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 and analyzing the tremendous volume of pictures and other data the probe has already sent to Earth.
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).
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.)
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.)
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.
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.
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
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.
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.
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.
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.
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.
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!
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.
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.
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, fifth, sixth 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.
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.)
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.
from the Sun (AU)
from the Sun (AU)
|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°|
|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°|
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.
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
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 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.
Control of Dawn’s orientation in the weightless conditions of spaceflight is the responsibility of the attitude control system. (To maintain a mystique about their work, engineers use the term “attitude” instead of “orientation.” This system also happens to have a very positive attitude about its work.) Dawn (and all other objects in three-dimensional space) can turn about three mutually perpendicular axes. The axes may be called pitch, roll and yaw; left/right, front/back and up/down; x, y and z; rock, paper and scissors; chocolate, vanilla and strawberry; Peter, Paul and Mary; etc., but whatever their names, attitude control has several different means to turn or to stabilize each axis. Earlier in its journey, the spacecraft depended on devices known as reaction wheels. As we have discussed in many Dawn Journals, that method is now used only rarely, because two of the four units have failed. The remaining two are being saved for the ultimate orbit at about 230 miles (375 kilometers), which Dawn will attain at the end of this year. Instead of reaction wheels, Dawn has been using its reaction control system, shooting puffs of hydrazine, a conventional rocket propellant, through small jets. (This is entirely different from the ion propulsion system, which expels high velocity xenon ions to change and control Dawn’s path through space. The reaction control system is used only to change and control attitude.)
Whenever Dawn is firing one of its three ion engines, its attitude control system uses still another method. The ship only operates one engine at a time, and attitude control swivels the mechanical gimbal system that holds that engine, thus imparting a small torque to the spacecraft, providing the means to control two axes (pitch and yaw, for example, or chocolate and strawberry). For the third axis (roll or vanilla), it still uses the hydrazine jets of the reaction control system.
On June 30, engine #3 came to life on schedule at 10:32:19 p.m. PDT to begin nearly five weeks of maneuvers. Attitude control deftly switched from using the reaction control system for all three axes to only one, and controlling the other two axes by tipping and tilting the engine with gimbal #3. But the control was not as effective as it should have been. Software monitoring the attitude recognized the condition but wisely avoided reacting too soon, instead giving attitude control time to try to rectify it. Nevertheless, the situation did not improve. Gradually the attitude deviated more and more from what it should have been, despite attitude control’s efforts. Seventeen minutes after thrusting started, the error had grown to 10 degrees. That’s comparable to how far the hour hand of a clock moves in 20 minutes, so Dawn was rotating only a little faster than an hour hand. But even that was more than the sophisticated probe could allow, so at 10:49:27 p.m., the main computer declared one of the “safe modes,” special configurations designed to protect the ship and the mission in uncertain, unexpected or difficult circumstances.
The spacecraft smoothly entered safe mode by turning off the ion engine, reconfiguring other systems, broadcasting a continuous radio signal through one of its antennas and then patiently awaiting further instructions. The radio transmission was received on a distant planet the next day. (It may yet be received on some other planets in the future, but we shall focus here on the response by Earthlings.) One of NASA’s Deep Space Network stations in Australia picked up the signal on July 1, and the mission control team at JPL began investigating immediately.
Engineers assessed the health of the spacecraft and soon started returning it to its normal configuration. By analyzing the myriad diagnostic details reported by the robot over the next few days, they determined that the gimbal mechanism had not operated correctly, so when attitude control tried to change the angle of the ion engine, it did not achieve the desired result.
Because Dawn had already accomplished more than 96 percent of the planned ion-thrusting for the entire mission (nearly 5.5 years so far), the remaining thrusting could easily be accomplished with only one of the ion engines. (Note that the 96 percent here is different from the 69 percent of the total time since launch mentioned above, simply because Dawn has been scheduled not to thrust some of the time, including when it takes data at Vesta and Ceres.) Similarly, of the ion propulsion system’s two computer controllers, two power units and two sets of valves and other plumbing for the xenon, the mission could be completed with only one of each. So although engineers likely could restore gimbal #3’s performance, they chose to switch to another gimbal (and thus another engine) and move on. Dawn’s goal is to explore a mysterious, fascinating world that used to be known as a planet, not to perform complex (and unnecessary) interplanetary gimbal repairs.
One of the benefits of being in orbit (besides it being an incredibly cool place to be) is that Dawn can linger at Ceres, studying it in great detail rather than being constrained by a fast flight and a quick glimpse. By the same principle, there was no urgency in resuming the spiral descent. The second mapping orbit was a perfectly fine place for the spacecraft, and it could circle Ceres there every 3.1 days as long as necessary. (Dawn consumed its hydrazine propellant at a very, very low rate while in that orbit, so the extra time there had a negligible cost, even as measured by the most precious resource.)
The operations team took the time to be cautious and to ensure that they understood the nature of the faulty gimbal well enough to be confident that the ship could continue its smooth sailing. They devised a test to confirm Dawn’s readiness to resume its spiral maneuvers. After swapping to gimbal #2 (and ipso facto engine #2), Dawn thrust from July 14 to 16 and demonstrated the excellent performance the operations team has seen so often from the veteran space traveler. Having passed its test with flying colors (or perhaps even with orbiting colors), Dawn is now well on its way to its third mapping orbit.
The gradual descent from the second mapping orbit to the third will require 25 revolutions. The maneuvers will conclude in about two weeks. (As always, you can follow the progress with your correspondent’s frequent and succinct updates here.) As in each mapping orbit, following arrival, a few days will be required in order to prepare for a new round of intensive observations. That third observing campaign will begin on August 17 and last more than two months.
Although this is the second lowest of the mapping orbits, it is also known as the high altitude mapping orbit (HAMO) for mysterious historical reasons. We presented an overview of the HAMO plans last year. Next month, we will describe how the flight team has built on a number of successes since then to make the plans even better.
The view of the landscapes on this distant and exotic dwarf planet from the third mapping orbit will be fantastic. How can we be so sure? The view in the second mapping orbit was fantastic, and it will be three times sharper in the upcoming orbit. Quod erat demonstrandum! To see the sights at Ceres, go there or go here.
Part of the flexibility built into the plans was to measure Ceres’ gravity field as accurately as possible in each mapping orbit and use that knowledge to refine the design for the subsequent orbital phase. Thanks to the extensive gravity measurements in the second mapping orbit in June, navigators were able not only to plot a spiral course but also to calculate the parameters for the next orbit to provide the views needed for the complex mapping activities.
We have discussed some of the difficulty in describing the orbital altitude, including variations in the elevation of the terrain, just as a plane flying over mountains and valleys does not maintain a fixed altitude. As you might expect on a world battered by more than four billion years in the main asteroid belt and with its own internal geological forces, Ceres has its ups and downs. (The topographical map above displays them, and you can see a cool animation of Ceres showing off its topography here.) In addition to local topographical features, its overall shape is not perfectly spherical, as we discussed in May. Ongoing refinements based on Dawn’s measurements now indicate the average diameter is 584 miles (940 kilometers), but the equatorial diameter is 599 miles (964 kilometers), whereas the polar diameter is 556 miles (894 kilometers). Moreover, the orbits themselves are not perfect circles, and irregularities in the gravitational field, caused by regions of lower and higher density inside the dwarf planet, tug less or more on the craft, making it move up and down somewhat. (By using that same principle, scientists learn about the interior structure of Ceres and Vesta with very accurate measurements of the subtleties in the spacecraft’s orbital motions.) Although Dawn’s average altitude will be 915 miles (1,470 kilometers), its actual distance above the ground will vary over a range of about 25 miles (40 kilometers).
In March we summarized the four Ceres mapping orbits along with a guarantee that the dates would change. In addition to delivering exciting interplanetary adventures to thrill anyone who has ever gazed at the night sky in wonder, Dawn delivers on its promises. Therefore, we present the updated table here. With such a long and complex mission taking place in orbit around the largest previously uncharted world in the inner solar system, further changes are highly likely. (Nevertheless, we would consider the probability to be low that changes will occur for the phases in the past.)
Click on the name of each orbit for a more detailed description. As a reminder, the last column illustrates how large Ceres appears to be from Dawn’s perspective by comparing it with a view of a soccer ball. (Note that Ceres is not only 4.4 million times the diameter of a soccer ball but it is a lot more fun to play with.)
Resolute and resilient, Dawn patiently continues its graceful spirals, propelled not only by its ion engine but also by the passions of everyone who yearns for new knowledge and noble adventures. Humankind’s robotic emissary is well on its way to providing more fascinating insights for everyone who longs to know the cosmos.
Dawn is 1,500 miles (2,400 kilometers) from Ceres. It is also 1.95 AU (181 million miles, or 291 million kilometers) from Earth, or 785 times as far as the moon and 1.92 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 32 minutes to make the round trip.
Dr. Marc D. Rayman
8:00 p.m. PDT July 29, 2015
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.
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.
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.)
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.
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.
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.
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
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.
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.
To gain the same view Dawn had, simply build your own ion-propelled spaceship, voyage deep into the main asteroid belt between Mars and Jupiter, take up residence at the giant orb and look out the window. Or go to the image gallery here.
Either way, the sights are spectacular. And they have already gotten even better. As Dawn has been descending to its second mapping orbit, it paused ion-thrusting on May 16 and May 22 to take more pictures, helping navigators get a tight fix on its orbital location. We explained this technique of optical navigation earlier, but now it is slightly different. Dawn is so close to Ceres that the behemoth fills the camera's field of view. No longer charting Ceres' location relative to background stars, navigators now use distinctive features on Ceres itself. It was an indistinct, fuzzy little blob just a few months ago, but now the maps are becoming detailed and accurate. Mathematical analyses of the locations of specific landmarks in each picture allow navigators to determine where Dawn was when the picture was taken.
Let's see how this works. Suppose I gave you a picture I had taken in your house. (The last time I was there, I opted for the cover of darkness rather than a more visible demonstration of optical navigation, but we can still imagine.) Because you know the positions of the doors, windows, furniture, impact craters, paintings, etc., you could establish where I had been when I took the photo. Now that they have charted the positions of the features at Dawn's new home, navigators can do virtually the same thing.
In addition to aiding in celestial navigation, the photos provided still better views of the world Dawn traveled so long and so far to explore. Greater and greater detail is visible as Dawn orbits closer, and a tremendous variety of intriguing sights are coming into view. It may well be that the most interesting discoveries have not even been made yet, but for now, what captivates most people (and other readers as well) are the bright spots.
We have discussed them here and there in recent months, and their luminous power continues to dazzle us. What appeared initially as one fuzzy spot proved to be two smaller spots and now many even smaller regions as the focus has become sharper. Why the ground there reflects so much sunlight remains elusive. Dawn's finer examinations with its suite of sophisticated instruments in the second, third and then final mapping orbits will provide scientists with data they need to unravel this marvelous mystery. For now, the enigmatic lights present an irresistible cosmic invitation to go closer and to scrutinize this strange and wonderful world, and we are eager to accept. After all, we explore to learn, to know the unknown, and the uniquely powerful scientific method will reveal the nature of the bright areas and what they can tell us about the composition and geology of this complex dwarf planet.
› Full image and caption
After having been viewed as little more than a smudge in telescopes for more than two centuries since its discovery, Ceres now is seen as a detailed, three-dimensional world. As promised, measurements from Dawn have revised the size to be about 599 miles (963 kilometers) across at the equator. Like Earth and other planets, Ceres is oblate, or slightly wider at the equator than from pole to pole. The polar diameter is 554 miles (891 kilometers). These dimensions are impressively close to what astronomers had determined from telescopic observations and confirm Ceres to be the colossus we have described.
Before Dawn, scientists had estimated Ceres' mass to be 1.04 billion billion tons (947 billion billion kilograms). Now it is measured to be 1.03 billion billion tons (939 billion billion kilograms), well within the previous margin of error. It is an impressive demonstration of the success of science that astronomers had been able to determine the heft of that point of light so accurately. Nevertheless, even this small change of less than one percent is important for planning the rest of Dawn’s mission as it orbits closer and closer, feeling the gravitational tug ever more strongly.
Let's put this change in context. Dawn has now refined the mass, making a proportionally small adjustment of about 0.01 billion billion tons (eight billion billion kilograms). Although no more than a tweak on the overall value, it is still significantly greater than the combined mass of all asteroids visited by all other spacecraft. Ceres is so immense, so massive that even if all those asteroids were added to it, the difference would hardly even have been noticeable. This serves as another reminder that the dwarf planet really is quite unlike the millions of small asteroids that constitute the main asteroid belt. This behemoth contains about 30 percent of all the mass in that entire vast region of space. Vesta, the protoplanet Dawn orbited and studied in 2011-2012, is the second most massive resident there, holding about 8 percent of the asteroid belt's mass. Dawn by itself is exploring around 40 percent of the asteroid belt's mass!
Upon concluding its first mapping orbit, Dawn powered on its remarkable ion propulsion system on May 9 to fly down to a lower altitude where it will gain a better view. We examined the nature of the spiral paths between mapping orbits last year (and at Vesta in 2011-2012).
› Larger image
In its first mapping orbit, Dawn was 8,400 miles (13,600 kilometers) high, revolving once in 15.2 days at a speed of 150 mph (240 kilometers per hour). By the time it completes this descent, the probe will be at an altitude of 2,700 miles (4,400 kilometers), orbiting Ceres every 3.1 days at 254 mph (408 kilometers per hour). (All of the mapping orbits were summarized in this table.) We have discussed that lower orbits require greater velocity to counterbalance the stronger gravitational hold.
Dawn's uniquely capable ion propulsion system, with its extraordinary combination of efficiency and gentleness, propels the ship to its new orbital destination in just under four weeks. The descent requires five revolutions, each one faster than the one before. The flight profile is complicated, and sometimes Dawn even dips below the final, planned altitude and then rises to greater heights as it flies on a path that is temporarily elliptical. The overall trend, of course, is downward. As Dawn heads for its targeted circular orbit, its maneuvering is also generally reducing the orbit period, the time required to make one complete revolution around Ceres. Indeed, if Dawn stopped thrusting now, its orbit period would be about 83 hours, or 3.5 days.
Dawn will complete ion-thrusting on June 3, but it will not be ready to begin its next science observations then. Rather, as in the other new mapping orbits, the first order of business will be for navigators to measure the new orbital parameters accurately. The flight team then will install in Dawn's main computer the details of the orbit it achieved so it will always know its location.
In addition, the intensive campaign of observations is planned to begin when the robotic explorer travels from the night side to the day side over the north pole. With the three-day orbit period, that will next occur on June 5. Controllers will take advantage of the intervening time to conduct other activities, including routine maintenance of the two reaction wheels that remain operable, although they are powered off most of the time. (Two of the four failed years ago. Dawn no longer relies on these devices to control its orientation, and it is remarkable that the mission can accomplish all of its original objectives without them. But if two do function in the final mapping orbit later this year, they will help extend the spacecraft's lifetime for bonus studies.)
We have already presented the ambitious plans for this second mapping orbit, sometimes known as "the second mapping orbit" and sometimes more succinctly and confusingly as "survey orbit." As with all four of Dawn's mapping orbits, it is designed to take the spacecraft over the poles, ensuring the best possible coverage. The ship will fly from the north pole to the south over the side of Ceres facing the sun, and then loop back to the north over the side hidden in the deep dark of night. On the day side, Dawn will aim its camera and spectrometers at the lit ground, filling its memory to capacity with the readings. On the night side, it will point its main antenna to distant Earth in order to radio its findings home. At Dawn's altitude, Ceres will appear twice as wide as the camera's view. (As illustrated in this table, it will look about the size of a soccer ball seen from a yard, or a meter, away.) But as the dwarf planet rotates on its axis and Dawn sails around in its more leisurely orbit, eventually all of the landscape will come within sight of the instruments.
Only one noteworthy change has been made in the intricate plans for survey orbit since May 2014's shocking exposé. With the observations starting on June 5, the subsequent complex orbital flight to the third mapping orbit (also known as HAMO) would have begun on June 27. As we have seen, the rapidly changing orbit in the spiral descents requires a great deal of effort by the small operations team on a rigid schedule. The capable men and women flying Dawn accomplished the maneuvers flawlessly at Vesta and are well prepared for the challenges at Ceres. The work is very demanding, however, and so, just as at Vesta, the team has built into the strategy the capability to make adjustments to align most of the tasks with a conventional work schedule. The technical plans (even including the exquisitely careful husbanding of hydrazine following the loss of the two reaction wheels) fully account for such human factors. It turns out that leaving survey orbit three days later shifts a significant amount of the following work off weekends, making it more comfortable for the team members. Three days is one complete revolution, and always extracting as much from the mission as possible, they have devised another full set of observations for an eighth orbit. As a result, survey orbit may be even more extensive and productive than originally anticipated.
What awaits Dawn in the next mapping phase? The views will be three times as sharp as in the previous orbit, and exciting new discoveries are sure to come. What answers will be revealed? And what new questions (besides this one) will arise? We will know soon, as we all share in the thrill of this grand adventure. To help you keep track of Dawn's progress as it powers its way down and then conducts further observations, your correspondent writes brief (hard to believe, isn't it?) mission status updates. And although in space no one can hear you tweet, terrestrial followers can get even more frequent updates with information he provides for Twitter @NASA_Dawn.
Dawn is 3,400 miles (5,500 kilometers) from Ceres. It is also 2.30 AU (214 million miles, or 345 million kilometers) from Earth, or 855 times as far as the moon and 2.27 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 38 minutes to make the round trip.
Dr. Marc D. Rayman
12:00 p.m. PDT May 28, 2015
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.
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).
As the table here shows, on April 14 (and extending into April 15), Dawn will obtain its last navigational fix before it finishes maneuvering. Should we look forward to catching sight of the bright spots then? In truth, we do not yet know. The spots surely will be there, but the uncertainty is exactly where “there” is. We still have much to learn about a dwarf planet that, until recently, was little more than a fuzzy patch of light among the glowing jewels of the night sky. (For example, only last month did we determine where Ceres’north and south poles point.) Astronomers had clocked the length of its day, the time it takes to turn once on its axis, at a few minutes more than nine hours. But the last time the spots were in view of Dawn’s camera was on Feb. 19. From then until April 14, while Earth rotates more than 54 times (at 24 hours per turn), Ceres will rotate more than 140 times, which provides plenty of time for a small discrepancy in the exact rate to build up. To illustrate this, if our knowledge of the length of a Cerean day were off by one minute (or less than 0.2 percent), that would translate into more than a quarter of a turn during this period, drastically shifting the location of the spots from Dawn’s point of view. So we are not certain exactly what range of longitudes will be within view in the scheduled OpNav 7 window. Regardless, the pictures will serve their intended purpose of helping navigators establish the probe’s location in relation to its gravitational captor.
Dawn’s gradual, graceful arc down to its first mapping orbit will take the craft from the night side to the day side over the north pole, and then it will travel south. It will conclude its powered flight over the sunlit terrain at about 60 degrees south latitude. The spacecraft will finish reshaping its orbit on April 23, and when it stops its ion engine on that date, it will be in its new circular orbit, designated RC3. (We will return to the confusing names of the different orbits at Ceres below.) Then it will coast, just as the moon coasts in orbit around Earth and Earth coasts around the sun. It will take Dawn just over 15 days to complete one revolution around Ceres at this height. We had a preview of RC3 last year, and now we can take an updated look at the plans.
The dwarf planet is around 590 miles (950 kilometers) in diameter (like Earth and other planets, however, it is slightly wider at the equator than from pole to pole). At the spacecraft’s orbital altitude, it will appear to be the same size as a soccer ball seen from 10 feet (3 meters) away. Part of the basis upon which mission planners chose this distance for the first mapping campaign is that the visible disc of Ceres will just fit in the camera’s field of view. All the pictures taken at lower altitudes will cover a smaller area (but will be correspondingly more detailed). The photos from RC3 will be 3.4 times sharper than those in RC2.
There will be work to do before photography begins however. The first order of business after concluding ion thrusting will be for the flight team to perform a quick navigational update (this time, using only the radio signal) and transmit any refinements (if necessary) in Dawn’s orbital parameters, so it always has an accurate knowledge of where it is. (These will not be adjustments to the orbit but rather a precise mathematical description of the orbit it achieved.) Controllers will also reconfigure the spacecraft for its intensive observations, which will commence on April 24 as it passes over the south pole and to the night side again.
As at Vesta, even though half of each circular orbit will be over the night side of Ceres, the spacecraft itself will never enter the shadows. The operations team has carefully designed the orbits so that at Dawn’s altitude, it remains illuminated by the sun, even when the land below is not.
It may seem surprising (or even be surprising) that Dawn will conduct measurements when the ground directly beneath it is hidden in the deep darkness of night. To add to the surprise, these observations were not even envisioned when Dawn’s mission was designed, and it did not perform comparable measurements during its extensive exploration of Vesta in 2011-2012.
The measurements on the night side will serve several purposes. One of the many sophisticated techniques scientists use to elucidate the nature of planetary surfaces is to measure how much light they reflect at different angles. Over the course of the next year, Dawn will acquire tens of thousands of pictures from the day side of Ceres, when, in essence, the sun is behind the camera. When it is over the night side in RC3, carefully designed observations of the lit terrain (with the sun somewhat in front of the camera, although still at a safe angle) will significantly extend the range of angles.
In December, we described the fascinating discovery of an extremely diffuse veil of water vapor around Ceres. How the water makes its way from the dwarf planet high into space is not known. The Dawn team has devised a plan to investigate this further, even though the tiny amount of vapor was sighted long after the explorer left Earth equipped with sensors designed to study worlds without atmospheres.
It is worth emphasizing that the water vapor is exceedingly tenuous. Indeed, it is much less dense than Earth’s atmosphere at altitudes above the International Space Station, which orbits in what most people consider to be the vacuum of space. Our hero will not need to deploy its umbrella. Even comets, which are miniscule in comparison with Ceres, liberate significantly more water.
There may not even be any water vapor at all now because Ceres is farther from the sun than when the Herschel Space Observatory saw it, but if there is, detecting it will be very challenging. The best method to glimpse it is to look for its subtle effects on light passing through it. Although Dawn cannot gaze directly at the sun, it can look above the lit horizon from the night side, searching intently for faint signs of sunlight scattered by sparse water molecules (or perhaps dust lofted into space with them).
For three days in RC3 after passing over the south pole, the probe will take many pictures and visible and infrared spectra as it watches the slowly shrinking illuminated crescent and the space over it. When the spacecraft has flown to about 29 degrees south latitude over the night side, it will no longer be safe to aim its sensitive instruments in that direction, because they would be too close to the sun. With its memory full of data, Dawn will turn to point its main antenna toward distant Earth. It will take almost two days to radio its findings to NASA’s Deep Space Network. Meanwhile, the spacecraft will continue northward, gliding silently high over the dark surface.
On April 28, it will rotate again to aim its sensors at Ceres and the space above it, resuming measurements when it is about 21 degrees north of the equator and continuing almost to the north pole on May 1. By the time it turns once again to beam its data to Earth, it will have completed a wealth of measurements not even considered when the mission was being designed.
Loyal readers will recall that Dawn has lost two of its four reaction wheels, gyroscope-like devices it uses to turn and to stabilize itself. Although such a loss could be grave for some missions, the operations team overcame this very serious challenge. They now have detailed plans to accomplish all of the original Ceres objectives regardless of the condition of the reaction wheels, even the two that have not failed (yet). It is quite a testament to their creativity and resourcefulness that despite the tight constraints of flying the spacecraft differently, the team has been able to add bonus objectives to the mission.
Dawn will finish transmitting its data after its orbit takes it over the north pole and to the day side of Ceres again. For three periods during its gradual flight of more than a week over the illuminated landscape, it will take pictures (in visible and near-infrared wavelengths) and spectra. Each time, it will look down from space for a full Cerean day, watching for more than nine hours as the dwarf planet pirouettes, as if showing off to her new admirer. As the exotic features parade by, Dawn will faithfully record the sites.
It is important to set the camera exposures carefully. Most of the surface reflects nine percent of the sunlight. (For comparison, the moon reflects 12 percent on average, although as many Earthlings have noticed, there is some variation from place to place. Mars reflects 17 percent, and Vesta reflects 42 percent. Many photos seem to show that your correspondent’s forehead reflects about 100 percent.) But there are some small areas that are significantly more reflective, including the two most famous bright spots. Each spot occupies only one pixel (2.7 miles, or 4.3 kilometers across) in the best pictures so far. If each bright area on the ground is the size of a pixel, then they reflect around 40 percent of the light, providing the stark contrast with the much darker surroundings. When Dawn’s pictures show more detail, it could be that they will turn out to be even smaller and even more reflective than they have appeared so far. In RC3, each pixel will cover 0.8 miles (1.3 kilometers). To ensure the best photographic results, controllers are modifying the elaborate instructions for the camera to take pictures of the entire surface with a wider range of exposures than previously planned, providing high confidence that all dark and all bright areas will be revealed clearly.
Dawn will observe Ceres as it flies from 45 degrees to 35 degrees north latitude on May 3-4. Of course, the camera’s view will extend well north and south of the point immediately below it. (Imagine looking at a globe. Even though you are directly over one point, you can see a larger area.) The territory it will inspect will include those intriguing bright spots. The explorer will report back to Earth on May 4-5. It will perform the same observations between 5 degrees north and 5 degrees south on May 5-6 and transmit those findings on May 6-7. To complete its first global map, it will make another full set of measurements for a Cerean day as it glides between 35 degrees and 45 degrees south on May 7.
By the time it has transmitted its final measurements on May 8, the bounty from RC3 may be more than 2,500 pictures and two million spectra. Mission controllers recognize that glitches are always possible, especially in such complex activities, and they take that into account in their plans. Even if some of the scheduled pictures or spectra are not acquired, RC3 should provide an excellent new perspective on the alien world, displaying details three times smaller than what we have discerned so far.
Dawn activated its gamma ray spectrometer and neutron spectrometer on March 12, but it will not detect radiation from Ceres at this high altitude. For now, it is measuring space radiation to provide context for later measurements. Perhaps it will sense some neutrons in the third mapping orbit this summer, but its primary work to determine the atomic constituents of the material within about a yard (meter) of the surface will be in the lowest altitude orbit at the end of the year.
Dawn will conduct its studies from three lower orbital altitudes after RC3, taking advantage of the tremendous maneuverability provided by ion propulsion to spiral from one to another. We presented previews last year of each phase, and as each approaches, we will give still more up-to-date details, but now that Dawn is in orbit, let’s summarize them here. Of course, with complicated operations in the forbidding depths of space, there are always possibilities for changes, especially in the schedule. The team has developed an intricate but robust and flexible plan to extract as many secrets from Ceres as possible, and they will take any changes in stride.
Each orbit is designed to provide a better view than the one before, and Dawn will map the orb thoroughly while at each altitude. The names for the orbits – rotation characterization 3 (RC3); survey; high altitude mapping orbit (HAMO); and low altitude mapping orbit (LAMO) – are based on ancient ideas, and the origins are (or should be) lost in the mists of time. Readers should avoid trying to infer anything at all meaningful in the designations. After some careful consideration, your correspondent chose to use the same names the Dawn team uses rather than create more helpful descriptors for the purposes of these blogs. That ensures consistency with other Dawn project communications. After all, what is important is not what the different orbits are called but rather what amazing new discoveries each one enables.
The robotic explorer will make many kinds of measurements with its suite of powerful instruments. As one indication of the improving view, this table includes the resolution of the photos, and the ever finer detail may be compared with the pictures during the approach phase. For another perspective, we extend the soccer ball analogy above to illustrate how large Ceres will appear to be from the spacecraft’s orbital vantage point.
As Dawn orbits Ceres, together they orbit the sun. Closer to the master of the solar system, Earth (with its own retinue, including the moon and many artificial satellites) travels faster in its heliocentric orbit because of the sun’s stronger gravitational pull at its location. In December, Earth was on the opposite side of the sun from Dawn, and now the planet’s higher speed is causing their separation to shrink. Earth will get closer and closer until July 22, when it will pass on the inside track, and the distance will increase again.
In the meantime, on April 12, Dawn will be equidistant from the sun and Earth. The spacecraft will be 2.89 AU or 269 million miles (433 million kilometers) from both. At the same time, Earth will be 1.00 AU or 93.2 million miles (150 million kilometers) from the sun.
It will be as if Dawn is at the tip of a giant celestial arrowhead, pointing the way to a remarkable solar system spectacle. The cosmos should take note! Right there, a sophisticated spaceship from Earth is gracefully descending on a blue-green beam of xenon ions. Finally, the dwarf planet beneath it, a remote remnant from the dawn of the solar system, is lonely no more. Almost 4.6 billion years after it formed, and 214 years after inquisitive creatures on a distant planet first caught sight of it, a mysterious world is still welcoming the new arrival. And as Dawn prepares to settle into its first close orbit, ready to discover secrets Ceres has kept for so long, everyone who shares in the thrill of this grand and noble adventure eagerly awaits its findings. Together, we look forward to the excitement of new knowledge, new insight and new fuel for our passionate drive to explore the universe.
Dawn is 35,000 miles (57,000 kilometers) from Ceres, or 15 percent of the average distance between Earth and the moon. It is also 3.04 AU (282 million miles, or 454 million kilometers) from Earth, or 1,120 times as far as the moon and 3.04 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.
Dr. Marc D. Rayman
6:00 p.m. PDT March 31, 2015
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.
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!)
What was the scene like at JPL for Dawn’s historic achievement? It’s easy to imagine the typical setting in mission control. The tension is overwhelming. Will it succeed or will it fail? Anxious people watch their screens, monitoring telemetry carefully, frustrated that there is nothing more they can do now. Nervously biting their nails, they are thinking of each crucial step, any one of which might doom the mission to failure. At the same time, the spacecraft is executing a bone-rattling, whiplash-inducing burn of its main engine to drop into orbit. When the good news finally arrives that orbit is achieved, the room erupts! People jump up and down, punch the air, shout, tweet, cry, hug and feel the tremendous relief of overcoming a huge risk. You can imagine all that, but that’s not what happened.
If you had been in Dawn mission control, the scene would have been different. You would mostly be in the dark. (For your future reference, the light switches are to the left of the door.) The computer displays would be off, and most of the illumination would be from the digital clock and the string of decorative blue lights that indicate the ion engine is scheduled to be thrusting. You also would be alone (at least until JPL Security arrived to escort you away, because you were not cleared to enter the room, and, for that matter, how did you get past the electronic locks?). Meanwhile, most of the members of the flight team were at home and asleep! (Your correspondent was too, rare though that is. When Dawn entered orbit around Vesta, he was dancing. Ceres’ arrival happened to be at a time less conducive to consciousness.)
Why was such a significant event treated with somnolence? It is because Dawn has a unique way of entering orbit, which is connected with the nature of the journey itself. We have discussed some aspects of getting into orbit before (with this update to the nature of the approach trajectory). Let’s review some of it here.
It may be surprising that prior to Dawn, no spacecraft had even attempted to orbit two distant targets. Who wouldn’t want to study two alien worlds in detail, rather than, as previous missions, either fly by one or more for brief encounters or orbit only one? A mission like Dawn’s is an obvious kind to undertake. It happens in science fiction often: go somewhere, do whatever you need to do there (e.g., beat someone up or make out with someone) and then boldly go somewhere else. However, science fact is not always as easy as science fiction. Such missions are far, far beyond the capability of conventional propulsion.
Deep Space 1 (DS1) blazed a new trail with its successful testing of ion propulsion, which provides 10 times the efficiency of standard propulsion, showing on an operational interplanetary mission that the advanced technology really does work as expected. (This writer was fortunate enough to work on DS1, and he even documented the mission in a series of increasingly wordy blogs. But he first heard of ion propulsion from the succinct Mr. Spock and subsequently followed its use by the less logical Darth Vader.)
Dawn’s ambitious expedition would be truly impossible without ion propulsion. (For a comparison of chemical and ion propulsion for entering orbit around Mars, an easier destination to reach than either Vesta or Ceres, visit this earlier log.) So far, our advanced spacecraft has changed its own velocity by 23,800 mph (38,400 kilometers per hour) since separating from its rocket, far in excess of what any other mission has achieved propulsively. (The previous record was held by DS1.)
Dawn is exceptionally frugal in its use of xenon propellant. In this phase of the mission, the engine expends only a quarter of a pound (120 grams) per day, or the equivalent of about 2.5 fluid ounces (75 milliliters) per day. So although the thrust is very efficient, it is also very gentle. If you hold a single sheet of paper in your hand, it will push on your hand harder than the ion engine pushes on the spacecraft at maximum thrust. At today’s throttle level, it would take the distant explorer almost 11 days to accelerate from zero to 60 mph (97 kilometers per hour). That may not evoke the concept of a drag racer. But in the zero-gravity, frictionless conditions of spaceflight, the effect of this whisper-like thrust can build up. Instead of thrusting for 11 days, if we thrust for a month, or a year, or as Dawn already has, for more than five years, we can achieve fantastically high velocity. Ion propulsion delivers acceleration with patience.
Most spacecraft coast most of the time, following their repetitive orbits like planets do. They may use the main engine for a few minutes or perhaps an hour or two throughout the entire mission. With ion propulsion, in contrast, the spacecraft may spend most of its time in powered flight. Dawn has flown for 69% of its time in space emitting a cool blue-green glow from one of its ion engines. (With three ion engines, Dawn outdoes the Star Wars TIE (twin ion engine) fighters.)
The robotic probe uses its gentle thrust to gradually reshape its path through space rather than simply following the natural course that a planet would. After it escaped from Vesta’s gravitational clutches, it slowly spiraled outward from the sun, climbing the solar system hill, making its heliocentric orbit more and more and more like Ceres’. By the time it was in the vicinity of the dwarf planet today, both were traveling around the sun at more than 38,600 mph (62,100 kilometers per hour). Their trajectories were nearly identical, however, so the difference in their speeds was only 100 mph (160 kilometers per hour), or less than 0.3 percent of the total. Flying like a crackerjack spaceship pilot, Dawn elegantly used the light touch of its ion engine to be at a position and velocity that it could ease gracefully into orbit. At a distance of 37,700 miles (60,600 kilometers), Ceres reached out and tenderly took the newcomer from Earth into its permanent gravitational embrace.
If you had been in space watching the event, you would have been cold, hungry and hypoxic. But it would not have looked much different from the 1,885 days of ion thrust that had preceded it. The spacecraft was perched atop its blue-green pillar of xenon ions, patiently changing its course, as it does for so much of quiet cruise. But now, at one moment it was flying too fast for Ceres’ gravity to hang on to it, and the next moment it had slowed just enough that it was in orbit. Had it stopped thrusting at that point, it would have continued looping around the dwarf planet. But it did not stop. Instead, it is working now to reshape its orbit around Ceres. As we saw in November, its orbital acrobatics first will take it up to an altitude of 47,000 miles (75,000 kilometers) on March 19 before it swoops down to 8,400 miles (13,500 kilometers) on April 23 to begin its intensive observations in the orbit designated RC3.
In fact, Dawn’s arrival today really is simply a consequence of the route it is taking to reach that lower orbit next month. Navigators did not aim for arriving today. Rather, they plotted a course that began at Vesta and goes to RC3 (with a new design along the way), and it happens that the conditions for capture into orbit occurred this morning. As promised last month, we present here a different view of the skillful maneuvering by this veteran space traveler.
If Dawn had stopped thrusting before Ceres could exert its gravitational control, it wouldn’t have flown very far away. The spacecraft had already made their paths around the sun very similar, and the ion propulsion system provides such exceptional flexibility to the mission that controllers could have guided it into orbit some other time. This was not a one-time, all-or-nothing event.
So the flight team was not tense. They had no need to observe it or make a spectacle out of it. Mission control remained quiet. The drama is not in whether the mission will succeed or fail, in whether a single glitch could cause a catastrophic loss, in whether even a tiny mistake could spell doom. Rather, the drama is in the opportunity to unveil the wonderful secrets of a fascinating relict from the dawn of the solar system more than 4.5 billion years ago, a celestial orb that has beckoned for more than two centuries, the first dwarf planet discovered.
Dawn usually flies with its radio transmitter turned off (devoting its electricity instead to the power-hungry ion engine), and so it entered orbit silently. As it happened, a routine telecommunications session was scheduled about an hour after attaining orbit, at 5:36 a.m. PST. (It’s only coincidence it was that soon. At Vesta, it was more than 25 hours between arrival and the next radio contact.) For primary communications, Dawn pauses thrusting to point its main antenna to Earth, but other times, as in this case, it is programmed to use one of its auxiliary antennas to transmit a weaker signal without stopping its engine, whispering just enough for engineers to verify that it remains healthy.
The Deep Space Network’s exquisitely sensitive 230-foot (70-meter) diameter antenna in Goldstone, Calif., picked up the faint signal from across the solar system on schedule and relayed it to Dawn mission control. One person was in the room (and yes, he was cleared to enter). He works with the antenna operator to ensure the communications session goes smoothly, and he is always ready to contact others on the flight team if any anomalies arise. In this case, none did, and it was a quiet morning as usual. The mission director checked in with him shortly after the data started to trickle in, and they had a friendly, casual conversation that included discussing some of the telemetry that indicated the spacecraft was still performing its routine ion thrusting. The determination that Dawn was in orbit was that simple. Confirming that it was following its flight plan was all that was needed to know it had entered orbit. This beautifully choreographed celestial dance is now a pas de deux.
As casual and tranquil as all that sounds, and as logical and systematic as the whole process is, the reality is that the mission director was excited. There was no visible hoopla, no audible fanfare, but the experience was powerful fuel for the passionate fires that burn within.
As soundlessly as a spacecraft gliding through the void, the realization emerges …
Dawn made it!!
It is in orbit around a distant world!!
Yes, it’s clear from the technical details, but it is more intensely reflected in the silent pounding of a heart that has spent a lifetime yearning to know the cosmos. Years and years of hard work devoted to this grand undertaking, constant hopes and dreams and fears of all possible futures, uncounted challenges (some initially appearing insurmountable) and a seeming infinitude of decisions along the way from early concepts through a real interplanetary spacecraft flying on an ion beam beyond the sun.
And then, a short, relaxed chat over a few bits of routine data that report the same conditions as usual on the distant robot. But today they mean something different.
They mean we did it!!
Everyone on the team will experience the news that comes in a congratulatory email in their own way, in the silence and privacy of their own thoughts. But it means the same to everyone.
We did it!!
And it’s not only the flight team. Humankind!! With our relentless curiosity, our insatiable hunger for knowledge, our noble spirit of adventure, we all share in the experience of reaching out from our humble home to the stars.
Together, we did it!!!
It was a good way to begin the day. It was Dawn at Ceres.
Let’s bring into perspective the cosmic landscape on which this remarkable adventure is now taking place. Imagine Earth reduced to the size of a soccer ball. On this scale, the International Space Station would orbit at an altitude of a bit more than one-quarter of an inch (seven millimeters). The moon would be a billiard ball almost 21 feet (6.4 meters) away. The sun, the conductor of the solar system orchestra, would be 79 feet (24 meters) across at a distance of 1.6 miles (2.6 kilometers). But even more remote, Dawn would be 5.3 miles (8.6 kilometers) away. (Just a few months ago, when the spacecraft was on the opposite side of the sun from Earth, it would have been more than six miles, or almost 10 kilometers, from the soccer ball.) Tremendously far now from its erstwhile home, it would be only a little over a yard (a meter) from its new residence. (By the end of this year, Dawn will be slightly closer to it than the space station is to Earth, a quarter of an inch, or six millimeters.) That distant world, Ceres, the largest object between Mars and Jupiter, would be five-eighths of an inch (1.6 centimeters) across, about the size of a grape. Of course a grape has a higher water content than Ceres, but we can be sure that exploring this intriguing world of rock and ice will be much sweeter!
As part of getting to know its new neighborhood, Dawn has been hunting for moons of Ceres. Telescopic studies had not revealed any, but if there were a moon smaller than about half a mile (one kilometer), it probably would not have been discovered. The spacecraft’s unique vantage point provides an opportunity to look for any that might have escaped detection. Many pictures have been taken specifically for this purpose, and scientists scrutinize them and all of the other photographs for any indication of moons. While the search will continue, so far, no picture has shown evidence of companions orbiting Ceres.
And yet we know that as of today, Ceres most certainly does have one. Its name is Dawn!
Dawn is 37,800 miles (60,800 kilometers) from Ceres, or 16 percent of the average distance between Earth and the moon. It is also 3.33 AU (310 million miles, or 498 million kilometers) from Earth, or 1,230 times as far as the moon and 3.36 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
6:00 a.m. PST March 6, 2015
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 amid 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.
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.
The positions of the spacecraft and dwarf planet are already determined well enough with the conventional navigation methods that controllers know which particular stars are near Ceres from Dawn’s perspective. It is the analysis of precisely where Ceres appears relative to those stars that will yield the necessary navigational refinement. Later, when Dawn is so close that the colossus occupies most of the camera’s view, stars will no longer be visible in the pictures. Then the optical navigation will be based on determining the location of the spacecraft with respect to specific surface features that have been charted in previous images.
To execute an OpNav, Dawn suspends ion thrusting and turns to point its camera at Ceres. It usually spends one or two hours taking photos (and bonus measurements with its visible and infrared mapping spectrometer). Then it turns to point its main antenna to Earth and transmits its findings across the solar system to the Deep Space Network.
While it is turning once again to resume ion thrusting, navigators are already starting to extract information from the images to calculate where the probe is relative to its destination. Experts update the design of the trajectory the spacecraft must follow to reach its intended orbital position and fine-tune the corresponding ion thrust flight plan. At the next communications session, the revised instructions are radioed back across the solar system, and then the reliable robot carries them out. This process is repeated throughout the approach phase.
Dawn turned to observe Vesta during that approach phase more often than it does on approach to Ceres, and the reason is simple. It has lost two of its four reaction wheels, devices used to help turn or stabilize the craft in the zero-gravity, frictionless conditions of spaceflight. (In full disclosure, the units aren’t actually lost. We know precisely where they are. But given that they stopped functioning, they might as well be elsewhere in the universe; they don’t do Dawn any good.)
Dawn’s sentient colleagues at JPL, along with excellent support from Orbital Sciences Corporation, have applied their remarkable creativity, tenacity and technical acumen to devise a strategy that allows all the original objectives of exploring Ceres to be met regardless of the condition of the wheels, even the (currently) healthy ones. Your correspondent refers to this as the “zero reaction wheel plan” One of the many methods that contributed to this surprising resilience was a substantial reduction in the number of turns during all remaining phases of the mission, thus conserving the precious hydrazine propellant used by the small jets of the reaction control system. Guided by their successful experience at Vesta, experts determined that they could accommodate fewer OpNavs during the approach to Ceres, thus saving turns. (We will return to the topic of hydrazine conservation below.)
The images serve several purposes besides navigation. Of course, they provide a tantalizing preview of the intriguing world observed from Earth since 1801. Each picture whets our appetite! What will Ceres look like as it comes into sharper focus? Will we see evidence of a subsurface ocean? What unexpected shapes and structures will we find? What strange new features will show up? Just what is that bright spot? Quite simply: we don’t know. It would be a pretty good idea to send a spacecraft there to find out!
Scientists scrutinize all the photos for moons of Ceres, and OpNavs 3-7 will include many extra images with exposures chosen to help reveal moons. In addition, hundreds more pictures will be taken of the space around Ceres in the hours before and after OpNav 3 to allow an even more thorough search.
On two occasions during the approach, Dawn will take images and spectra throughout a complete Ceres rotation of slightly over nine hours, or one Cerean day. During that time, Dawn’s position will not change significantly, so it will be almost as if the spacecraft hovers in place as the dwarf planet pirouettes beneath its watchful eye, exhibiting most of the surface. These “rotation characterizations” (known by the stirring names RC1 and RC2) will provide the first global perspectives.
As Dawn flies into orbit, it arcs around Ceres. In November, we described the route into orbit in detail, and one of the figures there is reproduced here. Dawn will slip into Ceres’ gravitational embrace on the night of March 5 (PST). But as the figure shows, its initial elliptical orbit will carry it to higher altitudes before it swoops back down. As a result, pictures of Ceres will grow for a while, then shrink and then grow again.
Because of the changing direction to Ceres, Dawn does not always see a fully illuminated disk, just as the moon goes through its familiar phases as its position relative to the sun changes. The hemisphere of the moon facing the sun is bright and the other is dark. The half facing Earth may include part of the lit side and part of the dark side. Sometimes we see a full moon, sometimes gibbous, and sometimes a thin crescent.
The table shows what fraction of Ceres is illuminated from Dawn’s perspective. Seeing a full moon would correspond to 100 percent illumination. A half moon would be 50 percent, and a new moon would be zero percent. In OpNav 6, when Ceres is 18 percent illuminated, it will be a delicate crescent, like the moon about four days after it’s new.
OpNav images of a narrow crescent won’t contain enough information to warrant the expenditure of hydrazine in all that turning. Moreover, the camera’s precision optics and sensitive detector, designed for revealing the landscapes of Vesta and Ceres, cannot tolerate looking too close to the sun, even as far from the brilliant star as it is now. Therefore, no pictures will be taken in March and early April when Dawn is far on the opposite side of Ceres from the sun. By the end of April, the probe will have descended to its first observational orbit (RC3), where it will begin its intensive observations.
The closer Dawn is to Ceres, the larger the orb appears to its camera, and the table includes the (approximate) diameter the full disk would be, measured in the number of camera pixels. To display greater detail, each pixel must occupy a smaller portion of the surface. So the “resolution” of the picture indicates how sharp Dawn’s view is.
We also describe the pictures in comparison to the best that have been obtained with Hubble Space Telescope. In Hubble’s pictures, each pixel covered about 19 miles (30 kilometers). Now, after a journey of more than seven years through the solar system, Dawn is finally close enough to Ceres that its view surpasses that of the powerful telescope. By the time Dawn is in its lowest altitude orbit at the end of this year, its pictures will be well over 800 times better than Hubble’s and more than 600 times better than the OpNav 2 pictures from Jan. 25. This is going to be a fantastic year of discovery!
|Beginning of activity in Pacific Time zone||Distance from Dawn to Ceres in miles (kilometers)||Ceres diameter in pixels||Resolution in miles (kilometers) per pixel||Resolution compared to Hubble||Illuminated portion of disk||Activity|
|Dec 1, 2014||740,000
|Jan 13, 2015||238,000
Some of the numbers may change slightly as Dawn’s trajectory is refined and even as estimates of the strength of Ceres’ gravitational tug improve. (Dawn is already feeling that pull, even though it is not yet in orbit.) Still, this should help you fill out your social calendar for the next few months.
To get views like those Dawn has, you can build your own spaceship and fly it deep into the heart of the main asteroid belt to this intriguing world of rock and ice. Or you can visit our Ceres image gallery to see pictures as soon as they are released. If you chose the first option, use your hydrazine wisely!
As we discussed above, to explore Ceres without the use of the reaction wheels that were essential to the original design, mission controllers have worked very hard to conserve hydrazine. Let’s see how productive that effort has been. (You should be able to follow the story here without careful focus on the numbers. They are here for the more technically oriented readers, accountants and our old friends the Numerivores.)
Dawn launched in Sept. 2007 with 101 pounds (45.6 kilograms) of hydrazine. The ship escaped from Vesta in Sept. 2012, four weeks after the second reaction wheel failed during the climb out of Vesta’s gravitational hole. (By the way, Dawn is now more than one thousand times farther from Vesta than it is from Ceres. It is even farther from Vesta than Earth is from the sun!) At the beginning of the long interplanetary flight to Ceres, it still had 71.2 pounds (32.3 kilograms) left. As it had expended less than one-third of the original supply through the end of the Vesta expedition, that might seem like plenty. But it was not. Without the reaction wheels, subsequent operations would consume much more hydrazine. Indeed, engineers determined that even if they still had the entire amount that had been onboard at launch, it would not be enough. The Ceres objectives were at serious risk!
The flight team undertook an aggressive campaign to conserve hydrazine. They conceived more than 50 new candidate techniques for reducing hydrazine consumption in the 30-month journey to Ceres and the 18 months of Ceres operations and systematically but quickly assessed every one of them.
The team initially calculated that the long interplanetary flight between the departure from Vesta and the beginning of the Ceres approach phase would consume 27.6 pounds (12.5 kilograms) of hydrazine even if there were no errors, no glitches, no problems and no changes in the plans. Following the intensive conservation work, they determined that the spacecraft might instead be able to complete all of its assignments for only 9.7 pounds (4.4 kilograms), an astonishing 65 percent reduction. (Keep track of that mass through the end of the next paragraph.) That would translate directly into more hydrazine being available for the exploration of Ceres. They devised many new methods of conducting the mission at Ceres as well, estimating today that it will cost less than 42.5 pounds (19.3 kilograms) with the zero reaction wheel plan. (If the two remaining wheels operate when called upon in the lowest orbit, they will provide a bonus reduction in hydrazine use.)
Dawn’s two years and four months of interplanetary cruise concluded on Dec. 26, 2014, when the approach phase began. Although the team had computed that they might squeeze the consumption down to as low as 9.7 pounds (4.4 kilograms), it’s one thing to predict it and it’s another to achieve it. Changes to plans become necessary, and not every detail can be foreseen. As recounted in October, the trip was not entirely free of problems, as a burst of cosmic radiation interrupted the smooth operations. Now that the cruise phase is complete, we can measure how well it really went. Dawn used 9.7 pounds (4.4 kilograms), exactly as predicted in 2012. Isn’t flying spacecraft through the forbidding depths of the interplanetary void amazing?
This success provides high confidence in our ability to accomplish all of the plans at Ceres (even if the remaining reaction wheels are not operable). Now that the explorer is so close, it is starting to reap the rewards of the daring 3.0-billion-mile (4.9-billion-kilometer) journey to an ancient world that has long awaited a terrestrial emissary. As Dawn continues its approach phase, our growing anticipation will be fueled by thrilling new pictures, each offering a new perspective on this relict from the dawn of the solar system. Very soon, patience, diligence and unwavering determination will be rewarded with new knowledge and new insight into the nature of the cosmos.
Dawn is 121,000 miles (195,000 kilometers) from Ceres, or half the average distance between Earth and the moon. It is also 3.63 AU (338 million miles, or 544 million kilometers) from Earth, or 1,390 times as far as the moon and 3.69 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour to make the round trip.
Dr. Marc D. Rayman
7:00 p.m. PST January 29, 2014
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.
There can be two accounts of Ceres: its own history, which dates back to near the dawn of the solar system almost 4.6 billion years ago, and its history in the scope of human knowledge, which is somewhat shorter. Both are rich topics, with much more than we can cover here (or in the first log for this entire mission), but let’s touch on a few tidbits. We begin with the latter history.
In 1800, the known solar system contained seven planets: Mercury, Venus, Earth (home to some of our readers), Mars, Jupiter, Saturn and Uranus. This reflected a new and sophisticated scientific understanding, because Uranus had first been noticed in telescopes not long before, in 1781. (The other planets had been known to ancient sky watchers.) Even before William Herschel’s fortuitous sighting of a planet beyond Saturn, astronomers had wondered about the gap between Mars and Jupiter and speculated about the possibility of a planet there. Although some astronomers had searched, their efforts had not yielded a new planet.
Astronomer Giuseppe Piazzi was not looking for a planet on Jan. 1, 1801, but he spotted an unfamiliar dot of light that moved slowly among the stars. He named it for Ceres, the Roman goddess of agriculture, and if you had cereal this morning, you have already had an etymological connection with the goddess.
The Dawn project worked with the International Astronomical Union (IAU) to formalize a plan for names on Ceres that builds upon and broadens Piazzi’s theme. Craters will be named for gods and goddesses of agriculture and vegetation from world mythology. Other features will be named for agricultural festivals.
Because Ceres was fainter than the other known planets, it was evident that it was smaller. Nevertheless, many astronomers considered it to be a planet too.
It is worth noting the significance of this. Modern astronomy had chanced upon only one other planet, so Piazzi’s discovery was A Big Deal. When a new chemical element was found a couple of years later, it was named cerium in tribute to the new planet Ceres. (Uranus had been similarly honored with the 1789 naming of uranium. That element’s peculiar property of emitting radiation would not be known for another century.)
In the six years following the discovery of Ceres, three more bodies were detected orbiting between Mars and Jupiter. (One of them is Vesta, now known in spectacular detail thanks to Dawn’s extensive exploration in 2011-2012.) There then ensued a gap of more than 38 years before another was found, so for well over a generation, the sun’s family of planets was unchanged.
So if you had been reading about all this 200 years ago, there would have been at least two important differences from now. One is that your Internet connection would have been considerably slower. The other is that you might have learned in school or elsewhere that Ceres was a planet.
In 1846, a planet was discovered beyond Uranus, and we call it Neptune. Nothing else of comparable size has subsequently been seen in our solar system.
With scientific knowledge and technology progressing in the middle of the nineteenth century, new objects were glimpsed between Mars and Jupiter. As more and more were seen over the years, what we now know as the main asteroid belt was gradually recognized. Terminology changed too. One of the great strengths of science is that it advances, and sometimes we have to modify our vocabulary to reflect the improved, refined view of the universe.
By the time Pluto was sighted in 1930, Ceres had long been known as a “minor planet” and an “asteroid.” For a while thereafter, Pluto enjoyed planetary status similar to what Ceres had had. In fact, in 1940, scientists named two more additions to the periodic table of the elements neptunium and plutonium. While the histories are not identical, there is a certain parallel, with more and more objects in Pluto’s part of the solar system later being found. Terminology changed again: Pluto was subsumed into the new category of “dwarf planets” defined by the IAU in 2006. Ceres was the first body to be discovered that met the criteria established by the IAU, and Pluto was the second. (Spacecraft are now on their way to both dwarf planets: Dawn to orbit Ceres 214 years after its discovery and the wonderful New Horizons mission to fly past Pluto 85 years after it was found.)
We discussed this new nomenclature in some detail shortly after it was adopted. We understand that the designation then, as now, is controversial among some scientists and the public, and there are strong emotions on this topic. We will not delve into it here (nor in the blog comments below), preferring instead to focus on the extraordinary successes of science, the great power of the scientific method and the thrill of bold adventures far from home. The Dawn team remains both unperturbed and confident in what to call this intriguing and alluring world: we call it “Ceres.” And our goal is to develop that faint smudge of light amidst the stars into a richly detailed portrait.
One of the advances of science was the recognition that Ceres really is entirely different from typical residents of the main asteroid belt. It is a colossus! There are millions upon millions of asteroids, and yet Ceres itself contains roughly 30 percent of the mass in that entire vast region of space. By the way, Vesta, the second most massive body there, constitutes about eight percent of the asteroid belt’s mass. It is remarkable that Dawn will single-handedly explore around 40 percent of the asteroid belt’s mass.
With an equatorial diameter of about 605 miles (975 kilometers), a value that Dawn will refine very soon, Ceres is the largest body between the sun and Pluto that a spacecraft has not yet visited. It is occasionally described as being comparable in size to Texas, which is like comparing a basketball to a flat sheet of paper. Ceres has a surface area 38 percent of that of the continental United States, or more than four times the area of Texas. (Nevertheless, until Dawn shows evidence to the contrary, we will assume Texas has more rodeos.) It is nearly a third of the area of Europe and larger than the combined lands of France, Germany, Italy, Norway, Spain, Sweden and the United Kingdom. Such a large place offers the promise of tremendous diversity and many marvelous and exciting sights to behold. Earth is about to be introduced to a fascinating new world.
How did Ceres come to be? And why is that being phrased as a question instead of a more declarative introduction to the history and nature of this dwarf planet? For that matter, why is this paragraph composed exclusively of questions? At least this sentence isn’t a question, right? OK, really, shouldn’t we stay more on topic?
At the dawn of the solar system almost 4.6 billion years ago, the young sun was surrounded by a swirling cloud of dust and gas. Sometimes some particles would happen to hit and stick together. Then more and more and more particles would stick to them, and eventually these agglomerations would grow so large that their gravity would pull in even more material. It was through mechanisms like this that the planets formed.
But when massive Jupiter developed, its powerful gravity terminated the growth of objects nearby, leaving bits and pieces as asteroids. Ceres and Vesta, already sizable by then, might have grown to become even larger, each incorporating still more of the nearby material, had Jupiter not deprived them of such an opportunity. Not having made it to full planetary proportions, Ceres and Vesta are known as protoplanets, and studying them provides scientists with insight into the largest building blocks of planets and into worlds that are intriguing in their own rights.
Ceres apparently formed far enough from the sun under conditions cool enough for it to hang on to water molecules. Indeed, scientists have good reason to believe that water (mostly in the form of ice) may make up an astonishing 30 percent of its mass. Ceres may contain more water than Mars or any other body in the inner solar system except Earth. (Comets, of course, have high proportions of water too, but they are so minuscule compared to this behemoth that each one harbors a quite negligible amount of water when measured against Ceres’ huge inventory.)
Although some of the moons of the outer planets also are ice and rock, and they display very interesting characteristics to the impressive and capable spacecraft that have flown past (in some cases repeatedly, as the craft orbited the host planet), no probe has had the capability to linger in orbit around any of them. Dawn’s in-depth exploration of Ceres will yield more detailed and complete views than we have obtained of any icy moon.
Radioactive elements incorporated into Ceres when it was forming would supply it with some heat, and its great bulk would provide thermal insulation, so it would take a very long time for the heat to escape into space. The sun, faraway though it is, adds still more heat. As a result, there may be some water warm enough to be liquid. (The concentration of any chemical impurities in the water that affect its freezing point, as salt does, may make an important difference in how much is liquid.) This distant, alien world may have lakes or even oceans of liquid water deep underground. What a fantastic possibility!
There will be no liquid on the frigid surface. Even ice on the surface, exposed to the cold vacuum of space, would sublimate before long. But ice could be just beneath the surface, perhaps well less than a yard (a meter) deep.
Ceres then may have a thin, dusty crust over a mantle rich in ice that might be more than 60 miles (100 kilometers) thick. Its warmer core is likely composed mostly of rock.
As heat dissipated from Ceres’ interior over the eons, it may have undergone convection, with the warmer material rising and cooler material sinking very slowly. This is reminiscent of what occurs in pot of heated water and in Earth’s interior. Even if it did occur at some time in Ceres’ history, it probably is not happening any longer, as too much heat would have been lost by now, so there would not be enough left to power the upward movement of warm material. But the convective process might have written its signature in structures or minerals left behind when ice sublimated after being pushed to the surface. Dawn’s photos of geological features and measurements of the composition may provide a window to forces in the interior of the protoplanet sometime in its past.
Even if convection is no longer occurring, Ceres is not entirely static. We have very tantalizing information from a marvelously productive far-infrared space telescope named for the only known astronomer who found a planet before Piazzi made his discovery. The Herschel Space Observatory recently detected a tiny amount of water vapor emanating from the distant dwarf planet. Scientists do not know how the water vapor makes it into space. It might be from ice sublimating (possibly following a powerful impact that exposed subsurface ice) or perhaps from geysers or even erupting cryovolcanoes (“cold volcanoes”) powered by heat that Ceres has retained since its formation. In any case, Herschel saw water, albeit in very, very small quantity.
It is not certain whether water vapor is there all the time. It is unknown whether, for example, it depends on solar heating and hence where Ceres is in its somewhat elliptical orbit around the sun (not as circular as Earth’s orbit but more circular than Mars’), which requires 4.6 years to complete.
Even if the water vapor is present during Dawn’s 1.3-year primary mission in orbit, it would be extremely difficult to detect. Herschel made its findings when our ship was already far, far from Earth, well along its interplanetary itinerary. The probe’s sensors were designed for studying the solid surfaces of airless bodies, not an exceedingly tenuous veil of water molecules. For context, the water vapor Herschel measured is significantly less dense than Earth’s atmosphere is even far above the International Space Station, which orbits in what most people consider to be the vacuum of space. Dawn will not need windshield wipers! Nevertheless, as we saw in February, the Dawn team, ever creative and dedicated to squeezing as much out of the mission as possible, investigated techniques this year that might be effective in searching for an exceptionally thin vapor. They have augmented the plan with many hours of observations of the space above Ceres when the spacecraft is over the night side during its first science orbit in April and May at an altitude of 8,400 miles (13,500 kilometers). It is possible that if there is some water vapor, the instruments may pick up a faint signature in the sunlight that passes through it.
Regardless of the possibility of detecting traces of water from Ceres, Dawn will focus its measurements on the uncharted surface and the interior, as it did at Vesta. Vesta displayed landscapes battered by craters from impacts during more than 4.5 billion years in the rough and tumble asteroid belt. Ceres has spent most or all of its history also in the asteroid belt, but it is possible it will not show its age so clearly. Ice, although very hard at such low temperatures, is not as hard as rock. So it may be that the surface gradually “relaxes” after an impact, just as your skin restores its shape after pressure has been removed. Craters older than a few tens of millions of years may have slowly disappeared. (That may sound old, but it is a small fraction of Ceres’ lifetime.) Near the poles, where it is colder so ice is harder, the scars of impact craters may be preserved for longer.
Ceres has more than water-ice and rock. It probably contains organic materials, some produced by chemical processes with the minerals already there and some delivered by asteroids that fell to its surface. This is noteworthy, because water and organic chemicals are ingredients for life. The combination of Ceres’ internal heat and the weak but persistent heating from the sun provides energy, which also is essential for life. Even if the possibility of life itself there is extremely remote (and it is beyond Dawn’s capability to detect), the conditions for “prebiotic” chemistry would be tremendously interesting. That is why, as we explained in August, we want to protect the special environment on the ground from contamination by the terrestrial chemicals in our orbiting spacecraft.
While there is more known about Ceres, there is much, much more that is unknown. Dawn seeks to discover many of the secrets of this unfamiliar, fascinating member of the solar system family. One of the measures of its success would be if, upon answering many of our questions about Ceres, we are left with even more questions. Now on the threshold of an old world which will be new to us, we do not have long to wait for the great rewards of new knowledge, new insight, new thrills and new mysteries to solve.
Dawn is 382,000 miles (614,000 kilometers) from Ceres, or 1.6 times the average distance between Earth and the moon. It is also 3.77 AU (351 million miles, or 564 million kilometers) from Earth, or 1,500 times as far as the moon and 3.84 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and three minutes to make the round trip.
Dr. Marc D. Rayman
8:00 a.m. PST December 29, 2014