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):

1. Things that can’t go on forever, go on much longer than you think they will.

2. Corollary 1 applies even after taking into account Corollary 1.

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

TAGS: DAWN, CERES, DWARF PLANET, VESTA, ASTEROID, SPACE TRAVEL

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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.

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

TAGS: DAWN, CERES, DWARF PLANET, VESTA, ASTEROID, SPACE TRAVEL, ION PROPULSION, ELECTRIC PROPULSION

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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

TAGS: DAWN, CERES, VESTA, DWARF PLANET, ASTEROID BELT, ASTEROIDS, SPACECRAFT, SOLAR SYSTEM

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

When Dawn left Earth in 2007, it was outfitted with four reaction wheels, devices that were considered indispensable for controlling its orientation on its long expedition in deep space. Despite the failures of reaction wheels in 2010, 2012 and 2017, the team has accomplished an extremely successful mission, yielding riches at Vesta and at Ceres far beyond what had been anticipated when the interplanetary journey began. But now the rapidly dwindling supply of hydrazine propellant the robot uses in place of the reaction wheels is nearly exhausted.

With no friction to stabilize it, the large ship, with electricity-generating solar arrays stretching 65 feet (19.7 meters) wingtip-to-wingtip, holds its orientation in space by firing hydrazine propellant from the small jets of its reaction control system. The orientation should not be confused with the position. In the zero-gravity of spaceflight, they are quite independent. Unlike an aircraft, a spacecraft's position and the direction it travels are largely unrelated to its orientation. The probe's position is dictated by the principles of orbital motion, whether in orbit around the Sun, Vesta or (now) Ceres, and the ion propulsion system is used to change its trajectory. We are concerned here about orientation.

Dawn can hold its orientation quite stable, but it still lazily oscillates a little bit in pitch, roll and yaw. When the spacecraft points its main antenna to Earth, for example, spending many hours radioing its findings to the Deep Space Network (DSN) as it travels around Ceres, it rotates back and forth, but the angular motion is both tiny and slow. The ship turns about a thousand times slower than the hour hand on a clock. The clock hand continues its steady motion, going all the way around, rotating through a full circle in 12 hours. Dawn needs to keep its antenna pointed at Earth, however. If Dawn were at the center of the clock and Earth were at the 12, it wouldn't let the antenna point any farther away than the hour hand gets from the 12 in about a minute. The tiny angle is only about a tenth of the way from the 12 to the adjacent ticks (both on the left and on the right) that mark one second for the second hand. When Dawn's orientation approaches the maximum allowed angular deviation, the main computer instructs a jet to puff out a little hydrazine to reverse the motion.

When the spacecraft follows its elliptical orbit down to a low altitude, only three times higher than you are when you fly on a commercial jet, it needs to expel hydrazine to keep aiming its camera and spectrometers down as it rushes over the ground. If this isn't clear, try pointing your finger at an object and then circling around it. You are constantly changing the direction you're pointing. For Dawn to do that, especially in its elliptical orbit, requires hydrazine. (If you think Dawn could simply start rotating with hydrazine and then just point without using more, there are some subtleties here we will not describe. It really does require extensive hydrazine.)

Whether pointing at the landscape beneath it or at Earth, it might seem that Dawn could remain perfectly steady, but there are always tiny forces acting on it that would compromise its pointing. One is caused by the difference between Ceres' gravitational pull on the two ends of the solar arrays that occurs when the wings are not perfectly level. (We described this gravity gradient torque when Dawn was orbiting Vesta.) Also, sunlight reflecting in different ways from different components (some with polished, mirror-like surfaces, others with matte finishes) can exert a very small torque. Dawn uses hydrazine to counter these and other slight disturbances in its orientation.

As we have discussed extensively, very soon, the hydrazine will be depleted. Most likely between the middle of September and the middle of October (although possibly earlier or later), the computer will tell a reaction control jet to emit a small burst of hydrazine, as it has myriad times before in the mission, but the jet will not be able to do so. There won't be any usable hydrazine left. It will be like opening the end of a completely deflated balloon. No gas will escape. There will be no action, so there will be no reaction. Dawn's very slow angular motion will not be reversed but rather will continue, and the orientation will slowly move out of the tight bounds the ship normally maintains.

The computer will quickly recognize that the intended effect was not achieved. It will send more signals to the jet to fire, but the result will be no different. On a mission often operating out of radio contact with Earth and always very, very far away, help can never be immediate (after all, radio signals travel at the universal limit of the speed of light), so the robot is programmed to deal with problems on its own. There are several possibilities for what actions Dawn will take, depending on details we will not delve into, but a likely one is to try switching from the primary reaction control jets to the backup reaction control jets. Of course, that won't fix the problem, because the jets will not be at fault. In fact, with no hydrazine available, none of its attempts to correct the problem will succeed.

When Dawn experiences problems it can't resolve on its own, it invokes one of its safe modes, standard responses the craft uses when it encounters conditions its programming and logic cannot accommodate. (We have described the safe modes a number of times before, with perhaps the most exciting time being here.) In this case, the safe mode it will chose will go through many steps to reconfigure the spacecraft and prepare to wait for help from humans on a faraway planet (or anyone else who happens to lend assistance).

One of the first steps will be to temporarily power off the radio transmitter, one of the biggest consumers of electrical power on the ship. Until Dawn can make all of the necessary changes, including turning to point the solar panels at the Sun, it will not want to devote precious energy to unnecessary systems. Electrical power is vital. Without it, the spacecraft will be completely inoperative, just as your car, computer, smartphone or lights do nothing at all when they are deprived of power.

Dawn will try to do all its work using only the energy stored in its battery (which it keeps charged, using excess power from the solar arrays). It knows that later, once the arrays are in sunlight, it will have plenty of power, but in the meantime, it needs to be parsimonious. The computer, heaters, motors to rotate the solar arrays, and some other devices are essential to getting into safe mode. The radio is needed only after the spacecraft has completed other steps.

The spacecraft will not complete those other steps. One of them is to turn to point at the Sun, ensuring that the large solar arrays are fully illuminated. But without hydrazine, it will have no means to accomplish the necessary turn.

So, Dawn will not be able to achieve the planned orientation with the solar arrays generating electrical power. The computer will stubbornly refuse to turn on the radio, instead continuing to try to turn so the Sun will light up the arrays and infuse the robot with its electrical lifeblood.

Dawn will continue to try as long as it has power, whether flowing from partially lit solar arrays or from the battery. All the while, the spacecraft will continue to rotate at the same leisurely speed it did when it had hydrazine. But instead of gently oscillating back and forth, it will simply keep going in the same direction, like a clock's hour hand slowed down to measure months instead of hours.

Some of the time, the solar arrays will face away from the Sun and the battery will drain. Some of the time the solar arrays will point at (or near) the Sun just by luck. But Dawn doesn't rely on luck. Until it has a stable orientation with the arrays reliably on the Sun, the computer will insist that power not be devoted to the radio. First things first: first achieve a condition that can be safe for days, weeks, or even months, and then radio Earth for help. The programming did not anticipate being completely unable to control orientation.

Engineers have analyzed what will happen and observed many examples of it in the spacecraft simulator at JPL. Eventually, the computer may make some other attempts. But Dawn's struggle will be brief, lasting only hours before the battery is exhausted. The seasoned adventurer will sink into unconsciousness. At some later time, as its stately rotation brings the solar arrays back into the light, it may well begin to revive, but the cycle will repeat. The newly awakened Dawn will try to point at the Sun and hold that position, taking advantage of the power from the fortuitously illuminated solar arrays. But soon its continuing rotation will point the arrays into the dark of space again. It might seem that half the time the arrays would receive light and so it should be able to operate at half power, but it doesn't work that way. At Dawn's distance from the Sun, a little bit of that faint light on the solar arrays is not sufficient.

After an extraordinary extraterrestrial expedition, more than a decade of interplanetary travels, unveiling two of the last uncharted worlds in the inner solar system, performing unique and complex maneuvers, encountering and overcoming a host of unanticipated problems, Dawn will be on the losing end of a battle with the cold, hard reality of operation in deep space. Its mission will be over.

The spacecraft will be well over a million times farther from Earth than the International Space Station. How will we know when it has run out of hydrazine if its radio is off? (The reaction control system is expected to operate normally as long as there is usable hydrazine, so there will be no prior indication that its exhaustion is imminent.) 

Even as it goes about trying to fix or recover from problems, the computer issues some brief status reports. (They often are more informative than the dialog boxes that pop up on your computer, and Dawn never asks you to click on something to proceed.) If the loss of hydrazine happens to occur while Dawn is communicating with Earth, one of those concise reports may be received before the computer turns off the transmitter. The short message would be like a farewell tweet that Dawn is ending its mission.

Most of the time, however, the probe does not point its main antenna at Earth. When it zips down to low altitude, it aims its sensors at the ground, so the antenna is pointed in an arbitrary direction. Dawn transmits a very broad radio signal through one of its auxiliary antennas so scientists and engineers can follow its motion very precisely. (We have explained before that this allows them to determine the interior structure of the dwarf planet.) That radio connection is too weak for anything else, so Dawn won't be able to tweet its news. If the last of the hydrazine is spent when Dawn's orbital motion is being tracked, the radio signal will simply disappear.

In its elliptical orbit, Dawn spends far less time traveling fast at low altitude than it does traveling slowly at high altitude, much as the girl on a swing we encountered in April. And when it is high up, we generally do not have radio contact at all. So it is more likely that the hydrazine will be depleted when Dawn is out of touch than when the DSN is recording its radio transmissions, through either the main antenna or an auxiliary antenna. Then the next time one of the antennas of the DSN aims at Dawn's location in the sky, it will strain to hear the faint radio whisper of the faraway probe, but all will be silent.

Dawn controllers and the DSN will work together to be sure the inability to detect the spacecraft isn't some other problem, perhaps in mission control or in the tremendously complex DSN. Over the course of a few days, they will use more than one antenna and will take a few other measures. After all, there could be other reasons for a temporary loss of signal, and engineers will work through the possibilities. But given Dawn's resilience and sophistication, if it remains uncommunicative during that time, the conclusion will not be in doubt. Even without a tweet, it will be clear Dawn has run out of hydrazine and is at the end of its operational life.

After conducting a systematic investigation, when the Dawn project is confident of the situation, we will announce the result. In the next Dawn Journal, we will consider a more personal side of this story.

But what of Dawn's long-term fate? Remember, its orientation in space is largely independent of its orbital motion. The spacecraft's inability to point where it wants, to power its systems, and to communicate with its human handlers will have virtually no effect on where it goes.

Dawn doesn't need propulsion to stay in orbit around Ceres, just as the Moon doesn't need it to stay in orbit around Earth and Earth doesn't need it to stay in orbit around the Sun. And that's important. We do not want Dawn to come into contact any time soon with the dwarf planet it orbits.

Ceres is subject to planetary protection, a set of standards designed to ensure the integrity of possible future "biological exploration" of the alien world. That terminology does not mean there is biology on Ceres but rather that that exotic world is of interest in the field of astrobiology. Ceres was once covered with an ocean and today harbors a vast inventory of water (mostly as ice but perhaps with some liquid still present underground). It also has a supply of heat (retained even now, long after radioactive elements decayed and warmed the interior), organics and a rich variety of other chemicals. With all these ingredients, Ceres could experience some of the chemistry related to the development of life. Scientists do not want to contaminate that pristine environment with Dawn's terrestrial materials.

Not all solar system bodies need such protection. The Moon, Mercury and Venus, for example, have not been of interest for searches for life or for prebiotic chemistry. For that reason, spacecraft are allowed to land or crash on those worlds because there is no expectation of subsequent biological exploration. Also exempt from such rules are tiny asteroids, including two that are being explored this year, Ryugu and Bennu. They are entirely unlike giant Ceres. They are often mistakenly thought of as being similar because of the oversimplified notion that all are asteroids. We will provide an illustration of the dramatic difference in the next Dawn Journal.

The planetary protection rules for Ceres specify that Dawn not be allowed to contact it for at least 20 years. There is a common misconception that the time is needed to allow the spacecraft to be sterilized by the radiation, vacuum and temperature extremes of spaceflight. That's not the case. Many terrestrial microbes are impressively hardy, and there is good reason to believe that some that have taken an unplanned interplanetary cruise with Dawn would remain viable after much longer than 20 years.

The requirement for 20 years is intended to allow enough time for a follow-up mission, if deemed of sufficiently high priority given the many goals NASA has for exploring the solar system. Two decades should be long enough to mount a mission that builds on Dawn's many discoveries. We would not want such a hypothetical mission to be misled by finding microorganisms or nonbiological organic chemicals that were deposited by our spacecraft. As we'll see below, the deadline for another mission to get there before Dawn contaminates Ceres is likely to be significantly more relaxed even than that.

Earlier this year, when the team was figuring out how to fly to and operate in an orbit like the one Dawn is in now, much of their work was guided by this planetary protection requirement. We did not want to enter an orbit that would not meet the 20-year lifetime. We could not take the chance of going to an orbit with a shorter lifetime and plan for subsequent maneuvers to increase the duration. We were not sufficiently confident Dawn would have enough hydrazine to remain operable long enough to make its observations and still be able to change its orbit.

The team studied elliptical orbits with different minimum altitudes. Trajectory experts investigated the long-term behavior of each orbit as Ceres' irregular gravity field tugs on the spacecraft revolution after revolution, year after year. Like Earth, Ceres has some regions of higher density and some of lower density. As Dawn orbits over these different regions, they gradually distort the orbit. The analyses also accounted for the slight pressure of sunlight, which not only can rotate the spacecraft but also can push it in its orbit. An orbit with a minimum of 22 miles (35 kilometers) was the lowest that the team was confident would comply with planetary protection, and that's why Dawn is now in just such an orbit.

And after 20 years? Calculations show that even over 50 years, the orbital perturbations are overwhelmingly likely to be too small to cause Dawn to crash. In fact, there is less than a one percent chance of the orbit being distorted enough that Dawn would hit Ceres. In other words, our analysis gives us more than 99 percent confidence that even in half a century, Dawn will still be revolving around Ceres, the largest object between Mars and Jupiter, the only dwarf planet in the inner solar system and the first dwarf planet discovered (129 years before Pluto).

Leaving the remarkable craft in orbit around the distant colossus will be a fitting and honorable conclusion to its historic journey of discovery at Vesta and Ceres. Dawn's scientific legacy is secure, having revealed myriad fascinating and exciting insights into two quite dissimilar and mysterious alien worlds. This interplanetary ambassador from Earth will be 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 1,400 miles (2,300 kilometers) from Ceres. It is also 3.46 AU (321 million miles, or 517 million kilometers) from Earth, or 1,270 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
7:00 pm PDT August 21, 2018

TAGS: DAWN, CERES, DWARF PLANET, ASTEROID BELT, SPACECRAFT, ASTROBIOLOGY

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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

TAGS: DAWN, CERES, VESTA, DWARF PLANET, JOURNAL

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Almost once a day in its next orbit, Dawn will dive from 2,500 miles (4,000 kilometers) down to only 22 miles (35 kilometers), speeding above the ground at 1,050 mph (1,690 kph), and then shoot back up again. (Warning: Do not try this at home! Dawn is a trained professional.)

Before we (and Dawn) get to this new and final orbit, let's review the outstanding accomplishments this month. Dawn used its ion engine in April and May to descend to an orbit creatively known as extended mission orbit 6 (XMO6). (We showed the flight path last month and tracked the progress in mission status updates.) Ion thrusting concluded on schedule on May 14 when Dawn was in the targeted elliptical orbit, which ranged from 280 miles (450 kilometers) to 2,900 miles (4,700 kilometers).

Each of the 10 loops around Ceres took one and a half days, and Dawn successfully performed all of its planned observations. Every time Dawn flew northward over the sunlit hemisphere, the spacecraft used its cameras and other sensors to collect new data. During some orbits, as it flew southward over the hemisphere opposite the Sun, it turned to point its main antenna at faraway Earth and then radioed its findings to NASA's Deep Space Network. On other orbits, Dawn patiently continued looking down at Ceres. Of course, with the ground there hidden in the deep black of night on a moonless world, there was nothing to see, but by not turning, the spacecraft could conserve precious hydrazine for later in the mission. (Dawn used this strategy in most of the other phases at Ceres as well, starting with the third mapping orbit of the prime mission in 2015.) We will discuss more about hydrazine below.

As we saw in March's preview, Dawn's primary goal in XMO6 was to take advantage of it being summer in the southern hemisphere by making extensive observations in the far south. We also explained that XMO6 provided an opportunity for collecting new data (including higher resolution color pictures), providing new perspectives closer to the equator and in the northern hemisphere as well. Dawn spotted sites we have discussed before, including Ernutet Crater with deposits of organic materials, the smooth landscape around Ikapati Crater showing a history of flowing material, the volcano Ahuna Mons and other locations pictured above and below. Prior to three years ago, these places were all quite unknown (at least to Earthlings). In the intervening time, Dawn has studied many of them in exquisite detail, and at each one has discovered new questions to ask. XMO6 may provide new answers (and probably still newer questions.)

In addition to its normal photography and spectroscopy, the spacecraft took long exposure pictures to investigate areas that are in shadow throughout the Cerean year. We described before how water can be trapped in such locations, but when we last touched on this topic in December 2016 (along with a cool animation), we also mentioned that the seasons had precluded a good study in the southern hemisphere. XMO6 has helped rectify that, illustrating one benefit of being able to stay in orbit rather than catching whatever is to be seen during a fast flyby.

Dawn had one more assignment in XMO6. After the primary scientific observations were complete on the first, third, and tenth orbits, the spacecraft turned from pointing at the ground beneath it to the horizon. (The amount of hydrazine needed for a turn depends on the direction. In each case, mission controllers selected the most hydrazine-efficient direction.) As it turned, Dawn continued taking pictures. This showed terrain at new angles, contributing to the collection of stereo pictures taken in the third and fourth mapping orbits. But in this case, the scientific benefit, while real, was secondary. The primary objective was to get some cool new views of the limb of Ceres, including the one above. Loyal readers (and some others as well) may know that your correspondent finds such perspectives especially appealing, as described here (with other fine examples here, there and elsewhere). He decided the pure coolness of these XMO6 pictures would be reason enough to instruct Dawn to take them.

By the time Dawn completed XMO6, it had collected 1,800 new photos of Ceres in addition to a wealth of infrared spectra and visible spectra. As soon as its bounty was safely on Earth, the itinerant adventurer was ready for its next great challenge.

And now the blue lights are on again in mission control at JPL, as they were at the end of last month. The illumination is not designed to alter the circadian rhythm of the flight team but rather to provide a visual connection with the distant spacecraft as its ion engine emits a steady bluish glow. Dawn is now spiraling down, tightening its elliptical loops, getting lower and lower and lower. We described the previous descent last month, and you can see the current trajectory in the figure below.

Dawn will spend the rest of its operational life in the target orbit, XMO7, and most future Dawn Journals will be devoted to it. How long will that be? That's a good question (in contrast, perhaps, to all the absurd questions posed in previous Dawn Journals), but the answer is not easy.

We have discussed many times (here is a summary) that Dawn's lifetime is limited by its hydrazine, a conventional rocket propellant expelled from reaction control system thrusters to control its orientation in space. When that dwindling supply is exhausted, the robot will no longer be able to point its solar arrays at the Sun, its antenna at Earth, its sensors at Ceres or its ion engines in the direction needed to travel elsewhere. The mission will end, and the ship will become an inert celestial monument to the power of human ingenuity, creativity and curiosity, a lasting reminder orbiting one of the solar system worlds it unveiled that our passion for bold adventures and our noble aspirations to extend our reach into the universe can take us very, very far beyond the confines of our humble planetary home.

The rate at which Dawn consumes hydrazine depends very strongly on the nature of the orbit. The lower the height, the faster it uses hydrazine, because it must rotate more quickly to keep its sensors pointed at the ground. In addition, it has to fight harder to resist Ceres’ relentless gravitational tug on the very large solar arrays, creating an unwanted torque on the ship. In XMO7, Dawn will dip to less than one-tenth of its lowest altitude so far. The hydrazine is going to go fast. But that's okay. The hydrazine is there to be used in service of accomplishing the mission, and Dawn is going to use it very well indeed as it pursues fabulous new goals.

Dawn engineers have sophisticated mathematical models to predict just how quickly the hydrazine will be spent, and those models have done an excellent job throughout the mission. Nevertheless, as in all realistic and complex systems, there remains some degree of uncertainty. (As a courtesy to most readers, we will not delve into the recondite details.) We can predict only approximately how fast Dawn will expend hydrazine as it carries out its intricate assignments in the coming months. Glitches, which are inevitable on such a complex mission, can both consume hydrazine and compel the flight team to change the schedule and the plans, introducing further uncertainty.

As it turns out, there are two more aspects of this problem. Not only are we limited in our ability to predict how much hydrazine each activity will require but our measurement of how much hydrazine Dawn has remaining is imperfect too. We know that when it left Earth, riding atop a Delta rocket, the 12-gallon (45-liter) hydrazine tank was filled with 99.8 pounds (45.3 kilograms) of the propellant. In the subsequent 11.5 years, every time it has fired a thruster, the spacecraft has dutifully recorded the duration (in milliseconds) and reported that to mission control at JPL. It has also sent telemetry on the temperature and pressure in the hydrazine tank. With that information, engineers can calculate how much hydrazine is expended in each pulse of a thruster and, more to the point, how much is left in the tank. It is now down to about 1.8 gallons (7 liters). But no physical measurement is perfectly accurate. As only one example, the sensors that read the temperature and pressure have been subjected to violent shaking during the rocket's fiery ascent as well as almost a dozen years in space. Their readings now may be off a little bit one way or the other. The determination of how much hydrazine is still onboard thus has some uncertainty.

So, it is not possible to predict exactly how much hydrazine Dawn will need nor exactly how much it has. There is still another source of uncertainty. There is a complex network of tubing, valves and a filter between the tank and each of the 12 thrusters located around the spacecraft. Once the pressure in the lines is too low for a thruster to operate, the remaining hydrazine cannot be expelled. Of course, engineers can calculate how much of the hydrazine will be trapped in the system (known as the unusable hydrazine). That turns out to be 1.7 pints (0.8 liters), but, as with these other problems, they cannot know the answer with absolute precision, so it could be a little more or a little less.

Taken together, all these reasons prevent controllers from being able to pin down the day and time that Dawn will deplete the usable hydrazine. Experienced interplanetary explorers, like the Dawn flight team at JPL, are accustomed to dealing with such uncertainty.

The team will continue to guide Dawn in squeezing as much out of its time at Ceres as possible, acquiring new data until the spacecraft is unable to comply because it has expended the last puff of hydrazine. Right now, that is deemed most likely to be in September of this year (with a smaller chance it will be in August or maybe even October). Once Dawn has settled in to XMO7, and engineers have operational experience in the new orbit, they will update their estimate, and they will continue to refine it as the mission progresses.

And when the last of the hydrazine is used up, the spacecraft will actuate valves and try to fire thrusters to control its orientation, but hydrazine will no longer flow, so the torque it wants to exert will not be achieved. The spacecraft will be impotent, its attempts to point correctly futile. The struggle will be brief, as it will soon run out of electrical power, and the central computer will cease operating. We will address the details of its final moments in a future Dawn Journal.

For now, we needn't anticipate the end with despair. Dawn has already succeeded beyond our wildest expectations. The prime mission accomplished far more than planned at Vesta and at Ceres even though it confronted completely unanticipated and daunting obstacles, like the failures of two reaction wheels. The first extended mission (in XMO1 through XMO5) yielded many additional impressive bonuses as well as another reaction wheel failure. Now the second extension has provided further rewards in XMO6. And as we look ahead to XMO7, we can expect even more riches and, of course, more challenges (although no more reaction wheel failures).

A daring and exciting interplanetary adventure, journeying through the solar system atop a bluish beam of xenon ions, soaring past Mars and flying well over one million times farther from Earth than the International Space Station, orbiting Vesta and Ceres, the two largest bodies in the main asteroid belt (together representing about 40 percent of the combined mass of the millions of objects between Mars and Jupiter), exploring these mysterious uncharted worlds, revealing dramatic alien landscapes, powered by the collective passions of everyone exhilarated by new knowledge and everyone who longs to know the cosmos, Dawn has already surpassed any reasonable expectation for what it might achieve. What more may come, we do not yet know. That's part of the thrill of exploration and discovery. But when the end does come, it will represent the culmination of a truly extraordinary extraterrestrial expedition.

Dawn is 1,800 miles (2,900 kilometers) from Ceres. It is also 2.73 AU (254 million miles, or 408 million kilometers) from Earth, or 1,010 times as far as the Moon and 2.69 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 45 minutes to make the round trip.

Dr. Marc D. Rayman
6:30 pm PDT May 31, 2018

TAGS: DAWN, CERES

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

The rope lights strung around the room bathe it in a gentle light reminiscent of the beam emitted by an ion engine on the faraway spacecraft as it maneuvers in orbit around Ceres. Dawn had not thrust since June, but it is now using ion engine #2 to fly to a new orbit around the dwarf planet. Thanks to its uniquely capable ion propulsion system, Dawn has accomplished far more powered flight than any other spacecraft, and more is ahead.

Dawn has spent most of the last year revolving around Ceres once every 30 days in extended mission orbit 5 (XMO5), a designation that illustrates the team's flair for the dramatic. (Your correspondent, as passionate as anyone about the exploration of the cosmos, can imagine only a few names more inspiring than that. Fortunately, one of them happens to be "XMO7." Read on!) As the probe followed that elliptical course, it reached down to a little less than 2,800 miles (4,400 kilometers) above the alien world and up to 24,300 miles (39,100 kilometers).

Dawn flew to high altitude late in 2016. Its work there is now complete, and defying expectations, the aged adventurer still has life left in it. As we saw in last month's overview of the two upcoming orbits, Dawn's next assignment is to go much, much lower.

XMO5 and the subsequent two orbits are elliptical, as shown in the illustrations last month and the new one below. Observing Ceres from a very low altitude is possible only in an elliptical orbit, not a circular one. Dawn was not designed to operate at low altitude, and its reaction wheels, which are so important for controlling its orientation, have failed, making the problem even more difficult. We have discussed this before and will address another aspect of it this month for the lowest orbit.

Although the elliptical orbits introduce many new technical challenges for the team, Dawn still takes a spiral route from each orbit to the next, just as it did earlier at Ceres and at Vesta when the orbits were circular. In essence, the ion engine smoothly shrinks the starting ellipse until the new ellipse is the size needed. These trajectories are very complicated to plan and to execute, but with the expert piloting of the experienced team, the maneuvering is going very well. (You can follow the progress with the mission status updates.)

Dawn began its descent on April 16. On May 15, with the blue lights turned off in mission control, the veteran explorer will begin its observations in XMO6. (As suggested last month, the targeted minimum and maximum altitudes for XMO6 are being updated slightly even as Dawn is on its way. In the next Dawn Journal, we will present the actual altitude range.) If all goes well, the control room will be lit up in blue again from May 31 to June 7, as the ship sails down to XMO7.

In XMO7, Dawn will swoop down to an incredibly low 22 miles (35 kilometers) above the exotic terrain of ice, rock and salt. The last time it was that close to a solar system body was when it rode a rocket from Cape Canaveral over the Atlantic Ocean more than a decade ago. (For readers unfamiliar with solar system geography, that was Earth.) The XMO7 ellipse will then take the spacecraft up to 2,500 miles (4,000 kilometers). Each revolution will last 27 hours and 13 minutes. In considerably less time than that (assuming you read at a typical speed), we will discuss why this orbital period is important.

Last month, we described some of Dawn's planned low-altitude measurements of nuclear radiation to reveal more about Ceres' composition. As a bonus objective, scientists would like to study the elements in one of their favorite places (and perhaps one of yours as well): Occator Crater, site of the highly reflective salt deposits, famous not only on Ceres but also on Earth and everywhere else that readers follow Dawn's discoveries. Studying this one crater and the area around it (together known as a geological unit) could reveal more about the complex geology there. But doing so is quite a challenge, as Dawn would need to pass over that region 20 times to allow the gamma ray and neutron detector (GRaND) to record enough of the faint nuclear radiation. This is the equivalent of taking a long exposure with a camera when photographing a very dim scene.

Attempting to repeatedly fly low over that geological unit presents daunting obstacles, as we will discuss. It may not work, but the team will try. That's part of what makes for a daring adventure! And accomplishing such a feat requires a special trick. Fortunately, the Dawn team has several at its disposal.

Recall that Dawn will loop around Ceres, going south to north at low altitude and back to the south again at high altitude. Meanwhile, Ceres will turn on its axis toward the east, completing one rotation in just over 9 hours, 4 minutes. (Note that Ceres turns quite a bit faster than Earth. A Cerean day is much closer in duration to a day on Jupiter, which is 9 hours, 56 minutes. All three turn east.) Therefore, the flight team will synchronize the orbit so that each time Dawn swoops down to low altitude, it does so at just the right time so that Ceres' rotation will place the Occator geological unit under the probe's flight path.

We mentioned above that Dawn's orbit will take 27 hours, 13 minutes. This period is chosen to be exactly three times Ceres' rotation period. Experts (now including you) describe this as a three-to-one resonant orbit, meaning that for every three times Ceres turns, Dawn turns around it once.

If this synchronization is clear, feel free to skip this paragraph. Perhaps get a snack until it's time for the next paragraph or, better yet, use this time to gaze at the mesmerizing beauty of the night sky and contemplate the magnificence of the cosmos. If the synchronization is not clear, find a globe of Earth. Now imagine a satellite circling it, flying from the south pole to the north pole over one hemisphere and back to the south pole over the opposite hemisphere. Suppose the first passage occurs over your location. If Earth didn't rotate, the second orbit would take it over the same place. (Of course, if Earth didn't rotate, you might run out of patience waiting for tomorrow.) Now rotate the globe a little bit while your imagined satellite goes through one revolution. If it flew over your location the first time, it will not the second time. And you can see that with Earth rotating at a constant speed, it requires a carefully chosen speed for the satellite to pass over the desired target on each revolution. The Dawn flight team will work very hard to help our distant explorer have the orbit needed to achieve the three-to-one resonance.

The accuracy necessary will be difficult to achieve, even for the Dawn flight team at JPL, where the best celestial navigators in the solar system get to work. The problems that must be overcome are manifold. One of them is that, lacking functioning reaction wheels, Dawn fires its small hydrazine-fueled thrusters to control its orientation in space. Whether to turn to keep its sensors trained on the ground, even with the constantly changing altitude and velocity in the elliptical orbit, or to point its main antenna at Earth, the reaction from a little burst of hydrazine not only rotates the spacecraft but also nudges it in its orbit. (We have described this several times in great detail before.) Each small push from the thrusters distorts the orbit a little bit, desynchronizing it from the three-to-one resonance.

Another difficulty is that, just like Earth, Mars, the Moon and other solar system residents (not to mention cookie dough ice cream), Ceres is not uniform inside. Its complex geology has produced some regions of higher density and some of lower density (although not with the same delectable composition as the ice cream). The total gravitational pull on the spacecraft depends on the dwarf planet's internal structure. We have described before how scientists take advantage of it to map the interior. But we have measured the gravity from 240 miles (385 kilometers) high. When Dawn swoops down much lower, our gravity map will not be accurate enough to predict all the subtle details of the mass distribution that may cause slightly larger or slightly smaller pulls at some locations. It will take quite a while to formulate the new gravity map. That new map may reveal more about what's underground, but until then, it will be harder to keep the orbit in sync.

On two occasions in mid-June Dawn will use its ion engine to tweak its orbit (in what we have described before as a trajectory correction maneuver) to help maintain the synchronization, but there will still be residual discrepancies.

We described and depicted last month how the low point of Dawn's orbit will gradually shift southward on each successive revolution. That means we will have only a limited number of opportunities to fly over Occator before the low point is too far south. Given the complexity of the operations, the planned measurements are not at all assured.

There are other aspects of this problem as well. While we will not delve into them here, engineers have been working hard on every one of them.

We have mentioned before that photography will be extremely challenging in XMO7, because of both the high speed so close to the ground and the difficulty pointing the camera accurately enough to capture a specific target. Let's take a more careful look at the nature of the orbit to understand more about the problem of trying to see any particular site.

You can think of the motion in an elliptical orbit as being somewhat like that of a swing. Imagine a girl named Dawn on a swing. Perhaps she is 10 and a half years old (like our spacecraft), usually (but not always) does what we instruct (like our spacecraft), feels energized by the light of the Sun (like our spacecraft), loves the idea of exploring uncharted worlds (like our spacecraft) and uses photomultiplier tubes coupled to a bismuth germanate crystal scintillator, lithiated glass and boron-loaded plastic to measure the spectra of nuclear radiation (okay, she is not like our spacecraft in every way).

When Dawn rides her swing, her speed is constantly changing. As she approaches the top of her arc, gravity slows her down and even brings her momentarily to a stop. She then begins to fall, accelerating as she gets lower. As soon as she passes the lowest point, her upward motion and the downward pull of gravity oppose each other, and once again she begins to slow. When her swing is pumped up (whether with her legs or by the push of her friend or her friendly ion engine), her arc will reach higher, and then she will speed through the low point even faster.

Of course, the swing does not trace out an ellipse, and the girl does not loop all the way around, but the fundamental principles of motion are the same, as methodically investigated by Galileo Galilei four centuries ago and explained by Isaac Newton in the second half of the 17th century. Dawn's elliptical orbit around Ceres will behave somewhat like the swing. At high altitude, far above the dwarf planet, the spacecraft will move at only about 120 mph (190 kph). Then, as gravity pulls it back down, the spacecraft will accelerate until it skims over the ground at 1,050 mph (1,690 kph) before starting to swing up again.

Dawn is much, much, much too far away for controllers to point its camera and other instruments as you might with a joystick or other controller in real time. Readers of the final paragraph of every Dawn Journal know that radio signals, traveling at the universal limit of the speed of light, usually take more than half an hour to complete the round trip. When Dawn is in XMO7 this summer, it will be about an hour. While the spacecraft is racing over the Cerean landscape, it can't wait for its radio signal to tell controllers what it sees and then, based on that, for a return radio signal to help it adjust the pointing of its camera. All the instructions from Earth have to be radioed in advance.

It is a very complicated process to go from measuring Dawn's orbit accurately to the probe actually aiming its camera and its spectrometers to collect new data, with many calculations and many steps in between, each of which has to be checked and double checked. The team has a special campaign planned for that purpose, and they will maneuver to XMO7 so that the best viewing will be in late June. But even when they work quickly for this dedicated attempt to get some bonus photographs of Occator, the entire process will take the better part of a week because of the spacecraft's orbital activities (e.g., while it observes Ceres, it cannot communicate with Earth), segments of its orbit where Ceres blocks its radio signal to Earth and so it is not possible to communicate, and the schedule for the large Deep Space Network antennas to shout so Dawn can then listen for what fades to become a long-distance radio whisper. Time needs to be allocated for computers and people to analyze data, to formulate and verify the new plans, to beam the instructions to Dawn and then Dawn finally to execute them. Meanwhile, even after the initial measurement of its orbit, while all this work is occurring on Earth, the ship will continue to be buffeted by the hydrazine winds and the gravitational currents, so its course will continue to change.

The consequence of all this is that by the time Dawn actually conducts its observations, its orbit will be different from what was measured days earlier. The carefully devised prediction that formed the basis of the plans could well be off one way or the other by four minutes or even more. (By the way, calculating now the credible magnitude of the error for this June campaign is a sophisticated science that, in itself, involves thousands and thousands of hours of computer calculations, performed on hundreds of computers working simultaneously. Epistemic knowledge does not come easily.)

From Dawn's perspective, descending and speeding north at 1,050 mph (1,690 kph) to the vicinity of Occator, faithfully pointing its sensors according to the plan worked out days before on a distant planet and stored in its computer, Ceres' rotation will carry the crater to the right at more than 190 mph (310 kph). Dawn's camera will take in a scene about 2.1 miles (3.4 kilometers) across, and at the spacecraft's high velocity, there won't be time to turn right and left to cover a broader swath. Even if the probe arrived at Occator's latitude a mere 20 seconds off schedule, a spot on the ground that was expected to be in the center of the camera would have moved entirely out of view and so would not even be glimpsed. If Dawn were four minutes too early or too late, the ground beneath the spacecraft (known as the ground track) would shift west or east by 13 miles (21 kilometers), and the terrain that's photographed could be entirely different from what was expected.

Occator Crater is 57 miles (92 kilometers) across, so all this work should allow GRaND, with its very wide field of view, to measure the composition in the geological unit that contains the crater. But the narrower view of the camera means we cannot be certain what features we will see. Fortunately, we already know that there is fascinating geology just about everywhere in and near Occator. Indeed, the dwarf planet is vast and varied, with a great many intriguing features. We are going to behold some amazing sights!

Before then, we will gain new perspectives from XMO6 in May. And as Dawn was getting closer to Ceres, together the pair were getting closer to the Sun until yesterday. Dawn isn't the only object in an elliptical orbit. Ceres, Earth, and all the other planets (whether dwarf or not) travel in elliptical orbits too, although they orbit the Sun. Ceres' orbit is more elliptical than Earth's but not as much as some of the other planets. The shape of Ceres' orbit is between that of Saturn's (which is more circular) and Mars' (which is more elliptical). (Of course, Ceres' orbit is larger than Mars' orbit -- it revolves farther from the Sun than the Red Planet does -- and smaller than Saturn's, but our focus here is on how much the orbit deviates from a perfect circle, regardless of the size.)

In its 4.6-year-long Cerean year, Ceres, with Dawn in tow, reached the minimum solar distance of just under 2.56 AU (238 million miles, or 383 million kilometers) on April 28. Dawn also was in residence at Ceres when they were at their maximum distance from the Sun in January 2016. Although the dwarf planet's orbit is not elliptical enough that the additional solar heating is expected to have much effect, the upcoming observations in XMO6 will provide scientists with the opportunity to look for any changes just in case. (The change Dawn detected at Juling Crater is more likely related to the seasonal change of the angle of the Sun rather than the distance to the Sun.)

The solar system constantly performs a complex and beautiful choreography, with everything in motion. Dawn will complete its current elegant spiral in another two weeks, and then it will be time for the next act, XMO6 and, after that, the finale, XMO7. A great many challenges are ahead but the allure of the rich rewards of new knowledge, new insight, and a new adventure is irresistible as Dawn delves further into the unknown.

Dawn is 1,400 miles (2,300 kilometers) from Ceres. It is also 2.34 AU (218 million miles, or 350 million kilometers) from Earth, or 900 times as far as the Moon and 2.32 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 39 minutes to make the round trip.

Dr. Marc D. Rayman
7:30 pm PDT April 29, 2018

TAGS: DAWN, CERES

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

For eight months in 2015-2016, Dawn circled Ceres once every 5.4 hours at only 240 miles (385 kilometers). (The orbit has been variously designated as LAMO, then XMO1, and often as "the lowest orbit.") It then flew higher to pursue new objectives. The probe's orbit now takes it from slightly under 2,800 miles (4,400 kilometers) up to 24,300 miles (39,100 kilometers) and then back down again. (These values are a little different from what we presented in December, principally because the Sun's gravity gradually alters the orbit.) The orbit is known to people who call it extended mission orbit 5, or XMO5, as "extended mission orbit 5" or "XMO5" (following the nomenclature described here). XMO5 is illustrated in a figure below.

In contrast to the distant, serene probe, the operations team has been working quite intensively to prepare for a bold new phase of the mission. They have been assiduously working through all the tasks necessary to prepare for piloting this unique spaceship, late in its life and low on supplies, through maneuvers it was never designed for and to conduct observations never conceived of prior to late last year. Since the previous Dawn Journal, the team has generated more than 45,000 trajectories to study how to fly Dawn to two new orbits. Often there are more than 100 computers operating simultaneously to perform the necessary calculations. Many thousands more trajectories are yet to be computed and analyzed. If all goes well, by June, the probe will have followed an intricate flight plan that will allow it to glide a mere 22 miles (35 kilometers) above the alien landscapes almost every day in an orbit dramatically and poetically designated XMO7 (but occasionally summarized as "Whoa, that's low!").

Let's take a look at some of the plans the flight team is developing. As always, we will provide more details when Dawn is executing its complex assignments. In addition, as some parts of the plan are still being refined, there may be a few changes, and we will keep you updated on those as well. But plans are firm enough now that a preview is warranted.

On April 17, the spacecraft will fire up ion engine #2 and begin a downward spiral, gradually shrinking its elliptical orbit. Along the way to its final space destination, XMO7, the ship will moor at an intermediate orbit. On May 14, when it is in an orbit that ranges from about 235 miles (375 kilometers) to almost 3,000 miles (4,800 kilometers), it will shut down the engine. (This orbit is illustrated in the next two figures below.)

It is only coincidental that the lowest altitude of this intermediate orbit, XMO6, is so close to height of the lowest orbit so far. Indeed, the lowest point is not the most important point. The motivation for stopping in XMO6 is to collect infrared spectra and take pictures in the southern hemisphere in a range of about 900 miles (1,500 kilometers) to 1,600 miles (2,500 kilometers). It just so happens that when flying from XMO5 to XMO7, an orbit that provides that viewing opportunity dips down to the height of LAMO/XMO1 elsewhere in the orbit.

The XMO6 altitude in the south was chosen to be comparable to the altitude from which Dawn observed Ceres so extensively in its third and fifth mapping orbits (known as HAMO and XMO2, respectively). XMO6 will afford the probe views of the terrain with the illumination of southern summer that will make for the best comparison with what it has already observed farther north on the dwarf planet. Dawn photographed all of Ceres in full color in those earlier orbits, but it was not possible then to cover the vast surface with the infrared mapping spectrometer, which has a much smaller field of view than the camera. Therefore, scientists had focused their spectral mapping in the northern hemisphere, taking advantage of the lighting then. While some of the southern hemisphere was studied in infrared as well, the opportunity now to observe more of it will allow a more complete understanding of the distribution of minerals.

In XMO6 the spacecraft will fly over the south pole and then head north over the hemisphere of Ceres facing the Sun. It will go lower and lower as it does so. The lowest point in the orbit will occur between 50° and 60°N. Dawn already mapped that territory from LAMO/XMO1, but now it will take advantage of being low again to acquire some new color photography in the northern hemisphere.

As the spacecraft continues farther north, the altitude will increase again. It will sail higher as it travels over the night side before beginning its fall back down. It will take about 37 hours to complete one elliptical revolution.

Some readers may recall that for all of the mapping orbits at Vesta and Ceres, Dawn traveled south over the sunlit side and north over the hemisphere shrouded in the dark of night. (Readers who don't recall that are invited to trust that it's true.) Experts readily recognize that it is very, very difficult to reverse the orbital direction. Dawn did so, however, with the extensive maneuvering in February-April 2017 that allowed it to make the unique observation of opposition. Those who are interested can review the skilled piloting that reversed the direction.

The explorer will observe Ceres on 10 consecutive orbits in XMO6. To conserve precious hydrazine, Dawn will turn to point its main antenna to Earth and radio its findings after every other transit over the sunlit landscapes. In the other orbits, it will wait patiently, saving both data and hydrazine onboard for later.

On May 31, the spaceship will resume maneuvering. It will take about a week of ion thrusting to push down to the final orbit of the mission.

In XMO7 (shown in the two figures below), Dawn will range from as high as 2,500 miles (4,000 kilometers) to as low as about 22 miles (35 kilometers). (The minimum altitude will vary by a few miles, or kilometers, from revolution to revolution, for reasons we will explain in a future Dawn Journal.) It will take a little more than a day to complete one loop.

We have described before that photography will be very challenging, both because of the difficulty pointing the camera accurately enough to capture specific targets and the high speed so close to the ground. We will return to this problem in an upcoming Dawn Journal.

At the high point of XMO7, Dawn will move at only about 120 mph (190 kph). Then as gravity pulls it back down, the spacecraft will accelerate until it streaks northward at 1,050 mph (1,690 kph) above a relatively narrow strip of ground before starting to soar up again. Dawn was designed for mapping uncharted worlds, not making specialized observations under such conditions, and traveling so fast and so low means it cannot take pictures as sharp as you might expect. Nevertheless, even with a little bit of motion-induced blur at low altitude, any sights we photograph certainly will reveal finer details than we have seen before. This is going to be exciting!

The highest priority measurements will be the nuclear spectra, giving scientists the opportunity to take a sharper picture of the elemental composition of the faraway world, making a more accurate map of the concentration of atomic species that are important for Ceres' geology and chemistry. Dawn's gamma ray and neutron detector (GRaND) is not subject to the limitations of pointing accuracy and blur that can affect the photography. You can think of GRaND's gamma ray vision and its neutron vision as being broader but less acute than the camera's visible-light vision. Getting closer to the ground will help ensure the instrument sees a stronger nuclear signal than ever before and takes a clearer picture.

As the spacecraft races over the ground, GRaND will measure gamma rays and neutrons escaping into space from the atoms down to about a yard (meter) underground. It collected a large volume of such data from LAMO/XMO1, but being so much lower in XMO7 will allow scientists to identify and locate elements more accurately.

There are several GRaND (if not grand) objectives for XMO7. One is to see how the elemental composition differs at different latitudes. The instrument has already revealed that water is more plentiful near the surface at higher latitudes than near the equator, and now it may be able to refine this finding. One of the properties of XMO7 is that the low point will shift almost 2° of latitude south on each revolution. That is, each time Dawn swoops down to its lowest point, it will be south of the low point on the previous orbit. That will provide GRaND the opportunity to survey the concentration and distribution of underground ice at different latitudes. GRaND also may tell us more about other constituents, providing clues about the geological processes that shaped this exotic world.

Of course, as Dawn orbits Ceres, Ceres turns on its axis, pirouetting beneath her admiring companion. So each time Dawn zooms down for a close look, it will not only be farther south than the time before but it will also be at a different longitude. The next Dawn Journal will focus on this and what it means for GRaND and for photography.

Controlling Dawn's orientation in the zero-gravity of spaceflight is harder at low altitude, where Ceres' gravitational pull is stronger. Dawn will use hydrazine much more quickly in XMO7 than at any other part of the mission, and the last of the propellant will be expended before the end of this year.

Dawn just celebrated the third anniversary of arriving at its permanent residence in the solar system. In the natural perspective of its current home, Dawn arrived about two-thirds of a Cerean year ago, or nearly 3,000 Cerean days ago. The explorer has now completed 1,600 orbits. Although hydrazine is dwindling, and the adventure is nearing its end, there is still plenty to look forward to. Stay onboard as Dawn prepares to delve further into the unknown. It's going to be a great ride!

Dawn is 10,800 miles (17,400 kilometers) from Ceres. It is also 1.87 AU (174 million miles, or 280 million kilometers) from Earth, or 740 times as far as the Moon and 1.88 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 31 minutes to make the round trip.

Dr. Marc D. Rayman
9:15 am PDT March 20, 2018

TAGS: DAWN, CERES

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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

TAGS: DAWN, CERES, VESTA, ASTEROID BELT

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Building on the successes of its primary mission and its first extended mission, NASA has approved the veteran explorer for a second extended mission. Dawn will undertake ambitious new investigations of dwarf planet Ceres, its permanent residence far from Earth.

It was not a foregone conclusion that Dawn would conduct further operations. In part, that's because it is only one of many exciting and important missions NASA has underway, and more are being designed and built. But the universe is a big place, as you may have noticed if you've ever gazed in awestruck reflection at the night sky (or had to search for a parking space in Los Angeles). It simply isn't possible to do everything we want. Entrusted with precious taxpayers' dollars, NASA has to make well-considered choices about what to do and what not to do.

In addition, as we have discussed in detail, Earth's ambassador to two giants in the main asteroid belt has had to contend with severe life-limiting problems. Dawn's reaction wheels have failed, and now it has consumed most of its original small supply of hydrazine that it uses in compensation. It has also expended most of the xenon propellant for its uniquely capable ion propulsion system. It was not clear that a truly productive future would be possible for this aged, damaged ship with some supplies that are so limited. Fortunately, Dawn has endless supplies of creativity, ingenuity, dedication and enthusiasm.

For several months, the flight team has been studying the feasibility of flying the spaceship closer to Ceres than had ever been seriously considered. Dawn spent more than eight months in 2015-2016 circling about 240 miles (385 kilometers) above the dwarf planet. It had spectacular views of mysterious landscapes and acquired a wealth of data far beyond what the team had anticipated. Then Dawn flew to a higher altitude during its first extended mission for new observations. Now engineers are making progress on ways to operate the spacecraft in an elliptical orbit that would allow it to swoop down to below 125 miles (200 kilometers) for a few minutes on each revolution. Their results so far are very encouraging. There are still many complex technical problems to solve, and months of additional work remain. Dawn can wait relatively patiently in its current orbit, where it expends hydrazine quite parsimoniously as it measures cosmic rays.

The promising potential for observing Ceres in elliptical orbits from closer than ever before makes a second extended mission there extremely attractive. NASA and the panel of scientists and engineers convened to provide an independent, objective assessment concluded that further exploration of Ceres would be the most valuable assignment for the spacecraft. It is noteworthy that Dawn is the only spacecraft ever to orbit two extraterrestrial destinations and even now, having significantly exceeded its original objectives, has the capability to leave Ceres and pay a brief visit to a third (although it does not have enough xenon left to orbit a third), but the prospects for new discoveries at Ceres are too great to pass up.

Ceres is not only the largest object between Mars and Jupiter but also certainly one of the most intriguing. In fact, motivated by what Dawn has found, there is now great interest in the possibility of sending a lander there someday. Anything more Dawn can do to learn about Ceres or to help pave the way for a subsequent mission will be of great importance.

Ceres is just too fascinating to abandon! Dawn has already revealed the dwarf planet to be an exotic world of ice, rock and salt, with organic materials and other chemical constituents, and now we can look forward to more discoveries. After all, the benefit of having the capability to orbit a distant destination, rather than being limited to a quick glimpse during a fleeting flyby, is that we can linger to scrutinize it and uncover even more of the secrets it holds. (Some readers may also draw inspiration from Ceres' ingredients to concoct recipes for treats to give out to Halloween visitors.)

In addition to the possibility of observing Ceres from unprecedentedly close, there are other benefits to keeping our sophisticated probe at work there. For now, let's consider two of them, both related to how long it takes Ceres to complete its stately orbit around the sun. One Cerean year is 4.6 terrestrial years.

The dwarf planet carries its robotic moon with it as it follows its elliptical path around the sun. In fact, all orbits, including Earth’s, are ellipses. Ceres’ orbit is more elliptical than Earth’s but not as much as some of the other planets. The shape of Ceres’ orbit is between that of Saturn (which is more circular) and Mars (which is more elliptical). (Of course, Ceres’ orbit is larger than Mars’ and smaller than Saturn’s, but here we are considering how much each orbit deviates from a perfect circle, regardless of the size.)

When Dawn arrived at Ceres in March 2015, they were 2.87 AU from the sun. That was well before the dwarf planet's orbit carried them to the maximum solar distance of 2.98 AU in January 2016. Now, with the second extended mission, the spacecraft will still be operating when Ceres reaches its minimum solar distance of 2.56 AU in April 2018. Dawn will keep a sharp eye out for any changes caused by being somewhat closer to the sun.

The extension also will give scientists the opportunity to examine Ceres with the different lighting caused by the change of seasons. Ceres' slower heliocentric orbit than Earth's means seasons last longer on that distant world. It was near the end of autumn in the southern hemisphere when Dawn took up residence at Ceres. Winter came to that hemisphere on July 24, 2015, when the sun reached its greatest northern latitude. The sun crossed the equator, bringing spring to the southern hemisphere, on Nov. 13, 2016, and summer begins on Dec. 22 of this year. Autumn, when the sun will leave the southern hemisphere, is more than one (terrestrial) year later. Most of Dawn's observations so far were made with the sun in the northern hemisphere. Now Dawn will have new opportunities to see the southern hemisphere with similar illumination.

In the coming months, as the team develops and refines its plans, we will describe how they will pilot the ship down to very low altitudes and what new measurements they will make. Before the new phase gets underway, however, you can explore Ceres (and other planets) yourself with Google maps (some functions don't work in some web browsers). Even though it does not use Dawn's sharpest photos, it should be more than adequate for most of your navigational needs. (It isn't quite adequate for Dawn's needs, but that's no cause for worry, because JPL navigators employ somewhat more sophisticated and accurate methods.)

What will Dawn find when it ventures closer to the ground than ever before? What will the new perspectives reveal about a strange world from the dawn of the solar system? What new challenges will the adventurer confront as it pushes further into uncharted territory? We don't know, but stay onboard as we find out together, for that is an essential element both of the tremendously successful process of science and the powerful thrill of exploration.

Dawn is 21,600 miles (34,700 kilometers) from Ceres. It is also 2.47 AU (229 million miles, or 369 million kilometers) from Earth, or 970 times as far as the moon and 2.49 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.

Dr. Marc D. Rayman
2:30 p.m. PDT October 31, 2017

TAGS: DAWN, CERES, EXTENDED MISSION

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.