore these two uncharted worlds, the largest, most massive residents of the main asteroid belt between Mars and Jupiter. And now, as of February 2016, the spacecraft has accomplished all of the objectives that NASA defined for it in 2004, even before construction began (and before the very first Dawn Journal, nearly a decade ago).

More than eight years after leaving its erstwhile planetary home behind for an ambitious deep space adventure, Dawn has now collected all of the data originally planned. Indeed, even prior to this third intercalary day of its expedition, the probe had already actually sent back a great deal more data for all investigations, significantly exceeding not only the original goals but also new ones added after the ship had set sail on the interplanetary seas. While scientists have a great deal of work still ahead to translate the bounty of data into knowledge, which is the greatest joy of science, the spacecraft can continue its work with the satisfaction that it has fulfilled its purpose and achieved an outstandingly successful mission.

Dawn LAMO Image 23

Dawn took this picture of the rim of Datan crater on Jan. 7 in its fourth mapping orbit at 240 miles (385 kilometers). It flew over the same location on Oct. 2, 2015, in its third mapping orbit at 915 miles (1,470 kilometers). To see the improvement in detail, compare this with the earlier image (presented fully in November but reproduced in part below to make comparison easier). The bright material to the right of the crater rim here may help you locate this area within the wider image. Full image and captionImage credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

HAMO Image 57

Dawn took this picture in its third mapping orbit at an altitude of 915 miles (1,470 kilometers) in mapping cycle #5 of its third mapping orbit. The prominent triplet of overlapping craters nicely displays relative ages, which are apparent by which ones affect others and hence which ones formed later. The largest crater, Geshtin, is 48 miles (77 kilometers) across and is the oldest. (Geshtin is a Sumerian and Assyro-Babylonian goddess of the vine.) A subsequent impact that excavated Datan crater, which is 37 miles (60 kilometers) in diameter, obliterated a large section of Geshtin’s rim and made its own crater wall in Geshtin’s interior. (Datan is one of the Polish gods who protect the fields but apparently not this crater.) Still later, Datan itself was the victim of a sizable impact on its rim (although not large enough to have merited an approved name this early in the geological studies of Ceres). Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, which would have been impossible without its advanced ion propulsion system. It is the only spacecraft ever to orbit an object in the main asteroid belt. It is also the only spacecraft ever to orbit massive bodies (apart from the sun and Earth) that had not been visited first by a flyby spacecraft to characterize the gravity and other properties. (By the way, Ceres is one of eight solar system bodies that operating spacecraft are orbiting now. The others are the sun, Venus, Earth, the moon, comet Churyumov-Gerasimenko, Mars and Saturn.)

Now in its fourth and final mapping orbit at Ceres, at an altitude of 240 miles (385 kilometers), Dawn is closer to the exotic terrain than the International Space Station is to Earth. The benefit of being in orbit is that the probe can linger rather than take only a brief look during a fast flyby. Even though Dawn has met its full list of objectives at Ceres, it continues to return new, valuable pictures and other measurements to provide even greater insight into this relict from the dawn of the solar system. For example, it is acquiring more nuclear spectra with its gamma ray and neutron detector, sharpening its picture of some atomic elements on Ceres. In addition, taking advantage of its unique vantage point, Dawn is collecting more infrared spectra of locations that are of special interest and soon will also take color photos and stereo photos (as it did in the third mapping orbit) of selected areas.

Dawn has completed more than 600 revolutions since taking up residence one year ago. The first few orbits took several weeks each, but as the spacecraft descended and Ceres’ gravitational embrace grew more firm, its orbital velocity increased and the orbital period decreased. Now circling in less than five and a half hours, Dawn has made 370 orbits since reaching this altitude on Dec. 7.

Dawn LAMO Image 23

On Jan. 1, Dawn observed this scene at 78 degrees south latitude. This deep in the southern hemisphere, the sun is low on the horizon (it is three degrees north of the equator). The long shadows emphasize the topography in this densely cratered (and therefore old) region. Landslides are evident in the large crater wall on the left. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The pace of observations here is higher than in the previous mapping orbits, where the orbital periods were longer. The spacecraft flies over the landscape faster now, and being closer to the ground, its instruments discern much more detail but capture a smaller area. Mission controllers have developed intricate plans for observing Ceres, but those plans depend on the spacecraft being at the right place at the right time. As we will see below, however, sometimes it may not be.

Suppose, for example, the intent is to observe a particular feature, perhaps the bright center of Occator crater, the lonely, towering mountain Ahuna Mons, the fractures in Dantu crater or artificial structures that definitively prove the existence of extraterrestrial intelligence, utterly transforming our understanding of the cosmos and shattering our naive perspectives on life in the universe. Trajectory analysis indicates when Dawn will fly over the designated location, and engineers will program it to take pictures or infrared spectra at that time. They will also include some margin, so they may program it to start 10 minutes before and end 10 minutes after. But they can’t afford to put in too much margin. Data storage on the spacecraft is limited, so other geological features could not be observed. Also, transmitting data to Earth requires pointing the main antenna at that distant planet instead of pointing sensors at Ceres, so it would be unwise to collect much more than is necessary.

Even if devoting additional time (and data) to trying to observe the desired place were feasible, it wouldn’t necessarily solve the problem. Dawn travels in a polar orbit, which is the only way to ensure that it passes over all latitudes. While Dawn soars from north to south over the sunlit hemisphere making its observations, the dwarf planet itself rotates on its axis, so the ground moves from east to west. If the spacecraft arrives at the planned orbital location a little early or a little late, the feature of interest may not even be beneath it but rather could be too far east or west, out of view of the instruments. In that case, increasing the duration of the observation period doesn’t help.

All of that is why, as we saw last month, it requires more pictures to fully map Ceres than you might expect. Many pictures may have to be taken in order to fill in gaps, and quite a few of the pictures overlap with others. Nevertheless, Dawn has done an excellent job. The spacecraft has photographed 99.6 percent of the dwarf planet from this low altitude. (If you aren’t regularly visiting the image gallery, you are missing out on some truly out-of-this-world scenes.)

Dawn LAMO Image 33

Dawn photographed this scene on Jan. 4 as it was looking toward the horizon (as explained last month). Fluusa, the large crater from the center to the upper left is 37 miles (60 kilometers) in diameter. (Fluusa was a goddess of flowers for the Oscans of southern Italy who honored her to make plants bloom and bear fruit.) Its degraded features and dense cratering show it is old. Full image and captionImage credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The flight team devises very detailed plans that tell the spacecraft what to do every second, including where to point and what data to collect with each sensor. When the observation plans are developed, they are checked and double-checked. Then they are translated into the appropriate software that the robotic ship will understand, and these instructions are checked and double-checked. That is integrated with all the other software that will be beamed to the spacecraft covering the same period of time, any conflicts are resolved and then the final version is checked and, well, you know.

This process is very involved, and it is usually well over a month between the formulation and the execution of the plan. During that time, Dawn’s orbit can deviate slightly from the expert navigators’ mathematical predictions, preventing the spacecraft from flying over the desired targets. There are several reasons the actual orbit may differ from the orbit used for developing the plan. (We have seen related examples of this, including as Dawn approached Mars, when it orbited Vesta and when it spiraled from one mapping orbit to another.) Let’s briefly consider two.

One reason is that we do not have perfect knowledge of the variations in the strength of Ceres’ gravitational pull from one location to another. We have discussed before that measuring these tiny irregularities in the gravity field provides insight into the distribution of mass within the dwarf planet that gives rise to them. The team has mapped the hills and valleys of the field quite well and even better than expected. Still, the remaining small uncertainty can lead to slight differences between what navigators calculate Dawn’s motion will be and what its actual motion will be as it is buffeted by the gravitational currents.

A second source of discrepancy is that Dawn’s own activities distort its orbit. Every time the reaction control system expels a tiny burst of hydrazine to control the spacecraft’s orientation, keeping it pointed at its target, the force not only affects the orientation but also nudges the probe in its orbit, slowing it down or speeding it up very slightly. It’s up to the spacecraft to decide exactly when to make these small adjustments, and it is not possible for controllers to predict their timing. (In a similar way, when you are driving, you occasionally move the steering wheel to keep going the direction you want, even if is straight ahead. It would be impossible to forecast each tiny movement, because they all depend on what has already happened plus the exact conditions at the moment.) The details of the reaction control system activity also depend on the use of the novel hybrid control scheme, which the joint Orbital/JPL team developed because of the failure of two of the spacecraft’s four reaction wheels. The effect of each small firing of hydrazine is very small, but they can add up.

Dawn LAMO Image 20

Dawn had this view of two unnamed craters on Jan. 1. The craters are about 10 miles (16 kilometers) and 3 miles (5 kilometers) in diameter. The distinct features show these are relatively young craters, not yet degraded by subsequent impacts or geological processes intrinsic to Ceres. The lighting in the craters shows that the sun is to the right, illuminating the left side of the depressions and missing the right side. Click on the image (or follow the link to the full image) and look carefully inside and around the larger crater. There are many small features that are light on the right and dark on the left. Therefore, they aren’t depressions like these two craters. Rather, they rise up, catching the light as it comes in from the right, and their left sides are in shadow. These are large blocks from the impact that excavated the crater. Each pixel in this picture is 120 feet (35 meters). Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

It took about a month in this mapping orbit to discover many of the subtleties of the gravity field and gain experience with how hybrid control affects the orbit. But even before descending to this altitude, the operations team understood the nature of these effects and was well prepared to deal with them.

They devised several strategies, all of which are being used to good effect. One of the ways to account for Dawn’s actual orbit differing from its planned orbit is simply to change the orbit. Simply? Well, not really. It turns out to that to analyze the orbit and then maneuver to correct it in a timely way is a surprisingly complicated process, but, come to think of it, what isn’t complicated when flying a spaceship around a distant, alien world? Nevertheless, every three weeks, the flight team makes a careful assessment of the orbit and determines whether a small refinement with the ion propulsion system is in order. For technical reasons, if maneuvers are needed, they will be executed in pairs, so mission planners have scheduled two windows (each 12 hours long and separated by eight days) about every 23 days.

Adjustments to resynchronize the actual orbit with the predicted orbit that formed the basis of the exploration plan are known as “orbit maintenance maneuvers.” Succumbing to instincts developed during their long evolutionary history, engineers refer to them by an acronym: OMM. (As the common thread among team members is their technical training and passion for the exploration of the cosmos, and not Buddhism, the term is spoken by naming the letters, not pronouncing it as if it were a means of achieving inner peace. Instead, it may be thought of as a means of achieving orbital tranquility and harmony.)

For both Vesta and Ceres, trajectory analyses long in advance determined that OMMs would not be needed in the higher orbits, so no windows were included in those schedules. There have been three OMM opportunities since arriving at the lowest altitude above Ceres, but only the first was needed. Dawn performed the pair on Dec. 31-Jan. 1 and on Jan. 8 with its famously efficient ion engine. The orbit was good enough the next two times that OMMs were deemed unnecessary. It is certain that some future OMMs will be required. Your faithful correspondent provides frequent (and uncharacteristically concise) reports on Dawn’s day-to-day activities, including OMMs.

By the end of the Jan. 8 OMM, Dawn’s ion propulsion system had accumulated 2,019 days of operation in space, more than 5.5 years. During that time, the effective change in speed was 24,600 mph (39,600 kilometers per hour). (We have discussed in detail that this is not Dawn’s current speed but rather the amount by which the ion engines have changed it.) This is uniquely high for a spacecraft to accomplish with its own propulsion system and validates our description of ion propulsion as delivering acceleration with patience. (The previous record holder, Deep Space 1, achieved 9,600 mph, or 15,000 kilometers per hour.)

The effect of Dawn’s gentle ion thrusting during its mission has been nearly the same as that of the entire Delta II 7925H-9.5 rocket, with its nine external rocket engines, first stage, second stage and third stage. To get started on its interplanetary adventure, Dawn’s rocket boosted it from Cape Canaveral to out of Earth orbit with only four percent higher velocity than Dawn subsequently added on its own with its ion engines.

As Dawn and Earth follow their own independent orbits around the sun (Dawn’s now tied permanently to its gravitational master, Ceres), next month they will reach their greatest separation of the entire mission. On March 4 (about one Earth year after Ceres took hold of Dawn), on opposite sides of the solar system, they will be 3.95278 AU (367.434 million miles, or 591.328 million kilometers) from each other. (For those of you with full schedules, note that the maximum separation will be 5:40 a.m. PST.) They won’t be this far apart again until Feb. 6, 2025, long after Dawn has ceased operating (as discussed below). The figure below depicts the arrangement next month.

March Geometry

Earth’s and Ceres’ orbits will bring them to their maximum separation on March 4. Earth’s orbit is shown in green and Ceres’ is in purple. Dawn’s interplanetary trajectory is in blue. Compare this figure with the ones depicting Dawn and Earth on opposite sides of the sun in December 2014, Dawn equidistant from Earth and the sun in April 2015, and Dawn and Earth at their minimum separation in July 2015. Also note that Earth has completed one full loop around the sun in the year since March 2015, when Dawn arrived at Ceres. During the same period, Ceres, traveling in a higher heliocentric orbit, has completed only about a fifth of a revolution. Credit: NASA/JPL-Caltech

Dawn has faced many challenges in its unique voyage in the forbidding depths of space, but it has surmounted all of them. It has even overcome the dire threat posed by the loss of two reaction wheels (the second failure occurring in orbit around Vesta 3.5 years and 1.3 billion miles, or 2.0 billion kilometers, ago). With only two operable reaction wheels (and those no longer trustworthy), the ship’s remaining lifetime is very limited.

A year ago, the team couldn’t count on Dawn even having enough hydrazine to last beyond next month. But the creative methods of conserving that precious resource have proved to be quite efficacious, and the reliable explorer still has enough hydrazine to continue to return bonus data for a while longer. Now it seems highly likely that the spacecraft will keep functioning through the scheduled end of its primary mission on June 30, 2016.

NASA may choose to continue the mission even after that. Such decisions are difficult, as there is literally an entire universe full of interesting subjects to study, but resources are more limited. In any case, even if NASA extended the mission, and even if the two wheels operated without faltering, and even if the intensive campaign of investigating Ceres executed flawlessly, losing not an ounce (or even a gram) of hydrazine to the kinds of glitches that can occur in such a complex undertaking, the hydrazine would be exhausted early in 2017. Clearly an earlier termination remains quite possible.

Regardless of when Dawn’s end comes, it will not be a time for regret. The mission has realized its raison d’être and is reaping rewards even beyond those envisioned when it was conceived. It has taken us all on a marvelous interplanetary journey and allowed us to behold previously unseen sights of distant lands. The conclusion of the mission will be a time for gratitude that it was so successful. And until then, every new picture or other measurement adds to the richly detailed portrait of a faraway, exotic world. There is plenty more still to do before this remarkable story draws to a close.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.95 AU (367 million miles, or 591 million kilometers) from Earth, or 1,475 times as far as the moon and 3.99 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and six minutes to make the round trip.

Dr. Marc D. Rayman
5:00 p.m. PST February 29, 2016

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31 Jan
2016
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | January 31

by Marc Rayman
 

Dear Spellbindawngs,

A veteran interplanetary traveler is writing the closing chapter in its long and storied expedition. In its final orbit, where it will remain even beyond the end of its mission, at its lowest altitude, Dawn is circling dwarf planet Ceres, gathering an album of spellbinding pictures and other data to reveal the nature of this mysterious world of rock and ice.

Kupalo Crater from LAMO

Dawn captured this view of Kupalo crater on Dec. 20, shortly after beginning the observations from its current low altitude mapping orbit at 240 miles (385 kilometers). (Kupalo is a Slavic harvest deity associated with love and fertility.) This is a relatively young crater, as seen by its sharp, clear features and the paucity of overlying smaller impact craters which would have formed later. Bright material on the rim and walls may be salts, as explained last month. The crater is 16 miles (26 kilometers) across. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Ceres turns on its axis in a little more than nine hours (one Cerean day). Meanwhile, its new permanent companion, a robotic emissary from Earth, revolves in a polar orbit, completing a loop in slightly under 5.5 hours. It flies from the north pole to the south over the side of Ceres facing the sun. Then when it heads north, the ground beneath it is cloaked in the deep dark of night on a world without a moon (save Dawn itself). As we discussed last month, Dawn’s primary measurements do not depend on illumination. It can sense the nuclear radiation (specifically, gamma rays and neutrons) and the gravity field regardless of the lighting. This month, let’s take a look at the other measurements our explorer is performing, most of which do depend on sunlight.

Of course the photographs do. Dawn had already mapped Ceres quite thoroughly from higher altitudes. The spacecraft acquired an extensive set of stereo and color pictures in its third mapping orbit. But now that Dawn is only about 240 miles (385 kilometers) high, its images are four times as sharp, revealing new details of the strange and beautiful landscapes.

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This is an excerpt from a much more extensive animation providing a colorful tour of some of the highlights on Ceres. It is made with the color and stereo pictures Dawn collected in its third mapping orbit 915 miles (1,470 kilometers) above the dwarf planet. Here we see Occator crater, with its famous bright regions. The full animation (in which both color and sound are exaggerated) also shows the strange, conical mountain Ahuna Mons plus Urvara, Haulani and Dantu (seen in more detail below) craters and more. The colors indicate different compositions, which may include salts and phyllosilicates, as explained last monthFull animation and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Our spaceship is closer to Ceres than the International Space Station is to Earth. At that short range, it takes a long time to capture all of the vast territory, because each picture covers a relatively small area. Dawn’s camera sees a square about 23 miles (37 kilometers) on a side, less than one twentieth of one percent of the more than one million square miles (nearly 2.8 million square kilometers). In an ideal world (which is not the one Dawn is in or at), it would take just over two thousand photos from this altitude to see all the sights. However, as we will discuss in more detail next month, it is not possible to control the orbital motion and the pointing of the camera accurately enough to manage without more photos than that.

Most of the time, Dawn is programmed to turn at just the right rate to keep looking at the ground beneath it as it travels, synchronizing its rotation with its revolution around Ceres. It photographs the passing scenery, storing the pictures for later transmission to Earth. But some of the time, it cannot take pictures, because to send its bounty of data, it needs to point its main antenna at that distant planet, home not only to its controllers but also to many others (including you, loyal reader) who share in the thrill of a bold cosmic adventure. Dawn spends about three and a half days (nine Cerean days) with its camera and other sensors pointed at Ceres. Then it radioes its findings home for a little more than one day (almost three Cerean days). During these communications sessions, even when it soars over lit terrain, it does not observe the sights below.

Mission planners have devised an intricate plan that should allow nearly complete coverage in about six weeks. To accomplish this, they guided Dawn to a carefully chosen orbit, and it has been doing an exceptionally good job there executing its complex activities.

Floor of Dantu Crater from LAMO

On Dec. 21, in its lowest orbit at about 240 miles (385 kilometers), Dawn flew over Dantu crater and obtained pictures with four times the clarity of the third mapping orbit, where we saw the entire crater. (Dantu is a timekeeper god who initiates the cycle of planting rites among the Ga people of the Accra Plains of southeastern Ghana.) The bright material here is at the 4:00 position, half way from the center to the rim, in the picture shown in November. The network of fractures may have formed when the ground cooled after being heated by the crater-forming impact, or perhaps later when other geological processes caused the crater floor to be uplifted. The crater is about 78 miles (126 kilometers) in diameter. The next picture below shows detail of another part of Dantu. The animation above includes Dantu (as seen from farther away). Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Last month, we marveled at a stunning view that was not the typical perspective of peering straight down from orbit. Sometimes controllers now program Dawn to take a few more pictures after it stops aiming its instruments down, while it starts to turn to aim its antenna to Earth. This clever idea provides bonus views of whatever happens to be in the camera’s sights as it slowly rotates from the point beneath the spacecraft off to the horizon. Who doesn’t feel the attraction of the horizon and long to know what lies beyond?

Another of Dawn’s scientific devices is two different sensors combined into one instrument. Like the camera, the visible and infrared mapping spectrometers (VIR) look at the sunlight reflected from the ground. (As we’ll see below, however, VIR also can detect something more.) A spectrometer breaks up light into its constituent colors, just as a prism or a droplet of water does when revealing, quite literally, all the colors of the rainbow. Dawn’s visible spectrometer would have a view very much like that. The infrared spectrometer, of course, looks at wavelengths of light our limited eyes cannot see, just as there are wavelengths of sound our limited ears cannot hear (consult with your dog for details).

A spectrometer does more than simply disperse the light into its components, however. It measures the intensity of that light at the different wavelengths. The materials on the surface leave their signature in the sunlight they reflect, making some wavelengths relatively brighter and some dimmer. That characteristic pattern is called a spectrum. By comparing these spectra with spectra measured in laboratories, scientists can infer the nature of the minerals on the ground. We described some of the intriguing conclusions last month.

Dawn LAMO Image 10

On Dec. 19, Dawn’s orbit took it over a different part of Dantu crater, showing more reflective material on the walls and floor. (This scene is from the right side of the crater as pictured in November.) More of the fractures visible in the picture above are in the upper left of this picture. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

VIR does still more. Rather than record the visible spectrum and the infrared spectrum from a single region, it takes spectra at 256 adjacent locations simultaneously. This would be like taking one column of 256 pixels in a picture and having a separate spectrum for each. By stitching columns together, you could construct the two dimensional picture but with the added dimension of an extensive spectrum at every location. (Because the extra information provides a sort of depth that flat pictures don’t have, the result is sometimes called an “image cube.”) This capability to build up an image with spectra everywhere is what makes it a mapping spectrometer. VIR produces a remarkably rich view of its targets!

VIR’s spectra contain much finer measurements of the colors and a wider range of wavelengths than the camera’s images. In exchange, the camera has sharper vision and so can discern smaller geological features. In more technical terms, VIR achieves better spectral resolution and the camera achieves better spatial resolution. Fortunately, it is not a competition, because Dawn has both, and the instruments yield complementary measurements.

VIR generates a very large volume of data in each snapshot. As a result, Dawn can only capture and store relatively small areas of the dwarf planet with the mapping spectrometers, especially at this low altitude. Scientists have recognized from the first design of the mission that it would not be possible to cover all of Ceres (or Vesta) with VIR from the closer orbits. Nevertheless, Dawn has far exceeded expectations, returning a great many more spectra than anticipated. Still, as long as the spacecraft operates in this final mapping orbit, there will continue to be interesting targets to study with VIR.

Based on the nearly 20 million spectra of Ceres that VIR acquired from higher altitudes, the team has determined that new infrared spectra will provide more insight into the dwarf planet’s character than the visible spectra. Because of their composition, the minerals display more salient signatures in infrared wavelengths than visible. The excellent visible spectra from the first three mapping orbits are deemed more than sufficient. Therefore, to make the best use of our faithful probe and to dedicate the resources to what is most likely to yield new knowledge about Ceres, VIR is devoting its share of the mission data in this final orbit to its infrared mapping spectrometer. We have many more exciting discoveries to look forward to!

Crater with Scarps in LAMO

Dawn photographed this unnamed crater on Dec. 23. It is 20 miles (32 kilometers) in diameter and is between Dantu and Rao craters. (See the map here.) Part of this crater is shown at the bottom left of the photo of Dantu we saw in November. The many ridges and steep slopes here may be the result of the crater partially collapsing during its formation. The complex geology evokes an image of a flower (at least for this writer). Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The infrared light Ceres reflects from the sun can tell scientists a great deal about the composition, but they can learn even more from analyzing VIR’s measurements. The sun isn’t the only source of infrared. Ceres itself is. Many people correctly associate infrared with heat, because warm objects emit infrared light, and the strength at different wavelengths depends on the temperature. That calls for measuring the spectrum! Distant from the sun though it is, Ceres is warmed slightly by the brilliant star, so it has a very faint infrared glow of its own. Scientists can distinguish in VIR’s observations between the reflected infrared sunlight and the infrared light Ceres radiates. In essence, VIR can function as a remote thermometer.

Last month, in one of Dawn’s best photos yet of Ceres, we considered planning a hike across a breathtaking landscape. In case we do, VIR has shown we should be prepared for chilly conditions. Observed temperatures (all rounded to the nearest multiple of five) during the day on the dwarf planet range from -135 degrees Fahrenheit (-95 degrees Celsius) to -30 degrees Fahrenheit (-35 degrees Celsius). (It is so cold in some locations and times, especially at night, that Ceres produces too little infrared light for VIR to measure. Temperatures below the coldest reported here actually don’t register.) This finding provides compelling support for this writer’s frequent claim that Ceres is really cool. In addition, knowing the temperatures will be very important for understanding geological processes on this icy, rocky world, just as we know the movement of terrestrial glaciers depends on temperature.

Your loyal correspondent can’t — or, at least, won’t — help but indulge his nerdiness with a brief tangent. The range of temperatures above represent the warmest on Ceres, given that VIR cannot measure lower values. It’s amusing, if you have a similar weird sense of humor, that Ceres’ average temperature apparently is not that far from what it would be for a black hole of the same mass. We won’t delve into the physics here, but such a black hole would be -225 degrees Fahrenheit (-140 degrees Celsius). OK, enough hilarity. Back to Dawn and Ceres…

Ever creative, scientists are attempting another clever method to gain insight into the nature of this exotic orb. When Dawn is at just the right position in its orbit on the far side of Ceres, so that a straight line to Earth passes very close to the limb of Ceres itself, the spacecraft’s radio signal will actually hit the dwarf planet. The radio waves interact with the materials on the surface, which can induce an exquisitely subtle distortion. After bouncing off the ground at a grazing angle, the radio signal continues on its way, heading toward Earth. The effect on the signal is much too small to affect the normal communications at all, but specialized equipment at NASA’s Deep Space Network designed for this purpose might still be able to detect the tiny changes. The fantastically sensitive antennas measure the properties of the radio waves, and by studying the details, scientists may be able to learn more about the properties of the surface of the distant world. For example, this could help them distinguish between different types of materials (such as ice, rocks, sand, etc.) as well as reveal how rough or smooth the ground is at scales far, far smaller than the camera can discern. This is an extremely challenging measurement, and no small distortions have been detected so far, but always making the best possible use of the resources, scientists continue to look for them.

In addition to those bonus measurements, Dawn remains very productive in acquiring infrared spectra, photographs, gamma ray spectra and neutron spectra plus conducting measurements of the massive body’s gravitational field, all of which contribute to unlocking the mysteries of the first dwarf planet ever discovered or explored. The venerable adventurer is in good condition and is operating flawlessly.

Dawn LAMO Image 5

Dawn observed Victa crater on Dec. 19. (Victa was a Roman goddess of food and nourishment.) The crater is 20 miles (32 kilometers) in diameter and so is the same size as the unnamed one shown above. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

We have discussed extensively the failures of two of the four reaction wheels, devices Dawn used to depend on to control its orientation in space. Without three healthy reaction wheels, the probe has had to rely instead on hydrazine propellant expelled from the small jets of the reaction control system. (When Dawn uses its ion engine, that remarkable system does double duty, reducing the need for the hydrazine.)

For most of the time since escaping from Vesta’s gravitational clutches in 2012, Dawn has kept the other two reaction wheels in reserve so any remaining lifetime from those devices could offset the high cost of hydrazine propellant to turn and point in this current tight orbit. Those two wheels have been on and functioning flawlessly since Dec. 14, 2015, and every day they operate, they keep the expenditure of the dwindling supply of hydrazine to half of what it would be without them. (Next month we will offer some estimates of how long Dawn might continue to operate.) But the ever-diligent team recognizes another wheel could falter at any moment, and they remain ready to continue the mission with pure hydrazine control after only a short recovery operation. If a third failure is at all like the two that have occurred already, the hapless wheel won’t give an indication of a problem until it’s too late. A reaction wheel failure evidently is entirely unpredictable. We’ll know about it only after it occurs in the remote depths of space where Dawn resides at an alien world.

Earth and Ceres are so far from each other that their motions are essentially independent. The planet and the dwarf planet follow their own separate repetitive paths around the sun. And each carries its own retinue: Earth has thousands of artificial satellites and one prominent natural one, the moon. Ceres has one known satellite. It arrived there in March 2015, and its name is Dawn.

Coincidentally, both reached extremes earlier this month in their elliptical heliocentric orbits. Earth, in its annual journey around our star, was at perihelion, or the closest point to the sun, on Jan. 2, when it was 0.98 AU (91.4 million miles, or 147 million kilometers) away. Ceres, which takes 4.6 years (one Cerean year) for each loop, attained its aphelion, or greatest distance from the sun, on Jan. 6. On that day, it was 2.98 AU (277 million miles, or 445 million kilometers) from the gravitational master of the solar system.

Far, far from the planet where its deep-space voyage began, Dawn is now bound to Ceres, held in a firm but gentle gravitational embrace. The spacecraft continues to unveil new and fascinating secrets there for the benefit of all those who remain with Earth but who still look to the sky with wonder, who feel the lure of the unknown, who are thrilled by new knowledge, and who yearn to know the cosmos.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.87 AU (360 million miles, or 580 million kilometers) from Earth, or 1,440 times as far as the moon and 3.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and four minutes to make the round trip.

Dr. Marc D. Rayman
4:30 p.m. PST January 31, 2016

All Dawn Journal entries

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30 Nov
2015
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | November 30

by Marc Rayman
 

Dear Superintendawnts and Assisdawnts,

An intrepid interplanetary explorer is now powering its way down through the gravity field of a distant alien world. Soaring on a blue-green beam of high-velocity xenon ions, Dawn is making excellent progress as it spirals closer and closer to Ceres, the first dwarf planet discovered. Meanwhile, scientists are progressing in analyzing the tremendous volume of pictures and other data the probe has already sent to Earth.

4th Mapping Orbit (LAMO)

Dawn’s spiral descent from its third mapping orbit (HAMO), at 915 miles (1,470 kilometers), to its fourth (LAMO), at 240 miles (385 kilometers). The two mapping orbits are shown in green. The color of Dawn’s trajectory progresses through the spectrum from blue, when it began ion-thrusting in HAMO, to red, when it arrives in LAMO. The red dashed sections show where Dawn is coasting for telecommunications. It requires 118 spiral revolutions around Ceres to reach the low altitude (and additional revolutions to prepare for and conduct the trajectory correction maneuver described below). Compare this to the previous spiral. (Readers with total recall will note that this is fewer loops than illustrated last year. The flight team has made several improvements in the complex design since then, shortening the time required and thus allowing more time for observing Ceres.) Image credit: NASA/JPL-Caltech

Dawn is flying down to an average altitude of about 240 miles (385 kilometers), where it will conduct wide-ranging investigations with its suite of scientific instruments. The spacecraft will be even closer to the rocky, icy ground than the International Space Station is to Earth’s surface. The pictures will be four times sharper than the best it has yet taken. The view is going to be fabulous!

Dawn will be so near the dwarf planet that its sensors will detect only a small fraction of the vast territory at a time. Mission planners have designed the complex itinerary so that every three weeks, Dawn will fly over most of the terrain while on the sunlit side. (The neutron spectrometer, gamma ray spectrometer and gravity measurements do not depend on illumination from the sun, but the camera, infrared mapping spectrometer and visible mapping spectrometer do.)

Obtaining the planned coverage of the exotic landscapes requires a delicate synchrony between Ceres’ and Dawn’s movements. Ceres rotates on its axis every nine hours and four minutes (one Cerean day). Dawn will revolve around it in a little less than five and a half hours, traveling from the north pole to the south pole over the hemisphere facing the sun and sailing northward over the hemisphere hidden in the darkness of night. Orbital velocity at this altitude is around 610 mph (980 kilometers per hour).

Last year we had a preview of the plans for this fourth and final mapping orbit (sometimes also known as the low altitude mapping orbit, or LAMO), and we will present an updated summary next month.

The planned altitude differs from the earlier, tentative value of 230 miles (375 kilometers) for several reasons. One is that the previous notion for the altitude was based on theoretical models of Ceres’ gravity field. Navigators measured the field quite accurately in the previous mapping orbit (using the method outlined here), and that has allowed them to refine the orbital parameters to choreograph Dawn’s celestial pas de deux with Ceres. In addition, prior to Dawn’s investigations, Ceres’ topography was a complete mystery. Hubble Space Telescope had shown the overall shape well enough to allow scientists to determine that Ceres qualifies as a dwarf planet, but the landforms were indiscernible and the range of relative elevations was simply unknown. Now that Dawn has mapped the topography, we can specify the spacecraft’s average height above the ground as it orbits. With continuing analyses of the thousands of stereo pictures taken in August – October and more measurements of the gravity field in the final orbit, we will further refine the average altitude. Finally, we round the altitude numbers to the nearest multiple of five (both for miles and kilometers), because, as we will discuss in a subsequent Dawn Journal, the actual orbit will vary in altitude by much more than that. (We described some of the ups and dawns of the corresponding orbit at Vesta here. The variations at Ceres will not be as large, but the principles are the same.)

Dawn HAMO Image 50

Dawn had this view of Urvara crater in mapping cycle #4 from an altitude of 915 miles (1,470 kilometers) during the third mapping orbit. (Urvara is a Vedic goddess associated with fertile lands and plants.) The crater is 101 miles (163 kilometers) in diameter. It displays a variety of features, including a particularly bright region on the peak at the center, ridges nearby, a network of fissures, some smooth regions and much rougher terrain. You can locate all the areas shown in this month’s photos on the Ceres map presented last month. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

To attain its new orbit, Dawn relies on its trusty and uniquely efficient ion engine, which has already allowed the spacecraft to accomplish what no other has even attempted in the 58-year history of space exploration. This is the only mission ever to orbit two extraterrestrial destinations. The spaceship orbited the protoplanet Vesta for 14 months in 2011-2012, revealing myriad fascinating details of the second most massive object in the main asteroid belt between Mars and Jupiter, before its March 2015 arrival in orbit around the most massive. Ion propulsion enables Dawn to undertake a mission that would be impossible without it.

While the ion engine provides 10 times the efficiency of conventional spacecraft propulsion, the engine expends the merest whisper of xenon propellant, delivering a remarkably gentle thrust. As a result, Dawn achieves acceleration with patience, and that patience is rewarded with the capability to explore two of the last uncharted worlds in the inner solar system. This raises an obvious question: How cool is that? Fortunately, the answer is equally obvious: Incredibly cool!

The efficiency of the ion engine enables Dawn not only to orbit two destinations but also to maneuver extensively around each one, optimizing its orbits to reap the richest possible scientific return at Vesta and Ceres. The gentleness of the ion engine makes the maneuvers gradual and graceful. The spiral descents are an excellent illustration of that.

Dawn began its elegant downward coils on Oct. 23 upon concluding more than two months of intensive observations of Ceres from an altitude of 915 miles (1,470 kilometers). At that height, Ceres’ gravitational hold was not as firm as it will be in Dawn’s lower orbit, so orbital velocity was slower. Circling at 400 mph (645 kilometers per hour), it took 19 hours to complete one revolution around Ceres. It will take Dawn more than six weeks to travel from that orbit to its new one. (You can track its progress and continue to follow its activities once it reaches its final orbit with the frequent mission status updates.)

PIA19993: Dawn HAMO Image 51

Dawn took this picture of Dantu crater from an altitude of 915 miles (1,470 kilometers) during the third mapping orbit, in mapping cycle #4. (Dantu is a timekeeper god who initiates the cycle of planting rites among the Ga people of the Accra Plains of southeastern Ghana. You can find Dantu, but not Ghana, on this map.) The crater is about 78 miles (126 kilometers) across. Note the isolated bright regions, the long fissures, and the zigzag structure at the center. Scientists are working to understand what these indicate about the geological processes on Ceres. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

On Nov. 16, at an altitude of about 450 miles (720 kilometers), Dawn circled at the same rate that Ceres turned. Now the spacecraft is looping around its home even faster than the world beneath it turns.

When ion-thrusting ends on Dec. 7, navigators will measure and analyze the orbital parameters to establish how close they are to the targeted values and whether a final adjustment is needed to fit with the intricate observing strategy. Several phenomena contribute to small differences between the planned orbit and the actual orbit. (See here and here for two of our attempts to elucidate this topic.) Engineers have already thoroughly assessed the full range of credible possibilities using sophisticated mathematical methods. This is a complex and challenging process, but the experienced team is well prepared. In case Dawn needs to execute an additional maneuver to bring its orbital motion into closer alignment with the plan, the schedule includes a window for more ion-thrusting on Dec. 11-13 (concluding on Dawn’s 2,999th day in space). In the parlance of spaceflight, this maneuver to adjust the orbit is a trajectory correction maneuver (TCM), and Dawn has experience with them.

The operations team takes advantage of every precious moment at Ceres they can, so while they are determining whether to perform the TCM and then developing the final flight plan to implement it, they will ensure the spacecraft continues to work productively. Dawn carries two identical cameras, a primary and a backup. Engineers occasionally operate the backup camera to verify that it remains healthy and ready to be put into service should the primary camera falter. On Dec. 10, the backup will execute a set of tests, and Dawn will transmit the results to Earth on Dec. 11. By then, the work on the TCM will be complete.

Although it is likely a TCM will be needed, if it turns out to be unnecessary, mission control has other plans for the spacecraft. In this final orbit, Dawn will resume using its reaction wheels to control its orientation. By electrically changing the speed at which these gyroscope-like devices rotate, the probe can control its orientation, stabilizing itself or turning. We have discussed their lamentable history on Dawn extensively, with two of the four having failed. Although such losses could have been ruinous, the flight team formulated and implemented very clever strategies to complete the mission without the wheels. Exceeding their own expectations in such a serious situation, Dawn is accomplishing even more observations at Ceres than had been planned when it was being built or when it embarked on its ambitious interplanetary journey in 2007.

 PIA20000: Dawn HAMO Image 57

Dawn took this picture in its third mapping orbit at an altitude of 915 miles (1,470 kilometers) in mapping cycle #5 of its third mapping orbit. The prominent triplet of overlapping craters nicely displays relative ages, which are apparent by which ones affect others and hence which ones formed later. The largest crater, Geshtin, is 48 miles (77 kilometers) across and is the oldest. (Geshtin is a Sumerian and Assyro-Babylonian goddess of the vine.) A subsequent impact that excavated Datan crater, which is 37 miles (60 kilometers) in diameter, obliterated a large section of Geshtin’s rim and made its own crater wall in Geshtin’s interior. (Datan is one of the Polish gods who protect the fields but apparently not this crater.) Still later, Datan itself was the victim of a sizable impact on its rim (although not large enough to have merited an approved name this early in the geological studies of Ceres). Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Now the mission lifetime is limited by the small supply of conventional rocket propellant, expelled from reaction control system thrusters strategically located around the spacecraft. When that precious hydrazine is exhausted, the robot will no longer be able to point its solar arrays at the sun, its antenna at Earth, its sensors at Ceres or its ion engines in the direction needed to travel elsewhere, so the mission will conclude. The lower Dawn’s orbital altitude, the faster it uses hydrazine, because it must rotate more quickly to keep its sensors pointed at the ground. In addition, it has to fight harder to resist Ceres’ relentless gravitational tug on the very large solar arrays, creating an unwanted torque on the ship.

Among the innovative solutions to the reaction wheel problems was the development of a new method of orienting the spacecraft with a combination of only two wheels plus hydrazine. In the final orbit, this “hybrid control” will use hydrazine at only half the rate that would be needed without the wheels. Therefore, mission controllers have been preserving the units for this final phase of the expedition, devoting the limited remaining usable life to the time that they can provide the greatest benefit in saving hydrazine. (The accuracy with which Dawn can aim its sensors is essentially unaffected by which control mode is used, so hydrazine conservation is the dominant consideration in when to use the wheels.) Apart from a successful test of hybrid control two years ago and three subsequent periods of a few hours each for biannual operation to redistribute internal lubricants, the two operable wheels have been off since August 2012, when Dawn was climbing away from Vesta on its way out of orbit.

Controllers plan to reactivate the wheels on Dec. 14. However, in the unlikely case that the TCM is deemed unnecessary, they will power the wheels on on Dec. 11. The reaction wheels will remain in use for as long as both function correctly. If either one fails, which could happen immediately or might not happen before the hydrazine is depleted next year, it and the other will be powered off, and the mission will continue, relying exclusively on hydrazine control.

PIA20124: Dawn HAMO Image 62

Dawn recorded this view in its third mapping orbit at an altitude of 915 miles (1,470 kilometers) in mapping cycle #5. The region shown is located between Fluusa and Toharu craters. The largest crater here is 16 miles (26 kilometers) across. The well defined features indicate the crater is relatively young, so subsequent small impacts have not degraded it significantly. As elsewhere on Ceres, some strikingly bright material is evident, particularly in the walls. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn will measure the energies and numbers of neutrons and gamma rays emanating from Ceres as soon as it arrives in its new orbit. With a month or so of these measurements, scientists will be able to determine the abundances of some of the elements that compose the material near the surface. Engineers and scientists also will collect new data on the gravity field at this low altitude right away, so they eventually can build up a profile of the dwarf planet’s interior structure. The other instruments (including the camera) have narrower fields of view and are more sensitive to small discrepancies in where they are aimed. It will take a few more days to incorporate the actual measured orbital parameters into the corresponding plans that controllers will radio to the spacecraft. Those observations are scheduled to begin on Dec. 18. But always squeezing as much as possible out of the mission, the flight team might actually begin some photography and infrared spectroscopy as early as Dec. 16.

Now closing in on its final orbit, the veteran space traveler soon will commence the last phase of its long and fruitful adventure, when it will provide the best views yet of Ceres. Known for more than two centuries as little more than a speck of light in the vast and beautiful expanse of the stars, the spacecraft has already transformed it into a richly detailed and fascinating world. Now Dawn is on the verge of revealing even more of Ceres’ secrets, answering more questions and, as is the marvelous nature of science and exploration, raising new ones.

Dawn is 295 miles (470 kilometers) from Ceres. It is also 3.33 AU (309 million miles, or 498 million kilometers) from Earth, or 1,270 times as far as the moon and 3.37 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 55 minutes to make the round trip.

Dr. Marc D. Rayman
5:00 p.m. PST November 30, 2015

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30 Oct
2015
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | October 30

by Marc Rayman
 

Dear Exuldawnt Readers,

Dawn has completed another outstandingly successful campaign to acquire a wealth of pictures and other data in its exploration of dwarf planet Ceres. Exultant residents of distant Earth now have the clearest and most complete view ever of this former planet.

The stalwart probe spent more than two months orbiting 915 miles (1,470 kilometers) above the alien world. We described the plans for this third major phase of Dawn’s investigation (also known as the high altitude mapping orbit, or HAMO) in August and provided a brief progress report in September. Now we can look back on its extremely productive work.

Ceres wuth planetary names

This map of Ceres shows the feature names approved by the International Astronomical Union. We described the naming convention in December, and the most up-to-date list of names is here. The small crater Kait (named for the ancient Hattic grain goddess) is used to define the location of the prime meridian. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Each revolution, flying over the north pole to the south pole and back to the north, took Dawn 19 hours. Mission planners carefully chose the orbital parameters to coordinate the spacecraft’s travels with the nine-hour rotation period of Ceres (one Cerean day) and with the field of view of the camera so that in 12 orbits over the lit hemisphere (one mapping “cycle”), Dawn could photograph all of the terrain.

In each of six mapping cycles, the robot held its camera and its infrared and visible mapping spectrometers at a different angle. For the first cycle (Aug. 17-26), Dawn looked straight down. For the second, it looked a little bit behind and to the left as it completed another dozen orbits. For the third map, it pointed the sensors a little behind and to the right. In its fourth cycle, it aimed ahead and to the left. When it made its fifth map, it peered immediately ahead, and for the sixth and final cycle (Oct. 12-21) it viewed terrain farther back than in the third cycle but not as far to the right.

The result of this extensive mapping is a very rich collection of photos of the fascinating scenery on a distant world. Think for a moment of the pictures not so much from the standpoint of the spacecraft but rather from a location on the ground. With the different perspectives in each mapping cycle, that location has been photographed from several different angles, providing stereo views. Scientists will use these pictures to make the landscape pop into its full three dimensionality.

Dawn’s reward for these two months of hard work is much more than revealing Ceres’ detailed topography, valuable though that is. During the first and fifth mapping cycles, it used the seven color filters in the camera, providing extensive coverage in visible and infrared wavelengths.

Hints at Ceres’ Composition from Color

This false-color map of Ceres was constructed using images taken in the first mapping cycle at an altitude of 915 miles (1,470 kilometers). It combines pictures taken in filters that admit light in what the human eye perceives as violet (440 nanometers), near the limit of visible red (750 nanometers), and invisible infrared (920 nanometers). Because humans are so good at processing visual information, depictions such as this are a helpful way to highlight and illustrate variations in the composition or other properties of the material on Ceres’ surface. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

In addition to taking more than 6,700 pictures, the spacecraft operated its visible and infrared mapping spectrometers to acquire in excess of 12.5 million spectra. Each spectrum contains much finer measurements of the colors and a wider range of wavelengths than the camera. In exchange, the camera has sharper vision and so can discern smaller geological features. As the nerdier among us would say, the spectrometers achieve better spectral resolution and the camera achieves better spatial resolution. Fortunately, it is not a competition, because Dawn has both, and the instruments yield complementary measurements.

Even as scientists are methodically analyzing the vast trove of data, turning it into knowledge, you can go to the Ceres image gallery to see some of Dawn’s pictures, exhibiting a great variety of terrain, smooth or rugged, strangely bright or dark, unique in the solar system or reminiscent of elsewhere spacecraft have traveled, and always intriguing.

Occator Mosaic

Ten photos from Dawn’s first mapping cycle were combined to make this view centered on Occator crater. Because of the range of brightness, pictures with two different exposures were required to record the details of the bright regions and the rest of the crater itself, as explained last month. Eight additional pictures show the area around the crater. Occator is almost 60 miles (more than 90 kilometers) in diameter. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Among the questions scientists are grappling with is what the nature of the bright regions is. There are many places on Ceres that display strikingly reflective material but nowhere as prominently as in Occator crater. Even as Dawn approached Ceres, the mysterious reflections shone out far into space, mesmerizing and irresistible, as if to guide or even seduce a passing ship into going closer. Our intrepid interplanetary adventurer, compelled not by this cosmic invitation but rather by humankind’s still more powerful yearning for new knowledge and new insights, did indeed venture in. Now it has acquired excellent pictures and beautiful spectra that will help determine the composition and perhaps even how the bright areas came to be. Thanks to the extraordinary power of the scientific method, we can look forward to explanations. (And while you wait, you can register your vote here for what the answer will be.)

Scientists also puzzle over the number and distribution of craters. We mentioned in December the possibility that ice being mixed in as a major component on or near the surface would cause the material to flow, albeit very slowly on the scale of a human lifetime. But over longer times, the glacially slow movement might prove significant. Most of Ceres’ craters are excavated by impacts from some of the many bodies that roam that part of the solar system. Ceres lives in a rough neighborhood, and being the most massive body between Mars and Jupiter does not give it immunity to assaults. Indeed, its gravity makes it even more susceptible, attracting passersby. But once a crater is formed, the scar might be expected to heal as the misshapen ground gradually recovers. In some ways this is similar to when you remove pressure from your skin. What may be a deep impression relaxes, and after a while, the original mark (or, one may hope, Marc) is gone. But Ceres has more craters than some scientists had anticipated, especially at low latitudes where sunlight provides a faint warming. Apparently the expectation of the gradual disappearance of craters was not quite right. Is there less evidence of flowing ground material because the temperature is lower than predicted (causing the flow to be even slower), because the composition is not quite what was assumed, or because of other reasons? Moreover, craters are not distributed as would be expected for random pummeling; some regions display significantly more craters than others. Investigating this heterogeneity may give further insight into the geological processes that have taken place and are occurring now on this dwarf planet.

Occator Topography

This color-coded topographic map of Occator crater is based on Dawn’s observations in its second mapping orbit at an altitude of 2,700 miles (4,400 kilometers). Of course there is no sea level on Ceres, but the deep blue here is 5,150 feet (1,570 meters) below a reference level, and brown is 14,025 feet (4,275 meters) above it. (Brown is used in place of white for the elevation, so white can show the bright regions.) Imagine the exotic scenery here, with strangely bright areas and towering crater walls. The stereo views acquired in the third mapping orbit will reveal finer detail in the topography. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn’s bounty from this third major science campaign includes even more than stereo and color pictures plus visible and infrared spectra. Precise tracking of the spacecraft as it moves in response to Ceres’ gravitational pull allows scientists to calculate the arrangement of mass in the behemoth. Performing such measurements will be among the top three priorities for the lowest altitude orbit, when Dawn experiences the strongest buffeting from the gravitational currents, but already the structure of the gravitational field is starting to be evident. We will see next month how this led to a small change in the choice of the altitude for this next orbit, which will be less than 235 miles (380 kilometers).

The other top two priorities for the final mission phase are the measurement of neutron spectra and the measurement of gamma ray spectra, both of which will help in establishing what species of atoms are present on and near the surface. The weak radiation from Ceres is difficult to measure from the altitudes at which Dawn has been operating so far. The gamma ray and neutron detector (GRaND) has been in use since March 12 (shortly after Dawn arrived in orbit), but that has been to prepare for the low orbit. Nevertheless, the sophisticated instrument did detect the dwarf planet’s faint nuclear emissions even in this third orbital phase. The signal was not strong enough to allow any conclusions about the elemental composition, but it is interesting to begin seeing the radiation which will help uncover more of Ceres’ secrets when Dawn is closer.

To scientists’ great delight, one of GRaND’s sensors even found an entirely unexpected signature of Ceres in Dawn’s second mapping orbit, where the spacecraft revolved every 3.1 days at an altitude of 2,700 miles (4,400 kilometers). In a nice example of scientific serendipity, it detected high energy electrons in the same region of space above Ceres on three consecutive orbits. Electrons and other subatomic particles stream outward from the sun in what is called the solar wind, and researchers understand how planets with magnetic fields can accelerate them to higher energy. Earth is an example of a planet with a magnetic field, but Ceres is thought not to be. So scientists now have the unanticipated joy not only of establishing the physical mechanism responsible for this discovery but also determining what it reveals about this dwarf planet.

Dawn HAMO Image 29

Dawn had this view near 0 degrees longitude in the northern hemisphere on Sept. 9 in its third mapping cycle at an altitude of 915 miles (1,470 kilometers). Oxo crater on the right, which shows bright material inside and out as well as a peculiar shape, is slightly over five miles (nearly nine kilometers) in diameter. The crater is named for the god of agriculture for the Yoruba people of Brazil. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Several times during each of the six mapping cycles, Dawn expended a few grams of its precious hydrazine propellant to rotate so it could aim its main antenna at Earth. While the craft soared high above ground cloaked in the deep black of night, it transmitted some of its findings to NASA’s Deep Space Network. But Dawn conducted so many observations that during half an orbit, or about 9.5 hours, it could not radio enough data to empty its memory. By the end of each mapping cycle, the probe had accumulated so much data that it fixed its antenna on Earth for about two days, or 2.5 revolutions, to send its detailed reports on Ceres to eager Earthlings.

Following the conclusion of the final mapping cycle, after transmitting the last of the information it had stored in its computer, the robotic explorer did not waste any time gloating over its accomplishments. There was still a great deal more work to do. On Oct. 23 at 3:30 p.m., it fired up ion engine #2 (the same one it used to descend from the second mapping orbit to the third) to begin more than seven weeks of spiraling down to its fourth orbit. (You can follow its progress here and on Twitter @NASA_Dawn.) Dawn has accomplished more than 5.4 years of ion thrusting since it left Earth, and the complex descent to less than 235 miles (380 kilometers) is the final thrusting campaign of the entire extraterrestrial expedition. (The ion propulsion system will be used occasionally to make small adjustments to the final orbit.)

The blue lights in Dawn mission control that indicate the spacecraft is thrusting had been off since Aug. 13. Now they are on again, serving as a constant (and cool) reminder that the ambitious mission is continuing to power its way to new (and cool) destinations.

Dawn is 740 miles (1,190 kilometers) from Ceres. It is also 2.91 AU (271 million miles, or 436 million kilometers) from Earth, or 1,165 times as far as the moon and 2.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 48 minutes to make the round trip.

Dr. Marc D. Rayman
3:00 p.m. PDT October 30, 2015

P.S. While the spacecraft is hard at work continuing its descent tomorrow, your correspondent will be hard at work dispensing treats to budding (but cute) extortionists at his front door. But zany and playful as ever, he will expand his delightful costume from last year by adding eight parts dark energy. Trick or treat!

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31 Mar
2015
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | March 31

by Marc Rayman
 

Dear Dawnticipating Explorers,

Now orbiting high over the night side of a dwarf planet far from Earth, Dawn arrived at its new permanent residence on March 6. Ceres welcomed the newcomer from Earth with a gentle but firm gravitational embrace. The goddess of agriculture will never release her companion. Indeed, Dawn will only get closer from now on. With the ace flying skills it has demonstrated many times on this ambitious deep-space trek, the interplanetary spaceship is using its ion propulsion system to maneuver into a circular orbit 8,400 miles (13,500 kilometers) above the cratered landscape of ice and rock. Once there, it will commence its first set of intensive observations of the alien world it has traveled for so long and so far to reach.

For now, however, Dawn is not taking pictures. Even after it entered orbit, its momentum carried it to a higher altitude, from which it is now descending. From March 2 to April 9, so much of the ground beneath it is cloaked in darkness that the spacecraft is not even peering at it. Instead, it is steadfastly looking ahead to the rewards of the view it will have when its long, leisurely, elliptical orbit loops far enough around to glimpse the sunlit surface again.

OP NAV 5 image

Dawn took this picture of Ceres on Feb. 19 at a distance of 28,000 miles (46,000 kilometers). Despite extensive speculation (see, for example, here), the nature of the bright spots is not yet known, but we can look forward to more detailed pictures and other data that will elucidate this mystery. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Among the many sights we eagerly anticipate are those captivating bright spots. Hinted at more than a decade ago by Hubble Space Telescope, Dawn started to bring them into sharper focus after an extraordinary journey of more than seven years and three billion miles (nearly five billion kilometers). Although the spots are reflections of sunlight, they seem almost to radiate from Ceres as cosmic beacons, drawing us forth, spellbound. Like interplanetary lighthouses, their brilliant glow illuminates the way for a bold ship from Earth sailing on the celestial seas to a mysterious, uncharted port. The entrancing lights fire our imagination and remind us of the irresistible lure of exploration and the powerful anticipation of an adventure into the unknown.

As we describe below, Dawn’s extensive photographic coverage of the sunlit terrain in early May will include these bright spots. They will not be in view, however, when Dawn spies the thin crescent of Ceres in its next optical navigation session, scheduled for April 10 (as always, all dates here are in the Pacific time zone).

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6 Mar
2015
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | March 6

by Marc Rayman
 

Dear Unprecedawnted Readers,

Since its discovery in 1801, Ceres has been known as a planet, then as an asteroid, and later as a dwarf planet. Now, after a journey of 3.1 billion miles (4.9 billion kilometers) and 7.5 years, Dawn calls it “home.”

Earth’s robotic emissary arrived at about 4:39 a.m. PST today. It will remain in residence at the alien world for the rest of its operational life, and long, long after.

Before we delve into this unprecedented milestone in the exploration of space, let’s recall that even before reaching orbit, Dawn started taking pictures of its new home. Last month we presented the updated schedule for photography. Each activity to acquire images (as well as visible spectra and infrared spectra) has executed smoothly and provided us with exciting and tantalizing new perspectives.

While there are countless questions about Ceres, the most popular now seems to be what the bright spots are. It is impossible not to be mesmerized by what appear to be glowing beacons, shining out across the cosmic seas from the uncharted lands ahead. But the answer hasn’t changed: we don’t know. There are many intriguing speculations, but we need more data, and Dawn will take photos and myriad other measurements as it spirals closer and closer during the year. For now, we simply know too little.

Animated Gif

Dawn observed Ceres for a full rotation of the dwarf planet, which lasts about nine hours. The photos were taken on Feb. 19 from a distance of about 28,000 miles (46,000 kilometers). A single mosaic made from the individual pictures is below. The imaging session was known as RC2. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

For example, some people ask if those spots might be lights from an alien city. That’s ridiculous! At this early stage, how could Dawn determine what kinds of groupings Cereans live in? Do they even have cities? For all we know, they may live only in rural communities, or perhaps they only have large states.

What we already know is that in more than 57 years of space exploration, Dawn is now the only spacecraft ever to orbit two extraterrestrial destinations. A true interplanetary spaceship, Dawn left Earth in Sep. 2007 and traveled on its own independent course through the solar system. It flew past Mars in Feb. 2009, robbing the red planet of some of its own orbital energy around the sun. In July 2011, the ship entered orbit around the giant protoplanet Vesta, the second most massive object in the main asteroid belt between Mars and Jupiter. (By the way, Dawn’s arrival at Vesta was exactly one Vestan year ago earlier this week.) It conducted a spectacular exploration of that fascinating world, showing it to be more closely related to the terrestrial planets (including Earth, home to many of our readers) than to the typical objects people think of as asteroids. After 14 months of intensive operations at Vesta, Dawn climbed out of orbit in Sep. 2012, resuming its interplanetary voyage. Today it arrived at its final destination, Ceres, the largest object between the sun and Pluto that had not previously been visited by a spacecraft. (Fortunately, New Horizons is soon to fly by Pluto. We are in for a great year!)

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25 Feb
2015
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | February 25

by Marc Rayman
 

Dear Fine and Dawndy Readers,

The Dawn spacecraft is performing flawlessly as it conducts the first exploration of the first dwarf planet. Each new picture of Ceres reveals exciting and surprising new details about a fascinating and enigmatic orb that has been glimpsed only as a smudge of light for more than two centuries. And yet as that fuzzy little blob comes into sharper focus, it seems to grow only more perplexing.

Dawn is showing us exotic scenery on a world that dates back to the dawn of the solar system, more than 4.5 billion years ago. Craters large and small remind us that Ceres lives in the rough and tumble environment of the main asteroid belt between Mars and Jupiter, and collectively they will help scientists develop a deeper understanding of the history and nature not only of Ceres itself but also of the solar system.

Ceres Op Nav 3 animated gif

Dawn observed Ceres for three hours, or one-third of a Cerean day, on Feb. 3-4. The spacecraft was 91,000 miles (146,000 kilometers) from the dwarf planet in this imaging session, known as OpNav 3. More detail on that one big bright spot is shown in another image below. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Even as we discover more about Ceres, some mysteries only deepen. It certainly does not require sophisticated scientific insight to be captivated by the bright spots. What are they? At this point, the clearest answer is that the answer is unknown. One of the great rewards of exploring the cosmos is uncovering new questions, and this one captures the imagination of everyone who gazes at the pictures sent back from deep space.

Other intriguing features newly visible on the unfamiliar landscape further assure us that there will be much more to see and to learn — and probably much more to puzzle over — when Dawn flies in closer and acquires new photographs and myriad other measurements. Over the course of this year, as the spacecraft spirals to lower and lower orbits, the view will continue to improve. In the lowest orbit, the pictures will display detail well over one hundred times finer than the RC2 pictures returned a few days ago (and shown below). Right now, however, Dawn is not getting closer to Ceres. On course and on schedule for entering orbit on March 6, Earth’s robotic ambassador is slowly separating from its destination.

“Slowly” is the key. Dawn is in the vicinity of Ceres and is not leaving. The adventurer has traveled more than 900 million miles (1.5 billion kilometers) since departing from Vesta in 2012, devoting most of the time to using its advanced ion propulsion system to reshape its orbit around the sun to match Ceres’ orbit. Now that their paths are so similar, the spacecraft is receding from the massive behemoth at the leisurely pace of about 35 mph (55 kilometers per hour), even as they race around the sun together at 38,700 mph (62,300 kilometers per hour). The probe is expertly flying an intricate course that would be the envy of any hotshot spaceship pilot. To reach its first observational orbit — a circular path from pole to pole and back at an altitude of 8,400 miles (13,500 kilometers) — Dawn is now taking advantage not only of ion propulsion but also the gravity of Ceres.

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29 Jan
2015
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | January 29

by Marc Rayman
 

Dear Abundawnt Readers,

The dwarf planet Ceres is a giant mystery. Drawn on by the irresistible lure of exploring this exotic, alien world, Dawn is closing in on it. The probe is much closer to Ceres than the moon is to Earth.

And now it is even closer…

And now it is closer still!

What has been glimpsed as little more than a faint smudge of light amidst the stars for more than two centuries is finally coming into focus. The first dwarf planet discovered (129 years before Pluto), the largest body between the sun and Pluto that a spacecraft has not yet visited, is starting to reveal its secrets. Dawn is seeing sights never before beheld, and all of humankind is along for the extraordinary experience.

We have had a preview of Dawn’s approach phase, and in November we looked at the acrobatics the spacecraft performs as it glides gracefully into orbit. Now the adventurer is executing those intricate plans, and it is flying beautifully, just the way a seasoned space traveler should.

Dawn’s unique method of patiently, gradually reshaping its orbit around the sun with its ion propulsion system is nearly at its end. Just as two cars may drive together at high speed and thus travel at low speed relative to each other, Dawn is now close to matching Ceres’ heliocentric orbital motion. Together, they are traveling around the sun at nearly 39,000 mph (almost 64,000 kilometers per hour), or 10.8 miles per second (17.4 kilometers per second). But the spaceship is closing in on the world ahead at the quite modest relative speed of about 250 mph (400 kilometers per hour), much less than is typical for interplanetary spaceflight.

Animated Gif of Ceres

Dawn observed Ceres for an hour on Jan. 13, from a distance of 238,000 miles (383,000 kilometers). A little more than half of the surface was revealed as Ceres rotated. This imaging session is known as OpNav 1. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Dawn has begun its approach imaging campaign, and the pictures are wonderfully exciting. This month, we will take a more careful look at the plans for photographing Ceres. Eager readers may jump directly to the summary table, but others may want to emulate the spacecraft by taking a more leisurely approach to it, which may aid in understanding some details.

While our faithful Dawn is the star of this bold deep-space adventure (along with protoplanet Vesta and dwarf planet Ceres), the real talent is behind the scenes, as is often the case with celebrities. The success of the mission depends on the dedication and expertise of the members of the Dawn flight team, no farther from Earth than the eighth floor of JPL’s building 264 (although occasionally your correspondent goes on the roof to enjoy the sights of the evening sky). They are carefully guiding the distant spacecraft on its approach trajectory and ensuring it accomplishes all of its tasks.

To keep Dawn on course to Ceres, navigators need a good fix on where the probe and its target are. Both are far, far from Earth, so the job is not easy. In addition to the extraordinarily sophisticated but standard methods of navigating a remote interplanetary spacecraft, using the radio signal to measure its distance and speed, Dawn’s controllers use another technique now that it is in the vicinity of its destination.

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29 Dec
2014
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | December 29

by Marc Rayman
 

Pardawn Me, Dear Readers,

Far away from Earthlings who look forward to a new year, Dawn looks forward to a new world. On the far side of the sun, the interplanetary explorer is closing in on Ceres, using its advanced ion propulsion system to match solar orbits with the dwarf planet.

Since breaking out of orbit around the giant protoplanet Vesta in September 2012, the spaceship has patiently flown in interplanetary cruise. That long mission phase is over, and now Dawn is starting the Ceres chapter of its extraordinary extraterrestrial expedition. Configured for its approach phase, the craft is following a new and carefully designed course described in detail last month. In March it will slip ever so gracefully into orbit for an ambitious and exciting exploration of the alien world ahead.

Over the past year, we have provided previews of the major activities during all the phases of Dawn’s mission at Ceres. This month, let’s take a look at Ceres itself, an intriguing and mysterious orb that has beckoned for more than two centuries. Now, finally, after so long, Earth is answering the cosmic invitation, and an ambassador from our planet is about to take up permanent residence there. Over the course of Dawn’s grand adventure, our knowledge will rocket far, far beyond all that has been learned before.

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