Following a successful mission, Dawn mission operations concluded successfully on Oct. 31. (Please note the understated elegance of that sentence.)
After more than 11 years in deep space, after unveiling the two largest uncharted worlds in the inner solar system, after overcoming myriad daunting obstacles, Dawn's interplanetary adventure came to an end.
We explained in detail in the two August Dawn Journals that the spaceship would deplete its supply of hydrazine, which was essential for controlling its orientation as it orbited dwarf planet Ceres. We predicted that the last of the hydrazine would be spent between mid-September and mid-October (although we acknowledged that it could be earlier or later). Dawn, ever the overachiever, held on until the end of October, and the explorer was productive to the very end. This was the best way to end a mission. It was good to the last drop!
Dawn took us on a truly amazing solar system journey. Never content simply to do the same thing over and over, the flight team kept coming up with new kinds of activities and new kinds of observations from new orbital perspectives. With such a long and constantly active mission, it may have seemed like it would just keep going. For readers who did not keep up with our recent forecasts of the end, it might be worth recognizing Stein's Law: “If something cannot go on forever, it will stop.” (The eponymous economist who both conceived of and named this epigram was Herbert Stein.)
Dawn demonstrates not only Stein's Law but also Davies’ Corollaries (proposed by Daniel Davies, a financial analyst):
When it stopped, Dawn was in its extended mission orbit 7 (XMO7). Every 27 hours, the intrepid ship dove from 2,500 miles (4,000 kilometers) to 22 miles (35 kilometers) above the ground, only three times higher than a typical passenger aircraft travels over Earth, and then soared back up again. It had a fantastic view as it streaked over the alien landscape at 1,050 mph (1,690 kph). (While the supply of hydrazine was very limited, it seemed there was no end to the adrenaline. Too bad Dawn's reaction control thrusters couldn't use that chemical instead.) The spacecraft expertly performed high-resolution measurements, providing scientists with a wealth of exquisite data and giving everyone incredibly detailed looks at the exotic sights.
On Oct. 31, Dawn flew down to peridemeter (the low point of its elliptical orbit) shortly after 5:00 am PDT. As always, to keep its solar arrays pointed at the Sun and its sensors pointed at the ground, it had to fire its hydrazine thrusters extensively. Using the thrusters was routine after having operated for more than half of its time in space without the use of the reaction wheels that were intended for controlling its orientation, but which had failed. While the spacecraft didn't know the hydrazine was about to run out, mission controllers had known for quite some time.
As is typical for missions in deep space, Dawn operated most of the time out of radio contact. NASA's Deep Space Network (DSN) cannot serve all missions simultaneously, and often spacecraft have tasks to perform that are incompatible with radio communications. As it turned out, however, Dawn's final moments happened to be while the largest antenna at the Goldstone Deep Space Communications Complex in California was tracking it. The 230-foot (70-meter) antenna thus allowed the flight team to hear Dawn's swan song.
We have described before that with Dawn broadcasting through an auxiliary antenna when it flew close to the ground, scientists and engineers could learn about Ceres' interior. The spacecraft's radio signal was too weak to carry data to the DSN. Rather, it was as if Dawn were playing a single note with no variation. That may not make for an especially imaginative or melodious performance, but as different regions underground exerted their gravitational pulls and accelerated and decelerated the probe, the Doppler shift was music to the ears of planetary geologists.
By observing changes in the strength and some other characteristics of the signal (and knowing the likely explanation), engineers were able to reconstruct some of the spacecraft's final actions. Around 20 minutes after it was at peridemeter, still quite low but with its momentum starting to carry it back up to high altitudes, the hydrazine thrusters became ineffective. Dawn recognized that it could no longer control its orientation (although it did not know the reason) and systematically proceeded through all the contingency procedures possible, such as swapping to backup equipment and even rebooting its main computer. It made valiant attempts and continued to operate with the professionalism of a dedicated, veteran space explorer, but without hydrazine, there was nothing it could do. The outcome was inevitable. Dawn was up against an unsolvable problem.
Although the spacecraft's depletion of hydrazine and subsequent inability to communicate had been predicted for quite some time, your correspondent considered it worthwhile to verify the diagnosis. It was possible, albeit highly unlikely, that some other problem had befallen Dawn and that after the scheduled session with the Goldstone antenna, the sophisticated robot would solve it and try to reestablish radio contact.
The plan then was for the flight team to look for Dawn at night. Hours after young trick-or-treaters everywhere had finished extorting sweets from their elders, when Earth had rotated so that another 230-foot (70-meter) antenna, the largest at the Madrid Deep Space Communications Complex, could point at Dawn's location in the sky, controllers listened again. Not even the faintest whisper was heard. The remote spacecraft was orbiting Ceres as silently as the cold vacuum of space itself.
After more than 11 years of an incredibly exciting, fantastically fruitful, extremely difficult, deeply rewarding, super fun and just totally awesome interplanetary adventure, your correspondent declared the mission over shortly before 1:30 am PDT on Nov. 1.
The mission had been a dream come true. Now the dream was over.
Somehow, the stark reality of the end of the phantasmagorical mission in the middle of the night seemed to turn upside down the meaning of a proverb commonly (but vaguely) attributed to Africa: "However long the night, dawn will break."
This video captures some of the highlights of Dawn's interplanetary adventure, as well as some personal reflections on it.
With a rare excursion into first person, I wrote in my Aug. 22 Dawn Journal about how I felt with the mission coming to an end (and offered a fanciful additional perspective at the end of my Sept. 27 Dawn Journal). My feelings were unchanged when the end came. Nevertheless, in the actual event, I wrote down some of my thoughts, because Dawn was such a significant part of my life, and I am well aware of the fallibility of human memory. Memories, however vivid, are often more of a reconstruction than people like to believe.
But I quickly realized that it didn't matter how I was feeling! Here is an unedited excerpt of what I wrote after declaring the mission to be over: "These feelings are transitory, and I don't need to remember them anyway. It would be a mistake to consider how I feel now as somehow representing my overall experience or feelings about the mission. Indeed, this is very much the wrong time to try to put it into perspective. It would make a good story if I had some revelation or profound description of my feelings at this point, but there's no reason I should. It takes time to gain a good perspective. People construct and then gradually change their memories, all without any awareness. And I should not think that somehow now I will be imbued with the wisdom, insight, or other capability to put this into perspective. If I feel sad, elated, disappointed, relieved, proud, empty, gratified or any of myriad other feelings -- and, more to the point, a combination of myriad feelings -- I won't feel that way again. The end of Dawn is not what's important. All that preceded it is. And I cannot so easily grasp it all right now, so my feelings now are not as special or as meaningful as one might be tempted to think."
Finally, you can't appreciate the end of the mission if you don't appreciate the rest of the mission. So, feel free to reread the previous 310,000 words in Dawn Journals to gain the full appreciation.
There will be future opportunities to address some of the overall accomplishments of the mission and discoveries about Ceres. For now, we will devote more attention to this final phase.
And there was no doubt about its finality. On Nov. 1-2, immediately after the official end of the mission, there was not enough time to reallocate previously scheduled DSN antenna time to other missions. So although confidence was high that Dawn would forever be silent, each of the three deep space communications complexes (Goldstone, then Canberra, and then Madrid) turned a sensitive ear to Ceres for one last time. No surprises occurred.
The final phase of Dawn's exploration began in June when it completed maneuvering to XMO7. We have explained that as the orbit precessed so that peridemeter gradually shifted from Ceres' day side to the night side, photography, infrared spectroscopy and visible spectroscopy became less valuable. The spacecraft had collected a tremendous number of such measurements earlier in the mission, so when it flew over illuminated terrain in XMO7 at higher altitudes than it had already been, new observations were not worthwhile. (Low altitude measurements of Ceres' nuclear radiation and gravity continued in darkness to the very end of the mission.)
Recognizing that the hydrazine would be long gone by the time peridemeter moved back to the day side, controllers took advantage of a nice opportunity at higher altitude for a last, fond look at Ceres on Sept. 1-2. As the dwarf planet pirouetted before the admiring eye of its permanent companion, Dawn recorded its final views of Ceres. One of them is shown above and another is below.
On Sept. 28 and 29, Dawn performed a calibration of the camera and the visible and infrared mapping spectrometer for one last time. They are precision scientific instruments, and the thorough analysis of their data depends on accurate knowledge of their sensitivity and other properties. The team has conducted calibrations throughout the mission so even slight changes could be detected and accounted for in interpreting the pictures and spectra and drawing conclusions about the nature of Vesta and Ceres. Dawn expended a little more of its remaining hydrazine to point the instruments at the stars Vega and Arcturus, which they had observed before. Indeed, the first time was less than three months after the journey began in 2007 (and Vega still holds special significance).
Even though Dawn took no more pictures nor infrared or visible spectra of Ceres after the beginning of September, it acquired a great many before that, far exceeding the team's expectations when planning this phase of the mission. In XMO7, the spacecraft sent more than 11,000 photographs of Ceres to Earth, almost all of them at very low altitude, revealing amazing new details. (This brought the total for Vesta plus Ceres to more than 100,000 pictures.) Also during XMO7, Dawn provided scientists with more than three million infrared spectra and almost 50,000 visible spectra.
We have explained before that Ceres' nuclear glow is very faint, so the gamma ray and neutron detector (GRaND) requires a great deal of data to make its measurements, just as a camera needs a long exposure to record a dark scene. Despite its name, GRaND is meek and unprepossessing, but the instrument does do a wonderful job revealing the atomic composition of the material down to about a yard (meter) underground. GRaND does not need illumination, so it continued to operate even as Dawn glided over ground cloaked in the deep dark of night.
In XMO7, GRaND acquired 140 hours of nuclear spectra from altitudes below Dawn's previous low altitude orbit, at 240 miles (385 kilometers) in 2015-2016. And it accumulated 50 hours of measurements of Ceres' radiation from within 60 miles (100 kilometers) of the ground. GRaND collected about four times as much data in XMO7 as scientists needed to meet their objectives. This will allow them to see Ceres' elemental abundances with much sharper resolution, like a close-up picture, than ever before.
Like the other investigations, gravity measurements far surpassed what the team expected not only when planning XMO7 but even when the spacecraft was there.
With the smooth and productive operations in XMO7, these successes may all seem pretty simple. After all, it's only the cutting edge of rocket science, operating an ion propelled spaceship at incredibly low altitude around a dwarf planet well over a million times farther away than the International Space Station. But there were a few challenges to overcome. The team confronted and solved myriad problems to accomplish so much.
Now, even if you don't have your own interplanetary spacecraft, you can explore Ceres and do so from the comfort of your home. Instead of going all the way to the main asteroid belt, bring that distant world to your computer with Cerestrek. You can also see all the sights on the first world Dawn unveiled with Vestatrek.
Dawn, however, will never again explore alien worlds. It will never again emit a bluish beam of xenon ions. It will never again communicate with beings on the faraway planet where its voyage began. It will never again perform any of the functions or tasks it executed so admirably on its remarkable journey. For decades, and quite possibly even for centuries, the ship that undertook a long, daring, difficult and successful deep-space expedition on behalf of humankind will remain silently in orbit around Ceres. It has become an inert celestial monument to the power of human ingenuity, creativity, and curiosity, a lasting reminder that our passion for bold adventures and our noble aspirations to know the cosmos can take us very, very far beyond the confines of our humble home.
Dawn is 1,800 miles (2,900 kilometers) from Ceres. It is also 3.53 AU (328 million miles, or 527 million kilometers) from Earth, or 1,320 times as far as the moon and 3.56 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, will never again make the round trip.
Dr. Marc D. Rayman
7:30 am PST November 11, 2018
After its short dawn ride to space on a Delta, Dawn began its long interplanetary expedition atop a cool blue beam from its ion engines. The spacecraft sailed past Mars in 2009 and in 2011 entered orbit around Vesta, the second largest body in the main asteroid belt. During 14 months there, it revealed Vesta to be more like the terrestrial planets, including the one on which Dawn was conceived and built (and where its controllers still reside), than like the much smaller chunks of rock we know as asteroids. In 2012, propelled by a zephyr of xenon ions, the ship set sail on the cosmic seas once again, its sights set on the largest uncharted world between the Sun and Pluto. Ceres took the traveler into a gentle but permanent gravitational hold in 2015. Thanks to curious and creative creatures on Earth, Ceres now has a moon named Dawn.
During its 11th year in deep space, the explorer undertook some of its most ambitious activities of the entire mission. Unlike missions limited to a brief glimpse of their targets during a flyby, Dawn has taken great advantage of being able to conduct comprehensive studies of Ceres (and Vesta). And thanks to the maneuverability afforded by its ion engines, the spacecraft has frequently changed its orbit to optimize its investigations. We described earlier this year how Dawn flew to extended mission orbit 6 (XMO6) for a new campaign of photography and other measurements. Following that, early in June the spaceship used its ion engine to descend to XMO7, in which it dives down to about 22 miles (35 kilometers) above the ground, only three times your altitude when you travel in a commercial jet. (Perhaps you ought to consider traveling by spaceship. Note that Dawn travels a little faster than a jet, although it does require some patience to reach the top speed. We will see more about this below.)
The orbit was initially aligned so the low point, known as peridemeter, would be in the vicinity of Occator Crater on the dayside of Ceres, allowing the spacecraft to obtain stunning pictures and other data. We discussed in March and in June that the peridemeter gradually shifts south. As it did so, the focus of the close-up observations moved to Urvara Crater. In late August, to the delight of everyone interested in the exploration of space, Dawn was still operating productively, and by then, the peridemeter had moved to its greatest southerly latitude of 84° (corresponding to the orbital inclination, for those who understand orbits). Since then, it has progressed north on the other side of Ceres, opposite the Sun. (If this progression isn't clear, see the diagram in March and imagine continuing the trend of the orbital precession it illustrates.) Now the peridemeter has moved so far to the nightside that throughout Dawn's time over illuminated terrain, it is higher than it was in previous orbits. There is little new to see now from higher up. Therefore, Dawn no longer conducts visible or infrared observations, but it is continuing to measure nuclear radiation and the gravity field, both of which provide valuable insight into the nature of the dwarf planet.
On every Sept. 27, we reflect on this unique interplanetary adventure. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the 11th annual summary, reusing text from previous years with updates where appropriate. Readers who wish to investigate Dawn’s ambitious journey in detail may find it helpful to compare this material with the Dawn Journals from its first, second, third, fourth, fifth, sixth, seventh, eighth, ninth and tenth anniversaries.
In its 11 years of interplanetary travels, the spacecraft has thrust with its ion engines for a total of 2,141 days (5.9 years), or 53 percent of the time (and 0.000000043 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 907 pounds (411 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sept. 27, 2007. The spacecraft has used 69 of the 71 gallons (261 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space. (Note that on the tenth anniversary, we actually gave a slightly higher xenon cost. Dawn has not refueled since then. For technical reasons we will not delve into, it is very difficult to compute the xenon consumption late in the mission. Engineers devoted extensive effort to refining their measurements of the xenon during the past year, resulting in a small change in their final calculation of how much xenon Dawn has used.) We saw in June that Dawn will never use its ion propulsion system again. We have the spacecraft right where we want it.
The thrusting since launch has achieved the equivalent of accelerating the probe by 25,700 mph (41,400 kph). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the Sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.) It is remarkable that Dawn’s ion propulsion system has provided nearly the same change in speed as the entire Delta rocket.
Since launch, our readers who have remained on or near Earth have completed 11 revolutions around the Sun, covering 69.1 AU (6.4 billion miles, or 10.3 billion kilometers). Orbiting farther from the Sun, and thus moving at a more leisurely pace, Dawn has traveled 46.4 AU (4.3 billion miles, or 6.9 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 11 years since Dawn began its voyage, Vesta has traveled only 44.9 AU (4.2 billion miles, or 6.7 billion kilometers), and the even more sedate Ceres has gone 41.8 AU (3.9 billion miles, or 6.3 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 11 years. You will see that as the strength of the Sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)
Comparing mileage with cars is highly misleading, but some readers can't help but try to make that comparison. The reason it is deceptive is that cars have to keep burning fuel to move as they overcome friction, but orbiting objects normally move without propulsion at all. Earth has completed its annual trip around the Sun (currently 584 million miles, or 940 million kilometers) for billions of years without requiring any propellant at all. Similarly, spacecraft coast most of the time. With ion propulsion, Dawn (and Deep Space 1 before it) were the exceptions, thrusting more often than coasting. But readers who require a comparison with their car (or their spaceship) can credit Dawn with 63 million miles per gallon (0.0000038 liters per 100 kilometers, or 3.8 liters per 100 million kilometers).
Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the Sun has changed. This discussion will culminate with even more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. (If you prefer not to skip it, click here.) In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.
Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family (including Earth, Dawn, Vesta and Ceres) follow their individual paths around the Sun, they sometimes move closer and sometimes move farther from it.
In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth’s orbit around the Sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft may travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body’s orbit around the Sun is the inclination of that orbit. Vesta and Ceres do not orbit the Sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and part of the arduousness of Dawn’s journey has been changing the inclination of its orbit, an energetically expensive task.)
Now we can see how Dawn has done by considering the size and shape (together expressed by the minimum and maximum distances from the Sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts’ websites when their readership extends to one more elliptical galaxy than ours does.)
The table below shows what the orbit would have been if the spacecraft had terminated ion thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on Sept. 27, 2007, its orbit around the Sun was exactly Earth’s orbit. After launch, it was in its own solar orbit.
from the Sun (AU)
from the Sun (AU)
|Dawn’s orbit on Sept. 27, 2007 (before launch)||0.98||1.02||0.0°|
|Dawn’s orbit on Sept. 27, 2007 (after launch)||1.00||1.62||0.6°|
|Dawn’s orbit on Sept. 27, 2008||1.21||1.68||1.4°|
|Dawn’s orbit on Sept. 27, 2009||1.42||1.87||6.2°|
|Dawn’s orbit on Sept. 27, 2010||1.89||2.13||6.8°|
|Dawn’s orbit on Sept. 27, 2011||2.15||2.57||7.1°|
|Dawn’s orbit on Sept. 27, 2012||2.17||2.57||7.3°|
|Dawn’s orbit on Sept. 27, 2013||2.44||2.98||8.7°|
|Dawn’s orbit on Sept. 27, 2014||2.46||3.02||9.8°|
|Dawn’s orbit on Sept. 27, 2015||2.56||2.98||10.6°|
|Dawn’s orbit on Sept. 27, 2016||2.56||2.98||10.6°|
|Dawn’s orbit on Sept. 27, 2017||2.56||2.98||10.6°|
|Dawn’s orbit on Sept. 27, 2018||2.56||2.98||10.6°|
For readers who are not overwhelmed by the number of numbers, investing the effort to study the table may help to demonstrate how Dawn patiently transformed its orbit during the course of its mission. Note that seven 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 exotic world in such detail. Dawn has long since gone well beyond that. Having discovered so many of Vesta’s secrets, the adventurer left it behind. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. From 2012 to 2015, the stalwart craft reshaped and tilted its orbit even more so that now it is identical to Ceres’. Once again, that was essential to accomplishing the intricate celestial choreography in which the behemoth reached out with its gravity and tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.
Dawn has had a long and productive life. Indeed, many readers might agree that Dawn has accomplished much, much more in its life than, say, a Nathusius' pipistrelle does in its life, which lasts about the same length of time. (And while Nathusius' pipistrelle was honored as the first-ever "Bat Species of the Year," Dawn has been honored for its accomplishments too, although somewhat different ones.) For our chiroptophobic readers, Dawn's lifetime also is about the same as a paradoxical frog, a magnificent hummingbird (recently renamed Rivoli's hummingbird), and a Taipan beauty snake. They also tend not to achieve nearly as much in their lifetimes as Dawn has, although it's nice that all those names have some connection with Dawn's magnificent exploration of two worlds, paradoxically at similar distances from the Sun and yet dramatically different, and each beautiful in its way.
Since the two August Dawn Journals, some people have expressed wishes that Dawn would live even longer. I would say it has already lasted longer! The hydrazine could have been depleted much earlier. Indeed, the mission could easily have ended years ago. Life is not easy in the forbidding depths of space, far from Earth. There are many reasons the mission could have concluded early, including the failures of the probe's reaction wheels. Dawn has flown more than half of its time in space without the use of those gyro-like devices, which had previously been considered indispensable for the mission. It is only through the near-heroic work of the flight team that, despite those failures and the many other challenges Dawn has faced, the prime mission concluded successfully in 2016. Dawn is now near the end of its second extension. One can even fantasize that Dawn did, in fact, fail early, succumbing to one of the risks during its unique and ambitious mission, dodging only 999 of 1,000 bullets, and that many of the fabulous pictures and other data from uncharted worlds were never acquired. And then somehow, we said, "If only..." with enough fervor, and we wished hard enough that the fatal problem had not occurred, and presto: we were granted a second chance! Then we could now be the beneficiaries. We might be living in that alternate universe, unaware that, in the original timeline, we were not so lucky. There is good reason not to believe that, but it may provide some perspective on our being fortunate that Dawn has lived so long and been so productive in its extraordinary extraterrestrial expedition.
Dr. Marc D. Rayman
4:34 am PDT September 27, 2018
People have been gazing in wonder and appreciation at the beauty of the night sky throughout the history of our species. The gleaming jewels in the seemingly infinite black of space ignite passions and stir myriad thoughts and feelings, from the trivial to the profound. Many people have been inspired to learn more, sometimes even devoting their lives to the pursuit of new knowledge. Since Galileo pointed his telescope up four centuries ago and beheld astonishing new sights, more and more celestial gems have been discovered, making us ever richer.
In a practical sense, Dawn brought two of those jewels down to Earth, or at least brought them more securely within the scope of Earthlings' knowledge. Science and technology together have uncloaked and explained aspects of the universe that would otherwise have seemed entirely inscrutable. Vesta and Ceres revealed little of themselves as they were observed with telescopes for more than two centuries. Throughout that time, they beckoned, waiting for a visitor from distant Earth. Finally their cosmic invitations were answered when Dawn arrived to introduce each of them to Earth, whereupon the two planet-like worlds gave up many of their secrets.
Even now, Ceres continues to do so, as it holds Dawn in its firm but gentle gravitational embrace. Every 27 hours, almost once a day, the orbiting explorer plunges from 2,500 miles (4,000 kilometers) high to as low as about 22 miles (35 kilometers) and then shoots back up again. Each time Dawn races over the alien landscapes, it gathers information to add to the detailed story it has been compiling on the dwarf planet.
Dawn began its ambitious mission in 2007. (And on Aug. 17, 2018, it passed a milestone: three Vestan years of being in space.) But the mission is rapidly approaching its conclusion. In the previous Dawn Journal, we began an in-depth discussion of the end, and we continue it here.
We described how the spacecraft will lose the ability to control its orientation, perhaps as soon as September. It will struggle for a short time, but it will be impotent. Unable to point its electricity-generating solar panels at the Sun or its radio antenna to Earth, the seasoned explorer will go silent and will explore no more. Its expedition will be over.
We also took a short look at the long-term fate of the spacecraft. To ensure the integrity of possible future exploration that may focus on the chemistry related to life, planetary protection protocols dictate that Dawn not contact Ceres for at least 20 years. Despite being in an orbit that regularly dips so low, the spaceship will continue to revolve around its gravitational master for at least that long and, with very high confidence, for more than 50 years. The terrestrial materials that compose the probe will not contaminate the alien world before another Earth ship could arrive.
Like its human colleagues, Dawn started out on Earth, but now its permanent residence in the solar system, Ceres, is far, far away. Let's bring this cosmic landscape into perspective.
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 (7 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). More remote even than that, when Dawn ceases operating, it would be more than 5.5 miles (9.0 kilometers) from the soccer ball. The ship will stay locked in orbit around Ceres, the only dwarf planet in the inner solar system. The largest object between Mars and Jupiter, that distant orb 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 exploring this fascinating world of ice, rock and salt has been so much sweeter!
Now let's take a less terrestrial viewpoint and shift our reference to Ceres. Suppose it were the size of a soccer ball. In Dawn's final, elliptical orbit, which it entered in June, the spacecraft would travel only 37 inches (94 centimeters) away at its farthest point. Then it would go in to skim a mere one-third of an inch (8 millimeters) from the ball.
Dawn is one mission among many to explore the solar system, dating back almost 60 years and (we hope) continuing and even accelerating for much longer into the future. Learning about the cosmos is not a competition but rather a collective effort of humankind to advance our understanding. And to clarify one of the many popular mistaken notions about the solar system, let's take advantage of reducing Ceres to the size of a soccer ball to put some other bodies in perspective.
Because it is in the main asteroid belt, there is a common misconception that Ceres is just another asteroid, somehow like the ones visited by other spacecraft. It is not. The dwarf planet is distinctly unlike the small chunks of rock that are more typical asteroids. We have discussed various aspects of Ceres' complex geology, and much more remains to be gleaned from Dawn's data. Vesta too has a rich and complicated geology, and it is more akin to the terrestrial planets (including Earth) than to asteroids. But for now, let's focus simply on the size in order to make for an easy comparison. Of course, size is not a measure of interest or importance, but it will illustrate how dramatically different these objects are.
With a soccer-ball-sized Ceres, Vesta would be nearly five inches (more than 12 centimeters) in diameter. (This writer's comprehensive knowledge of sports inspires him to describe this as a ball nearly five inches, or more than 12 centimeters, in diameter.)
What about some of the asteroids being explored as Dawn's mission winds to an end? There are two wonderfully exciting missions with major events at asteroids (albeit ones much closer to Earth than the main asteroid belt) in the second half of 2018. Your correspondent, a lifelong space enthusiast, is as hopeful for success as anyone! Hayabusa2 is revealing Ryugu and OSIRIS-REx is on the verge of unveiling Bennu.
Ryugu and Bennu are more irregular in shape than Ceres and Vesta, but they would both be so small compared to the soccer ball that their specific shapes wouldn't matter. Ryugu would be less than a hundredth of an inch (a quarter of a millimeter) across. Bennu would be about half that size. They would be like two grains of sand compared to the soccer ball. In the first picture of the June Dawn Journal, we remarked on the detail visible in a feature photographed on one of Dawn's low streaks over the alien terrain. It is also visible in the first two pictures above. That one structure on Ceres is only a part of Cerealia Facula, which is the bright center of the much larger Occator Crater. Occator is a good-sized crater, but not even among the 10 largest on Ceres. Yet that one bright feature in the high-resolution photo is larger than either of these small asteroids. In many of Dawn's pictures that show the entire disk of the dwarf planet (like the rotation movie and the color picture here), Ryugu and Bennu would be less than a pixel, undetectably small, just as invisible specks of dust on a soccer ball.
The tremendous difference in size between Ceres (and Vesta) and small asteroids illustrates a widely unappreciated diversity in the solar system. Of course, that is part of the motivation for continuing to explore. There is a great deal yet to be learned!
Although Ryugu and Bennu aren't in the main asteroid belt, the belt contains many more Lilliputian asteroids closer in size to them than to the Brobdingnagian Ceres and Vesta. In fact, of the millions of objects in the main asteroid belt, Ceres by itself contains 35 percent of the total mass. Vesta has 10 percent of the total.
Readers with perfect memories may note that we used slightly smaller fractions in earlier Dawn Journals. Science advances! More recent estimates of the mass of the asteroid belt are slightly lower, so these percentages are now correspondingly higher. The difference is not significant, but the small increase only emphasizes how different Vesta and Ceres are from typical residents of the asteroid belt. It's also noteworthy -- or, at least, pretty cool -- that Dawn has single-handedly explored 45 percent of the mass between Mars and Jupiter.
Dawn will end its mission in the same orbit it is in now, looping around from a fraction of an inch (fraction of a centimeter) to a yard (a meter) from the soccer-ball-sized Ceres. In the previous Dawn Journal, we described what will happen onboard the spacecraft. We also saw that the most likely indication controllers will have that Dawn has run out of hydrazine will be its radio silence. They will take some carefully considered steps to verify that that is the correct conclusion.
But it is certain that emotions will be ahead of rationality. Even as team members are narrowing down the causes for the disappearance of the radio signal, many strong feelings about the end of the mission will arise. And they will be as varied as the people on the Dawn team, every one of whom has worked long and hard to make the mission so successful. Your correspondent can make reasonable guesses about their feelings but won't be so presumptuous as to do so.
As for my own feelings, well, I won't know until it happens, but I'm not too presumptuous to guess now. Long-time readers may recognize that your correspondent has avoided writing anything about himself (with a few rare exceptions), or even using first person, in his Dawn Journals. They are meant to be a record of a mission undertaken by humankind, for everyone who longs for knowledge and for adventures in the cosmos. But now I will devote a few words to my own perspective.
My love affair with the universe began when I was four, and my passion has burned brighter and brighter ever since. I knew when I was a starry-eyed nine-year-old that I wanted to get a Ph.D. in physics and work for NASA, although it was a few more years before I did. I had my own Galileo moment of discovery and awe when I first turned a telescope to the sky. Science and space exploration are part of me. They make me who I am. (My friend Mat Kaplan at The Planetary Society described me in the beginning of this video as "the ultimate space nerd." He's too kind!) Adding to my own understanding and contributing to humankind's knowledge are among my greatest rewards.
Passion and dedication are not the whole story. I recognize how incredibly lucky I am to be doing what I have loved for so long. I am lucky to have had access to the resources I have needed. I am lucky that I was able to do well in my formal education and in my own informal (but extensive) studies. I am lucky I could find the discipline and motivation within myself. For that matter, I am lucky to be able to communicate in terms that appeal to you, dear readers (or, at least, to some of you). My innate abilities and capabilities, and even many acquired ones, are, to a large extent, the product of factors out of my control, like my cognitive and psychological constitution.
That luck has paid off throughout my time at JPL. Working there has been a dream come true for me. It is so cool! I often have what amount to out-of-body experiences. When I am discussing a scientific or engineering point, or when I am explaining a conclusion or decision, sometimes a part of me pulls back and looks at the whole scene. Gosh! Listen to the cool things I get to say! Look at the cool things I get to do! Look at the cool things I know and understand! Imagine the cool spacecraft I'm working with and the cool world it is orbiting! I am still that starry-eyed kid, yet somehow, through luck and coincidence, I am doing the kind of things I love and once could only have dreamed of.
Dawn will continue to be exciting to the very end, performing new and valuable observations as it skims incredibly low over the dwarf planet on every orbital revolution. The spacecraft has almost always either been collecting new data or, thanks to the amazing ion propulsion, flying on a blue beam of xenon ions to somewhere else to gain a new perspective, see new sights and make more discoveries. Whether in orbit around Vesta or Ceres or traveling through the solar system between worlds, the mission was rarely anything like routine.
I love working on Dawn (although it was not my first space love). You won't be surprised that I think it is really cool. I could not be happier with its successes. I am not sad it is ending. I am thrilled beyond belief that it achieved so much!
I was very saddened in graduate school when my grandfather died. When I said something about it in my lab to a scientist from Shanghai I was working with, he asked how old my grandfather was. When I said he was 85, the wiser gentleman's smile lit up and he said, "Oh, you should be happy." And immediately I was! Of course I should be happy -- my grandfather had lived a long (and happy) life.
And so has Dawn. It has overcome problems not even imagined when we were designing and building it. It not only exceeded all of its original goals, but it has accomplished ambitious objectives not even conceived of until after it had experienced what could have been mission-ending failures. It has carried me, and uncounted others (including, I hope, you), on a truly amazing and exciting deep-space adventure with spectacular discoveries. Dawn is an extraordinary success by any measure.
It did not come easily. Dawn has consumed a tremendous amount of my life energy, many times at the expense of other desires and interests. (Perhaps ironically, it even comes at the expense of my many other deep interests in space exploration and in science, such as cosmology and particle physics, interests shared by my cats Quark and Lepton. Also, writing these Dawn Journals and doing my other outreach activities take up a very large fraction of what would otherwise be my personal time. As a result, I always write these in haste, and I'm never satisfied with them. That applies to this one as well. But I must rush ahead.) The challenges and the demands have been enormous, sometimes feeling insurmountable. That would not have been my preference, of course, yet it makes the endeavor's successful outcome that much more gratifying.
At the same time I have felt all the pressure, I have long been so overjoyed with the nature of the mission, I will miss it. There is nothing quite like controlling a spacecraft well over a thousand times farther than the Moon, farther even than the Sun. Silly, trite, perhaps even mawkish though it may seem, when spacecraft I have been responsible for have passed on the far side of the Sun, I have taken those opportunities to use that blinding signpost to experience some of the awe of the missions. I block the Sun with my hand and contemplate the significance, both to this particular big, starry-eyed kid and to humankind, of such an alignment. I -- we -- have a spacecraft on the far side of the Sun!
Every day I feel exhilarated knowing that, as my car's license plate frame proclaims, my other vehicle is in the main asteroid belt. It won't be the same when that vehicle is no longer operating.
But I will always have the memories, the thrills, the deep and powerful personal gratification. And I have good reason to believe they will persist, just as some prior space experiences still fill me with gratitude, pride, excitement and pure joy. (I also hope to have many more cool out-of-body experiences.)
And long after I'm gone and forgotten, Dawn’s successes will still be important. Its place in the annals of space exploration will be secure: a wealth of marvelous scientific discoveries, the first spacecraft to orbit an object in the asteroid belt, the first spacecraft to visit a dwarf planet (indeed, the first spacecraft to visit the first dwarf planet that was discovered), the first spacecraft to orbit a dwarf planet, the first spacecraft to orbit any two extraterrestrial destinations, and more.
For now, Dawn is continuing to operate beautifully (and you can read about it in subsequent Dawn Journals). The end of the mission, when it comes, will be bittersweet for me, a time to reflect and rejoice at how fantastically well it has gone, and a time to grieve that it is no more. I will have many powerful and conflicting feelings. Like Walt Whitman, I am large, I contain multitudes.
Thanks to Dawn, we now have Vesta and we now have Ceres. Soon, very soon, Dawn will be only a memory (save for those who visit Ceres and find it still in orbit) but the worlds it revealed will forever be a part of our intellectual universe, and the capabilities to explore the solar system that it advanced and devised will be applied to exciting new missions. And the experience of being intimately involved in this grand adventure will remain with me for as long as I am able to see the night sky and marvel at the mysteries of the universe that captivated me even as a starry-eyed child.
Dawn is 1,500 miles (2,400 kilometers) from Ceres. It is also 3.46 AU (322 million miles, or 518 million kilometers) from Earth, or 1,275 times as far as the Moon and 3.42 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 58 minutes to make the round trip.
Dr. Marc D. Rayman
10:00 pm PDT August 22, 2018
Dawn is going out on a high! Or maybe a low. Rapidly nearing the end of a unique decade-long interplanetary expedition, Dawn is taking phenomenal pictures of dwarf planet Ceres as it swoops closer to the ground than ever before. While the pictures are too new for compelling scientific conclusions to be reached, a clear consensus has already emerged: Wow!!!
Every 27 hours, the bold adventurer plunges from 2,500 miles (4,000 kilometers) down to just 22 miles (35 kilometers) above the alien world, accelerating to 1,050 mph (1,690 kph), and then shoots back up to do it all over again. (Try that, bungee jumpers!)
When Dawn dives low, it takes spectacular pictures, and you can see some of them here and more in the image gallery. But that's not all it does. The spacecraft also collects a trove of data on the nuclear radiation emanating from Ceres (which can reveal some of the atomic elements that are present), the gravity field (which can reveal the distribution of mass underground) and the infrared and visible light (which can reveal the minerals on the ground). Dawn has made all these kinds of measurements before, not only during more than three years at Ceres, the largest object in the main asteroid belt, but also during its 2011-2012 studies of Vesta, the second largest. But prior to this month, Dawn had never been this close and so never had such breathtaking sights and never been able to gather such high-resolution information.
We described the nature of this orbit in the three previous Dawn Journals. It is known as extended mission orbit 7 (XMO7) because Dawn's computer program for generating really cool and dramatic names was offline when it was time to come up with the name. Ever resourceful, the team activated the backup software that generates accurate but uninspiring names.
That kind of resourcefulness has served Dawn very well. Despite critical hardware failures that could have been disastrous for the mission, the flight team has accomplished success after success. The difficulty of flying so low -- only three times your altitude when you travel in a commercial jet -- and actually collecting useful data there seemed unachievable as recently as late last year. And now Dawn is doing it regularly.
Before XMO7, the spacecraft's lowest orbit around Ceres was 240 miles (385 kilometers), about the same height as the International Space Station is above Earth. Dawn spent eight months in 2015-2016 at that altitude, providing an exquisite view of the dwarf planet. It subsequently flew higher to pursue other scientific objectives.
Now Dawn is observing Ceres from as low as about 22 miles (35 kilometers). That tremendous reduction in altitude, a factor of 11, is the largest of the entire mission. At no other time at Vesta or Ceres did Dawn move in that much closer from its previous best vantage point. For those of you who enjoy the numbers, the table here has the distances for each of Dawn's observations of Ceres before the comprehensive mapping began, and this table shows the altitudes of the four mapping orbits of the prime mission, the last being the lowest. In those tables, we compared Dawn's view of Ceres to a view of a soccer ball. The low point of XMO7 would be like looking at a soccer ball from only one-third of an inch (eight millimeters) away. This is truly in-your-face exploration.
And the jump in resolution is amazing. With the fantastic new details, it seems we are seeing a whole new Ceres. A picture is worth a thousand words, but these pictures also merit a few exclamation points!!!
Dawn completed ion thrusting to XMO7 on June 6 and began its new observations of Ceres on schedule on June 9. Everywhere the spacecraft looked, it had fascinating new views. But the team had one special site in mind, and you might too. (Maybe it's even the same site.)
One of the bonus objectives was to try to get photos of Cerealia Facula, the mesmerizingly bright center of Occator Crater. We have explained why targeting a specific location is so hard. One of the attractive features of XMO7 was that it allowed two specially targeted attempts, thus increasing the chances that at least one would work. The team worked very hard to devise methods to take full advantage of that, while always quite well aware that it might not work.
Before we proceed, let's recall some terminology and introduce a new term. The high point in Dawn's orbit, 2,500 miles (4,000 kilometers), is known as apodemeter, analogous to the more common term apogee, which applies for orbits around Earth. (Demeter is the Greek counterpart of the Roman goddess Ceres.) The low point, 22 miles (35 kilometers), is peridemeter. Each 27.2-hour orbital revolution has one apodemeter and one peridemeter.
In April we discussed that Dawn travels much faster near peridemeter than near apodemeter, just as a swing moves faster at its low point than at its high point. As a fun fact, which does not bear on any of the following discussion, Dawn spends less than two hours over the dayside of Ceres (including peridemeter) and more than 25 hours over the nightside (including apodemeter). That may be surprising, but if you contemplate the illustrations of the elliptical XMO7 below and in March and think about the constantly changing velocity, it may make sense. (Or you may decide that it doesn't matter, accept it and move on.)
Mission planners had windows in the schedule for using the ion propulsion system to adjust the orbit. They instructed Dawn to fire its ion engine for 2 hours and 7 minutes on June 20 as the ship sailed upward. Fifteen hours later, on June 21, after it had crested in its orbit and was descending, it performed a second burn for 1 hour and 11 minutes.
The purpose of this pair of maneuvers was to bring Dawn's flight path at peridemeter right over Cerealia Facula. But the experienced explorers in mission control recognized that even with all their careful planning and Dawn's faithful execution of its assignments, there was a good chance the probe would not fly directly above that unique site as it sped northward. Therefore, they had also worked out plans to quickly determine how far east or west it would be at peridemeter and radio a (nearly) last minute adjustment in the angle it would point its sensors.
After the second segment of ion maneuvering, Dawn's orbit took it down to peridemeter again on June 21 for another intensive period of close-up observations. Even before it had time to finish radioing those findings to Earth the next day, the team began preparing for the next dive down. On June 22, they made their final calculations of the orbital path and predicted that Dawn would fly a little west of Cerealia Facula that night and a little east of it the next time around. That afternoon, they transmitted instructions to Dawn to aim its camera and spectrometers just a little to the right the first time and just a little to the left the second time. (Sophisticated and capable though Dawn is, the instructions controllers sent were a little more specific and quantitative than the descriptions here.)
The team would have considered their extensive efforts successful if the spacecraft had photographed part of Cerealia Facula once. (Dawn flies so close to the ground that it would be impossible to photograph all of Cerealia Facula on any one orbit; its camera's view is simply not wide enough.) As it turned out, Dawn managed to get pictures of Cerealia Facula on three consecutive orbits, each time seeing different parts, yielding far better coverage of this exotic landscape than we had even hoped for.
Flying to this incredibly low orbit, getting such a wealth of data and even managing to photograph a good portion of Cerealia Facula truly tested the very limits of the mission's capabilities. Dawn has surpassed all expectations, accomplishing feats not even considered when it was designed.
In order to prepare for the long shot of attempting to capture Cerealia Facula, Dawn rotated to point its main antenna to Earth relatively often, sometimes after each peridemeter or after two or three. That allowed the flight team to work more closely with the spacecraft. Then it would turn again to bring its sensors to bear on Ceres shortly before soaring through the next peridemeter. But all that turning costs Dawn hydrazine, the resource that limits its operational life to only another few months. (We outlined this situation last month and will delve into it more fully next month.) Now Dawn will observe Ceres on five consecutive orbits, filling its memory with data, and then spend almost two full days, including one peridemeter, transmitting that valuable information back to Earth. While its antenna is trained on Earth, the spacecraft cannot simultaneously direct its sensors at Ceres. That actually yields especially good gravity measurements, which use the Doppler shift of the radio signal, because the signal is much stronger with the main antenna than with one of the auxiliary antennas. Pictures and spectra, however, cannot be acquired on that one peridemeter in every six during which Dawn sends its results to Earth. The flight team determined that the benefit of turning less often and thus reducing hydrazine consumption yields the best scientific return. (This savings was already accounted for when we described the end of the mission as likely being between August and October.)
We saw in March that the latitude at which Dawn reaches peridemeter shifts south with every revolution. That is, the low point of each orbit is about 2° south of the one before. As a result, each time the spacecraft flies over Occator Crater now, it is higher than the previous time. Occator is at 20°N. Now the peridemeter is close to the equator, and soon Dawn's best views of Ceres will be in the region of Urvara Crater.
Firing ion engine #2 on June 21 accomplished more than the orbital adjustment that allowed the ship to spot Cerealia Facula at three consecutive peridemeters. It also completed the planned use of the ion propulsion system for the entire mission.
Dawn's ion engines have enabled this interplanetary spaceship to accomplish a journey unique in humankind's exploration of the solar system. After departing Earth with the help of a conventional rocket, Dawn used those engines to fly past Mars in 2009, to travel to Vesta and enter orbit in 2011, to maneuver extensively in orbit to optimize its observations there, to break out of orbit in 2012, to travel to Ceres and slip into orbit in 2015, and to perform even more maneuvering there than at Vesta. No other spacecraft has ever orbited two extraterrestrial destinations, and Dawn's mission to do so would have been impossible without ion propulsion.
We summarize the mission's ion thrusting on every Dawnniversary of launch, but since no further use is planned, we can give some final numbers here. Dawn thrust for a total of 2,141 days (5.9 years), or 55 percent of the time it has been in space (and 0.000000043 percent of the time since the Big Bang). The thrusting has achieved the equivalent of accelerating the probe by 25,700 mph (41,400 kilometers per hour). As we have often explained (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the Sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft's propulsive work. Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier Dawn Journal.)
The engines have done their job admirably, and now we have no further use for them. As a reminder, they are not needed for Dawn to stay in orbit around Ceres, just as the Moon doesn't need propulsion to stay in orbit around Earth and Earth doesn't need propulsion to say in orbit around the Sun. Next month we will discuss what will happen to Dawn's orbit after the mission ends.
When the ion engine was programmed to stop thrusting on June 21, some Dawn team members gathered in mission control to mark the occasion. Dawn was busy and was not communicating with Earth at the time. Even if it had been, a radio signal confirming the end of thrust would have taken almost 25 minutes to reach our planet. But the team decided to neglect the limitation of the speed of light and mark the moment (1:15:03 pm PDT) that the blue glow on the distant ship's engine would extinguish for the last time. And at that same moment, the blue lights in mission control were turned off for the last time as well.
It's natural to feel some sadness or loss now that the engines will not fire again. After all, ion propulsion is cool, especially for those of us who first heard of it in science fiction. It is even cooler for those who appreciate its tremendous capability and how valuable that is for deep-space missions. We can feel wistful, of course, but the last use of the ion engines was a direct result of their great success. After a truly stupendous interplanetary mission, we have Dawn right where we want it: in an orbit optimized for getting the last, best data at the endlessly fascinating dwarf planet it has unveiled. We can be grateful the ion engines allowed Dawn to explore two of the last uncharted worlds in the inner solar system and that they captivated our imagination as the distant spacecraft traveled through the solar system on a blue-green beam of xenon ions. Not too long ago, ion propulsion was mostly in the domain of science fiction. NASA's Deep Space 1 put it firmly into the realm of science fact. Building on DS1, Dawn has rocketed far beyond, accomplishing a space trek that would have been impossible without ion propulsion. Its mission was to boldly go where -- well, you know. And it has! Dawn's engines will never emit their cool blue glow again, but their legacy will not fade.
Dawn is 100 miles (160 kilometers) from Ceres (and headed for peridemeter). It is also 3.06 AU (284 million miles, or 457 million kilometers) from Earth, or 1,125 times as far as the Moon and 3.01 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
7:00 pm PDT June 30, 2018
Dawn has now logged 4 billion miles (6.4 billion kilometers) on its unique deep-space adventure. Sailing on a gentle breeze of xenon ions, the ambitious explorer journeyed for nearly four years to what had been only a small, fuzzy orb for over two centuries of terrestrial observations. Dawn spent more than a year there transforming it into a vast, complex protoplanet. Having sent its Vestan riches safely back to distant Earth, Dawn devoted another 2.5 years to reaching another blank canvas and there created another masterpiece of otherworldly beauty. Permanently in residence at dwarf planet Ceres, Dawn is now preparing to add some finishing touches.
The Dawn flight team at JPL did not even take notice as the odometer rolled over to 4,000,000,000. They have been focused on intensive investigations of how to maneuver the spaceship to lower altitudes than ever anticipated and operate there. For more than eight months in 2015-2016, Dawn circled 240 miles (385 kilometers) above the exotic Cerean landscape. From there, the team piloted the probe to higher orbits to undertake new studies, not anticipating that they might devise new methods to safely go much lower.
There are many challenges to overcome in flying closer to the dwarf planet, and although progress has been excellent, much more work lies ahead before maneuvering can begin. Indeed, even as some team members took time off in December, work never stopped. Many computers operated continuously, running sophisticated trajectory calculations. Engineers will assess the results when they return at the dawn of the new year and then set the computers to work on the next set of problems.
Meanwhile, Dawn waits patiently, safe and healthy in an orbit that ranges from a little more than 3,000 miles (4,800 kilometers) to nearly 24,000 miles (39,000 kilometers). It takes 30 days to complete one revolution. The spacecraft will continue operating in this elliptical orbit at least until April, the earliest opportunity to start its descent.
Having lost the use of the reaction wheels that controlled its orientation, Dawn now relies on hydrazine propellant fired from the small jets of its reaction control system. But after years of interplanetary travels and extensive maneuvering to observe Ceres, the remaining supply is very low. There simply is not enough left for a circular orbit lower than the one the spacecraft has already operated in. Dawn has plenty of xenon propellant to perform all the thrusting with its ion engine to change its orbit, but the available hydrazine is insufficient to perform all the necessary turns and to maintain a stable orientation for pointing its ion engine, solar arrays, antenna and sensors.
To fly low with a paucity of hydrazine, controllers are devising plans for an elliptical orbit. In the previous Dawn Journal, we saw that they might try to steer Dawn down to less than 125 miles (200 kilometers). While more work remains (including all those calculations that are occupying a cluster of computers), the progress has been encouraging. They are now analyzing orbits in which Dawn might even dive below 30 miles (50 kilometers) and then glide up to about 2,500 miles (4,000 kilometers) almost once a day. With many analyses still to perform and plans to refine, engineers anticipate that Dawn has enough hydrazine to maneuver to and operate in such an orbit for two months, and perhaps even a little longer.
If Dawn does go so low, it will be an exciting ride. How cool to skim so close to an alien world! But controllers must be careful that the spaceship doesn't dip too low. We have described before that Dawn complies with a set of protocols called planetary protection (not entirely unrelated to the Prime Directive). The team must ensure that the final orbit is stable enough that Dawn will not contaminate the astrobiologically interesting Ceres even for decades after the mission concludes.
The primary reason to plunge down so close to the mysterious landscapes of rock, ice and salt -- apart from pure awesomeness -- is to sense the nuclear radiation emanating from Ceres with greater clarity than ever before. With its gamma ray and neutron detector (GRaND), Dawn's measurements of this radiation provide insight into the atomic constituents down to about a yard (meter) underground. We have discussed this before in detail, including how the measurements work and why after operating so close to Ceres, Dawn flew to a higher orbit to improve its data.
The radiation is so faint, however, that some elements can only be detected from much closer range than Dawn has been. This is akin to looking at a very dim object or taking a picture of it. From far away, where little light reaches your eyes or your camera, colors are difficult to discern, so the view may be nearly black and white. But if you could move in close enough to capture much more light, you could see more colors. If Dawn can move in much closer to capture more of Ceres' nuclear glow, it may be able to see more of the elements of the periodic table -- in effect, taking a more colorful picture.
We see most objects by reflected light that originates either on the sun or artificial light sources. The nuclear radiation Dawn sees from Ceres is principally caused by cosmic rays. Cosmic rays are a form of radiation that fills space and originates far outside our solar system, mostly from supernovas elsewhere in the Milky Way Galaxy. The brighter the cosmic rays, the brighter Ceres will seem to be. The atoms on and underground don't reflect cosmic rays that strike them. Rather, the cosmic rays cause them to emit neutrons and gamma rays that escape back into space and carry with them the identities of the atoms. So, we can think of this as cosmic rays illuminating a scene, and Dawn will make nuclear photographs, revealing more details of Ceres' composition.
In addition to the advantage of going very low, it turns out that there is a special benefit to performing these measurements in 2018. The sun's magnetic field, which reaches out far beyond the planets, weakens cosmic rays entering our solar system, partially dimming the illumination. But our star's magnetism waxes and wanes in a cycle of 11 years. The sun now is entering the part of this regular cycle in which the magnetic field is weak. And it just so happens that this is an unusually weak solar cycle, so the sun's ability to hold cosmic rays at bay is less than at any time in the history of space exploration. Cosmic rays will be copious in the solar system. This won't matter much for people on or near Earth, because our planet's magnetic field (which extends well above where astronauts, cosmonauts and taikonauts work) resists most of the cosmic rays, and the thick blanket of atmosphere stops the rest. Ceres, like most residents of the solar system, does not have such protections. Thanks to the combination of the forecast of uniquely bright cosmic rays and the latest technology, 2018 will the best year so far in the history of solar system exploration to measure gamma rays or neutrons. Flying so close to the ground, Dawn should get superb readings.
In a future Dawn Journal we will discuss more of the specific objectives for the measurements and what they may reveal about Ceres, but now let's not forget about Dawn's other sensors. What about photography, infrared spectroscopy, visible spectroscopy, and gravity measurements?
We can look forward to some remarkable pictures. Some will be sharper than the best so far, but not by as much as you might expect. When it is in the low altitude segment of its orbit, Dawn will be moving faster than ever at Ceres. If you were in a plane traveling hundreds of miles (kilometers) per hour, it would not be hard to take a picture of the ground six miles (10 kilometers) beneath you. But if you were in a car driving at that speed or even faster, despite being closer to the ground, your pictures might not be better. (That wouldn't be the greatest of your worries, but the Dawn team is devoting a great deal of work to ensuring the ship's safety, as we'll discuss below.) The situation on Dawn isn't that severe, so the photography certainly will improve somewhat on what we already have.
Because the camera's field of view is so small and the hydrazine imposes such a stubborn limitation on Dawn's lifetime, we will see only a very small fraction of the dwarf planet's vast landscape with the improved clarity of low altitude.
In previous Dawn Journals (see, for example, this one), we have delved into details of how difficult it can be to predict the orbit with great accuracy. The dominant (but not exclusive) cause is that every time the hydrazine jets fire, whether to maintain a stable orientation or to turn (including to keep the sensors pointed at Ceres while Dawn swoops by in its elliptical orbit), they push the probe a little and so distort its orbit slightly. Predicting the subtleties of the changes in the spacecraft's orbit is a very complex problem. Although the outcome is not yet clear, the flight team is making progress in investigating methods to manage these orbital perturbations well enough to be able to have some control over where GRaND measures the atomic composition, because its gamma ray spectrometer and neutron spectrometer have broad views. They can tolerate the deviations in the orbit. But Dawn probably will not have the capability to capture any specific targets with its other spectrometers or cameras. Rather, controllers will take pictures of whatever terrain happens to be in view of the cameras. But on a world with as much fascinating diversity as Ceres, intriguing new details are likely to be discovered.
Along with studying the potential for improvements in pictures and spectra, the team is investigating refinements in Ceres' gravity field. They have already measured the gravity much more accurately than expected before Dawn arrived. Whether flying very close to some regions will allow them to improve their determination of the structure deep underground is the subject of ongoing work.
We will see in a Dawn Journal in a few months that the team will try to use certain properties of the orbit besides low altitude to provide attractive scientific opportunities. Nevertheless, it is clear that some goals simply will not be possible to achieve. To accomplish other objectives that are not feasible in that low ellipse, the team is analyzing the merits of pausing the ion-propelled spiral descent for a few weeks before reaching the final orbit. This could allow the spacecraft to view some regions of Ceres with the illumination of southern hemisphere summer, as we described in the previous Dawn Journal.
To ensure our distant ship remains ready to undertake extensive new observations, the infrared spectrometer, visible spectrometer, primary camera and backup camera each will be activated in January and run through their standard health checks and calibrations. For many of the observations in 2018, the two cameras will be used simultaneously to take as many pictures as possible, just as they were for special observations in 2017. Prior to this year, Dawn never used them concurrently.
With the help of a team of dedicated controllers, Dawn has shown itself to be a fantastically capable and resourceful explorer. Many new questions have to be answered and many challenges overcome for it to undertake another (and final) year in its bold expedition. But we can be hopeful that the creativity, ingenuity, and passion for knowledge and adventure that have propelled Dawn so very far already will soon allow it to add rich new details to what is already a celestial masterpiece.
Dawn is 17,200 miles (27,700 kilometers) from Ceres. It is also 1.77 AU (165 million miles, or 265 million kilometers) from Earth, or 705 times as far as the moon and 1.80 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 30 minutes to make the round trip.
Dr. Marc D. Rayman
4:30pm PST December 27, 2017
A decade after leaving its first home in the solar system, Dawn is healthy and successful at its current residence.
Even as the veteran explorer orbits high over dwarf planet Ceres and looks forward to continuing its mission, today it can reflect upon 10 exciting and productive years (or equivalently, with its present perspective, 2.17 exciting and productive Cerean years).
The ambitious adventurer embarked on an extraordinary extraterrestrial expedition on Sept. 27, 2007. With its advanced ion propulsion system, Dawn soared past Mars in 2009. The spacecraft took some of the Red Planet’s orbital energy around the sun to boost itself on its journey. (Nevertheless, this extra energy amounts to less than a quarter of what the ion engines have provided.) Ever a responsible citizen of the cosmos, Dawn fully adheres to the principle of the conservation of energy. So to compensate for speeding up, it slowed Mars down.
In 2011, the spacecraft arrived at Vesta, the second largest object in the main asteroid belt between Mars and Jupiter. Dawn gracefully entered into Vesta’s firm but gentle gravitational embrace. The probe maneuvered extensively in orbit, optimizing its views to get the best return possible from its photography and other observations. During 14 months in orbit, Dawn completed 1,298 revolutions around Vesta, taking nearly 31,000 pictures and collecting a wealth of other scientific measurements. From the perspective it had then, Dawn was in residence for nearly a third of a Vestan year (or almost 1,900 Vestan days). The explorer revealed a strange, ancient protoplanet, now recognized to be more closely related to the terrestrial planets (including the one Dawn left 10 years ago) than to the typical and smaller asteroids.
Unlike all other deep-space missions, Dawn had the capability to leave its first orbital destination and voyage to and enter orbit around another. After smoothly disengaging from Vesta, the interplanetary spaceship flew more than 900 million miles (1.5 billion kilometers) in 2.5 years to Ceres, the largest object in the asteroid belt. Indeed, prior to Dawn’s arrival, that dwarf planet was the largest body between the sun and dwarf planet Pluto that a spacecraft had not yet visited. And just as at Vesta, thanks to the maneuverability of ion propulsion, Dawn did not have to be content with a one-time flyby, gathering only as much data as possible during a brief encounter. By going into orbit around Ceres, the spacecraft could linger to scrutinize the exotic, alien world. And that is exactly what it has done.
Both Vesta and Ceres have held secrets since the dawn of the solar system, and both have beckoned since they were first spotted in telescopes at the dawn of the 19th century. For the next two centuries, they appeared as little more than faint smudges of light amidst myriad glittering stellar jewels, waiting for an inquisitive and admiring visitor from Earth. Finally, Dawn answered their cosmic invitations and eventually developed richly detailed, intimate portraits of each.
As the last stop on a unique interplanetary journey of discovery, Ceres has proven well worth the wait. Since arriving in March 2015 (more than half a Cerean year ago, or nearly 2,500 Cerean days ago), Dawn has completed 1,595 revolutions. It has beheld mysterious and fascinating landscapes and unveiled a complex world of rock, ice and salt, along with organic compounds and other intriguing constituents. The dwarf planet may have been covered by an ocean long ago, and there might even be liquid water underground now. The 57,000 pictures and numerous other measurements with the sophisticated sensors will keep scientists busy for many years (both terrestrial and Cerean).
By early 2016, during its ninth year in space, Dawn had accomplished so much that it exceeded all of the original objectives established for it by NASA before the ship set sail. Along the way, Dawn encountered and ultimately overcame many obstacles, including equipment failures that could well have sunk the mission. Against all odds and expectations, however, when its prime mission concluded in June 2016, the spacecraft was still healthy enough that NASA decided to extend the mission to learn still more about Ceres. Since then, Dawn has conducted many investigations that had never even been considered prior to last year. Now it has successfully achieved all of the extended mission objectives. And, once again defying predictions thanks to expert piloting by the flight team (and a small dose of good luck), Dawn still has some life left in it. Before the end of the year, NASA will formulate another new set of objectives that will take it to the end of its operational life.
Dawn has flown to many different orbital altitudes and orientations to examine Ceres. Now the probe is in an elliptical orbit, ranging from less than 3,200 miles (5,100 kilometers) up to 23,800 miles (38,300 kilometers). At these heights, it is measuring cosmic rays. Scientists mathematically remove the cosmic ray noise from Dawn’s 2015-2016 recordings of atomic elements from a low, tight orbit at only 240 miles (385 kilometers).
In its present orbit, Dawn can make these measurements to clarify Ceres’ nuclear signals while being very frugal with its precious hydrazine, which is so crucial because of the loss of three reaction wheels. (The small supply was not loaded onboard with the intention of compensating for failed reaction wheels.) When the hydrazine is expended, the mission will end. So this high elliptical orbit is a very good place to be while NASA and the Dawn project are determining how best to use the spacecraft in the future.
Meanwhile, this anniversary presents a convenient opportunity to look back on a remarkable spaceflight. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the tenth annual summary, reusing text from previous years with updates where appropriate. Readers who wish to investigate Dawn’s ambitious journey in detail may find it helpful to compare this material with the Dawn Journals from its first, second, third, fourth, fifth, sixth, seventh, eighth and ninth anniversaries.
In its 10 years of interplanetary travels, the spacecraft has thrust with its ion engines for a total of 2,109 days (5.8 years), or 58 percent of the time (and 0.000000042 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 908 pounds (412 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sept. 27, 2007. The spacecraft has used 69 of the 71 gallons (262 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space.
The thrusting since then has achieved the equivalent of accelerating the probe by 25,400 mph (40,900 kilometers per hour). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.) It is remarkable that Dawn’s ion propulsion system has provided nearly the same change in speed as the entire Delta rocket.
Since launch, our readers who have remained on or near Earth have completed 10 revolutions around the sun, covering 62.8 AU (5.8 billion miles, or 9.4 billion kilometers). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 42.4 AU (3.9 billion miles, or 6.3 billion kilometers). As it climbed away from the sun, up the solar system hill to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It had to go even slower to perform its graceful rendezvous with Ceres. In the 10 years since Dawn began its voyage, Vesta has traveled only 40.5 AU (3.8 billion miles, or 6.1 billion kilometers), and the even more sedate Ceres has gone 37.8 AU (3.5 billion miles, or 5.7 billion kilometers). (To develop a feeling for the relative speeds, you might reread this paragraph while paying attention to only one set of units, whether you choose AU, miles, or kilometers. Ignore the other two scales so you can focus on the differences in distance among Earth, Dawn, Vesta and Ceres over the 10 years. You will see that as the strength of the sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)
Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with even more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. (If you prefer not to skip it, click here.) In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.
Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family (including Earth, Dawn, Vesta and Ceres) follow their individual paths around the sun, they sometimes move closer and sometimes move farther from it.
In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth’s orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft may travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body’s orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and part of the arduousness of Dawn’s journey has been changing the inclination of its orbit, an energetically expensive task.)
Now we can see how Dawn has done by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts’ websites when their readership extends to one more elliptical galaxy than ours does.)
The table below shows what the orbit would have been if the spacecraft had terminated ion thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on Sept. 27, 2007, its orbit around the sun was exactly Earth’s orbit. After launch, it was in its own solar orbit.
from the Sun (AU)
from the Sun (AU)
|Dawn’s orbit on Sept. 27, 2007 (before launch)||0.98||1.02||0.0°|
|Dawn’s orbit on Sept. 27, 2007 (after launch)||1.00||1.62||0.6°|
|Dawn’s orbit on Sept. 27, 2008||1.21||1.68||1.4°|
|Dawn’s orbit on Sept. 27, 2009||1.42||1.87||6.2°|
|Dawn’s orbit on Sept. 27, 2010||1.89||2.13||6.8°|
|Dawn’s orbit on Sept. 27, 2011||2.15||2.57||7.1°|
|Dawn’s orbit on Sept. 27, 2012||2.17||2.57||7.3°|
|Dawn’s orbit on Sept. 27, 2013||2.44||2.98||8.7°|
|Dawn’s orbit on Sept. 27, 2014||2.46||3.02||9.8°|
|Dawn’s orbit on Sept. 27, 2015||2.56||2.98||10.6°|
|Dawn’s orbit on Sept. 27, 2016||2.56||2.98||10.6°|
|Dawn’s orbit on Sept. 27, 2017||2.56||2.98||10.6°|
For readers who are not overwhelmed by the number of numbers, investing the effort to study the table may help to demonstrate how Dawn patiently transformed its orbit during the course of its mission. Note that six years ago, the spacecraft’s path around the sun was exactly the same as Vesta’s. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to slip into orbit around Vesta. While simply flying by it would have been far easier, matching orbits with Vesta required the exceptional capability of the ion propulsion system. Without that technology, NASA’s Discovery Program would not have been able to afford a mission to explore the massive protoplanet in such detail. Dawn has long since gone well beyond that. Having discovered so many of Vesta’s secrets, the adventurer left it behind. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. From 2012 to 2015, the stalwart craft reshaped and tilted its orbit even more so that now it is identical to Ceres’. Once again, that was essential to accomplishing the intricate celestial choreography in which the behemoth reached out with its gravity and tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.
Even after a decade of daring space travel, flying in deep space atop a blue-green pillar of xenon ions, exploring two of the last uncharted worlds in the inner solar system, overcoming the loss of three reaction wheels, working hard to stretch its shrinking supply of hydrazine, Dawn is ready for more. And so is everyone who yearns for new knowledge, everyone who is curious about the cosmos, and everyone who is exhilarated by bold adventures into the unknown. More is to come. Dawn -- and all those who find the lure of space irresistible -- can look forward to whatever lies ahead for this unique mission.
Dawn is 16,600 miles (26,700 kilometers) from Ceres. It is also 2.92 AU (271 million miles, or 437 million kilometers) from Earth, or 1,080 times as far as the moon and 2.91 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 49 minutes to make the round trip.
Dr. Marc D. Rayman
4:34 am PDT September 27, 2017
Now in its third year of orbiting a distant dwarf planet, a spacecraft from Earth is as active as ever. Like a master artist, Dawn is working hard to add fine details to its stunning portrait of Ceres.
In this phase of its extended mission, the spacecraft’s top priority is to record space radiation (known as cosmic rays) in order to refine its earlier measurements of the atomic species down to about a yard (meter) underground. The data Dawn has been collecting are excellent.
As we explained in January, the ambitious mission has added a complex bonus to its plans. The team is piloting the ship through an intricate set of space maneuvers to dramatically shift its orbit around Ceres. They are now about halfway through, and it has been smooth sailing. Dawn is on course and on schedule. (If you happen to be one of the few readers for whom it isn’t second nature to plan how to change a spacecraft’s orbit around a dwarf planet by 90 degrees and then fly it under control of ion engine, last month’s Dawn Journal presents a few of the details that may not be obvious. And you can follow the adventurer’s orbital progress with the regular mission status updates.)
If all goes well, on April 29 the new orbit will take Dawn exactly between the sun and the famous bright region at the center of Occator Crater. Named Cerealia Facula, the area is composed largely of salts. (Based on infrared spectra, the strongest candidate for the primary constituent is sodium carbonate). The probe will be at an altitude of about 12,400 miles (20,000 kilometers), or more than 50 times higher than it was in 2016 when it captured its sharpest photos of Occator (as well as the rest of Ceres’ 1.1 million square miles, or 2.8 million square kilometers). But the objective of reaching a position at which the sun and Ceres are in opposite directions, a special alignment known as opposition, is not to take pictures that display more details to our eyes. In fact, however, the pictures will contain intriguing new details that are not readily discerned by visual inspection. Dawn will take pictures as it gets closer and closer to opposition, covering a range of angles. In each image, scientists will scrutinize the handful of pixels on Cerealia Facula to track how the brightness changes as Dawn’s vantage point changes.
We described the opposition surge, in which the reflected sunlight at opposition may be significantly brighter than it is in any other geometrical arrangement. A few degrees or even a fraction of a degree can make a large difference. But why is that? What is the underlying reason for the opposition surge? What can we learn by measuring it? And is the best cake better than the best candy?
Those are all interesting and important questions. We will address some of them here and leave the rest for your own thorough investigation.
There are at least three separate physical effects that may contribute to the opposition surge. One of them is known as shadow hiding. When the sun shines on the ground, tiny irregularities in the surface, even at the microscopic level, will cast shadows. When you look at the ground, those shadows collectively detract from its overall brightness, even if each individual shadow is too small for you to see. The total amount of light reflected off the ground and into your eyes (or your camera) is less than it would be if every point, no matter how small, were well lit. However, if you look along the same direction as the incoming light, then all the shadows will be hidden. They will all be on the opposite side of those tiny irregularities, out of reach of both the incident light and your sight. In that case, anything you can see will be illuminated, and the scene will be brighter. The figure below is intended to illustrate this phenomenon of shadow hiding (and excluding the caption, the picture is probably worth almost 480 words).
The opposition surge was first described scientifically in 1887 by Hugo von Seeliger, an accomplished astronomer and highly esteemed teacher of astronomers. He analyzed data collected by Gustav Müller when Earth’s and Saturn’s orbits around the sun brought Saturn into opposition, and the brightness of the rings increased unexpectedly. Seeliger realized that shadow hiding among the myriad particles in the rings could explain Müller’s observations. The opposition surge is occasionally known as the Seeliger effect. (Although astronomers had been observing the rings for more than two centuries by then, a careful scientific analysis to show that the rings were not solid but rather composed of many small particles had only been completed about 30 years before Seeliger’s advance.)
Now astronomers recognize the opposition surge on many solar system bodies, including Earth’s moon and the moons of other planets, as well as Mars and asteroids. In fact, it also occurs on many materials on Earth, including vegetation. Scientists exploit the phenomenon to determine the character of materials at a distance when they can make careful measurements at opposition.
For many solar system objects, however, it is difficult or impossible to position the observer along the line between the sun and the target. But thanks to the extraordinary maneuverability provided by Dawn’s ion engine, we may be able to perform the desired measurement in Occator Crater.
It was nearly a century after Seeliger’s description of shadow hiding before scientists realized that there is another contributor to the opposition surge, which we mention only briefly here. It depends on the principle of constructive interference, which applies more in physics than in politics. Waves (in this case, light waves) that have their crests at the same places can add up to be especially strong (which makes the light bright). (Destructive interference, which may be more evident outside of the physics realm, occurs when troughs of one wave cancel crests of another.) We will not delve into why constructive interference tends to occur at opposition, but anyone with a thorough understanding of classical electromagnetic theory can work it out, as physicists did in the 1960s to 1980s. (More properly, it should be formulated not classically but quantum mechanically, but we recognize that some readers will prefer the former methodology because it is, as one physicist described it in 1968, "much simpler and more satisfying to the physical intuition." So, why make it hard?) For convenient use to ruin parties, the most common term for constructive interference in the opposition surge is coherent backscatter, but it sometimes goes by the other comparably self-explanatory terms weak photon localization and time reversal symmetry. Regardless of the name, as the light waves interact with the material they are illuminating at opposition, constructive interference can produce a surge in brightness.
The intensity of the opposition surge depends on the details of the material reflecting the light. Even the relative contributions of shadow hiding and coherent backscatter depend on the properties of the materials. (While both cause the reflected light to grow stronger as the angle to opposition shrinks, coherent backscatter tends to dominate at the very smallest angles.)
Especially sensitive laboratory measurements show that sometimes shadow hiding and coherent backscatter together are not sufficient to explain the result, so there must be even more to the opposition surge. The unique capability of science to explain the natural world, shown over and over and over again during the last half millennium, provides confidence that a detailed theoretical understanding eventually will be attained.
Part of science’s success derives from its combination of experiment and theory. For now, however, the opposition surge is more in the domain of the former than the latter. In other words, translating any opposition surge observation into a useful description of the properties of the reflecting material requires controlled laboratory measurements of well characterized materials. They provide the basis for interpreting the observation.
If Dawn accomplishes the tricky measurements (which we will describe next month), scientists will compare the Cerealia Facula opposition surge with lab measurements of the opposition surge. As always in good science, to establish the details of the experiments, they will start by integrating the knowledge already available, including the tremendous trove of data Dawn has already collected -- spectra of neutrons, gamma rays, visible light and infrared light plus extensive color and stereo photography and gravity measurements. In the context of their understanding of physics, chemistry and geology throughout the solar system, scientists will determine not only the mixtures of chemicals to test but also the properties such as grain sizes and how densely packed the particles are. They will perform experiments then on many combinations of credible facular composition and properties. Comparing those results with Dawn’s findings, they will be able to elucidate more about what really is on the ground in that mesmerizing crater. For example, if they determine the salt crystals are small, that may mean that salty water had been on the ground and sublimated quickly in the vacuum of space. But if the salt came out of solution more slowly underground and was later pushed to the surface by other geological processes, the crystals would be larger.
It is an impressive demonstration of the power of science that we can navigate an interplanetary spaceship to a particular location high above the mysterious, lustrous landscape of a distant alien world and gain insight into some details that would be too fine for you to see even if you were standing on the ground. Using the best of science, Dawn is teasing every secret it can from a relict from the dawn of the solar system. On behalf of everyone who appreciates the majesty of the cosmos, our dedicated, virtuoso artist is adding exquisite touches to what is already a masterpiece.
Dawn is 31,400 miles (50,500 kilometers) from Ceres. It is also 3.48 AU (324 million miles, or 521 million kilometers) from Earth, or 1,430 times as far as the moon and 3.48 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 58 minutes to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PDT March 30, 2017
Dear Glutdawnous Readers,
The distant dwarf planet that Dawn is circling is full of mystery and yet growing ever more familiar.
Ceres, which only last year was hardly more than a fuzzy blob against the stars, is now a richly detailed world, and our portrait grows more elaborate every day. Having greatly surpassed all of its original objectives, the reliable explorer is gathering still more data from its unique vantage point. Everyone who hungers for new knowledge about the cosmos or for bold adventures far from Earth can share in the sumptuous feast Dawn has been serving.
One of the major objectives of the mission was to photograph 80 percent of Ceres' vast landscape with a resolution of 660 feet (200 meters) per pixel. That would provide 150 times the clarity of the powerful Hubble Space Telescope. Dawn has now photographed 99.8 percent with a resolution of 120 feet (35 meters) per pixel.
This example of Dawn's extraordinary productivity may appear to be the limit of what it could achieve. After all, the spaceship is orbiting at an altitude of only 240 miles (385 kilometers), closer to the ground than the International Space Station is to Earth, and it will never go lower for more pictures. But it is already doing more.
Since April 11, instead of photographing the scenery directly beneath it, Dawn has been aiming its camera to the left and forward as it orbits and Ceres rotates. By May 25, it will have mapped most of the globe from that angle. Then it will start all over once more, looking instead to the right and forward from May 27 through July 10. The different perspectives on the terrain make stereo views, which scientists can combine to bring out the full three dimensionality of the alien world. Dawn already accomplished this in its third mapping orbit from four times its current altitude, but now that it is seeing the sights from so much lower, the new topographical map will be even more accurate.
Dawn is also earning extra credit on its assignment to measure the energy of gamma rays and neutrons. We have discussed before how the gamma ray and neutron detector (GRaND) can reveal the atomic composition down to about a yard (meter) underground, and last month we saw initial findings about the distribution of hydrogen. However, Ceres' nuclear glow is very faint. Scientists already have three times as much GRaND data from this low altitude as they had required, and both spectrometers in the instrument will continue to collect data. In effect, Dawn is achieving a longer exposure, making its nuclear picture of Ceres brighter and sharper.
In December we explained how using the radio signal to track the probe's movements allows scientists to chart the gravity field and thereby learn about the interior of Ceres, revealing regions of higher and lower density. Once again, Dawn performed even better than expected and achieved the mission's planned accuracy in the third mapping orbit. Because the strength of the dwarf planet's gravitational tug depends on the distance, even finer measurements of how it varies from location to location are possible in this final orbit. Thanks to the continued smooth operation of the mission, scientists now have a gravitational map fully twice as accurate as they had anticipated. With additional measurements, they may be able to squeeze out a little more detail, perhaps improving it by another 20 percent before reaching the method's limit.
Dawn has dramatically overachieved in acquiring spectra at both visible and infrared wavelengths. We have previously delved into how these measurements reveal the minerals on the ground and what some of the interesting discoveries are. Having already acquired more than seven times as many visible spectra and 21 times as many infrared spectra as originally called for, the spacecraft is adding to its riches with additional measurements. We saw in January that VIR has such a narrow view that it will never see all of Ceres from this close, so it is programmed to observe features that have caught scientists' interest based on the broad coverage from higher altitudes.
Dawn's remarkable success at Ceres was not a foregone conclusion. Of course, the flight team has confronted the familiar challenges people encounter every day in the normal routine of piloting an ion-propelled spaceship on a multibillion-mile (multibillion-kilometer) interplanetary journey to orbit and explore two uncharted worlds. But the mission was further complicated by the loss of two of the spacecraft's four reaction wheels, as we have recounted before. (In full disclosure, the devices aren’t actually lost. We know precisely where they are. But given that one stopped functioning in 2010 and the other in 2012, they might as well be elsewhere in the universe; they don’t do Dawn any good.) Without three of these units to control its orientation in space, the robot has relied on its limited supply of hydrazine, which was not intended to serve this function. But the mission's careful stewardship of the precious propellant has continued to exceed even the optimistic predictions, allowing Dawn good prospects for carrying on its fruitful work. In an upcoming Dawn Journal, we will discuss how the last of the dwindling supply of hydrazine may be used for further discoveries.
In the meantime, Dawn is continuing its intensive campaign to reveal the dwarf planet's secrets, and as it does so, it is passing several milestones. The adventurer has now been held in Ceres' tender but firm gravitational embrace longer than it was in orbit around Vesta. (Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, and its mission would have been impossible without ion propulsion.) The spacecraft provided us with about 31,000 pictures of Vesta, and it has now acquired the same number of Ceres.
For an interplanetary traveler, terrestrial days have little meaning. They are merely a memory of how long a faraway planet takes to turn on its axis. Dawn left that planet long ago, and as one of Earth's ambassadors to the cosmos, it is an inhabitant of deep space. But for those who keep track of its progress yet are still tied to Earth, on May 3 the journey will be pi thousand days long. (And for our nerdier friends and selves, it will be shortly after 6:47 p.m. PDT.)
By any measure, Dawn has already accomplished an extraordinary mission, and there is more to look forward to as its ambitious expedition continues.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.73 AU (346 million miles, or 558 million kilometers) from Earth, or 1,455 times as far as the moon and 3.70 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and two minutes to make the round trip.
Dear Resplendawnt Readers,
One year after taking up its new residence in the solar system, Dawn is continuing to witness extraordinary sights on dwarf planet Ceres. The indefatigable explorer is carrying out its intensive campaign of exploration from a tight orbit, circling its gravitational master at an altitude of only 240 miles (385 kilometers).
Even as we marvel at intriguing pictures and other discoveries, scientists are still in the early stages of putting together the pieces of the big puzzle of how (and where) Ceres formed, what its subsequent history has been, what geological processes are still occurring on this alien world and what all that reveals about the solar system.
For many readers who have not visited Ceres on their own, Occator Crater is the most mysterious and captivating feature. (To resolve the mystery of how to pronounce it, listen to the animation below.) As Dawn peered ahead at its destination in the beginning of 2015, the interplanetary traveler observed what appeared to be a bright spot, a shining beacon guiding the way for a ship sailing on the celestial seas. With its mesmerizing glow, the uncharted world beckoned, and Dawn answered the cosmic invitation by venturing in for a closer look, entering into Ceres' gravitational embrace. The latest pictures are one thousand times sharper than those early views. What was not so long ago a single bright spot has now come into focus as a complex distribution of reflective material in a 57-mile (92-kilometer) crater.
Scientists are still working on refining their understanding of this striking region. As we described in December, it seems that following the powerful impact that excavated Occator Crater, underground briny water reached the surface. The detailed photographs show many fractures cutting across the bright areas, and perhaps they provided a conduit. Water, whether as liquid or ice, would not last long there in the cold vacuum, eventually subliming. When the water molecules disperse, either escaping from Ceres into space or falling back to settle elsewhere, the dissolved salts are left behind. This reflective residue covers the ground, making the spellbinding and beautiful display Dawn now reveals.
While the crater is estimated to be a geological youngster at 80 million years old, that is an extremely long time for the material to remain so reflective. Exposed for so long to cosmic radiation and pelting from the rain of debris from space, it should have darkened. Scientists don't know (yet) what physical process are responsible, but perhaps it was replenished long after the crater itself formed, with more water, carrying dissolved salts, finding its way to the surface. As their analyses of the photos and spectra continue, scientists will gain a clearer picture and be able to answer this and other questions.
These latest Occator pictures did not come easily. Orbiting so close to Ceres, the adventurer’s camera captures only a small scene at a time, and it is challenging to cover the entirety of the expansive terrain. (Perhaps it comes as a surprise to those who have not read at least a few of the 123 Dawn Journals that precede this one that operating a spacecraft closer to a faraway dwarf planet than the International Space Station is to Earth is not as easy as, say, thinking about it.) But the patience and persistence in photographing the exotic landscapes have paid off handsomely.
We now have high resolution pictures of essentially all of Ceres save the small area around the south pole cloaked in the deep dark of a long winter night. Seasons last longer on Ceres than on Earth, and Dawn may not operate there long enough for the sun to rise at the south pole. By the beginning of southern hemisphere spring in November 2016, Dawn's mission to explore the first dwarf planet discovered may have come to its end.
In addition to photographing Ceres, Dawn conducts many other scientific observations, as we described in December and January. Among the probe's objectives at Ceres is to provide information for scientists to understand how much water is there, where it is, what form it is in and what role it plays in the geology.
We saw that extensive measurements of the faint nuclear radiation can help identify the atomic constituents. While the analysis of the data is complicated, and much more needs to be done, a picture is beginning to emerge from Dawn's neutron spectrometer (part of the gamma ray and neutron detector, GRaND). These subatomic particles are emitted from the nuclei of atoms buried within about a yard (meter) of the surface. Some manage to penetrate the material above them and fly into space, and the helpful ones then meet their fate upon hitting GRaND in orbit above. (Most others, however, will continue to fly through interplanetary space, decaying into a trio of other subatomic particles in less than an hour.) Before it escapes from the ground, a neutron's energy (and, equivalently, its speed) is strongly affected by any encounters with the nuclei of hydrogen atoms (although other atomic interactions can change the energy too). Therefore, the neutron energies can indicate to scientists the abundance of hydrogen. Among the most common forms in which hydrogen is found is water (composed of two hydrogen atoms and one oxygen atom), which can occur as ice or tied up in hydrated minerals.
GRaND shows Ceres is rich in hydrogen. Moreover, it detects more neutrons in an important energy range near the equator than near the poles, likely indicating there is more hydrogen, and hence more (frozen) water, in the ground at the high latitudes. Although Ceres is farther from the sun than Earth, and you would not consider it balmy there, it still receives some warmth. Just as at Earth, the sun's heating is less effective closer to the poles than at low latitudes, so this distribution of ice in the ground may reflect the temperature differences. Where it is warmer, ice close to the surface would have sublimed more quickly, thus depleting the inventory compared to the cooler ground far to the north or south.
Dawn spends most of its time measuring neutrons (and gamma rays), so it is providing a great deal of new data. And as scientists conduct additional analyses, they will learn more about the ice and other materials beneath the surface.
Another spectrometer is providing more tantalizing clues about the composition of Ceres, which is seen to vary widely. As the dwarf planet is not simply a huge rock but is a geologically active world, it is no surprise that it is not homogeneous. We discussed in December that the infrared mapping spectrometer had shown that minerals known as phyllosilicates are common on Ceres. Further studies of the data show evidence for the presence of two types: ammoniated phyllosilicates (described in December) and magnesium phyllosilicates. Scientists also find evidence of compounds known as carbonates, minerals that contain carbon and oxygen. There is also a dark substance in the mix that has not been identified yet.
And in one place (so far) on Ceres, this spectrometer has directly observed water, not below the surface but on the ground. The infrared signature shows up in a small crater named Oxo. (For the pronunciation, listen to the animation below.) As with the neutron spectra, it is too soon to know whether the water is in the form of ice or is chemically bound up in minerals.
At six miles (10 kilometers) in diameter, Oxo is small in comparison to the largest craters on Ceres, which are more than 25 times wider. (While geologists consider it a small crater, you might not agree if it formed in your backyard. Also note that when we showed Oxo Crater before, the diameter was slightly different. The crater's size has not changed since then, but as we receive sharper pictures, our measurements of feature sizes do change.) Dawn's first orbital destination, the fascinating protoplanet Vesta, is smaller than Ceres and yet has two craters far broader than the largest on Ceres. Based on studies of craters observed throughout the solar system, scientists have established methods of calculating the number and sizes of craters that could be formed on planetary surfaces. Those techniques show that Ceres is deficient in large craters. That is, more should have formed than appear in Dawn's pictures. Many other bodies (including Vesta and the moon) seem to preserve their craters for much longer, so this may be a clue about internal geological processes on Ceres that gradually erase the large craters.
Scientists are still in the initial stages of digesting and absorbing the tremendous wealth of data Dawn has been sending to Earth. The benefit of lingering in orbit (enabled by the remarkable ion propulsion system), rather than being limited to a brief glimpse during a fast flyby, is that the explorer can undertake much more thorough studies, and Dawn is continuing to make new measurements.
As recently as one year ago, controllers (and this writer) had great concern about the spacecraft's longevity given the loss of two reaction wheels, which are used for controlling the ship's orientation. And in 2014, when the flight team worked out the intricate instructions Dawn would follow in this fourth and final mapping orbit, they planned for three months of operation. That was deemed to be more than enough, because Dawn only needed half that time to accomplish the necessary measurements. Experienced spacecraft controllers recognize that there are myriad ways beautiful plans could go awry, so they planned for more time in order to ensure that the objectives would be met even if anomalies occurred. They also were keenly aware that the mission could very well conclude after three months of low altitude operations, with Dawn using up the last of its hydrazine. But their efforts since then to conserve hydrazine proved very effective. In addition, the two remaining wheels have been operating well since they were powered on in December, further reducing the consumption of the precious propellant.
As it turned out, operations have been virtually flawless in this orbit, and the first three months yielded a tremendous bounty, even including some new measurements that had not been part of the original plans. And because the entire mission at Ceres has gone so well, Dawn has not expended as much hydrazine as anticipated.
Dawn is now performing measurements that were not envisioned long in advance but rather developed only in the past two months, when it was apparent that the expedition could continue. And since March 19, Dawn has been following a new strategy to use even less hydrazine. Instead of pointing its sensors straight down at the scenery passing beneath it as the spacecraft orbits and Ceres rotates, the probe looks a little to the left. The angle is only five degrees (equal to the angle the minute hand of a clock moves in only 50 seconds, or less than the interval between adjacent minute tick marks), but that is enough to decrease the use of hydrazine and thus extend the spacecraft's lifetime. (We won't delve into the reason here. But for fellow nerds, it has to do with the alignment of the axes of the operable reaction wheels with the plane in which Dawn rotates to keep its instruments pointed at Ceres and its solar arrays pointed at the sun. The hydrazine saving depends on the wheels' ability to store angular momentum and applies only in hybrid control, not in pure hydrazine control. Have fun figuring out the details. We did!)
The angle is small enough now that the pictures will not look substantially different, but they will provide data that will help determine the topography. (Measurements of gravity and the neutron, gamma ray and infrared spectra are insensitive to this angle.) Dawn took pictures at a variety of angles during the third mapping orbit at Ceres (and in two of the mapping orbits at Vesta, HAMO1 and HAMO2) in order to get stereo views for topography. That worked exceedingly well, and photos from this lower altitude will allow an even finer determination of the three dimensional character of the landscape in selected regions. Beginning on April 11, Dawn will look at a new angle to gain still another perspective. That will actually increase the rate of hydrazine expenditure, but the savings now help make that more affordable. Besides, this is a mission of exploration and discovery, not a mission of hydrazine conservation. We save hydrazine when we can in order to spend it when we need it. Dawn's charge is to use the hydrazine to accomplish important scientific objectives and to pursue bold, exciting goals that lift our spirits and fuel our passion for knowledge and adventure. And that is exactly what it is has done and what it will continue to do.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.90 AU (362 million miles, or 583 million kilometers) from Earth, or 1,505 times as far as the moon and 3.90 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and five minutes to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PDT March 31, 2016
Dawn is now performing the final act of its remarkable celestial choreography, held close in Ceres’ firm gravitational embrace. The distant explorer is developing humankind’s most intimate portrait ever of a dwarf planet, and it likely will be a long, long time before the level of detail is surpassed.
The spacecraft is concluding an outstandingly successful year 1,500 times nearer to Ceres than it began. More important, it is more than 1.4 million times closer to Ceres than Earth is today. From its uniquely favorable vantage point, Dawn can relay to us spectacular views that would otherwise be unattainable. At an average altitude of only 240 miles (385 kilometers), the spacecraft is closer to Ceres than the International Space Station is to Earth. From that tight orbit, the dwarf planet looks the same size as a soccer ball seen from only 3.5 inches (9.0 centimeters) away. This is in-your-face exploration.
The spacecraft has returned more than 16,000 pictures of Ceres this year (including more than 2,000 since descending to its low orbit this month). One of your correspondent’s favorites (below) was taken on Dec. 10 when Dawn was verifying the condition of its backup camera. Not only did the camera pass its tests, but it yielded a wonderful, dramatic view not far from the south pole. It is southern hemisphere winter on Ceres now, with the sun north of the equator. From the perspective of the photographed location, the sun is near the horizon, creating the long shadows that add depth and character to the scene. And usually in close-in orbits, we look nearly straight down. Unlike such overhead pictures typical of planetary spacecraft (including Dawn), this view is mostly forward and shows a richly detailed landscape ahead, one you can imagine being in — a real place, albeit an exotic one. This may be like the breathtaking panorama you could enjoy with your face pressed to the porthole of your spaceship as you are approaching your landing sight. You are right there. It looks — it feels! — so real and physical. You might actually plan a hike across some of the terrain. And it may be that a visiting explorer or even a colonist someday will have this same view before setting off on a trek through the Cerean countryside.
Of course, Dawn's objectives include much more than taking incredibly neat pictures, a task at which it excels. It is designed to collect scientifically meaningful photos and other valuable measurements. We'll see more below about what some of the images and spectra from higher altitudes have revealed about Ceres, but first let's take a look at the three highest priority investigations Dawn is conducting now in its final orbit, sometimes known as the low altitude mapping orbit (LAMO). While the camera, visible mapping spectrometer and infrared mapping spectrometer show the surface, these other measurements probe beneath.
With the spacecraft this close to the ground, it can measure two kinds of nuclear radiation that come from as much as a yard (meter) deep. The radiation carries the signatures of the atoms there, allowing scientists to inventory some of the key chemical elements of geological interest. One component of this radiation is gamma ray photons, a high energy form of electromagnetic radiation with a frequency beyond visible light, beyond ultraviolet, even beyond X-rays. Neutrons in the radiation are entirely different from gamma rays. They are particles usually found in the nuclei of atoms (for those of you who happen to look there). Indeed, outweighing protons, and outnumbering them in most kinds of atoms, they constitute most of the mass of atoms other than hydrogen in Ceres (and everywhere else in the universe, including in your correspondent).
To tell us what members of the periodic table of the elements are present, Dawn's gamma ray and neutron detector (GRaND) does more than detect those two kinds of radiation. Despite its name, GRaND is not at all pretentious, but its capabilities are quite impressive. Consisting of 21 sensors, the device measures the energy of each gamma ray photon and of each neutron. (That doesn't lend itself to as engaging an acronym.) It is these gamma ray spectra and neutron spectra that reveal the identities of the atomic species in the ground.
Some of the gamma rays are produced by radioactive elements, but most of them and the neutrons are generated as byproducts of cosmic rays impinging on Ceres. Space is pervaded by cosmic radiation, composed of a variety of subatomic particles that originate outside our solar system. Earth's atmosphere and magnetic field protect the surface (and those who dwell there) from cosmic rays, but Ceres lacks such defenses. The cosmic rays interact with nuclei of atoms, and some of the gamma rays and neutrons that are released escape back into space where they are intercepted by GRaND on the orbiting Dawn.
Unlike the relatively bright light reflected from Ceres's surface that the camera, infrared spectrometer and visible spectrometer record, the radiation GRaND measures is very faint. Just as a picture of a dim object requires a longer exposure than for a bright subject, GRaND's "pictures" of Ceres require very long exposures, lasting weeks, but mission planners have provided Dawn with the necessary time. Because the equivalent of the illumination for the gamma ray and neutron pictures is cosmic rays, not sunlight, regions in darkness are no fainter than those illuminated by the sun. GRaND works on both the day side and the night side of Ceres.
In addition to the gamma ray spectra and neutron spectra, Dawn's other top priority now is measuring Ceres' gravity field. The results will help scientists infer the interior structure of the dwarf planet. The measurements made in the higher altitude orbits turned out to be even more accurate than the team had expected, but now that the probe is as close to Ceres as it will ever go, and so the gravitational pull is the strongest, they can obtain still better measurements.
Gravity is one of four fundamental forces in nature, and its extreme weakness is one of the fascinating mysteries of how the universe works. It feels strong to us (well, most of us) because we don't so easily sense the two kinds of nuclear forces, both of which extend only over extremely short distances, and we generally don't recognize the electromagnetic force. With both positive and negative electrical charges, attractive and repulsive electromagnetic forces often cancel. Not so with gravity. All matter exerts attractive gravity, and it can all add up. The reason gravity -- by far the weakest of the four forces -- is so salient for those of you on or near Earth is that there is such a vast amount of matter in the planet and it all pulls together to hold you down. Dawn overcame that pull with its powerful Delta rocket. Now the principal gravitational force acting on it is the cumulative effect of all the matter in Ceres, and that is what determines its orbital motion.
The spacecraft experiences a changing force both as the inhomogeneous dwarf planet beneath it rotates on its axis and as the craft circles that massive orb. When Dawn is closer to locations within Ceres with greater density (i.e., more matter), the ship feels a stronger tug, and when it is near regions with lower density, and hence less powerful gravity, the attraction is weaker. The spacecraft accelerates and decelerates very slightly as its orbit carries it closer to and farther from the volumes of different density. By carefully and systematically plotting the exquisitely small variations in the probe's motion, navigators can calculate how the mass is distributed inside Ceres, essentially creating an interior map. This technique allowed scientists to establish that Vesta, the protoplanet Dawn explored in 2011-2012, has a dense core (composed principally of iron and nickel) surrounded by a less dense mantle and crust. (That is one of the reasons scientists now consider Vesta to be more closely related to Earth and the other terrestrial planets than to typical asteroids.)
Mapping the orbit requires systems both on Dawn and on Earth. Using the large and exquisitely sensitive antennas of NASA's Deep Space Network (DSN), navigators measure tiny changes in the frequency, or pitch, of the spacecraft's radio signal, and that reveals changes in the craft's velocity. This technique relies on the Doppler effect, which is familiar to most terrestrial readers as they hear the pitch of a siren rise as it approaches and fall as it recedes. Other readers who more commonly travel at speeds closer to that of light recognize that the well-known blueshift and redshift are manifestations of the same principle, applied to light waves rather than sound waves. Even as Dawn orbits Ceres at 610 mph (980 kilometers per hour), engineers can detect changes in its speed of only one foot (0.3 meters) per hour, or one five-thousandth of a mph (one three-thousandth of a kilometer per hour). Another way to track the spacecraft is to measure the distance very accurately as it revolves around Ceres. The DSN times a radio signal that goes from Earth to Dawn and back. As you are reminded at the end of every Dawn Journal, those signals travel at the universal limit of the speed of light, which is known with exceptional accuracy. Combining the speed of light with the time allows the distance to be pinpointed. These measurements with Dawn's radio, along with other data, enable scientists to peer deep into the dwarf planet
Although it is not among the highest scientific priorities, the flight team is every bit as interested in the photography as you are. We are visual creatures, so photographs have a special appeal. They transport us to mysterious, faraway worlds more effectively than any propulsion system. Even as Dawn is bringing the alien surface into sharper focus now, the pictures taken in higher orbits have allowed scientists to gain new insights into this ancient world. Geologists have located more than 130 bright regions, none being more striking than the mesmerizing luster in Occator crater. The pictures taken in visible and infrared wavelengths have helped them determine that the highly reflective material is a kind of salt.
It is very difficult to pin down the specific composition with the measurements that have been analyzed so far. Scientists compare how reflective the scene is at different wavelengths with the reflective properties of likely candidate materials studied in laboratories. So far, magnesium sulfate yields the best match (although it is not definitive). That isn't the type of salt you normally put on your food (or if it is, I'll be wary about accepting the kind invitation to dine in your home), but it is very similar (albeit not identical) to Epsom salts, which have many other familiar uses.
Scientists' best explanation now for the deposits of salt is that when asteroids crash into Ceres, they excavate underground briny water-ice. Once on the surface and exposed to the vacuum of space, even in the freezing cold so far from the sun, the ice sublimes, the water molecules going directly from the solid ice to gas without an intermediate liquid stage. Left behind are the materials that had been dissolved in the water. The size and brightness of the different regions depend in part on how long ago the impact occurred. A very preliminary estimate is that Occator was formed by a powerful collision around 80 million years ago, which is relatively recent in geological times. (We will see in a future Dawn Journal how scientists estimate the age and why the pictures in this low altitude mapping orbit will help refine the value.)
As soon as Dawn's pictures of Ceres arrived early this year, many people referred to the bright regions as "white spots," although as we opined then, such a description was premature. The black and white pictures revealed nothing about the color, only the brightness. Now we know that most have a very slight blue tint. For reasons not yet clear, the central bright area of Occator is tinged with more red. Nevertheless, the coloration is subtle, and our eyes would register white.
Measurements with both finer wavelength discrimination and broader wavelength coverage in the infrared have revealed still more about the nature of Ceres. Scientists using data from one of the two spectrometers in the visible and infrared mapping spectrometer instrument (VIR) have found that a class of minerals known as phyllosilicates is common on Ceres. As with the magnesium sulfate, the identification is made by comparing Dawn's detailed spectral measurements with laboratory spectra of a great many different kinds of minerals. This technique is a mainstay of astronomy (with both spacecraft and telescopic observations) and has a solid foundation of research that dates to the nineteenth century, but given the tremendous variety of minerals that occur in nature, the results generally are neither absolutely conclusive nor extremely specific.
There are dozens of phyllosilicates on Earth (one well known group is mica). Ceres too likely contains a mixture of at least several. Other compounds are evident as well, but what is most striking is the signature of ammonia in the minerals. This chemical is manufactured extensively on Earth, but few industries have invested in production plants so far from their home offices. (Any corporations considering establishing Cerean chemical plants are invited to contact the Dawn project. Perhaps, however, mining would be a more appropriate first step in a long-term business plan.)
Ammonia's presence on Ceres is important. This simple molecule would have been common in the material swirling around the young sun almost 4.6 billion years ago when planets were forming. (Last year we discussed this period at the dawn of the solar system.) But at Ceres' present distance from the sun, it would have been too warm for ammonia to be caught up in the planet-forming process, just as it was even closer to the sun where Earth resides. There are at least two possible explanations for how Ceres acquired its large inventory of ammonia. One is that it formed much farther from the sun, perhaps even beyond Neptune, where conditions were cool enough for ammonia to condense. In that case, it could easily have incorporated ammonia. Subsequent gravitational jostling among the new residents of the solar system could have propelled Ceres into its present orbit between Mars and Jupiter. Another possibility is that Ceres formed closer to where it is now but that debris containing ammonia from the outer solar system drifted inward and some of it ultimately fell onto the dwarf planet. If enough made its way to Ceres, the ground would be covered with the chemical, just as VIR observed.
Scientists continue to analyze the thousands of photos and millions of infrared and visible spectra even as Dawn is now collecting more precious data. Next month, we will summarize the intricate plan that apportions time among pointing the spacecraft's sensors at Ceres to perform measurements, its main antenna at Earth to transmit its findings and receive new instructions and its ion engine in the direction needed to adjust its orbit.
The plans described last month for getting started in this fourth and final mapping orbit worked out extremely well. You can follow Dawn's activities with the status reports posted at least twice a week here. And you can see new pictures regularly in the Ceres image gallery.
We will be treated to many more marvelous sights on Ceres now that Dawn's pictures will display four times the detail of the views from its third mapping orbit. The mapping orbits are summarized in the following table, updated from what we have presented before. (This fourth orbit is listed here as beginning on Dec. 16. In fact, the highest priority work, which is obtaining the gamma ray spectra, neutron spectra and gravity measurements, began on Dec. 7, as explained last month. But Dec. 16 is when the spacecraft started its bonus campaign of measuring infrared spectra and taking pictures. Recognizing that what most readers care about is the photography, regardless of the scientific priorities, that is the date we use here.
|Mapping orbit||Dawn code name||Dates||Altitude in miles (kilometers)||Resolution in feet (meters) per pixel||Resolution compared to Hubble||Orbit period||Equivalent distance of a soccer ball|
|1||RC3||April 23 - May 9||8,400 (13,600)||4,200 (1,300)||24||15 days||10 feet (3.2 meters)|
|2||Survey||June 6-30||2,700 (4,400)||1,400 (410)||73||3.1 days||3.4 feet (1.0 meters)|
|3||HAMO||Aug 17 - Oct 23||915 (1,470)||450 (140)||217||19 hours||14 inches (34 cm)|
|4||LAMO||Dec 16 - end of mission||240 (385)||120 (35)||830||5.4 hours||3.5 inches (9.0 cm)|
Dawn is now well-positioned to make many more discoveries on the first dwarf planet discovered. Jan. 1 will be the 215th anniversary of Giuseppe Piazzi's first glimpse of that dot of light from his observatory in Sicily. Even to that experienced astronomer, Ceres looked like nothing other than a star, except that it moved a little bit from night to night like a planet, whereas the stars were stationary. (For more than a generation after, it was called a planet.) He could not imagine that more than two centuries later, humankind would dispatch a machine on a cosmic journey of more than seven years and three billion miles (five billion kilometers) to reach the distant, uncharted world he descried. Dawn can resolve details more than 60 thousand times finer than Piazzi's telescope would allow. Our knowledge, our capabilities, our reach and even our ambition all are far beyond what he could have conceived, and yet we can apply them to his discovery to learn more, not only about Ceres itself, but also about the dawn of the solar system.
On a personal note, I first saw Ceres through a telescope even smaller than Piazzi's when I was 12 years old. As a much less experienced observer of the stars than he was, and with the benefit of nearly two centuries of astronomical studies between us, I was thrilled! I knew that what I was seeing was the behemoth of the main asteroid belt. But it never occurred to me when I was only a starry-eyed youth that I would be lucky enough to follow up on Piazzi's discovery as a starry-eyed adult, responsible for humankind's first visitor to that fascinating alien world, answering a celestial invitation that was more than 200 years old.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.66 AU (340 million miles, or 547 million kilometers) from Earth, or 1,360 times as far as the moon and 3.72 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and one minute to make the round trip.