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.

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

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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.

Even as the veteran explorer orbits high over dwarf planet Ceres and looks forward to continuing its mission, today it can reflect upon 10 exciting and productive years (or equivalently, with its present perspective, 2.17 exciting and productive Cerean years).

The ambitious adventurer embarked on an extraordinary extraterrestrial expedition on Sept. 27, 2007. With its advanced ion propulsion system, Dawn soared past Mars in 2009. The spacecraft took some of the Red Planet’s orbital energy around the sun to boost itself on its journey. (Nevertheless, this extra energy amounts to less than a quarter of what the ion engines have provided.) Ever a responsible citizen of the cosmos, Dawn fully adheres to the principle of the conservation of energy. So to compensate for speeding up, it slowed Mars down.

In 2011, the spacecraft arrived at Vesta, the second largest object in the main asteroid belt between Mars and Jupiter. Dawn gracefully entered into Vesta’s firm but gentle gravitational embrace. The probe maneuvered extensively in orbit, optimizing its views to get the best return possible from its photography and other observations. During 14 months in orbit, Dawn completed 1,298 revolutions around Vesta, taking nearly 31,000 pictures and collecting a wealth of other scientific measurements. From the perspective it had then, Dawn was in residence for nearly a third of a Vestan year (or almost 1,900 Vestan days). The explorer revealed a strange, ancient protoplanet, now recognized to be more closely related to the terrestrial planets (including the one Dawn left 10 years ago) than to the typical and smaller asteroids.

Unlike all other deep-space missions, Dawn had the capability to leave its first orbital destination and voyage to and enter orbit around another. After smoothly disengaging from Vesta, the interplanetary spaceship flew more than 900 million miles (1.5 billion kilometers) in 2.5 years to Ceres, the largest object in the asteroid belt. Indeed, prior to Dawn’s arrival, that dwarf planet was the largest body between the sun and dwarf planet Pluto that a spacecraft had not yet visited. And just as at Vesta, thanks to the maneuverability of ion propulsion, Dawn did not have to be content with a one-time flyby, gathering only as much data as possible during a brief encounter. By going into orbit around Ceres, the spacecraft could linger to scrutinize the exotic, alien world. And that is exactly what it has done.

Both Vesta and Ceres have held secrets since the dawn of the solar system, and both have beckoned since they were first spotted in telescopes at the dawn of the 19th century. For the next two centuries, they appeared as little more than faint smudges of light amidst myriad glittering stellar jewels, waiting for an inquisitive and admiring visitor from Earth. Finally, Dawn answered their cosmic invitations and eventually developed richly detailed, intimate portraits of each.

As the last stop on a unique interplanetary journey of discovery, Ceres has proven well worth the wait. Since arriving in March 2015 (more than half a Cerean year ago, or nearly 2,500 Cerean days ago), Dawn has completed 1,595 revolutions. It has beheld mysterious and fascinating landscapes and unveiled a complex world of rock, ice and salt, along with organic compounds and other intriguing constituents. The dwarf planet may have been covered by an ocean long ago, and there might even be liquid water underground now. The 57,000 pictures and numerous other measurements with the sophisticated sensors will keep scientists busy for many years (both terrestrial and Cerean).

By early 2016, during its ninth year in space, Dawn had accomplished so much that it exceeded all of the original objectives established for it by NASA before the ship set sail. Along the way, Dawn encountered and ultimately overcame many obstacles, including equipment failures that could well have sunk the mission. Against all odds and expectations, however, when its prime mission concluded in June 2016, the spacecraft was still healthy enough that NASA decided to extend the mission to learn still more about Ceres. Since then, Dawn has conducted many investigations that had never even been considered prior to last year. Now it has successfully achieved all of the extended mission objectives. And, once again defying predictions thanks to expert piloting by the flight team (and a small dose of good luck), Dawn still has some life left in it. Before the end of the year, NASA will formulate another new set of objectives that will take it to the end of its operational life.

Dawn has flown to many different orbital altitudes and orientations to examine Ceres. Now the probe is in an elliptical orbit, ranging from less than 3,200 miles (5,100 kilometers) up to 23,800 miles (38,300 kilometers). At these heights, it is measuring cosmic rays. Scientists mathematically remove the cosmic ray noise from Dawn’s 2015-2016 recordings of atomic elements from a low, tight orbit at only 240 miles (385 kilometers).

In its present orbit, Dawn can make these measurements to clarify Ceres’ nuclear signals while being very frugal with its precious hydrazine, which is so crucial because of the loss of three reaction wheels. (The small supply was not loaded onboard with the intention of compensating for failed reaction wheels.) When the hydrazine is expended, the mission will end. So this high elliptical orbit is a very good place to be while NASA and the Dawn project are determining how best to use the spacecraft in the future.

Meanwhile, this anniversary presents a convenient opportunity to look back on a remarkable spaceflight. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the tenth annual summary, reusing text from previous years with updates where appropriate. Readers who wish to investigate Dawn’s ambitious journey in detail may find it helpful to compare this material with the Dawn Journals from its first, second, third, fourth, fifth, sixth, seventh, eighth and ninth anniversaries.

In its 10 years of interplanetary travels, the spacecraft has thrust with its ion engines for a total of 2,109 days (5.8 years), or 58 percent of the time (and 0.000000042 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 908 pounds (412 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sept. 27, 2007. The spacecraft has used 69 of the 71 gallons (262 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space.

The thrusting since then has achieved the equivalent of accelerating the probe by 25,400 mph (40,900 kilometers per hour). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.) It is remarkable that Dawn’s ion propulsion system has provided nearly the same change in speed as the entire Delta rocket.

Since launch, our readers who have remained on or near Earth have completed 10 revolutions around the sun, covering 62.8 AU (5.8 billion miles, or 9.4 billion kilometers). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 42.4 AU (3.9 billion miles, or 6.3 billion kilometers). As it climbed away from the sun, up the solar system hill to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It had to go even slower to perform its graceful rendezvous with Ceres. In the 10 years since Dawn began its voyage, Vesta has traveled only 40.5 AU (3.8 billion miles, or 6.1 billion kilometers), and the even more sedate Ceres has gone 37.8 AU (3.5 billion miles, or 5.7 billion kilometers). (To develop a feeling for the relative speeds, you might reread this paragraph while paying attention to only one set of units, whether you choose AU, miles, or kilometers. Ignore the other two scales so you can focus on the differences in distance among Earth, Dawn, Vesta and Ceres over the 10 years. You will see that as the strength of the sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)

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

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

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

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

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

For readers who are not overwhelmed by the number of numbers, investing the effort to study the table may help to demonstrate how Dawn patiently transformed its orbit during the course of its mission. Note that six years ago, the spacecraft’s path around the sun was exactly the same as Vesta’s. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to slip into orbit around Vesta. While simply flying by it would have been far easier, matching orbits with Vesta required the exceptional capability of the ion propulsion system. Without that technology, NASA’s Discovery Program would not have been able to afford a mission to explore the massive protoplanet in such detail. Dawn has long since gone well beyond that. Having discovered so many of Vesta’s secrets, the adventurer left it behind. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. From 2012 to 2015, the stalwart craft reshaped and tilted its orbit even more so that now it is identical to Ceres’. Once again, that was essential to accomplishing the intricate celestial choreography in which the behemoth reached out with its gravity and tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.

Even after a decade of daring space travel, flying in deep space atop a blue-green pillar of xenon ions, exploring two of the last uncharted worlds in the inner solar system, overcoming the loss of three reaction wheels, working hard to stretch its shrinking supply of hydrazine, Dawn is ready for more. And so is everyone who yearns for new knowledge, everyone who is curious about the cosmos, and everyone who is exhilarated by bold adventures into the unknown. More is to come. Dawn -- and all those who find the lure of space irresistible -- can look forward to whatever lies ahead for this unique mission.

Dawn is 16,600 miles (26,700 kilometers) from Ceres. It is also 2.92 AU (271 million miles, or 437 million kilometers) from Earth, or 1,080 times as far as the moon and 2.91 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 49 minutes to make the round trip.

Dr. Marc D. Rayman
4:34 am PDT September 27, 2017

TAGS: DAWN, CERES, VESTA, ASTEROID BELT, ION 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.