People have been gazing in wonder and appreciation at the beauty of the night sky throughout the history of our species. The gleaming jewels in the seemingly infinite black of space ignite passions and stir myriad thoughts and feelings, from the trivial to the profound. Many people have been inspired to learn more, sometimes even devoting their lives to the pursuit of new knowledge. Since Galileo pointed his telescope up four centuries ago and beheld astonishing new sights, more and more celestial gems have been discovered, making us ever richer.
In a practical sense, Dawn brought two of those jewels down to Earth, or at least brought them more securely within the scope of Earthlings' knowledge. Science and technology together have uncloaked and explained aspects of the universe that would otherwise have seemed entirely inscrutable. Vesta and Ceres revealed little of themselves as they were observed with telescopes for more than two centuries. Throughout that time, they beckoned, waiting for a visitor from distant Earth. Finally their cosmic invitations were answered when Dawn arrived to introduce each of them to Earth, whereupon the two planet-like worlds gave up many of their secrets.
Even now, Ceres continues to do so, as it holds Dawn in its firm but gentle gravitational embrace. Every 27 hours, almost once a day, the orbiting explorer plunges from 2,500 miles (4,000 kilometers) high to as low as about 22 miles (35 kilometers) and then shoots back up again. Each time Dawn races over the alien landscapes, it gathers information to add to the detailed story it has been compiling on the dwarf planet.
Dawn began its ambitious mission in 2007. (And on Aug. 17, 2018, it passed a milestone: three Vestan years of being in space.) But the mission is rapidly approaching its conclusion. In the previous Dawn Journal, we began an in-depth discussion of the end, and we continue it here.
We described how the spacecraft will lose the ability to control its orientation, perhaps as soon as September. It will struggle for a short time, but it will be impotent. Unable to point its electricity-generating solar panels at the Sun or its radio antenna to Earth, the seasoned explorer will go silent and will explore no more. Its expedition will be over.
We also took a short look at the long-term fate of the spacecraft. To ensure the integrity of possible future exploration that may focus on the chemistry related to life, planetary protection protocols dictate that Dawn not contact Ceres for at least 20 years. Despite being in an orbit that regularly dips so low, the spaceship will continue to revolve around its gravitational master for at least that long and, with very high confidence, for more than 50 years. The terrestrial materials that compose the probe will not contaminate the alien world before another Earth ship could arrive.
Like its human colleagues, Dawn started out on Earth, but now its permanent residence in the solar system, Ceres, is far, far away. Let's bring this cosmic landscape into perspective.
Imagine Earth reduced to the size of a soccer ball. On this scale, the International Space Station would orbit at an altitude of a bit more than one-quarter of an inch (7 millimeters). The moon would be a billiard ball almost 21 feet (6.4 meters) away. The Sun, the conductor of the solar system orchestra, would be 79 feet (24 meters) across at a distance of 1.6 miles (2.6 kilometers). More remote even than that, when Dawn ceases operating, it would be more than 5.5 miles (9.0 kilometers) from the soccer ball. The ship will stay locked in orbit around Ceres, the only dwarf planet in the inner solar system. The largest object between Mars and Jupiter, that distant orb would be five-eighths of an inch (1.6 centimeters) across, about the size of a grape. Of course, a grape has a higher water content than Ceres, but exploring this fascinating world of ice, rock and salt has been so much sweeter!
Now let's take a less terrestrial viewpoint and shift our reference to Ceres. Suppose it were the size of a soccer ball. In Dawn's final, elliptical orbit, which it entered in June, the spacecraft would travel only 37 inches (94 centimeters) away at its farthest point. Then it would go in to skim a mere one-third of an inch (8 millimeters) from the ball.
Dawn is one mission among many to explore the solar system, dating back almost 60 years and (we hope) continuing and even accelerating for much longer into the future. Learning about the cosmos is not a competition but rather a collective effort of humankind to advance our understanding. And to clarify one of the many popular mistaken notions about the solar system, let's take advantage of reducing Ceres to the size of a soccer ball to put some other bodies in perspective.
Because it is in the main asteroid belt, there is a common misconception that Ceres is just another asteroid, somehow like the ones visited by other spacecraft. It is not. The dwarf planet is distinctly unlike the small chunks of rock that are more typical asteroids. We have discussed various aspects of Ceres' complex geology, and much more remains to be gleaned from Dawn's data. Vesta too has a rich and complicated geology, and it is more akin to the terrestrial planets (including Earth) than to asteroids. But for now, let's focus simply on the size in order to make for an easy comparison. Of course, size is not a measure of interest or importance, but it will illustrate how dramatically different these objects are.
With a soccer-ball-sized Ceres, Vesta would be nearly five inches (more than 12 centimeters) in diameter. (This writer's comprehensive knowledge of sports inspires him to describe this as a ball nearly five inches, or more than 12 centimeters, in diameter.)
What about some of the asteroids being explored as Dawn's mission winds to an end? There are two wonderfully exciting missions with major events at asteroids (albeit ones much closer to Earth than the main asteroid belt) in the second half of 2018. Your correspondent, a lifelong space enthusiast, is as hopeful for success as anyone! Hayabusa2 is revealing Ryugu and OSIRIS-REx is on the verge of unveiling Bennu.
Ryugu and Bennu are more irregular in shape than Ceres and Vesta, but they would both be so small compared to the soccer ball that their specific shapes wouldn't matter. Ryugu would be less than a hundredth of an inch (a quarter of a millimeter) across. Bennu would be about half that size. They would be like two grains of sand compared to the soccer ball. In the first picture of the June Dawn Journal, we remarked on the detail visible in a feature photographed on one of Dawn's low streaks over the alien terrain. It is also visible in the first two pictures above. That one structure on Ceres is only a part of Cerealia Facula, which is the bright center of the much larger Occator Crater. Occator is a good-sized crater, but not even among the 10 largest on Ceres. Yet that one bright feature in the high-resolution photo is larger than either of these small asteroids. In many of Dawn's pictures that show the entire disk of the dwarf planet (like the rotation movie and the color picture here), Ryugu and Bennu would be less than a pixel, undetectably small, just as invisible specks of dust on a soccer ball.
The tremendous difference in size between Ceres (and Vesta) and small asteroids illustrates a widely unappreciated diversity in the solar system. Of course, that is part of the motivation for continuing to explore. There is a great deal yet to be learned!
Although Ryugu and Bennu aren't in the main asteroid belt, the belt contains many more Lilliputian asteroids closer in size to them than to the Brobdingnagian Ceres and Vesta. In fact, of the millions of objects in the main asteroid belt, Ceres by itself contains 35 percent of the total mass. Vesta has 10 percent of the total.
Readers with perfect memories may note that we used slightly smaller fractions in earlier Dawn Journals. Science advances! More recent estimates of the mass of the asteroid belt are slightly lower, so these percentages are now correspondingly higher. The difference is not significant, but the small increase only emphasizes how different Vesta and Ceres are from typical residents of the asteroid belt. It's also noteworthy -- or, at least, pretty cool -- that Dawn has single-handedly explored 45 percent of the mass between Mars and Jupiter.
Dawn will end its mission in the same orbit it is in now, looping around from a fraction of an inch (fraction of a centimeter) to a yard (a meter) from the soccer-ball-sized Ceres. In the previous Dawn Journal, we described what will happen onboard the spacecraft. We also saw that the most likely indication controllers will have that Dawn has run out of hydrazine will be its radio silence. They will take some carefully considered steps to verify that that is the correct conclusion.
But it is certain that emotions will be ahead of rationality. Even as team members are narrowing down the causes for the disappearance of the radio signal, many strong feelings about the end of the mission will arise. And they will be as varied as the people on the Dawn team, every one of whom has worked long and hard to make the mission so successful. Your correspondent can make reasonable guesses about their feelings but won't be so presumptuous as to do so.
As for my own feelings, well, I won't know until it happens, but I'm not too presumptuous to guess now. Long-time readers may recognize that your correspondent has avoided writing anything about himself (with a few rare exceptions), or even using first person, in his Dawn Journals. They are meant to be a record of a mission undertaken by humankind, for everyone who longs for knowledge and for adventures in the cosmos. But now I will devote a few words to my own perspective.
My love affair with the universe began when I was four, and my passion has burned brighter and brighter ever since. I knew when I was a starry-eyed nine-year-old that I wanted to get a Ph.D. in physics and work for NASA, although it was a few more years before I did. I had my own Galileo moment of discovery and awe when I first turned a telescope to the sky. Science and space exploration are part of me. They make me who I am. (My friend Mat Kaplan at The Planetary Society described me in the beginning of this video as "the ultimate space nerd." He's too kind!) Adding to my own understanding and contributing to humankind's knowledge are among my greatest rewards.
Passion and dedication are not the whole story. I recognize how incredibly lucky I am to be doing what I have loved for so long. I am lucky to have had access to the resources I have needed. I am lucky that I was able to do well in my formal education and in my own informal (but extensive) studies. I am lucky I could find the discipline and motivation within myself. For that matter, I am lucky to be able to communicate in terms that appeal to you, dear readers (or, at least, to some of you). My innate abilities and capabilities, and even many acquired ones, are, to a large extent, the product of factors out of my control, like my cognitive and psychological constitution.
That luck has paid off throughout my time at JPL. Working there has been a dream come true for me. It is so cool! I often have what amount to out-of-body experiences. When I am discussing a scientific or engineering point, or when I am explaining a conclusion or decision, sometimes a part of me pulls back and looks at the whole scene. Gosh! Listen to the cool things I get to say! Look at the cool things I get to do! Look at the cool things I know and understand! Imagine the cool spacecraft I'm working with and the cool world it is orbiting! I am still that starry-eyed kid, yet somehow, through luck and coincidence, I am doing the kind of things I love and once could only have dreamed of.
Dawn will continue to be exciting to the very end, performing new and valuable observations as it skims incredibly low over the dwarf planet on every orbital revolution. The spacecraft has almost always either been collecting new data or, thanks to the amazing ion propulsion, flying on a blue beam of xenon ions to somewhere else to gain a new perspective, see new sights and make more discoveries. Whether in orbit around Vesta or Ceres or traveling through the solar system between worlds, the mission was rarely anything like routine.
I love working on Dawn (although it was not my first space love). You won't be surprised that I think it is really cool. I could not be happier with its successes. I am not sad it is ending. I am thrilled beyond belief that it achieved so much!
I was very saddened in graduate school when my grandfather died. When I said something about it in my lab to a scientist from Shanghai I was working with, he asked how old my grandfather was. When I said he was 85, the wiser gentleman's smile lit up and he said, "Oh, you should be happy." And immediately I was! Of course I should be happy -- my grandfather had lived a long (and happy) life.
And so has Dawn. It has overcome problems not even imagined when we were designing and building it. It not only exceeded all of its original goals, but it has accomplished ambitious objectives not even conceived of until after it had experienced what could have been mission-ending failures. It has carried me, and uncounted others (including, I hope, you), on a truly amazing and exciting deep-space adventure with spectacular discoveries. Dawn is an extraordinary success by any measure.
It did not come easily. Dawn has consumed a tremendous amount of my life energy, many times at the expense of other desires and interests. (Perhaps ironically, it even comes at the expense of my many other deep interests in space exploration and in science, such as cosmology and particle physics, interests shared by my cats Quark and Lepton. Also, writing these Dawn Journals and doing my other outreach activities take up a very large fraction of what would otherwise be my personal time. As a result, I always write these in haste, and I'm never satisfied with them. That applies to this one as well. But I must rush ahead.) The challenges and the demands have been enormous, sometimes feeling insurmountable. That would not have been my preference, of course, yet it makes the endeavor's successful outcome that much more gratifying.
At the same time I have felt all the pressure, I have long been so overjoyed with the nature of the mission, I will miss it. There is nothing quite like controlling a spacecraft well over a thousand times farther than the Moon, farther even than the Sun. Silly, trite, perhaps even mawkish though it may seem, when spacecraft I have been responsible for have passed on the far side of the Sun, I have taken those opportunities to use that blinding signpost to experience some of the awe of the missions. I block the Sun with my hand and contemplate the significance, both to this particular big, starry-eyed kid and to humankind, of such an alignment. I -- we -- have a spacecraft on the far side of the Sun!
Every day I feel exhilarated knowing that, as my car's license plate frame proclaims, my other vehicle is in the main asteroid belt. It won't be the same when that vehicle is no longer operating.
But I will always have the memories, the thrills, the deep and powerful personal gratification. And I have good reason to believe they will persist, just as some prior space experiences still fill me with gratitude, pride, excitement and pure joy. (I also hope to have many more cool out-of-body experiences.)
And long after I'm gone and forgotten, Dawn’s successes will still be important. Its place in the annals of space exploration will be secure: a wealth of marvelous scientific discoveries, the first spacecraft to orbit an object in the asteroid belt, the first spacecraft to visit a dwarf planet (indeed, the first spacecraft to visit the first dwarf planet that was discovered), the first spacecraft to orbit a dwarf planet, the first spacecraft to orbit any two extraterrestrial destinations, and more.
For now, Dawn is continuing to operate beautifully (and you can read about it in subsequent Dawn Journals). The end of the mission, when it comes, will be bittersweet for me, a time to reflect and rejoice at how fantastically well it has gone, and a time to grieve that it is no more. I will have many powerful and conflicting feelings. Like Walt Whitman, I am large, I contain multitudes.
Thanks to Dawn, we now have Vesta and we now have Ceres. Soon, very soon, Dawn will be only a memory (save for those who visit Ceres and find it still in orbit) but the worlds it revealed will forever be a part of our intellectual universe, and the capabilities to explore the solar system that it advanced and devised will be applied to exciting new missions. And the experience of being intimately involved in this grand adventure will remain with me for as long as I am able to see the night sky and marvel at the mysteries of the universe that captivated me even as a starry-eyed child.
Dawn is 1,500 miles (2,400 kilometers) from Ceres. It is also 3.46 AU (322 million miles, or 518 million kilometers) from Earth, or 1,275 times as far as the Moon and 3.42 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 58 minutes to make the round trip.
Dr. Marc D. Rayman
10:00 pm PDT August 22, 2018
A fantastic story of adventure, exploration and discovery is reaching its denouement. In the final phase of its long and productive deep-space mission, Dawn is operating flawlessly in orbit around dwarf planet Ceres.
As described in the previous Dawn Journal, every 27 hours the spacecraft swoops as low as about 22 miles (35 kilometers) above the ground, taking stunning pictures and making other unique, valuable measurements with its suite of sophisticated sensors. It then soars up to 2,500 miles (4,000 kilometers) over the alien world before diving down again.
While it is too soon to reach clear conclusions from the wealth of high-resolution data, some of the questions already raised are noteworthy: Are the new pictures totally awesome or are they insane? Are they incredible or are they unbelievable? Are they amazing or are they spectacular? It may take years to resolve such questions. The mission will end long before then, indeed very soon. In this Dawn Journal and the next one (which will be posted in about three Cerean days), we will preview the end.
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
Dear Glutdawnous Readers,
The distant dwarf planet that Dawn is circling is full of mystery and yet growing ever more familiar.
Ceres, which only last year was hardly more than a fuzzy blob against the stars, is now a richly detailed world, and our portrait grows more elaborate every day. Having greatly surpassed all of its original objectives, the reliable explorer is gathering still more data from its unique vantage point. Everyone who hungers for new knowledge about the cosmos or for bold adventures far from Earth can share in the sumptuous feast Dawn has been serving.
One of the major objectives of the mission was to photograph 80 percent of Ceres' vast landscape with a resolution of 660 feet (200 meters) per pixel. That would provide 150 times the clarity of the powerful Hubble Space Telescope. Dawn has now photographed 99.8 percent with a resolution of 120 feet (35 meters) per pixel.
This example of Dawn's extraordinary productivity may appear to be the limit of what it could achieve. After all, the spaceship is orbiting at an altitude of only 240 miles (385 kilometers), closer to the ground than the International Space Station is to Earth, and it will never go lower for more pictures. But it is already doing more.
Since April 11, instead of photographing the scenery directly beneath it, Dawn has been aiming its camera to the left and forward as it orbits and Ceres rotates. By May 25, it will have mapped most of the globe from that angle. Then it will start all over once more, looking instead to the right and forward from May 27 through July 10. The different perspectives on the terrain make stereo views, which scientists can combine to bring out the full three dimensionality of the alien world. Dawn already accomplished this in its third mapping orbit from four times its current altitude, but now that it is seeing the sights from so much lower, the new topographical map will be even more accurate.
Dawn is also earning extra credit on its assignment to measure the energy of gamma rays and neutrons. We have discussed before how the gamma ray and neutron detector (GRaND) can reveal the atomic composition down to about a yard (meter) underground, and last month we saw initial findings about the distribution of hydrogen. However, Ceres' nuclear glow is very faint. Scientists already have three times as much GRaND data from this low altitude as they had required, and both spectrometers in the instrument will continue to collect data. In effect, Dawn is achieving a longer exposure, making its nuclear picture of Ceres brighter and sharper.
In December we explained how using the radio signal to track the probe's movements allows scientists to chart the gravity field and thereby learn about the interior of Ceres, revealing regions of higher and lower density. Once again, Dawn performed even better than expected and achieved the mission's planned accuracy in the third mapping orbit. Because the strength of the dwarf planet's gravitational tug depends on the distance, even finer measurements of how it varies from location to location are possible in this final orbit. Thanks to the continued smooth operation of the mission, scientists now have a gravitational map fully twice as accurate as they had anticipated. With additional measurements, they may be able to squeeze out a little more detail, perhaps improving it by another 20 percent before reaching the method's limit.
Dawn has dramatically overachieved in acquiring spectra at both visible and infrared wavelengths. We have previously delved into how these measurements reveal the minerals on the ground and what some of the interesting discoveries are. Having already acquired more than seven times as many visible spectra and 21 times as many infrared spectra as originally called for, the spacecraft is adding to its riches with additional measurements. We saw in January that VIR has such a narrow view that it will never see all of Ceres from this close, so it is programmed to observe features that have caught scientists' interest based on the broad coverage from higher altitudes.
Dawn's remarkable success at Ceres was not a foregone conclusion. Of course, the flight team has confronted the familiar challenges people encounter every day in the normal routine of piloting an ion-propelled spaceship on a multibillion-mile (multibillion-kilometer) interplanetary journey to orbit and explore two uncharted worlds. But the mission was further complicated by the loss of two of the spacecraft's four reaction wheels, as we have recounted before. (In full disclosure, the devices aren’t actually lost. We know precisely where they are. But given that one stopped functioning in 2010 and the other in 2012, they might as well be elsewhere in the universe; they don’t do Dawn any good.) Without three of these units to control its orientation in space, the robot has relied on its limited supply of hydrazine, which was not intended to serve this function. But the mission's careful stewardship of the precious propellant has continued to exceed even the optimistic predictions, allowing Dawn good prospects for carrying on its fruitful work. In an upcoming Dawn Journal, we will discuss how the last of the dwindling supply of hydrazine may be used for further discoveries.
In the meantime, Dawn is continuing its intensive campaign to reveal the dwarf planet's secrets, and as it does so, it is passing several milestones. The adventurer has now been held in Ceres' tender but firm gravitational embrace longer than it was in orbit around Vesta. (Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, and its mission would have been impossible without ion propulsion.) The spacecraft provided us with about 31,000 pictures of Vesta, and it has now acquired the same number of Ceres.
For an interplanetary traveler, terrestrial days have little meaning. They are merely a memory of how long a faraway planet takes to turn on its axis. Dawn left that planet long ago, and as one of Earth's ambassadors to the cosmos, it is an inhabitant of deep space. But for those who keep track of its progress yet are still tied to Earth, on May 3 the journey will be pi thousand days long. (And for our nerdier friends and selves, it will be shortly after 6:47 p.m. PDT.)
By any measure, Dawn has already accomplished an extraordinary mission, and there is more to look forward to as its ambitious expedition continues.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.73 AU (346 million miles, or 558 million kilometers) from Earth, or 1,455 times as far as the moon and 3.70 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and two minutes to make the round trip.
Dear Resplendawnt Readers,
Scientists are still working on refining their understanding of this striking region.
One year after taking up its new residence in the solar system, Dawn is continuing to witness extraordinary sights on dwarf planet Ceres. The indefatigable explorer is carrying out its intensive campaign of exploration from a tight orbit, circling its gravitational master at an altitude of only 240 miles (385 kilometers).
Even as we marvel at intriguing pictures and other discoveries, scientists are still in the early stages of putting together the pieces of the big puzzle of how (and where) Ceres formed, what its subsequent history has been, what geological processes are still occurring on this alien world and what all that reveals about the solar system.
For many readers who have not visited Ceres on their own, Occator Crater is the most mysterious and captivating feature. (To resolve the mystery of how to pronounce it, listen to the animation below.) As Dawn peered ahead at its destination in the beginning of 2015, the interplanetary traveler observed what appeared to be a bright spot, a shining beacon guiding the way for a ship sailing on the celestial seas. With its mesmerizing glow, the uncharted world beckoned, and Dawn answered the cosmic invitation by venturing in for a closer look, entering into Ceres' gravitational embrace. The latest pictures are one thousand times sharper than those early views. What was not so long ago a single bright spot has now come into focus as a complex distribution of reflective material in a 57-mile (92-kilometer) crater.
As we described in December, it seems that following the powerful impact that excavated Occator Crater, underground briny water reached the surface. The detailed photographs show many fractures cutting across the bright areas, and perhaps they provided a conduit. Water, whether as liquid or ice, would not last long there in the cold vacuum, eventually subliming. When the water molecules disperse, either escaping from Ceres into space or falling back to settle elsewhere, the dissolved salts are left behind. This reflective residue covers the ground, making the spellbinding and beautiful display Dawn now reveals.
While the crater is estimated to be a geological youngster at 80 million years old, that is an extremely long time for the material to remain so reflective. Exposed for so long to cosmic radiation and pelting from the rain of debris from space, it should have darkened. Scientists don't know (yet) what physical process are responsible, but perhaps it was replenished long after the crater itself formed, with more water, carrying dissolved salts, finding its way to the surface. As their analyses of the photos and spectra continue, scientists will gain a clearer picture and be able to answer this and other questions.
These latest Occator pictures did not come easily. Orbiting so close to Ceres, the adventurer’s camera captures only a small scene at a time, and it is challenging to cover the entirety of the expansive terrain. (Perhaps it comes as a surprise to those who have not read at least a few of the 123 Dawn Journals that precede this one that operating a spacecraft closer to a faraway dwarf planet than the International Space Station is to Earth is not as easy as, say, thinking about it.) But the patience and persistence in photographing the exotic landscapes have paid off handsomely.
We now have high resolution pictures of essentially all of Ceres save the small area around the south pole cloaked in the deep dark of a long winter night. Seasons last longer on Ceres than on Earth, and Dawn may not operate there long enough for the sun to rise at the south pole. By the beginning of southern hemisphere spring in November 2016, Dawn's mission to explore the first dwarf planet discovered may have come to its end.
In addition to photographing Ceres, Dawn conducts many other scientific observations, as we described in December and January. Among the probe's objectives at Ceres is to provide information for scientists to understand how much water is there, where it is, what form it is in and what role it plays in the geology.
We saw that extensive measurements of the faint nuclear radiation can help identify the atomic constituents. While the analysis of the data is complicated, and much more needs to be done, a picture is beginning to emerge from Dawn's neutron spectrometer (part of the gamma ray and neutron detector, GRaND). These subatomic particles are emitted from the nuclei of atoms buried within about a yard (meter) of the surface. Some manage to penetrate the material above them and fly into space, and the helpful ones then meet their fate upon hitting GRaND in orbit above. (Most others, however, will continue to fly through interplanetary space, decaying into a trio of other subatomic particles in less than an hour.) Before it escapes from the ground, a neutron's energy (and, equivalently, its speed) is strongly affected by any encounters with the nuclei of hydrogen atoms (although other atomic interactions can change the energy too). Therefore, the neutron energies can indicate to scientists the abundance of hydrogen. Among the most common forms in which hydrogen is found is water (composed of two hydrogen atoms and one oxygen atom), which can occur as ice or tied up in hydrated minerals.
GRaND shows Ceres is rich in hydrogen. Moreover, it detects more neutrons in an important energy range near the equator than near the poles, likely indicating there is more hydrogen, and hence more (frozen) water, in the ground at the high latitudes. Although Ceres is farther from the sun than Earth, and you would not consider it balmy there, it still receives some warmth. Just as at Earth, the sun's heating is less effective closer to the poles than at low latitudes, so this distribution of ice in the ground may reflect the temperature differences. Where it is warmer, ice close to the surface would have sublimed more quickly, thus depleting the inventory compared to the cooler ground far to the north or south.
Dawn spends most of its time measuring neutrons (and gamma rays), so it is providing a great deal of new data. And as scientists conduct additional analyses, they will learn more about the ice and other materials beneath the surface.
Another spectrometer is providing more tantalizing clues about the composition of Ceres, which is seen to vary widely. As the dwarf planet is not simply a huge rock but is a geologically active world, it is no surprise that it is not homogeneous. We discussed in December that the infrared mapping spectrometer had shown that minerals known as phyllosilicates are common on Ceres. Further studies of the data show evidence for the presence of two types: ammoniated phyllosilicates (described in December) and magnesium phyllosilicates. Scientists also find evidence of compounds known as carbonates, minerals that contain carbon and oxygen. There is also a dark substance in the mix that has not been identified yet.
And in one place (so far) on Ceres, this spectrometer has directly observed water, not below the surface but on the ground. The infrared signature shows up in a small crater named Oxo. (For the pronunciation, listen to the animation below.) As with the neutron spectra, it is too soon to know whether the water is in the form of ice or is chemically bound up in minerals.
At six miles (10 kilometers) in diameter, Oxo is small in comparison to the largest craters on Ceres, which are more than 25 times wider. (While geologists consider it a small crater, you might not agree if it formed in your backyard. Also note that when we showed Oxo Crater before, the diameter was slightly different. The crater's size has not changed since then, but as we receive sharper pictures, our measurements of feature sizes do change.) Dawn's first orbital destination, the fascinating protoplanet Vesta, is smaller than Ceres and yet has two craters far broader than the largest on Ceres. Based on studies of craters observed throughout the solar system, scientists have established methods of calculating the number and sizes of craters that could be formed on planetary surfaces. Those techniques show that Ceres is deficient in large craters. That is, more should have formed than appear in Dawn's pictures. Many other bodies (including Vesta and the moon) seem to preserve their craters for much longer, so this may be a clue about internal geological processes on Ceres that gradually erase the large craters.
Scientists are still in the initial stages of digesting and absorbing the tremendous wealth of data Dawn has been sending to Earth. The benefit of lingering in orbit (enabled by the remarkable ion propulsion system), rather than being limited to a brief glimpse during a fast flyby, is that the explorer can undertake much more thorough studies, and Dawn is continuing to make new measurements.
As recently as one year ago, controllers (and this writer) had great concern about the spacecraft's longevity given the loss of two reaction wheels, which are used for controlling the ship's orientation. And in 2014, when the flight team worked out the intricate instructions Dawn would follow in this fourth and final mapping orbit, they planned for three months of operation. That was deemed to be more than enough, because Dawn only needed half that time to accomplish the necessary measurements. Experienced spacecraft controllers recognize that there are myriad ways beautiful plans could go awry, so they planned for more time in order to ensure that the objectives would be met even if anomalies occurred. They also were keenly aware that the mission could very well conclude after three months of low altitude operations, with Dawn using up the last of its hydrazine. But their efforts since then to conserve hydrazine proved very effective. In addition, the two remaining wheels have been operating well since they were powered on in December, further reducing the consumption of the precious propellant.
As it turned out, operations have been virtually flawless in this orbit, and the first three months yielded a tremendous bounty, even including some new measurements that had not been part of the original plans. And because the entire mission at Ceres has gone so well, Dawn has not expended as much hydrazine as anticipated.
Dawn is now performing measurements that were not envisioned long in advance but rather developed only in the past two months, when it was apparent that the expedition could continue. And since March 19, Dawn has been following a new strategy to use even less hydrazine. Instead of pointing its sensors straight down at the scenery passing beneath it as the spacecraft orbits and Ceres rotates, the probe looks a little to the left. The angle is only five degrees (equal to the angle the minute hand of a clock moves in only 50 seconds, or less than the interval between adjacent minute tick marks), but that is enough to decrease the use of hydrazine and thus extend the spacecraft's lifetime. (We won't delve into the reason here. But for fellow nerds, it has to do with the alignment of the axes of the operable reaction wheels with the plane in which Dawn rotates to keep its instruments pointed at Ceres and its solar arrays pointed at the sun. The hydrazine saving depends on the wheels' ability to store angular momentum and applies only in hybrid control, not in pure hydrazine control. Have fun figuring out the details. We did!)
The angle is small enough now that the pictures will not look substantially different, but they will provide data that will help determine the topography. (Measurements of gravity and the neutron, gamma ray and infrared spectra are insensitive to this angle.) Dawn took pictures at a variety of angles during the third mapping orbit at Ceres (and in two of the mapping orbits at Vesta, HAMO1 and HAMO2) in order to get stereo views for topography. That worked exceedingly well, and photos from this lower altitude will allow an even finer determination of the three dimensional character of the landscape in selected regions. Beginning on April 11, Dawn will look at a new angle to gain still another perspective. That will actually increase the rate of hydrazine expenditure, but the savings now help make that more affordable. Besides, this is a mission of exploration and discovery, not a mission of hydrazine conservation. We save hydrazine when we can in order to spend it when we need it. Dawn's charge is to use the hydrazine to accomplish important scientific objectives and to pursue bold, exciting goals that lift our spirits and fuel our passion for knowledge and adventure. And that is exactly what it is has done and what it will continue to do.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.90 AU (362 million miles, or 583 million kilometers) from Earth, or 1,505 times as far as the moon and 3.90 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and five minutes to make the round trip.
Dear Indawnbitably Successful Readers,
A story of intense curiosity about the cosmos, passionate perseverance and bold ingenuity, a story more than two centuries in the making, has reached an extraordinary point. It begins with the discovery of dwarf planet Ceres in 1801 (129 years before its sibling Pluto; each was designated a planet for a time). Protoplanet Vesta was discovered in 1807. Following 200 years of telescopic observations, Dawn's daring mission was to explore these two uncharted worlds, the largest, most massive residents of the main asteroid belt between Mars and Jupiter. And now, as of February 2016, the spacecraft has accomplished all of the objectives that NASA defined for it in 2004, even before construction began (and before the very first Dawn Journal, nearly a decade ago).More than eight years after leaving its erstwhile planetary home behind for an ambitious deep space adventure, Dawn has now collected all of the data originally planned. Indeed, even prior to this third intercalary day of its expedition, the probe had already actually sent back a great deal more data for all investigations, significantly exceeding not only the original goals but also new ones added after the ship had set sail on the interplanetary seas. While scientists have a great deal of work still ahead to translate the bounty of data into knowledge, which is the greatest joy of science, the spacecraft can continue its work with the satisfaction that it has fulfilled its purpose and achieved an outstandingly successful mission.
Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, which would have been impossible without its advanced ion propulsion system. It is the only spacecraft ever to orbit an object in the main asteroid belt. It is also the only spacecraft ever to orbit massive bodies (apart from the sun and Earth) that had not been visited first by a flyby spacecraft to characterize the gravity and other properties. (By the way, Ceres is one of eight solar system bodies that operating spacecraft are orbiting now. The others are the sun, Venus, Earth, the moon, comet Churyumov-Gerasimenko, Mars and Saturn.)
Now in its fourth and final mapping orbit at Ceres, at an altitude of 240 miles (385 kilometers), Dawn is closer to the exotic terrain than the International Space Station is to Earth. The benefit of being in orbit is that the probe can linger rather than take only a brief look during a fast flyby. Even though Dawn has met its full list of objectives at Ceres, it continues to return new, valuable pictures and other measurements to provide even greater insight into this relict from the dawn of the solar system. For example, it is acquiring more nuclear spectra with its gamma ray and neutron detector, sharpening its picture of some atomic elements on Ceres. In addition, taking advantage of its unique vantage point, Dawn is collecting more infrared spectra of locations that are of special interest and soon will also take color photos and stereo photos (as it did in the third mapping orbit) of selected areas.
Dawn has completed more than 600 revolutions since taking up residence one year ago. The first few orbits took several weeks each, but as the spacecraft descended and Ceres' gravitational embrace grew more firm, its orbital velocity increased and the orbital period decreased. Now circling in less than five and a half hours, Dawn has made 370 orbits since reaching this altitude on Dec. 7.
The pace of observations here is higher than in the previous mapping orbits, where the orbital periods were longer. The spacecraft flies over the landscape faster now, and being closer to the ground, its instruments discern much more detail but capture a smaller area. Mission controllers have developed intricate plans for observing Ceres, but those plans depend on the spacecraft being at the right place at the right time. As we will see below, however, sometimes it may not be.
Suppose, for example, the intent is to observe a particular feature, perhaps the bright center of Occator crater, the lonely, towering mountain Ahuna Mons, the fractures in Dantu crater or artificial structures that definitively prove the existence of extraterrestrial intelligence, utterly transforming our understanding of the cosmos and shattering our naive perspectives on life in the universe. Trajectory analysis indicates when Dawn will fly over the designated location, and engineers will program it to take pictures or infrared spectra at that time. They will also include some margin, so they may program it to start 10 minutes before and end 10 minutes after. But they can't afford to put in too much margin. Data storage on the spacecraft is limited, so other geological features could not be observed. Also, transmitting data to Earth requires pointing the main antenna at that distant planet instead of pointing sensors at Ceres, so it would be unwise to collect much more than is necessary.
Even if devoting additional time (and data) to trying to observe the desired place were feasible, it wouldn't necessarily solve the problem. Dawn travels in a polar orbit, which is the only way to ensure that it passes over all latitudes. While Dawn soars from north to south over the sunlit hemisphere making its observations, the dwarf planet itself rotates on its axis, so the ground moves from east to west. If the spacecraft arrives at the planned orbital location a little early or a little late, the feature of interest may not even be beneath it but rather could be too far east or west, out of view of the instruments. In that case, increasing the duration of the observation period doesn't help.
All of that is why, as we saw last month, it requires more pictures to fully map Ceres than you might expect. Many pictures may have to be taken in order to fill in gaps, and quite a few of the pictures overlap with others. Nevertheless, Dawn has done an excellent job. The spacecraft has photographed 99.6 percent of the dwarf planet from this low altitude. (If you aren't regularly visiting the image gallery, you are missing out on some truly out-of-this-world scenes.)
The flight team devises very detailed plans that tell the spacecraft what to do every second, including where to point and what data to collect with each sensor. When the observation plans are developed, they are checked and double-checked. Then they are translated into the appropriate software that the robotic ship will understand, and these instructions are checked and double-checked. That is integrated with all the other software that will be beamed to the spacecraft covering the same period of time, any conflicts are resolved and then the final version is checked and, well, you know.
This process is very involved, and it is usually well over a month between the formulation and the execution of the plan. During that time, Dawn's orbit can deviate slightly from the expert navigators' mathematical predictions, preventing the spacecraft from flying over the desired targets. There are several reasons the actual orbit may differ from the orbit used for developing the plan. (We have seen related examples of this, including as Dawn approached Mars, when it orbited Vesta and when it spiraled from one mapping orbit to another.) Let's briefly consider two.
One reason is that we do not have perfect knowledge of the variations in the strength of Ceres' gravitational pull from one location to another. We have discussed before that measuring these tiny irregularities in the gravity field provides insight into the distribution of mass within the dwarf planet that gives rise to them. The team has mapped the hills and valleys of the field quite well and even better than expected. Still, the remaining small uncertainty can lead to slight differences between what navigators calculate Dawn's motion will be and what its actual motion will be as it is buffeted by the gravitational currents.
A second source of discrepancy is that Dawn's own activities distort its orbit. Every time the reaction control system expels a tiny burst of hydrazine to control the spacecraft's orientation, keeping it pointed at its target, the force not only affects the orientation but also nudges the probe in its orbit, slowing it down or speeding it up very slightly. It's up to the spacecraft to decide exactly when to make these small adjustments, and it is not possible for controllers to predict their timing. (In a similar way, when you are driving, you occasionally move the steering wheel to keep going the direction you want, even if is straight ahead. It would be impossible to forecast each tiny movement, because they all depend on what has already happened plus the exact conditions at the moment.) The details of the reaction control system activity also depend on the use of the novel hybrid control scheme, which the joint Orbital/JPL team developed because of the failure of two of the spacecraft's four reaction wheels. The effect of each small firing of hydrazine is very small, but they can add up.
It took about a month in this mapping orbit to discover many of the subtleties of the gravity field and gain experience with how hybrid control affects the orbit. But even before descending to this altitude, the operations team understood the nature of these effects and was well prepared to deal with them.
They devised several strategies, all of which are being used to good effect. One of the ways to account for Dawn's actual orbit differing from its planned orbit is simply to change the orbit. Simply? Well, not really. It turns out to that to analyze the orbit and then maneuver to correct it in a timely way is a surprisingly complicated process, but, come to think of it, what isn't complicated when flying a spaceship around a distant, alien world? Nevertheless, every three weeks, the flight team makes a careful assessment of the orbit and determines whether a small refinement with the ion propulsion system is in order. For technical reasons, if maneuvers are needed, they will be executed in pairs, so mission planners have scheduled two windows (each 12 hours long and separated by eight days) about every 23 days.
Adjustments to resynchronize the actual orbit with the predicted orbit that formed the basis of the exploration plan are known as “orbit maintenance maneuvers.” Succumbing to instincts developed during their long evolutionary history, engineers refer to them by an acronym: OMM. (As the common thread among team members is their technical training and passion for the exploration of the cosmos, and not Buddhism, the term is spoken by naming the letters, not pronouncing it as if it were a means of achieving inner peace. Instead, it may be thought of as a means of achieving orbital tranquility and harmony.)
For both Vesta and Ceres, trajectory analyses long in advance determined that OMMs would not be needed in the higher orbits, so no windows were included in those schedules. There have been three OMM opportunities since arriving at the lowest altitude above Ceres, but only the first was needed. Dawn performed the pair on Dec. 31-Jan. 1 and on Jan. 8 with its famously efficient ion engine. The orbit was good enough the next two times that OMMs were deemed unnecessary. It is certain that some future OMMs will be required. Your faithful correspondent provides frequent (and uncharacteristically concise) reports on Dawn's day-to-day activities, including OMMs.
By the end of the Jan. 8 OMM, Dawn's ion propulsion system had accumulated 2,019 days of operation in space, more than 5.5 years. During that time, the effective change in speed was 24,600 mph (39,600 kilometers per hour). (We have discussed in detail that this is not Dawn's current speed but rather the amount by which the ion engines have changed it.) This is uniquely high for a spacecraft to accomplish with its own propulsion system and validates our description of ion propulsion as delivering acceleration with patience. (The previous record holder, Deep Space 1, achieved 9,600 mph, or 15,000 kilometers per hour.)
The effect of Dawn's gentle ion thrusting during its mission has been nearly the same as that of the entire Delta II 7925H-9.5 rocket, with its nine external rocket engines, first stage, second stage and third stage. To get started on its interplanetary adventure, Dawn's rocket boosted it from Cape Canaveral to out of Earth orbit with only four percent higher velocity than Dawn subsequently added on its own with its ion engines.
As Dawn and Earth follow their own independent orbits around the sun (Dawn's now tied permanently to its gravitational master, Ceres), next month they will reach their greatest separation of the entire mission. On March 4 (about one Earth year after Ceres took hold of Dawn), on opposite sides of the solar system, they will be 3.95278 AU (367.434 million miles, or 591.328 million kilometers) from each other. (For those of you with full schedules, note that the maximum separation will be 5:40 a.m. PST.) They won't be this far apart again until Feb. 6, 2025, long after Dawn has ceased operating (as discussed below). The figure below depicts the arrangement next month.
Dawn has faced many challenges in its unique voyage in the forbidding depths of space, but it has surmounted all of them. It has even overcome the dire threat posed by the loss of two reaction wheels (the second failure occurring in orbit around Vesta 3.5 years and 1.3 billion miles, or 2.0 billion kilometers, ago). With only two operable reaction wheels (and those no longer trustworthy), the ship's remaining lifetime is very limited.
A year ago, the team couldn't count on Dawn even having enough hydrazine to last beyond next month. But the creative methods of conserving that precious resource have proved to be quite efficacious, and the reliable explorer still has enough hydrazine to continue to return bonus data for a while longer. Now it seems highly likely that the spacecraft will keep functioning through the scheduled end of its primary mission on June 30, 2016.
NASA may choose to continue the mission even after that. Such decisions are difficult, as there is literally an entire universe full of interesting subjects to study, but resources are more limited. In any case, even if NASA extended the mission, and even if the two wheels operated without faltering, and even if the intensive campaign of investigating Ceres executed flawlessly, losing not an ounce (or even a gram) of hydrazine to the kinds of glitches that can occur in such a complex undertaking, the hydrazine would be exhausted early in 2017. Clearly an earlier termination remains quite possible.
Regardless of when Dawn's end comes, it will not be a time for regret. The mission has realized its raison d'être and is reaping rewards even beyond those envisioned when it was conceived. It has taken us all on a marvelous interplanetary journey and allowed us to behold previously unseen sights of distant lands. The conclusion of the mission will be a time for gratitude that it was so successful. And until then, every new picture or other measurement adds to the richly detailed portrait of a faraway, exotic world. There is plenty more still to do before this remarkable story draws to a close.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.95 AU (367 million miles, or 591 million kilometers) from Earth, or 1,475 times as far as the moon and 3.99 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and six minutes to make the round trip.
A veteran interplanetary traveler is writing the closing chapter in its long and storied expedition. In its final orbit, where it will remain even beyond the end of its mission, at its lowest altitude, Dawn is circling dwarf planet Ceres, gathering an album of spellbinding pictures and other data to reveal the nature of this mysterious world of rock and ice.
Ceres turns on its axis in a little more than nine hours (one Cerean day). Meanwhile, its new permanent companion, a robotic emissary from Earth, revolves in a polar orbit, completing a loop in slightly under 5.5 hours. It flies from the north pole to the south over the side of Ceres facing the sun. Then when it heads north, the ground beneath it is cloaked in the deep dark of night on a world without a moon (save Dawn itself). As we discussed last month, Dawn's primary measurements do not depend on illumination. It can sense the nuclear radiation (specifically, gamma rays and neutrons) and the gravity field regardless of the lighting. This month, let's take a look at the other measurements our explorer is performing, most of which do depend on sunlight.
Of course the photographs do. Dawn had already mapped Ceres quite thoroughly from higher altitudes. The spacecraft acquired an extensive set of stereo and color pictures in its third mapping orbit. But now that Dawn is only about 240 miles (385 kilometers) high, its images are four times as sharp, revealing new details of the strange and beautiful landscapes.
Our spaceship is closer to Ceres than the International Space Station is to Earth. At that short range, it takes a long time to capture all of the vast territory, because each picture covers a relatively small area. Dawn’s camera sees a square about 23 miles (37 kilometers) on a side, less than one twentieth of one percent of the more than one million square miles (nearly 2.8 million square kilometers). In an ideal world (which is not the one Dawn is in or at), it would take just over two thousand photos from this altitude to see all the sights. However, as we will discuss in more detail next month, it is not possible to control the orbital motion and the pointing of the camera accurately enough to manage without more photos than that.
Most of the time, Dawn is programmed to turn at just the right rate to keep looking at the ground beneath it as it travels, synchronizing its rotation with its revolution around Ceres. It photographs the passing scenery, storing the pictures for later transmission to Earth. But some of the time, it cannot take pictures, because to send its bounty of data, it needs to point its main antenna at that distant planet, home not only to its controllers but also to many others (including you, loyal reader) who share in the thrill of a bold cosmic adventure. Dawn spends about three and a half days (nine Cerean days) with its camera and other sensors pointed at Ceres. Then it radios its findings home for a little more than one day (almost three Cerean days). During these communications sessions, even when it soars over lit terrain, it does not observe the sights below.
Mission planners have devised an intricate plan that should allow nearly complete coverage in about six weeks. To accomplish this, they guided Dawn to a carefully chosen orbit, and it has been doing an exceptionally good job there executing its complex activities.
Last month, we marveled at a stunning view that was not the typical perspective of peering straight down from orbit. Sometimes controllers now program Dawn to take a few more pictures after it stops aiming its instruments down, while it starts to turn to aim its antenna to Earth. This clever idea provides bonus views of whatever happens to be in the camera's sights as it slowly rotates from the point beneath the spacecraft off to the horizon. Who doesn't feel the attraction of the horizon and long to know what lies beyond?
Another of Dawn's scientific devices is two different sensors combined into one instrument. Like the camera, the visible and infrared mapping spectrometers (VIR) look at the sunlight reflected from the ground. (As we'll see below, however, VIR also can detect something more.) A spectrometer breaks up light into its constituent colors, just as a prism or a droplet of water does when revealing, quite literally, all the colors of the rainbow. Dawn's visible spectrometer would have a view very much like that. The infrared spectrometer, of course, looks at wavelengths of light our limited eyes cannot see, just as there are wavelengths of sound our limited ears cannot hear (consult with your dog for details).
A spectrometer does more than simply disperse the light into its components, however. It measures the intensity of that light at the different wavelengths. The materials on the surface leave their signature in the sunlight they reflect, making some wavelengths relatively brighter and some dimmer. That characteristic pattern is called a spectrum. By comparing these spectra with spectra measured in laboratories, scientists can infer the nature of the minerals on the ground. We described some of the intriguing conclusions last month.
VIR does still more. Rather than record the visible spectrum and the infrared spectrum from a single region, it takes spectra at 256 adjacent locations simultaneously. This would be like taking one column of 256 pixels in a picture and having a separate spectrum for each. By stitching columns together, you could construct the two dimensional picture but with the added dimension of an extensive spectrum at every location. (Because the extra information provides a sort of depth that flat pictures don't have, the result is sometimes called an “image cube.”) This capability to build up an image with spectra everywhere is what makes it a mapping spectrometer. VIR produces a remarkably rich view of its targets!
VIR's spectra contain much finer measurements of the colors and a wider range of wavelengths than the camera's images. In exchange, the camera has sharper vision and so can discern smaller geological features. In more technical terms, VIR achieves better spectral resolution and the camera achieves better spatial resolution. Fortunately, it is not a competition, because Dawn has both, and the instruments yield complementary measurements.
VIR generates a very large volume of data in each snapshot. As a result, Dawn can only capture and store relatively small areas of the dwarf planet with the mapping spectrometers, especially at this low altitude. Scientists have recognized from the first design of the mission that it would not be possible to cover all of Ceres (or Vesta) with VIR from the closer orbits. Nevertheless, Dawn has far exceeded expectations, returning a great many more spectra than anticipated. Still, as long as the spacecraft operates in this final mapping orbit, there will continue to be interesting targets to study with VIR.
Based on the nearly 20 million spectra of Ceres that VIR acquired from higher altitudes, the team has determined that new infrared spectra will provide more insight into the dwarf planet's character than the visible spectra. Because of their composition, the minerals display more salient signatures in infrared wavelengths than visible. The excellent visible spectra from the first three mapping orbits are deemed more than sufficient. Therefore, to make the best use of our faithful probe and to dedicate the resources to what is most likely to yield new knowledge about Ceres, VIR is devoting its share of the mission data in this final orbit to its infrared mapping spectrometer. We have many more exciting discoveries to look forward to!
The infrared light Ceres reflects from the sun can tell scientists a great deal about the composition, but they can learn even more from analyzing VIR's measurements. The sun isn't the only source of infrared. Ceres itself is. Many people correctly associate infrared with heat, because warm objects emit infrared light, and the strength at different wavelengths depends on the temperature. That calls for measuring the spectrum! Distant from the sun though it is, Ceres is warmed slightly by the brilliant star, so it has a very faint infrared glow of its own. Scientists can distinguish in VIR's observations between the reflected infrared sunlight and the infrared light Ceres radiates. In essence, VIR can function as a remote thermometer.
Last month, in one of Dawn's best photos yet of Ceres, we considered planning a hike across a breathtaking landscape. In case we do, VIR has shown we should be prepared for chilly conditions. Observed temperatures (all rounded to the nearest multiple of five) during the day on the dwarf planet range from -135 degrees Fahrenheit (-95 degrees Celsius) to -30 degrees Fahrenheit (-35 degrees Celsius). (It is so cold in some locations and times, especially at night, that Ceres produces too little infrared light for VIR to measure. Temperatures below the coldest reported here actually don't register.) This finding provides compelling support for this writer's frequent claim that Ceres is really cool. In addition, knowing the temperatures will be very important for understanding geological processes on this icy, rocky world, just as we know the movement of terrestrial glaciers depends on temperature.
Your loyal correspondent can't -- or, at least, won't -- help but indulge his nerdiness with a brief tangent. The range of temperatures above represent the warmest on Ceres, given that VIR cannot measure lower values. It's amusing, if you have a similar weird sense of humor, that Ceres' average temperature apparently is not that far from what it would be for a black hole of the same mass. We won't delve into the physics here, but such a black hole would be -225 degrees Fahrenheit (-140 degrees Celsius). OK, enough hilarity. Back to Dawn and Ceres...
Ever creative, scientists are attempting another clever method to gain insight into the nature of this exotic orb. When Dawn is at just the right position in its orbit on the far side of Ceres, so that a straight line to Earth passes very close to the limb of Ceres itself, the spacecraft's radio signal will actually hit the dwarf planet. The radio waves interact with the materials on the surface, which can induce an exquisitely subtle distortion. After bouncing off the ground at a grazing angle, the radio signal continues on its way, heading toward Earth. The effect on the signal is much too small to affect the normal communications at all, but specialized equipment at NASA's Deep Space Network designed for this purpose might still be able to detect the tiny changes. The fantastically sensitive antennas measure the properties of the radio waves, and by studying the details, scientists may be able to learn more about the properties of the surface of the distant world. For example, this could help them distinguish between different types of materials (such as ice, rocks, sand, etc.) as well as reveal how rough or smooth the ground is at scales far, far smaller than the camera can discern. This is an extremely challenging measurement, and no small distortions have been detected so far, but always making the best possible use of the resources, scientists continue to look for them.
In addition to those bonus measurements, Dawn remains very productive in acquiring infrared spectra, photographs, gamma ray spectra and neutron spectra plus conducting measurements of the massive body's gravitational field, all of which contribute to unlocking the mysteries of the first dwarf planet ever discovered or explored. The venerable adventurer is in good condition and is operating flawlessly.
We have discussed extensively the failures of two of the four reaction wheels, devices Dawn used to depend on to control its orientation in space. Without three healthy reaction wheels, the probe has had to rely instead on hydrazine propellant expelled from the small jets of the reaction control system. (When Dawn uses its ion engine, that remarkable system does double duty, reducing the need for the hydrazine.)
For most of the time since escaping from Vesta's gravitational clutches in 2012, Dawn has kept the other two reaction wheels in reserve so any remaining lifetime from those devices could offset the high cost of hydrazine propellant to turn and point in this current tight orbit. Those two wheels have been on and functioning flawlessly since Dec. 14, 2015, and every day they operate, they keep the expenditure of the dwindling supply of hydrazine to half of what it would be without them. (Next month we will offer some estimates of how long Dawn might continue to operate.) But the ever-diligent team recognizes another wheel could falter at any moment, and they remain ready to continue the mission with pure hydrazine control after only a short recovery operation. If a third failure is at all like the two that have occurred already, the hapless wheel won't give an indication of a problem until it's too late. A reaction wheel failure evidently is entirely unpredictable. We'll know about it only after it occurs in the remote depths of space where Dawn resides at an alien world.
Earth and Ceres are so far from each other that their motions are essentially independent. The planet and the dwarf planet follow their own separate repetitive paths around the sun. And each carries its own retinue: Earth has thousands of artificial satellites and one prominent natural one, the moon. Ceres has one known satellite. It arrived there in March 2015, and its name is Dawn.
Coincidentally, both reached extremes earlier this month in their elliptical heliocentric orbits. Earth, in its annual journey around our star, was at perihelion, or the closest point to the sun, on Jan. 2, when it was 0.98 AU (91.4 million miles, or 147 million kilometers) away. Ceres, which takes 4.6 years (one Cerean year) for each loop, attained its aphelion, or greatest distance from the sun, on Jan. 6. On that day, it was 2.98 AU (277 million miles, or 445 million kilometers) from the gravitational master of the solar system.
Far, far from the planet where its deep-space voyage began, Dawn is now bound to Ceres, held in a firm but gentle gravitational embrace. The spacecraft continues to unveil new and fascinating secrets there for the benefit of all those who remain with Earth but who still look to the sky with wonder, who feel the lure of the unknown, who are thrilled by new knowledge, and who yearn to know the cosmos.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.87 AU (360 million miles, or 580 million kilometers) from Earth, or 1,440 times as far as the moon and 3.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and four minutes to make the round trip.
Dawn is now performing the final act of its remarkable celestial choreography, held close in Ceres’ firm gravitational embrace. The distant explorer is developing humankind’s most intimate portrait ever of a dwarf planet, and it likely will be a long, long time before the level of detail is surpassed.
The spacecraft is concluding an outstandingly successful year 1,500 times nearer to Ceres than it began. More important, it is more than 1.4 million times closer to Ceres than Earth is today. From its uniquely favorable vantage point, Dawn can relay to us spectacular views that would otherwise be unattainable. At an average altitude of only 240 miles (385 kilometers), the spacecraft is closer to Ceres than the International Space Station is to Earth. From that tight orbit, the dwarf planet looks the same size as a soccer ball seen from only 3.5 inches (9.0 centimeters) away. This is in-your-face exploration.
The spacecraft has returned more than 16,000 pictures of Ceres this year (including more than 2,000 since descending to its low orbit this month). One of your correspondent’s favorites (below) was taken on Dec. 10 when Dawn was verifying the condition of its backup camera. Not only did the camera pass its tests, but it yielded a wonderful, dramatic view not far from the south pole. It is southern hemisphere winter on Ceres now, with the sun north of the equator. From the perspective of the photographed location, the sun is near the horizon, creating the long shadows that add depth and character to the scene. And usually in close-in orbits, we look nearly straight down. Unlike such overhead pictures typical of planetary spacecraft (including Dawn), this view is mostly forward and shows a richly detailed landscape ahead, one you can imagine being in — a real place, albeit an exotic one. This may be like the breathtaking panorama you could enjoy with your face pressed to the porthole of your spaceship as you are approaching your landing sight. You are right there. It looks — it feels! — so real and physical. You might actually plan a hike across some of the terrain. And it may be that a visiting explorer or even a colonist someday will have this same view before setting off on a trek through the Cerean countryside.
Of course, Dawn's objectives include much more than taking incredibly neat pictures, a task at which it excels. It is designed to collect scientifically meaningful photos and other valuable measurements. We'll see more below about what some of the images and spectra from higher altitudes have revealed about Ceres, but first let's take a look at the three highest priority investigations Dawn is conducting now in its final orbit, sometimes known as the low altitude mapping orbit (LAMO). While the camera, visible mapping spectrometer and infrared mapping spectrometer show the surface, these other measurements probe beneath.
With the spacecraft this close to the ground, it can measure two kinds of nuclear radiation that come from as much as a yard (meter) deep. The radiation carries the signatures of the atoms there, allowing scientists to inventory some of the key chemical elements of geological interest. One component of this radiation is gamma ray photons, a high energy form of electromagnetic radiation with a frequency beyond visible light, beyond ultraviolet, even beyond X-rays. Neutrons in the radiation are entirely different from gamma rays. They are particles usually found in the nuclei of atoms (for those of you who happen to look there). Indeed, outweighing protons, and outnumbering them in most kinds of atoms, they constitute most of the mass of atoms other than hydrogen in Ceres (and everywhere else in the universe, including in your correspondent).
To tell us what members of the periodic table of the elements are present, Dawn's gamma ray and neutron detector (GRaND) does more than detect those two kinds of radiation. Despite its name, GRaND is not at all pretentious, but its capabilities are quite impressive. Consisting of 21 sensors, the device measures the energy of each gamma ray photon and of each neutron. (That doesn't lend itself to as engaging an acronym.) It is these gamma ray spectra and neutron spectra that reveal the identities of the atomic species in the ground.
Some of the gamma rays are produced by radioactive elements, but most of them and the neutrons are generated as byproducts of cosmic rays impinging on Ceres. Space is pervaded by cosmic radiation, composed of a variety of subatomic particles that originate outside our solar system. Earth's atmosphere and magnetic field protect the surface (and those who dwell there) from cosmic rays, but Ceres lacks such defenses. The cosmic rays interact with nuclei of atoms, and some of the gamma rays and neutrons that are released escape back into space where they are intercepted by GRaND on the orbiting Dawn.
Unlike the relatively bright light reflected from Ceres's surface that the camera, infrared spectrometer and visible spectrometer record, the radiation GRaND measures is very faint. Just as a picture of a dim object requires a longer exposure than for a bright subject, GRaND's "pictures" of Ceres require very long exposures, lasting weeks, but mission planners have provided Dawn with the necessary time. Because the equivalent of the illumination for the gamma ray and neutron pictures is cosmic rays, not sunlight, regions in darkness are no fainter than those illuminated by the sun. GRaND works on both the day side and the night side of Ceres.
In addition to the gamma ray spectra and neutron spectra, Dawn's other top priority now is measuring Ceres' gravity field. The results will help scientists infer the interior structure of the dwarf planet. The measurements made in the higher altitude orbits turned out to be even more accurate than the team had expected, but now that the probe is as close to Ceres as it will ever go, and so the gravitational pull is the strongest, they can obtain still better measurements.
Gravity is one of four fundamental forces in nature, and its extreme weakness is one of the fascinating mysteries of how the universe works. It feels strong to us (well, most of us) because we don't so easily sense the two kinds of nuclear forces, both of which extend only over extremely short distances, and we generally don't recognize the electromagnetic force. With both positive and negative electrical charges, attractive and repulsive electromagnetic forces often cancel. Not so with gravity. All matter exerts attractive gravity, and it can all add up. The reason gravity -- by far the weakest of the four forces -- is so salient for those of you on or near Earth is that there is such a vast amount of matter in the planet and it all pulls together to hold you down. Dawn overcame that pull with its powerful Delta rocket. Now the principal gravitational force acting on it is the cumulative effect of all the matter in Ceres, and that is what determines its orbital motion.
The spacecraft experiences a changing force both as the inhomogeneous dwarf planet beneath it rotates on its axis and as the craft circles that massive orb. When Dawn is closer to locations within Ceres with greater density (i.e., more matter), the ship feels a stronger tug, and when it is near regions with lower density, and hence less powerful gravity, the attraction is weaker. The spacecraft accelerates and decelerates very slightly as its orbit carries it closer to and farther from the volumes of different density. By carefully and systematically plotting the exquisitely small variations in the probe's motion, navigators can calculate how the mass is distributed inside Ceres, essentially creating an interior map. This technique allowed scientists to establish that Vesta, the protoplanet Dawn explored in 2011-2012, has a dense core (composed principally of iron and nickel) surrounded by a less dense mantle and crust. (That is one of the reasons scientists now consider Vesta to be more closely related to Earth and the other terrestrial planets than to typical asteroids.)
Mapping the orbit requires systems both on Dawn and on Earth. Using the large and exquisitely sensitive antennas of NASA's Deep Space Network (DSN), navigators measure tiny changes in the frequency, or pitch, of the spacecraft's radio signal, and that reveals changes in the craft's velocity. This technique relies on the Doppler effect, which is familiar to most terrestrial readers as they hear the pitch of a siren rise as it approaches and fall as it recedes. Other readers who more commonly travel at speeds closer to that of light recognize that the well-known blueshift and redshift are manifestations of the same principle, applied to light waves rather than sound waves. Even as Dawn orbits Ceres at 610 mph (980 kilometers per hour), engineers can detect changes in its speed of only one foot (0.3 meters) per hour, or one five-thousandth of a mph (one three-thousandth of a kilometer per hour). Another way to track the spacecraft is to measure the distance very accurately as it revolves around Ceres. The DSN times a radio signal that goes from Earth to Dawn and back. As you are reminded at the end of every Dawn Journal, those signals travel at the universal limit of the speed of light, which is known with exceptional accuracy. Combining the speed of light with the time allows the distance to be pinpointed. These measurements with Dawn's radio, along with other data, enable scientists to peer deep into the dwarf planet
Although it is not among the highest scientific priorities, the flight team is every bit as interested in the photography as you are. We are visual creatures, so photographs have a special appeal. They transport us to mysterious, faraway worlds more effectively than any propulsion system. Even as Dawn is bringing the alien surface into sharper focus now, the pictures taken in higher orbits have allowed scientists to gain new insights into this ancient world. Geologists have located more than 130 bright regions, none being more striking than the mesmerizing luster in Occator crater. The pictures taken in visible and infrared wavelengths have helped them determine that the highly reflective material is a kind of salt.
It is very difficult to pin down the specific composition with the measurements that have been analyzed so far. Scientists compare how reflective the scene is at different wavelengths with the reflective properties of likely candidate materials studied in laboratories. So far, magnesium sulfate yields the best match (although it is not definitive). That isn't the type of salt you normally put on your food (or if it is, I'll be wary about accepting the kind invitation to dine in your home), but it is very similar (albeit not identical) to Epsom salts, which have many other familiar uses.
Scientists' best explanation now for the deposits of salt is that when asteroids crash into Ceres, they excavate underground briny water-ice. Once on the surface and exposed to the vacuum of space, even in the freezing cold so far from the sun, the ice sublimes, the water molecules going directly from the solid ice to gas without an intermediate liquid stage. Left behind are the materials that had been dissolved in the water. The size and brightness of the different regions depend in part on how long ago the impact occurred. A very preliminary estimate is that Occator was formed by a powerful collision around 80 million years ago, which is relatively recent in geological times. (We will see in a future Dawn Journal how scientists estimate the age and why the pictures in this low altitude mapping orbit will help refine the value.)
As soon as Dawn's pictures of Ceres arrived early this year, many people referred to the bright regions as "white spots," although as we opined then, such a description was premature. The black and white pictures revealed nothing about the color, only the brightness. Now we know that most have a very slight blue tint. For reasons not yet clear, the central bright area of Occator is tinged with more red. Nevertheless, the coloration is subtle, and our eyes would register white.
Measurements with both finer wavelength discrimination and broader wavelength coverage in the infrared have revealed still more about the nature of Ceres. Scientists using data from one of the two spectrometers in the visible and infrared mapping spectrometer instrument (VIR) have found that a class of minerals known as phyllosilicates is common on Ceres. As with the magnesium sulfate, the identification is made by comparing Dawn's detailed spectral measurements with laboratory spectra of a great many different kinds of minerals. This technique is a mainstay of astronomy (with both spacecraft and telescopic observations) and has a solid foundation of research that dates to the nineteenth century, but given the tremendous variety of minerals that occur in nature, the results generally are neither absolutely conclusive nor extremely specific.
There are dozens of phyllosilicates on Earth (one well known group is mica). Ceres too likely contains a mixture of at least several. Other compounds are evident as well, but what is most striking is the signature of ammonia in the minerals. This chemical is manufactured extensively on Earth, but few industries have invested in production plants so far from their home offices. (Any corporations considering establishing Cerean chemical plants are invited to contact the Dawn project. Perhaps, however, mining would be a more appropriate first step in a long-term business plan.)
Ammonia's presence on Ceres is important. This simple molecule would have been common in the material swirling around the young sun almost 4.6 billion years ago when planets were forming. (Last year we discussed this period at the dawn of the solar system.) But at Ceres' present distance from the sun, it would have been too warm for ammonia to be caught up in the planet-forming process, just as it was even closer to the sun where Earth resides. There are at least two possible explanations for how Ceres acquired its large inventory of ammonia. One is that it formed much farther from the sun, perhaps even beyond Neptune, where conditions were cool enough for ammonia to condense. In that case, it could easily have incorporated ammonia. Subsequent gravitational jostling among the new residents of the solar system could have propelled Ceres into its present orbit between Mars and Jupiter. Another possibility is that Ceres formed closer to where it is now but that debris containing ammonia from the outer solar system drifted inward and some of it ultimately fell onto the dwarf planet. If enough made its way to Ceres, the ground would be covered with the chemical, just as VIR observed.
Scientists continue to analyze the thousands of photos and millions of infrared and visible spectra even as Dawn is now collecting more precious data. Next month, we will summarize the intricate plan that apportions time among pointing the spacecraft's sensors at Ceres to perform measurements, its main antenna at Earth to transmit its findings and receive new instructions and its ion engine in the direction needed to adjust its orbit.
The plans described last month for getting started in this fourth and final mapping orbit worked out extremely well. You can follow Dawn's activities with the status reports posted at least twice a week here. And you can see new pictures regularly in the Ceres image gallery.
We will be treated to many more marvelous sights on Ceres now that Dawn's pictures will display four times the detail of the views from its third mapping orbit. The mapping orbits are summarized in the following table, updated from what we have presented before. (This fourth orbit is listed here as beginning on Dec. 16. In fact, the highest priority work, which is obtaining the gamma ray spectra, neutron spectra and gravity measurements, began on Dec. 7, as explained last month. But Dec. 16 is when the spacecraft started its bonus campaign of measuring infrared spectra and taking pictures. Recognizing that what most readers care about is the photography, regardless of the scientific priorities, that is the date we use here.
|Mapping orbit||Dawn code name||Dates||Altitude in miles (kilometers)||Resolution in feet (meters) per pixel||Resolution compared to Hubble||Orbit period||Equivalent distance of a soccer ball|
|1||RC3||April 23 - May 9||8,400 (13,600)||4,200 (1,300)||24||15 days||10 feet (3.2 meters)|
|2||Survey||June 6-30||2,700 (4,400)||1,400 (410)||73||3.1 days||3.4 feet (1.0 meters)|
|3||HAMO||Aug 17 - Oct 23||915 (1,470)||450 (140)||217||19 hours||14 inches (34 cm)|
|4||LAMO||Dec 16 - end of mission||240 (385)||120 (35)||830||5.4 hours||3.5 inches (9.0 cm)|
Dawn is now well-positioned to make many more discoveries on the first dwarf planet discovered. Jan. 1 will be the 215th anniversary of Giuseppe Piazzi's first glimpse of that dot of light from his observatory in Sicily. Even to that experienced astronomer, Ceres looked like nothing other than a star, except that it moved a little bit from night to night like a planet, whereas the stars were stationary. (For more than a generation after, it was called a planet.) He could not imagine that more than two centuries later, humankind would dispatch a machine on a cosmic journey of more than seven years and three billion miles (five billion kilometers) to reach the distant, uncharted world he descried. Dawn can resolve details more than 60 thousand times finer than Piazzi's telescope would allow. Our knowledge, our capabilities, our reach and even our ambition all are far beyond what he could have conceived, and yet we can apply them to his discovery to learn more, not only about Ceres itself, but also about the dawn of the solar system.
On a personal note, I first saw Ceres through a telescope even smaller than Piazzi's when I was 12 years old. As a much less experienced observer of the stars than he was, and with the benefit of nearly two centuries of astronomical studies between us, I was thrilled! I knew that what I was seeing was the behemoth of the main asteroid belt. But it never occurred to me when I was only a starry-eyed youth that I would be lucky enough to follow up on Piazzi's discovery as a starry-eyed adult, responsible for humankind's first visitor to that fascinating alien world, answering a celestial invitation that was more than 200 years old.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.66 AU (340 million miles, or 547 million kilometers) from Earth, or 1,360 times as far as the moon and 3.72 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and one minute to make the round trip.
Dear Superintendawnts and Assisdawnts,
An intrepid interplanetary explorer is now powering its way down through the gravity field of a distant alien world. Soaring on a blue-green beam of high-velocity xenon ions, Dawn is making excellent progress as it spirals closer and closer to Ceres, the first dwarf planet discovered. Meanwhile, scientists are progressing and analyzing the tremendous volume of pictures and other data the probe has already sent to Earth.
Dawn is flying down to an average altitude of about 240 miles (385 kilometers), where it will conduct wide-ranging investigations with its suite of scientific instruments. The spacecraft will be even closer to the rocky, icy ground than the International Space Station is to Earth's surface. The pictures will be four times sharper than the best it has yet taken. The view is going to be fabulous!
Dawn will be so near the dwarf planet that its sensors will detect only a small fraction of the vast territory at a time. Mission planners have designed the complex itinerary so that every three weeks, Dawn will fly over most of the terrain while on the sunlit side. (The neutron spectrometer, gamma ray spectrometer and gravity measurements do not depend on illumination from the sun, but the camera, infrared mapping spectrometer and visible mapping spectrometer do.)
Obtaining the planned coverage of the exotic landscapes requires a delicate synchrony between Ceres' and Dawn's movements. Ceres rotates on its axis every nine hours and four minutes (one Cerean day). Dawn will revolve around it in a little less than five and a half hours, traveling from the north pole to the south pole over the hemisphere facing the sun and sailing northward over the hemisphere hidden in the darkness of night. Orbital velocity at this altitude is around 610 mph (980 kilometers per hour).
The planned altitude differs from the earlier, tentative value of 230 miles (375 kilometers) for several reasons. One is that the previous notion for the altitude was based on theoretical models of Ceres’ gravity field. Navigators measured the field quite accurately in the previous mapping orbit (using the method outlined here), and that has allowed them to refine the orbital parameters to choreograph Dawn’s celestial pas de deux with Ceres. In addition, prior to Dawn’s investigations, Ceres’ topography was a complete mystery. Hubble Space Telescope had shown the overall shape well enough to allow scientists to determine that Ceres qualifies as a dwarf planet, but the landforms were indiscernible and the range of relative elevations was simply unknown. Now that Dawn has mapped the topography, we can specify the spacecraft’s average height above the ground as it orbits. With continuing analyses of the thousands of stereo pictures taken in August – October and more measurements of the gravity field in the final orbit, we will further refine the average altitude. Finally, we round the altitude numbers to the nearest multiple of five (both for miles and kilometers), because, as we will discuss in a subsequent Dawn Journal, the actual orbit will vary in altitude by much more than that. (We described some of the ups and dawns of the corresponding orbit at Vesta here. The variations at Ceres will not be as large, but the principles are the same.)
To attain its new orbit, Dawn relies on its trusty and uniquely efficient ion engine, which has already allowed the spacecraft to accomplish what no other has even attempted in the 58-year history of space exploration. This is the only mission ever to orbit two extraterrestrial destinations. The spaceship orbited the protoplanet Vesta for 14 months in 2011-2012, revealing myriad fascinating details of the second most massive object in the main asteroid belt between Mars and Jupiter, before its March 2015 arrival in orbit around the most massive. Ion propulsion enables Dawn to undertake a mission that would be impossible without it.
While the ion engine provides 10 times the efficiency of conventional spacecraft propulsion, the engine expends the merest whisper of xenon propellant, delivering a remarkably gentle thrust. As a result, Dawn achieves acceleration with patience, and that patience is rewarded with the capability to explore two of the last uncharted worlds in the inner solar system. This raises an obvious question: How cool is that? Fortunately, the answer is equally obvious: Incredibly cool!
The efficiency of the ion engine enables Dawn not only to orbit two destinations but also to maneuver extensively around each one, optimizing its orbits to reap the richest possible scientific return at Vesta and Ceres. The gentleness of the ion engine makes the maneuvers gradual and graceful. The spiral descents are an excellent illustration of that.
Dawn began its elegant downward coils on Oct. 23 upon concluding more than two months of intensive observations of Ceres from an altitude of 915 miles (1,470 kilometers). At that height, Ceres' gravitational hold was not as firm as it will be in Dawn's lower orbit, so orbital velocity was slower. Circling at 400 mph (645 kilometers per hour), it took 19 hours to complete one revolution around Ceres. It will take Dawn more than six weeks to travel from that orbit to its new one. (You can track its progress and continue to follow its activities once it reaches its final orbit with the frequent mission status updates.)
On Nov. 16, at an altitude of about 450 miles (720 kilometers), Dawn circled at the same rate that Ceres turned. Now the spacecraft is looping around its home even faster than the world beneath it turns.
When ion-thrusting ends on Dec. 7, navigators will measure and analyze the orbital parameters to establish how close they are to the targeted values and whether a final adjustment is needed to fit with the intricate observing strategy. Several phenomena contribute to small differences between the planned orbit and the actual orbit. (See here and here for two of our attempts to elucidate this topic.) Engineers have already thoroughly assessed the full range of credible possibilities using sophisticated mathematical methods. This is a complex and challenging process, but the experienced team is well prepared. In case Dawn needs to execute an additional maneuver to bring its orbital motion into closer alignment with the plan, the schedule includes a window for more ion-thrusting on Dec. 11-13 (concluding on Dawn's 2,999th day in space). In the parlance of spaceflight, this maneuver to adjust the orbit is a trajectory correction maneuver (TCM), and Dawn has experience with them.
The operations team takes advantage of every precious moment at Ceres they can, so while they are determining whether to perform the TCM and then developing the final flight plan to implement it, they will ensure the spacecraft continues to work productively. Dawn carries two identical cameras, a primary and a backup. Engineers occasionally operate the backup camera to verify that it remains healthy and ready to be put into service should the primary camera falter. On Dec. 10, the backup will execute a set of tests, and Dawn will transmit the results to Earth on Dec. 11. By then, the work on the TCM will be complete.
Although it is likely a TCM will be needed, if it turns out to be unnecessary, mission control has other plans for the spacecraft. In this final orbit, Dawn will resume using its reaction wheels to control its orientation. By electrically changing the speed at which these gyroscope-like devices rotate, the probe can control its orientation, stabilizing itself or turning. We have discussed their lamentable history on Dawn extensively, with two of the four having failed. Although such losses could have been ruinous, the flight team formulated and implemented very clever strategies to complete the mission without the wheels. Exceeding their own expectations in such a serious situation, Dawn is accomplishing even more observations at Ceres than had been planned when it was being built or when it embarked on its ambitious interplanetary journey in 2007.
Now the mission lifetime is limited by the small supply of conventional rocket propellant, expelled from reaction control system thrusters strategically located around the spacecraft. When that precious hydrazine is exhausted, the robot will no longer be able to point its solar arrays at the sun, its antenna at Earth, its sensors at Ceres or its ion engines in the direction needed to travel elsewhere, so the mission will conclude. The lower Dawn's orbital altitude, the faster it uses hydrazine, because it must rotate more quickly to keep its sensors pointed at the ground. In addition, it has to fight harder to resist Ceres' relentless gravitational tug on the very large solar arrays, creating an unwanted torque on the ship.
Among the innovative solutions to the reaction wheel problems was the development of a new method of orienting the spacecraft with a combination of only two wheels plus hydrazine. In the final orbit, this "hybrid control" will use hydrazine at only half the rate that would be needed without the wheels. Therefore, mission controllers have been preserving the units for this final phase of the expedition, devoting the limited remaining usable life to the time that they can provide the greatest benefit in saving hydrazine. (The accuracy with which Dawn can aim its sensors is essentially unaffected by which control mode is used, so hydrazine conservation is the dominant consideration in when to use the wheels.) Apart from a successful test of hybrid control two years ago and three subsequent periods of a few hours each for biannual operation to redistribute internal lubricants, the two operable wheels have been off since August 2012, when Dawn was climbing away from Vesta on its way out of orbit.
Controllers plan to reactivate the wheels on Dec. 14. However, in the unlikely case that the TCM is deemed unnecessary, they will power the wheels on on Dec. 11. The reaction wheels will remain in use for as long as both function correctly. If either one fails, which could happen immediately or might not happen before the hydrazine is depleted next year, it and the other will be powered off, and the mission will continue, relying exclusively on hydrazine control.
Dawn will measure the energies and numbers of neutrons and gamma rays emanating from Ceres as soon as it arrives in its new orbit. With a month or so of these measurements, scientists will be able to determine the abundances of some of the elements that compose the material near the surface. Engineers and scientists also will collect new data on the gravity field at this low altitude right away, so they eventually can build up a profile of the dwarf planet's interior structure. The other instruments (including the camera) have narrower fields of view and are more sensitive to small discrepancies in where they are aimed. It will take a few more days to incorporate the actual measured orbital parameters into the corresponding plans that controllers will radio to the spacecraft. Those observations are scheduled to begin on Dec. 18. But always squeezing as much as possible out of the mission, the flight team might actually begin some photography and infrared spectroscopy as early as Dec. 16.
Now closing in on its final orbit, the veteran space traveler soon will commence the last phase of its long and fruitful adventure, when it will provide the best views yet of Ceres. Known for more than two centuries as little more than a speck of light in the vast and beautiful expanse of the stars, the spacecraft has already transformed it into a richly detailed and fascinating world. Now Dawn is on the verge of revealing even more of Ceres' secrets, answering more questions and, as is the marvelous nature of science and exploration, raising new ones.
Dawn is 295 miles (470 kilometers) from Ceres. It is also 3.33 AU (309 million miles, or 498 million kilometers) from Earth, or 1,270 times as far as the moon and 3.37 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 55 minutes to make the round trip.
Dr. Marc D. Rayman
5:00 p.m. PST, November 30, 2015
Dear Exuldawnt Readers,
Dawn has completed another outstandingly successful campaign to acquire a wealth of pictures and other data in its exploration of dwarf planet Ceres. Exultant residents of distant Earth now have the clearest and most complete view ever of this former planet.
The stalwart probe spent more than two months orbiting 915 miles (1,470 kilometers) above the alien world. We described the plans for this third major phase of Dawn's investigation (also known as the high altitude mapping orbit, or HAMO) in August and provided a brief progress report in September. Now we can look back on its extremely productive work.
Each revolution, flying over the north pole to the south pole and back to the north, took Dawn 19 hours. Mission planners carefully chose the orbital parameters to coordinate the spacecraft's travels with the nine-hour rotation period of Ceres (one Cerean day) and with the field of view of the camera so that in 12 orbits over the lit hemisphere (one mapping "cycle"), Dawn could photograph all of the terrain.
In each of six mapping cycles, the robot held its camera and its infrared and visible mapping spectrometers at a different angle. For the first cycle (Aug. 17-26), Dawn looked straight down. For the second, it looked a little bit behind and to the left as it completed another dozen orbits. For the third map, it pointed the sensors a little behind and to the right. In its fourth cycle, it aimed ahead and to the left. When it made its fifth map, it peered immediately ahead, and for the sixth and final cycle (Oct. 12-21) it viewed terrain farther back than in the third cycle but not as far to the right.
The result of this extensive mapping is a very rich collection of photos of the fascinating scenery on a distant world. Think for a moment of the pictures not so much from the standpoint of the spacecraft but rather from a location on the ground. With the different perspectives in each mapping cycle, that location has been photographed from several different angles, providing stereo views. Scientists will use these pictures to make the landscape pop into its full three dimensionality.
Dawn's reward for these two months of hard work is much more than revealing Ceres' detailed topography, valuable though that is. During the first and fifth mapping cycles, it used the seven color filters in the camera, providing extensive coverage in visible and infrared wavelengths.
In addition to taking more than 6,700 pictures, the spacecraft operated its visible and infrared mapping spectrometers to acquire in excess of 12.5 million spectra. Each spectrum contains much finer measurements of the colors and a wider range of wavelengths than the camera. In exchange, the camera has sharper vision and so can discern smaller geological features. As the nerdier among us would say, the spectrometers achieve better spectral resolution and the camera achieves better spatial resolution. Fortunately, it is not a competition, because Dawn has both, and the instruments yield complementary measurements.
Even as scientists are methodically analyzing the vast trove of data, turning it into knowledge, you can go to the Ceres image gallery to see some of Dawn's pictures, exhibiting a great variety of terrain, smooth or rugged, strangely bright or dark, unique in the solar system or reminiscent of elsewhere spacecraft have traveled, and always intriguing.
Among the questions scientists are grappling with is what the nature of the bright regions is. There are many places on Ceres that display strikingly reflective material but nowhere as prominently as in Occator crater. Even as Dawn approached Ceres, the mysterious reflections shone out far into space, mesmerizing and irresistible, as if to guide or even seduce a passing ship into going closer. Our intrepid interplanetary adventurer, compelled not by this cosmic invitation but rather by humankind's still more powerful yearning for new knowledge and new insights, did indeed venture in. Now it has acquired excellent pictures and beautiful spectra that will help determine the composition and perhaps even how the bright areas came to be. Thanks to the extraordinary power of the scientific method, we can look forward to explanations. (And while you wait, you can register your vote here for what the answer will be.)
Scientists also puzzle over the number and distribution of craters. We mentioned in December the possibility that ice being mixed in as a major component on or near the surface would cause the material to flow, albeit very slowly on the scale of a human lifetime. But over longer times, the glacially slow movement might prove significant. Most of Ceres' craters are excavated by impacts from some of the many bodies that roam that part of the solar system. Ceres lives in a rough neighborhood, and being the most massive body between Mars and Jupiter does not give it immunity to assaults. Indeed, its gravity makes it even more susceptible, attracting passersby. But once a crater is formed, the scar might be expected to heal as the misshapen ground gradually recovers. In some ways this is similar to when you remove pressure from your skin. What may be a deep impression relaxes, and after a while, the original mark (or, one may hope, Marc) is gone. But Ceres has more craters than some scientists had anticipated, especially at low latitudes where sunlight provides a faint warming. Apparently the expectation of the gradual disappearance of craters was not quite right. Is there less evidence of flowing ground material because the temperature is lower than predicted (causing the flow to be even slower), because the composition is not quite what was assumed, or because of other reasons? Moreover, craters are not distributed as would be expected for random pummeling; some regions display significantly more craters than others. Investigating this heterogeneity may give further insight into the geological processes that have taken place and are occurring now on this dwarf planet.
Dawn's bounty from this third major science campaign includes even more than stereo and color pictures plus visible and infrared spectra. Precise tracking of the spacecraft as it moves in response to Ceres' gravitational pull allows scientists to calculate the arrangement of mass in the behemoth. Performing such measurements will be among the top three priorities for the lowest altitude orbit, when Dawn experiences the strongest buffeting from the gravitational currents, but already the structure of the gravitational field is starting to be evident. We will see next month how this led to a small change in the choice of the altitude for this next orbit, which will be less than 235 miles (380 kilometers).
The other top two priorities for the final mission phase are the measurement of neutron spectra and the measurement of gamma ray spectra, both of which will help in establishing what species of atoms are present on and near the surface. The weak radiation from Ceres is difficult to measure from the altitudes at which Dawn has been operating so far. The gamma ray and neutron detector (GRaND) has been in use since March 12 (shortly after Dawn arrived in orbit), but that has been to prepare for the low orbit. Nevertheless, the sophisticated instrument did detect the dwarf planet's faint nuclear emissions even in this third orbital phase. The signal was not strong enough to allow any conclusions about the elemental composition, but it is interesting to begin seeing the radiation which will help uncover more of Ceres' secrets when Dawn is closer.
To scientists' great delight, one of GRaND's sensors even found an entirely unexpected signature of Ceres in Dawn's second mapping orbit, where the spacecraft revolved every 3.1 days at an altitude of 2,700 miles (4,400 kilometers). In a nice example of scientific serendipity, it detected high energy electrons in the same region of space above Ceres on three consecutive orbits. Electrons and other subatomic particles stream outward from the sun in what is called the solar wind, and researchers understand how planets with magnetic fields can accelerate them to higher energy. Earth is an example of a planet with a magnetic field, but Ceres is thought not to be. So scientists now have the unanticipated joy not only of establishing the physical mechanism responsible for this discovery but also determining what it reveals about this dwarf planet.
Several times during each of the six mapping cycles, Dawn expended a few grams of its precious hydrazine propellant to rotate so it could aim its main antenna at Earth. While the craft soared high above ground cloaked in the deep black of night, it transmitted some of its findings to NASA's Deep Space Network. But Dawn conducted so many observations that during half an orbit, or about 9.5 hours, it could not radio enough data to empty its memory. By the end of each mapping cycle, the probe had accumulated so much data that it fixed its antenna on Earth for about two days, or 2.5 revolutions, to send its detailed reports on Ceres to eager Earthlings.
Following the conclusion of the final mapping cycle, after transmitting the last of the information it had stored in its computer, the robotic explorer did not waste any time gloating over its accomplishments. There was still a great deal more work to do. On Oct. 23 at 3:30 p.m., it fired up ion engine #2 (the same one it used to descend from the second mapping orbit to the third) to begin more than seven weeks of spiraling down to its fourth orbit. (You can follow its progress here and on Twitter @NASA_Dawn.) Dawn has accomplished more than 5.4 years of ion thrusting since it left Earth, and the complex descent to less than 235 miles (380 kilometers) is the final thrusting campaign of the entire extraterrestrial expedition. (The ion propulsion system will be used occasionally to make small adjustments to the final orbit.)
The blue lights in Dawn mission control that indicate the spacecraft is thrusting had been off since Aug. 13. Now they are on again, serving as a constant (and cool) reminder that the ambitious mission is continuing to power its way to new (and cool) destinations.
Dawn is 740 miles (1,190 kilometers) from Ceres. It is also 2.91 AU (271 million miles, or 436 million kilometers) from Earth, or 1,165 times as far as the moon and 2.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 48 minutes to make the round trip.
Dr. Marc D. Rayman
3:00 p.m. PDT October 30, 2015
P.S. While the spacecraft is hard at work continuing its descent tomorrow, your correspondent will be hard at work dispensing treats to budding (but cute) extortionists at his front door. But zany and playful as ever, he will expand his delightful costume from last year by adding eight parts dark energy. Trick or treat!
Most people know about the scenes, greetings, music and sounds from Earth found on the Voyager Golden Record. They may not know that there is also a handwritten message etched into the surface of the record. Timothy Ferris, who worked with Carl Sagan and the rest of the team that produced the record, wanted something done directly by a human hand to appear on the record. “To the makers of music – all worlds, all times” appears on the finished record, in between the photoengraved label and the record grooves.
The inscription can also be seen on some of the 14-inch recording masters that are found in the JPL Archives. There are several sets of the records, with a metal core and lacquer surface. From these masters, the copper records (“mothers”) were cut, then they were gold plated, etched, enclosed in aluminum containers and mounted on the sides of the Voyager 1 and 2 spacecraft.