30 Nov
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 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 77 miles (125 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. 12-14 (concluding on Dawn’s 3000th 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. 15. 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 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
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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27 Sep
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | September 27

by Marc Rayman

Dear Dawnniversaries,

Eight years ago today, Dawn was gravitationally bound to a planet. It was conceived and built there by creatures curious and bold, with an insatiable yearning to reach out and know the cosmos. Under their guidance, it left Earth behind as its Delta rocket dispatched it on an ambitious mission to explore two of the last uncharted worlds in the inner solar system. As Earth continued circling the sun once a year, now having completed eight revolutions since its celestial ambassador departed, Dawn has accomplished a remarkable interplanetary journey. The adventurer spent most of its anniversaries powering its way through the solar system, using its advanced and uniquely capable ion propulsion system to reshape its orbit around the sun. On its way to the main asteroid belt, it sailed past Mars, taking some of the that red planet’s orbital energy to boost its own solar orbit. On its fourth anniversary, the probe was locked in orbit around the giant protoplanet Vesta, the second most massive object between Mars and Jupiter. Dawn’s pictures and other data showed it to be a complex, fascinating world, more closely related to the terrestrial planets (including one on which it began its mission and another from which it stole some energy) than to the much smaller asteroids.

Dawn launch, JSC, Sept. 27. 2007

Dawn launched at dawn (7:34 a.m. EDT) from Cape Canaveral Air Force Station, Sep. 27, 2007. Its mission is to learn about the dawn of the solar system by studying Vesta and Ceres. The intricate sequence of activities between the time this photo was taken and Dawn’s separation from the rocket to fly on its own is described here. Credit: KSC/NASA

Today, on the eighth anniversary of venturing into the cosmos, Dawn is once again doing what it does best. In the permanent gravitational embrace of dwarf planet Ceres, orbiting at an altitude of 915 miles (1,470 kilometers), Dawn is using its suite of sophisticated sensors to scrutinize this mysterious, alien orb. Ceres was the first dwarf planet ever sighted (and was called a planet for more than a generation after its discovery), but it had to wait more than two centuries before Earth accepted its celestial invitation. The only spacecraft ever to orbit two extraterrestrial destinations, this interplanetary spaceship arrived at Ceres in March to take up residence.

Although this is the final anniversary during its scheduled primary mission, Dawn will remain in orbit around its new home far, far into the future. Later this year it will spiral down to its fourth and final orbital altitude at about 230 miles (375 kilometers). Once there, it will record spectra of neutrons, gamma rays, and visible and infrared light, measure the distribution of mass inside Ceres, and take pictures. Then when it exhausts its supply of hydrazine next year, as it surely will, the mission will end. We have discussed before that despite the failure of two reaction wheels, devices previously considered indispensable for the expedition, the hardy ship has excellent prospects now for fulfilling and even exceeding its many goals in exploring Ceres.

Last month we described the plans for Dawn’s penultimate mapping phase at the dwarf planet, and it is going very well. The probe is already more than halfway through this third orbital phase at Ceres, which is divided into six mapping cycles. Each 11-day cycle requires a dozen flights over the illuminated hemisphere to allow the camera to map the entire surface. Each map is made by looking at a different angle. Taken together then, they provide stereo views, so scientists gain perspectives that allow them to construct topographical maps. The camera’s internal computer detected an unexpected condition in the third cycle of this phase, and that caused the loss of some of the pictures. But experienced mission planners had designed all of the major mapping phases (summarized here) with more observations than are needed to meet their objectives, so the deletion of those images was not significant. At this moment, the spacecraft is nearing the end of its fourth mapping cycle, making its tenth flight over the side of Ceres lit by the sun.

You can follow Dawn’s progress by using your own interplanetary spaceship to snoop into its activities in orbit around the distant world, by tapping into the radio signals beamed back and forth across the solar system between Dawn and the giant antennas of NASA’s Deep Space Network, or by checking the frequent mission status reports.

You also can see the marvelous sights by visiting the Ceres image gallery. Among the most captivating is Occator crater (see the picture below). As the spacecraft has produced ever finer pictures this year, starting with its distant observations in January, the light reflecting from the interior of this crater has dazzled us. The latest pictures show 260 times as much detail. Dawn has transformed what was so recently just a bright spot into a complex and beautiful gleaming landscape. Last month we asked what these mesmerizing features would reveal when photographed from this the present altitude, and now we know.

Dawn Takes a Closer Look at Occator

Dawn’s view of Occator crater from an altitude of 915 miles (1,470 kilometers). This is a composite of two photos taken on Aug. 22. Because of the large range in brightness, controllers modified Dawn’s observation plan to take pictures with different exposures: a normal exposure for most of the scene, and a short exposure to capture the details of the brightest areas. Occator is almost 60 miles (more than 90 kilometers) in diameter. Following the theme established last year for naming features on Ceres, the International Astronomical Union named this crater for a Roman deity of harrowing. Whatever the geochemical reason for the stunning bright regions turns out to be, it’s unlikely to be related to that agricultural technique of breaking up soil and covering seeds. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Scientists are continuing to analyze Dawn’s pictures and other data not only from Occator but all of Ceres to learn more about the nature of this exotic relict from the dawn of the solar system. Many deep questions are unanswered and remain mystifying, but of one point there can be no doubt: the scenery is beautiful. Even now, the photos speak for themselves, displaying wondrous sights on a world shaped both by its own complex internal geological processes as well as by external forces from more than 4.5 billion years in the rough and tumble main asteroid belt.

Because the pictures speak for themselves, your correspondent will speak for the mission. So now, as every Sep. 27, let’s take a broader look at Dawn’s deep-space trek. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the eighth annual summary, reusing text from previous years with updates where appropriate. Readers who wish to reflect upon Dawn’s ambitious journey may find it helpful to compare this material with the logs from its first, second, third, fourth, fifthsixth and seventh anniversaries.

In its eight years of interplanetary travels, the spacecraft has thrust for a total of 1,976 days, or 68 percent of the time (and about 0.000000039 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 873 pounds (396 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sep. 27, 2007. The spacecraft has used 66 of the 71 gallons (252 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space.

The thrusting since then has achieved the equivalent of accelerating the probe by 24,400 mph (39,200 kilometers per hour). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Having accomplished 98 percent of the thrust time planned for its entire mission, Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.) The principal ion thrusting that remains is to maneuver from the present orbit to the final one from late October to mid-December.


Dawn’s interplanetary trajectory (in blue). The dates in white show Dawn’s location every Sep. 27, starting on Earth in 2007. Note that Earth returns to the same location, taking one year to complete each revolution around the sun. When Dawn is farther from the sun, it orbits more slowly, so the distance from one Sep. 27 to the next is shorter. Credit: NASA/JPL

Since launch, our readers who have remained on or near Earth have completed eight revolutions around the sun, covering 50.3 AU (4.7 billion miles, or 7.5 billion kilometers). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 35.0 AU (3.3 billion miles, or 5.2 billion kilometers). As it climbed away from the sun, up the solar system hill, to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It had to go even slower to perform its graceful rendezvous with Ceres. In the eight years since Dawn began its voyage, Vesta has traveled only 32.7 AU (3.0 billion miles, or 4.9 billion kilometers), and the even more sedate Ceres has gone 26.8 AU (2.5 billion miles, or 4.0 billion kilometers). (To develop a feeling for the relative speeds, you might reread this paragraph while paying attention to only one set of units, whether you choose AU, miles, or kilometers. Ignore the other two scales so you can focus on the differences in distance among Earth, Dawn, Vesta and Ceres over the eight years. You will see that as the strength of the sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)

The Lonely Mountain

Dawn had this view on Aug. 18 from an altitude of 915 miles (1,470 kilometers). The unnamed mountain to the right of center reaches a height of 4 miles (6 kilometers) or 20,000 feet. This curious cone, showing prominent bright streaks, has a sharply defined base with virtually no accumulated debris. We have seen this huge feature from other perspectives in previous months. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

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

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

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

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

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

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

For readers who are not overwhelmed by the number of numbers, investing the effort to study the table may help to demonstrate how Dawn has patiently transformed its orbit during the course of its mission. Note that four years ago, the spacecraft’s path around the sun was exactly the same as Vesta’s. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to slip into orbit around Vesta. While simply flying by it would have been far easier, matching orbits with Vesta required the exceptional capability of the ion propulsion system. Without that technology, NASA’s Discovery Program would not have been able to afford a mission to explore the massive protoplanet in such detail. But now, Dawn has gone even beyond that. Having discovered so many of Vesta’s secrets, the stalwart adventurer left it behind in 2012. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. Dawn devoted another 2.5 years to reshaping and tilting its orbit even more so that now it is identical to Ceres’. Once again, that was essential to the intricate celestial choreography in March, when the behemoth reached out with its gravity and tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.

Dawn takes great advantage of being able to orbit its two targets by performing extensive measurements that would not be feasible with a fleeting visit at high speed. As its detailed inspection of a strange and distant world continues, we can look forward to more intriguing perspectives and exciting insights into our solar system. On its eighth anniversary of setting sail on the cosmic seas for an extraordinary voyage, the faithful ship is steadily accumulating great treasures.

NASA's Dawn spacecraft took this image that shows a mountain ridge, near lower left, that lies in the center of Urvara crater on Ceres. Urvara is an Indian and Iranian deity of plants and fields. The crater's diameter is 101 miles (163 kilometers).

Dawn observed this region inside Urvara crater on Aug. 19. The crater is about 100 miles (160 kilometers) in diameter and is named for an Indian and Iranian deity of plants and fields. Although many craters have a mountain in the center, as we explained when we saw the entire crater from three times farther away in the second mapping orbit, Urvara has an interesting ridge, visible at lower left. Full image and caption. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn is 915 miles (1,470 kilometers) from Ceres. It is also 2.45 AU (228 million miles, or 367 million kilometers) from Earth, or 1,025 times as far as the moon and 2.45 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.

Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2015

All Dawn Journal entries

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21 Aug
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | August 21

by Marc Rayman

Dear Unhesidawntingly Enthusiastic Readers,

 Striking 3-D detail highlights a towering mountain, the brightest spots and other features on dwarf planet Ceres in a new video from NASA's Dawn mission

This is a very brief clip from an animation of Ceres based on Dawn’s observations through the second mapping orbit. The entire animation (along with a recording of your correspondent’s informal commentary) is here. This excerpt shows the conical mountain, and you can see more about it in pictures below. The complete animation also shows the bright spots and a 3-D view of the dwarf planet. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/LPI

An ambitious explorer from Earth is gaining the best views ever of dwarf planet Ceres. More than two centuries after its discovery, this erstwhile planet is now being mapped in great detail by Dawn.

The spacecraft is engaged in some of the most intensive observations of its entire mission at Ceres, using its camera and other sensors to scrutinize the alien world with unprecedented clarity and completeness. At an average altitude of 915 miles (1,470 kilometers) and traveling at 400 mph (645 kilometers per hour), Dawn completes an orbit every 19 hours. The pioneer will be here for more than two months before descending to its final orbit.

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29 Jul
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | July 29

by Marc Rayman

Dear Descendawnts,

Flying on a blue-green ray of xenon ions, Dawn is gracefully descending toward dwarf planet Ceres. Even as Dawn prepares for a sumptuous new feast in its next mapping orbit, scientists are continuing to delight in the delicacies Ceres has already served. With a wonderfully rich bounty of pictures and other observations already secured, the explorer is now on its way to an even better vantage point.

Dawn Survey Orbit Image 31 This image, taken by NASA's Dawn spacecraft, shows dwarf planet Ceres from an altitude of 2,700 miles (4,400 kilometers). The image, with a resolution of 1,400 feet (410 meters) per pixel, was taken on June 25, 2015.

Dawn was in its second mapping orbit at an altitude of 2,700 miles (4,400 kilometers) when it took this picture of Ceres. This area shows relatively few craters, suggesting it is younger than some other areas on Ceres. Some bright spots are visible, although they are not as prominent as the most famous bright spots. Scientists do not yet have a clear explanation for them, but you can register your vote here. Click on the picture (or follow the link to the full image) for a better view of some interesting narrow, straight features in the lower left. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

Dawn takes great advantage of its unique ion propulsion system to maneuver extensively in orbit, optimizing its views of the alien world that beckoned for more than two centuries before a terrestrial ambassador arrived in March. Dawn has been in powered flight for most of its time in space, gently thrusting with its ion engine for 69 percent of the time since it embarked on its bold interplanetary adventure in 2007. Such a flight profile is entirely different from the great majority of space missions. Most spacecraft coast most of the time (just as planets do), making only brief maneuvers that may add up to just a few hours or even less over the course of a mission of many years. But most spacecraft could not accomplish Dawn’s ambitious mission. Indeed, no other spacecraft could. The only ship ever to orbit two extraterrestrial destinations, Dawn accomplishes what would be impossible with conventional technology. With the extraordinary capability of ion propulsion, it is truly an interplanetary spaceship.

In addition to using its ion engine to travel to Vesta, enter into orbit around the protoplanet in 2011, break out of orbit in 2012, travel to Ceres and enter into orbit there this year, Dawn relies on the same system to fly to different orbits around these worlds it unveils, executing complex and graceful spirals around its gravitational master. After conducting wonderfully successful observation campaigns in its preantepenultimate Ceres orbit 8,400 miles (13,600 kilometers) high in April and May and its antepenultimate orbit at 2,700 miles (4,400 kilometers) in June, Dawn commenced its spiral descent to the penultimate orbit at 915 miles (1,470 kilometers) on June 30. (We will discuss this orbital altitude in more detail below.) A glitch interrupted the maneuvering almost as soon as it began, when protective software detected a discrepancy in the probe’s orientation. But thanks to the exceptional flexibility built into the plans, the mission could easily accommodate the change in schedule that followed. It will have no effect on the outcome of the exploration of Ceres. Let’s see what happened.

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29 Jun
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | June 29

by Marc Rayman

Dear Evidawnce-Based Readers,

Dawn is continuing to unveil a Ceres of mysteries at the first dwarf planet discovered. The spacecraft has been extremely productive, returning a wealth of photographs and other scientific measurements to reveal the nature of this exotic alien world of rock and ice. First glimpsed more than 200 years ago as a dot of light among the stars, Ceres is the only dwarf planet between the sun and Neptune.

Dawn has been orbiting Ceres every 3.1 days at an altitude of 2,700 miles (4,400 kilometers). As described last month, the probe aimed its powerful sensors at the strange landscape throughout each long, slow passage over the side of Ceres facing the sun. Meanwhile, Ceres turned on its axis every nine hours, presenting itself to the ambassador from Earth. On the half of each revolution when Dawn was above ground that was cloaked in the darkness of night, it pointed its main antenna to that planet far, far away and radioed its precious findings to eager Earthlings (although the results will be available for others throughout the cosmos as well). Dawn began this second mapping campaign (also known as “survey orbit”) on June 5, and tomorrow it will complete its eighth and final revolution.

The spacecraft made most of its observations by looking straight down at the terrain directly beneath it. During portions of its first, second and fourth orbits, however, Dawn peered at the limb of Ceres against the endless black of space, seeing the sights from a different perspective to gain a better sense of the lay of the land.

The largest is about 6 miles across. Each pixel is a quarter of a mile.

The brightest spots on Ceres. The largest is about four miles (seven kilometers) across. While a picture is worth a thousand words, “wow” might summarize this picture pretty well. The same spots can be seen on the limb in a picture below. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

And what marvels Dawn has beheld! How can you not be mesmerized by the luminous allure of the famous bright spots? They are not, in fact, a source of light, but for a reason that remains elusive, the ground there reflects much more sunlight than elsewhere. Still, it is easy to imagine them as radiating a light all their own, summoning space travelers from afar, beckoning the curious and the bold to venture closer in return for an attractive reward. And that is exactly what we will do, as we seek the rewards of new knowledge and new insights into the cosmos.

Although scientists have not yet determined what minerals are there, Dawn will gather much more data. As summarized in this table, our explorer will map Ceres again from much closer during the course of its orbital mission. New bright areas have shown up in other locations too, in some places as relatively small spots, in others as larger areas (as in the photo below), and all of them will come into sharper focus when Dawn descends further.

limb with crater and bright materials inside and out

There is bright material easily visible inside and around the crater near the upper right. Did the powerful impact that excavated the crater deposit bright material that it brought from elsewhere in space, excavate bright material from underground or create the conditions that subsequently caused some material to become bright? The reason for the greater reflectivity is not yet known. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

In the meantime, you can register your opinion for what the bright spots are. Join more than 100 thousand others who have voted for an explanation for this enigma. Of course, Ceres will be the ultimate arbiter, and nature rarely depends upon public opinion, but the Dawn project will consider sending the results of the poll to Ceres, courtesy of our team member on permanent assignment there.

In addition to the bright spots, Dawn’s views from its present altitude have included a wide range of other intriguing sights, as one would expect on a world of more than one million square miles (nearly 2.8 million square kilometers). There are myriad craters excavated by objects falling from space, inevitable scars from inhabiting the main asteroid belt for more than four billion years, even for the largest and most massive resident there.

The craters exhibit a wide range of appearances, not only in size but also in how sharp and fresh or how soft and aged they look. Some display a peak at the center. A crater can form from such a powerful punch that the hard ground practically melts and flows away from the impact site. Then the material rebounds, almost as if it sloshes back, while already cooling and then solidifying again. The central peak is like a snapshot, preserving a violent moment in the formation of the crater. By correlating the presence or absence of central peaks with the sizes of the craters, scientists can infer properties of Ceres’ crust, such as how strong it is. Rather than a peak at the center, some craters contain large pits, depressions that may be a result of gasses escaping after the impact. (Craters elsewhere in the solar system, including on Vesta and Mars, also have pits.)

crater with terraced walls, a central peak and ridge, smooth areas at top of picture and more rugged terrain at bottom

Several craters here have central peaks. The largest also has a ridge at the center. Note other intriguing geological structures, including the terraced walls of that crater and the contrast between the smooth area in the top half of the picture and the more rugged terrain at the bottom. The picture below overlaps the top of this view. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

Dawn also has spied many long, straight or gently curved canyons. Geologists have yet to determine how they formed, and it is likely that several different mechanisms are responsible. For example, some might turn out to be the result of the crust of Ceres shrinking as the heat and other energy accumulated upon formation gradually radiated into space. When the behemoth slowly cooled, stresses could have fractured the rocky, icy ground. Others might have been produced as part of the devastation when a space rock crashed, rupturing the terrain.

Bright spots on the limb plus canyons

Several long canyons are evident in this view. The large crater that extends off the bottom of the picture is in the center of the picture above. Also notice the bright spots, just visible on the limb at upper left. The first picture above shows them from overhead. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

Ceres shows other signs of an active past rather than that of a static chunk of inert material passing the eons with little notice. Some areas are less densely cratered than others, suggesting that there are geological processes that erase the craters. Indeed, some regions look as if something has flowed over them, as if perhaps there was mud or slush on the surface.

In addition to evidence of aging and renewal, some powerful internal forces have uplifted mountains. One particularly striking structure is a steep cone that juts three miles (five kilometers) high in an otherwise relatively smooth area, looking to an untrained (but transfixed) eye like a volcanic cone, a familiar sight on your home planet (or, at least, on mine). No other isolated, prominent protuberance has been spotted on Ceres.

limb with conical mountain above and to the right of center plus a few other bright areas

The conical mountain is above and to the right of center. With the solar illumination from the top of the picture, note how crater walls are brighter on the bottom (facing the sun) and darker on the top (shaded by the ground they sink into). The cone stands out because it is brighter on the top (facing the sun), and the opposite side is in the shade. (In addition, the material in some places on the cone is brighter than in other places on the same structure.) This view also show several bright spots and larger areas. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption


another view of the limb with the same conical mountain and a few other bright areas.

The conical feature in the previous picture is visible here on the limb at bottom center. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption

It is too soon for scientists to understand the intriguing geology of this ancient world, but the prolific adventurer is providing them with the information they will use. The bounty from this second mapping phase includes more than 1,600 pictures covering essentially all of Ceres, well over five million spectra in visible and infrared wavelengths and hundreds of hours of gravity measurements.

The spacecraft has performed its ambitious assignments quite admirably. Only a few deviations from the very elaborate plans occurred. On June 15 and 27, during the fourth and eighth flights over the dayside, the computer in the combination visible and infrared mapping spectrometer (VIR) detected an unexpected condition, and it stopped collecting data. When the spacecraft’s main computer recognized the situation, it instructed VIR to close its protective cover and then power down. The unit dutifully did so. Also on June 27, about three hours before VIR’s interruption, the camera’s computer experienced something similar.

Most of the time that Dawn points its sensors at Ceres, it simultaneously broadcasts through one of its auxiliary radio antennas, casting a very wide but faint signal in the general direction of Earth. (As Dawn progresses in its orbit, the direction to Earth changes, but the spacecraft is equipped with three of these auxiliary antennas, each pointing in a different direction, and mission controllers program it to switch antennas as needed.) The operations team observed what had occurred in each case and recognized there was no need to take immediate action. The instruments were safe and Dawn continued to carry out all of its other tasks.

When Dawn subsequently flew to the nightside of Ceres and pointed its main antenna to Earth, it transmitted much more detailed telemetry. As engineers and scientists continue their careful investigations, they recognize that in many ways, these events appear very similar to ones that have occurred at other times in the mission.

Four years ago, VIR’s computer reset when Dawn was approaching Vesta, and the most likely cause was deemed to be a cosmic ray strike. That’s life in deep space! It also reset twice in the survey orbit phase at Vesta. The camera reset three times in the first three months of the low altitude mapping orbit at Vesta.

Even with the glitches in this second mapping orbit, Dawn’s outstanding accomplishments represent well more than was originally envisioned or written into the mission’s scientific requirements for this phase of the mission. For those of you who have not been to Ceres or aren’t going soon (and even those of you who want to plan a trip there of your own), you can see what Dawn sees by going to the image gallery.

Although Dawn already has revealed far, far more about Ceres in the last six months than had been seen in the preceding two centuries of telescopic studies, the explorer is not ready to rest on its laurels. It is now preparing to undertake another complex spiral descent, using its sophisticated ion propulsion system to maneuver to a circular orbit three times as close to the dwarf planet as it is now. It will take five weeks to perform the intricate choreography needed to reach the third mapping altitude, starting tomorrow night. You can keep track of the spaceship’s flight as it propels itself to a new vantage point for observing Ceres by visiting the mission status page or following it on Twitter @NASA_Dawn.

As Dawn moves closer to Ceres, Earth will be moving closer as well. Earth and Ceres travel on independent orbits around the sun, the former completing one revolution per year (indeed, that’s what defines a year) and the latter completing one revolution in 4.6 years (which is one Cerean year). (We have discussed before why Earth revolves faster in its solar orbit, but in brief it is because being closer to the sun, it needs to move faster to counterbalance the stronger gravitational pull.) Of course, now that Dawn is in a permanent gravitational embrace with Ceres, where Ceres goes, so goes Dawn. And they are now and forever more so close together that the distance between Earth and Ceres is essentially equivalent to the distance between Earth and Dawn.

On July 22, Earth and Dawn will be at their closest since June 2014. As Earth laps Ceres, they will be 1.94 AU (180 million miles, or 290 million kilometers) apart. Earth will race ahead on its tight orbit around the sun, and they will be more than twice as far apart early next year.


Earth’s and Ceres’ orbits will bring them to their minimum separation on July 22. 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 and showing Dawn equidistant from Earth and the sun in April 2015. Credit: NASA/JPL-Caltech

Although Dawn communicates regularly with Earth, it left that planet behind nearly eight years ago and will keep its focus now on its new residence. With two very successful mapping campaigns complete, its next priority is to work its way down through Ceres’ gravitational field to an altitude of about 900 miles (less than 1,500 kilometers). With sharper views and new kinds of observations (including stereo photography), the treasure trove obtained by this intrepid extraterrestrial prospector will only be more valuable. Everyone who longs for new understandings and new perspectives on the cosmos will grow richer as Dawn continues to pioneer at a mysterious and distant dwarf planet.

Dawn is 2,700 miles (4,400 kilometers) from Ceres. It is also 2.01 AU (187 million miles, or 301 million kilometers) from Earth, or 785 times as far as the moon and 1.98 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 33 minutes to make the round trip.

Dr. Marc D. Rayman
10:00 p.m. PDT June 29, 2015

All Dawn Journal entries

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28 May
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | May 28, 2015

by Marc Rayman

Dear Emboldawned Readers,

A bold adventurer from Earth is gracefully soaring over an exotic world of rock and ice far, far away. Having already obtained a treasure trove from its first mapping orbit, Dawn is now seeking even greater riches at dwarf planet Ceres as it maneuvers to its second orbit.

Animated image of Ceres

In Dawn’s first mapping orbit, it watched Ceres rotate for one full Cerean day (about nine hours) on May 3-4. The spacecraft was 8,400 miles (13,600 kilometers) over the dwarf planet’s northern hemisphere. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Full image and caption.

The first intensive mapping campaign was extremely productive. As the spacecraft circled 8,400 miles (13,600 kilometers) above the alien terrain, one orbit around Ceres took 15 days. During its single revolution, the probe observed its new home on five occasions from April 24 to May 8. When Dawn was flying over the night side (still high enough that it was in sunlight even when the ground below was in darkness), it looked first at the illuminated crescent of the southern hemisphere and later at the northern hemisphere.

When Dawn traveled over the sunlit side, it watched the northern hemisphere, then the equatorial regions, and finally the southern hemisphere as Ceres rotated beneath it each time. One Cerean day, the time it takes the globe to turn once on its axis, is about nine hours, much shorter than the time needed for the spacecraft to loop around its orbit. So it was almost as if Dawn hovered in place, moving only slightly as it peered down, and its instruments could record all of the sights as they paraded by.

We described the plans in much more detail in March, and they executed beautifully, yielding a rich collection of photos in visible and near infrared wavelengths, spectra in visible and infrared, and measurements of the strength of Ceres’ gravitational attraction and hence its mass.

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29 Apr
Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL

Dawn Journal | April 29

by Marc Rayman

Let’s Get Dawn to Business, Dear Readers,

Dawn’s assignment when it embarked on its extraordinary extraterrestrial expedition in 2007 can be described quite simply: explore the two most massive uncharted worlds in the inner solar system. It conducted a spectacular mission at Vesta, orbiting the giant protoplanet for 14 months in 2011-2012, providing a wonderfully rich and detailed view. Now the sophisticated probe is performing its first intensive investigation of dwarf planet Ceres. Dawn is slowly circling the alien world of rock and ice, far from Earth and far from the sun, executing its complex operations with the prowess it has demonstrated throughout its ambitious journey.

Analglyph of Ceres

This anaglyph of Ceres is part of a sequence of images taken by NASA’s Dawn spacecraft April 24 to 26, 2015, from a distance of 8,400 miles (13,600 kilometers). Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI | ›Full image and caption

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31 Mar
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
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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