Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL
Dawn Journal | November 28

by Marc Rayman


Dear Unidawntified Flying Objects,

Flying silently and smoothly through the main asteroid belt between Mars and Jupiter, Dawn emits a blue-green beam of high velocity xenon ions. On the opposite side of the sun from Earth, firing its uniquely efficient ion propulsion system, the distant adventurer is continuing to make good progress on its long trek from the giant protoplanet Vesta to dwarf planet Ceres.

This month, let’s look ahead to some upcoming activities. You can use the sun in December to locate Dawn in the sky, but before we describe that, let’s see how Dawn is looking ahead to Ceres, with plans to take pictures on the night of Dec. 1.

The robotic explorer’s sensors are complex devices that perform many sensitive measurements. To ensure they yield the best possible scientific data, their health must be carefully monitored and maintained, and they must be accurately calibrated. The sophisticated instruments are activated and tested occasionally, and all remain in excellent condition. One final calibration of the science camera is needed before arrival at Ceres. To accomplish it, the camera needs to take pictures of a target that appears just a few pixels across. The endless sky that surrounds our interplanetary traveler is full of stars, but those beautiful pinpoints of light, while easily detectable, are too small for this specialized measurement. But there is an object that just happens to be the right size. On Dec. 1, Ceres will be about nine pixels in diameter, nearly perfect for this calibration.

The images will provide data on very subtle optical properties of the camera that scientists will use when they analyze and interpret the details of some of the pictures returned from orbit. At 740,000 miles (1.2 million kilometers), Dawn’s distance to Ceres will be about three times the separation between Earth and the moon. Its camera, designed for mapping Vesta and Ceres from orbit, will not reveal anything new. It will, however, reveal something cool! The pictures will be the first extended view for the first probe to reach the first dwarf planet discovered. They will show the largest body between the sun and Pluto that has not yet been visited by a spacecraft, Dawn’s destination since it climbed out of Vesta’s gravitational grip more than two years ago.

Image of Ceres

Dawn’s first photo of Ceres, taken on July 20, 2010. Credit: NASA/JPL-Caltech/MPS/DLR/IDA

This will not be the first time Dawn has spotted Ceres. In a different calibration of the camera more than four years ago, the explorer descried its faint destination, far away in both time and space. Back then, still a year before arriving at Vesta, Dawn was more than 1,300 times farther from Ceres than it will be for this new calibration. The giant of the main asteroid belt was an indistinct dot in the vast cosmic landscape. Now Ceres is the brightest object in Dawn’s sky save the distant sun. When it snaps the photos, Ceres will be as bright as Venus sometimes appears from Earth (what astronomers would call visual magnitude -3.6).

First image of Vesta

Dawn’s first extended picture of Ceres will be only slightly larger than this image of Vesta taken on May 3, 2011, at the beginning of the Vesta approach phase. The inset shows the pixelated Vesta, extracted from the main picture in which the overexposed Vesta can be seen against the background of stars. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

To conserve hydrazine, a precious resource following the loss of two reaction wheels, Dawn will thrust with its ion propulsion system when it performs this calibration, which requires long exposures. In addition to moving the spacecraft along in its trajectory, the ion engine stabilizes the ship, enabling it to point steadily in the zero-gravity of spaceflight. (Dawn’s predecessor, Deep Space 1, used the same trick of ion thrusting in order to be as stable as possible for its initial photos of comet Borrelly.)

As Dawn closes in on its quarry, Ceres will grow brighter and larger. Last month we summarized the plan for photographing Ceres during the first part of the approach phase, yielding views in January comparable to the best we currently have (from Hubble Space Telescope) and in February significantly better. The principal purpose of the pictures is to help navigators steer the ship into this uncharted, final port following a long voyage on the interplanetary seas. The camera serves as the helmsman’s eyes. Ceres has been observed with telescopes from (or near) Earth for more than two centuries, but it has appeared as little more than a faint, fuzzy blob farther away than the sun. But not for much longer!

The only spaceship ever built to orbit two extraterrestrial destinations, Dawn’s advanced ion propulsion system enables its ambitious mission. Providing the merest whisper of thrust, the ion engine allows Dawn to maneuver in ways entirely different from conventional spacecraft. In January, we presented in detail Dawn’s unique way of slipping into orbit. In September, a burst of space radiation disrupted the thrust profile. As we saw, the flight team responded swiftly to a very complex problem, minimizing the duration of the missed thrust. One part of their contingency operations was to design a new approach trajectory, accounting for the 95 hours that Dawn coasted instead of thrust. Let’s take a look now at how the resulting trajectory differs from what we discussed at the beginning of this year.

In the original approach, Dawn would follow a simple spiral around Ceres, approaching from the general direction of the sun, looping over the south pole, going beyond to the night side, and coming back above the north pole before easing into the targeted orbit, known by the stirring name RC3, at an altitude of 8,400 miles (13,500 kilometers). Like a pilot landing a plane, flying this route required lining up on a particular course and speed well in advance. The ion thrusting this year had been setting Dawn up to get on that approach spiral early next year.

The change in its flight profile following the September encounter with a rogue cosmic ray meant the spiral path would be markedly different and would require significantly longer to complete. While the flight team certainly is patient — after all, Earth’s robotic ambassador won’t reach Ceres until 214 years after its discovery and more than seven years after launch — the brilliantly creative navigators devised an entirely new approach trajectory that would be shorter. Demonstrating the extraordinary flexibility of ion propulsion, the spacecraft now will take a completely different path but will wind up in exactly the same orbit.

Dawn Trajectory

In this view, looking down on the north pole of Ceres, the sun is off the figure to the left and Ceres’ counterclockwise orbital motion around the sun takes it from the bottom of the figure to the top. Dawn flies in from the left, traveling behind Ceres, and then is captured on the way to the apex of its orbit. The white circles are at one-day intervals, illustrating how Dawn slows down gradually at first. (When the circles are closer together, Dawn is moving more slowly.) After capture, both Ceres’ gravity and the ion thrust slow it even more before the craft accelerates to the end of the approach phase. (You can think of this perspective as being from above. Then the next figure shows the view from the side, which here would mean looking toward the action from a location off the bottom of the graphic.) Credit: NASA/JPL

The spacecraft will allow itself to be captured by Ceres on March 6, only about half a day later than the trajectory it was pursuing before the hiatus in thrust, but the geometry both before and after will be quite different. Instead of flying south of Ceres, Dawn is now targeted to trail a little behind it, letting the dwarf planet lead as they both orbit the sun, and then the spacecraft will begin to gently curve around it. (You can see this in the figure to the left.) Dawn will come to 24,000 miles (38,000 kilometers) and then will slowly arc away. But thanks to the remarkable design of the thrust profile, the ion engine and the gravitational pull from the behemoth of rock and ice will work together. At a distance of 41,000 miles (61,000 kilometers), Ceres will reach out and tenderly take hold of its new consort, and they will be together evermore. Dawn will be in orbit, and Ceres will forever be accompanied by this former resident of Earth.

If the spacecraft stopped thrusting just when Ceres captured it, it would continue looping around the massive body in a high, elliptical orbit, but its mission is to scrutinize the mysterious world. Our goal is not to be in just any arbitrary orbit but rather in the particular orbits that have been chosen to provide the best scientific return for the probe’s camera and other sensors. So it won’t stop but instead will continue maneuvering to RC3.

Ever graceful, Dawn will gently thrust to counter its orbital momentum, keeping it from swinging up to the highest altitude it would otherwise attain. On March 18, nearly two weeks after it is captured by Ceres’ gravity, Dawn will arc to the crest of its orbit. Like a ball thrown high that slows to a momentary stop before falling back, Dawn’s orbital ascent will end at an altitude of 47,000 miles (75,000 kilometers), and Ceres’ relentless pull (aided by the constant, gentle thrust) will win out. As it begins descending toward its gravitational master, it will continue working with Ceres. Rather than resist the fall, the spacecraft will thrust to accelerate itself, quickening the trip down to RC3.

There is more to the specification of the orbit than the altitude. One of the other attributes is the orientation of the orbit in space. (Imagine an orbit as a ring around Ceres, but that ring can be tipped and tilted in many ways.) To provide a view of the entire surface as Ceres rotates underneath it, Dawn needs to be in a polar orbit, flying over the north pole as it travels from the nightside to the dayside, moving south as it passes over the equator, sailing back to the unilluminated side when it reaches the south pole, and then heading north above terrain in the dark of night. To accomplish the earlier part of its new approach trajectory, however, Dawn will stay over lower latitudes, very high above the mysterious surface but not far from the equator. Therefore, as it races toward RC3, it will orient its ion engine not only to shorten the time to reach that orbital altitude but also to tip the plane of its orbit so that it encircles the poles (and tilts the plane to be at a particular orientation relative to the sun). Then, finally, as it gets closer still, it will turn to use that famously efficient glowing beam of xenon ions against Ceres’ gravity, acting as a brake rather than an accelerator. By April 23, this first act of a beautiful new celestial ballet will conclude. Dawn will be in the originally intended orbit around Ceres, ready for its next act: the intensive observations of RC3 we described in February.

Comparison of Trajectories

North is at the top of this figure and the sun is far to the left. Ceres orbital motion around the sun carries it straight into the figure. The original approach took Dawn over Ceres’ south pole as it spiraled directly into RC3. On the new approach, it looks here as if it flies in over the north pole, but that is because of the flat depiction. As the previous figure shows, the approach takes Dawn well behind Ceres in their progression around the sun. The upper part of the green trajectory is not in the same plane as the original approach and RC3; rather, it is in the foreground, in front of the graphic. As Dawn flies to the right side of the diagram, it also moves back into the plane of the figure to align with the targeted RC3. As before, the circles, spaced at intervals of one day, indicate the spacecraft’s speed; where they are closer together, the ship travels more slowly. (You can think of this perspective as being from the side and the previous figure as showing the view from above, off the top of this graphic.) Credit: NASA/JPL

Dawn’s route to orbit is no more complex and elegant than what any crackerjack spaceship pilot would execute. However, one of the key differences between what our ace will perform and what often happens in science fiction movies is that Dawn’s maneuvers will comply with the laws of physics. And if that’s not gratifying enough, perhaps the fact that it’s real makes it even more impressive. A spaceship sent from Earth more than seven years ago, propelled by electrically accelerated ions, having already maneuvered extensively in orbit around the giant protoplanet Vesta to reveal its myriad secrets, soon will bank and roll, arc and turn, ascend and descend, and swoop into its planned orbit.

Earth, Sun, & Dawn

Illustration of the relative locations (but not sizes) of Earth, the sun, and Dawn in early December 2014. (Earth and the sun are at that location every December.) The images are superimposed on the trajectory for the entire mission, showing the positions of Earth, Mars, Vesta, and Ceres at milestones during Dawn’s voyage. Credit: NASA/JPL

And all this will take place far, far from Earth. Indeed, Dawn is on a very different heliocentric orbit from that of the planet it left behind in 2007. In December, their separate paths will take them to opposite sides of the sun. We will not have a similar celestial arrangement until 2016, by which time the craft will be in its lowest altitude orbit at Ceres. (We invite our future selves to return to the past to tell us here how the view is. __ )  From our terrestrial perspective this year, Dawn will appear to be less than one solar diameter from the sun’s limb on Dec. 9 and 10.

As Earth, the sun, and the spacecraft come closer into alignment, radio signals that go back and forth must pass near the sun. The solar environment is fierce indeed, and it will interfere with those radio waves. While some signals will get through, communication will not be reliable. Therefore, controllers plan to send no messages to the spacecraft from Dec. 4 through Dec. 15; all instructions needed during that time will be stored onboard beforehand. Occasionally Deep Space Network antennas, pointing near the sun, will listen through the roaring noise for the faint whisper of the spacecraft, but the team will consider any communication to be a bonus.

Dawn is big for an interplanetary spacecraft (or for an otherworldly dragonfly, for that matter), with a wingspan of nearly 65 feet (19.7 meters). However, more than 3.8 times as far as the sun, 352 million miles (567 million kilometers) away, humankind lacks any technology even remotely capable of glimpsing it. But we can bring to bear something more powerful than our technology: our mind’s eye. From Dec. 8 to 11, if you block the sun’s blazing light with your thumb, you will also be covering Dawn’s location. There, in that direction, is our faraway emissary to new worlds. It has traveled three billion miles (4.8 billion kilometers) already on its extraordinary extraterrestrial expedition, and some of the most exciting miles are still ahead as it nears Ceres. You can see right where it is. It is now on the far side of the sun.

The sun!

This is the same sun that is more than 100 times the diameter of Earth and a third of a million times its mass. This is the same sun that has been the unchallenged master of our solar system for more than 4.5 billion years. This is the same sun that has shone down on Earth all that time and has been the ultimate source of so much of the heat, light and other energy upon which the planet’s residents have been so dependent. This is the same sun that has so influenced human expression in art, literature, mythology and religion for uncounted millennia. This is the same sun that has motivated scientific studies for centuries. This is the same sun that is our signpost in the Milky Way galaxy. And humans have a spacecraft on the far side of it. We may be humbled by our own insignificance in the universe, yet we still undertake the most valiant adventures in our attempts to comprehend its majesty.

Dawn is 780,000 miles (1.3 million kilometers) from Ceres, or 3.3 times the average distance between Earth and the moon. It is also 3.77 AU (350 million miles, or 564 million kilometers) from Earth, or 1,525 times as far as the moon and 3.82 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and three minutes to make the round trip.

Dr. Marc D. Rayman
5:00 p.m. PST November 28, 2014

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34 Responses to “Dawn Journal | November 28”

  1. Ibni says:

    great example of extreme flight operations. nice

  2. Kirk Sorensen says:

    Merry Christmas Marc! I sure love reading your updates and check on Dawn several times each day. I am very excited to see Ceres next year!

    • Marc Rayman says:

      Thank you very much, Kirk. I appreciate your good wishes and your interest in Dawn. To further increase our shared excitement about Ceres, my December Dawn Journal will present a little more about the mysterious dwarf planet.

      I hope you and all other readers enjoy the holiday season too (assuming, of course, your planet is experiencing some seasons worth being festive about).


  3. Çağrı says:

    Dear Dr. Rayman

    I am an Electrical and Electronics Engineering student at Celal Bayar University, TURKEY. I keep track of the project named in “Dawn Mission” at NASA. I have been working on the final project which is about ion engines and your work is really charming and interesting for me. I follow your website and blog, but I couldn’t find the answers of couple questions in my mind. How much time is needed that a spacecraft used an ion engine achieves to its maximum speed? How much Xenon is disposed of until the spacecraft gets its maximum speed?

    Thank you so much for all.


    • Marc Rayman says:

      Dear Çağrı,

      I appreciate your interest!

      The time needed for a spacecraft using an ion engine to achieve its maximum speed depends on how much propellant it carries. The more propellant there is to expend, the longer it will take to attain the highest speed, because the rate of propellant use is (approximately) constant. (I have written about aspects of the ion propulsion system related to this a number of times, including here.) Of course, carrying more propellant incurs a penalty, because that extra propellant mass itself needs to be propelled. That is, it takes more xenon to push around the heavier load of xenon! Objects in orbit don’t have weight, but they do have mass, and overcoming that resistance to changes in motion is the job of the propulsion system.

      As an aside, I have written before about how, quite contrary to intuition, Dawn actually does not use its ion propulsion system to speed up. Going to planets farther from the sun requires a spacecraft not to speed up but rather to climb what I call the solar system hill.

      Whether with ion propulsion or conventional propulsion, the decision for how much propellant to carry depends on the objectives and constraints of the mission, and this is an analysis that engineers perform for every space mission. Nevertheless, in basic terms, with more propellant, you can achieve a higher speed (if that’s your objective) and it takes more time to reach that top speed. Therefore, there are no unique answers to your questions.

      I hope this answer (and the links I provided) gives you some greater insight into your questions.

      I also hope you will continue to stay with the mission as we take advantage of the ion propulsion system to explore distant lands!


  4. Romana Starfield says:

    Wow, I never considered that Ceres might have a moon. But come to think of it, why not? Ida has Dactyl, Mars has captured asteroids. Most planets in the solar system have moons. So there is a chance Ceres may have one. Though I would have expected it would have been spotted by now. I am trying to get a grasp of how good the resolution of the telescopes we have available is. Keeping in mind that the best images we have of Ceres are only a few dozen pixels across, with ground features barely discernible, could a small moon of Ceres be detected?

    With Christmas approaching, Dawn and Ceres are like a conspicuous present sitting under the tree. You know it is there, you can see vaguely what it is, and can guess what it may be, but until the day, you won’t really know what it is. I feel like I’m on a road trip and keep getting the urge to ask “Are we there yet?” Of course we’ll need to wait till March to unwrap the present that is Ceres.

    On somewhat of a different train of thought, how do you people know in which direction Dawn is pointing? Or, to rephrase that question, how does Dawn navigate? Out in micro gravity there is essentially only a down towards the biggest gravity well, the sun. So there’s no up, down, or air to help navigate, and certainly no GPS network. With a curved course, Dawn must slowly but surely be changing its angle towards both the sun and Ceres. If I understand what I have read right, there is a bunch of sensors that point towards the sun and stars and compares their spectrum to known star spectrums and basically says “If we have star X in sensor 1, and the spectrum of the sun in sensor 2 and so on, then triangulation suggests we are pointed in some direction.” Or have I got that all wrong? How do you keep Dawn orientated in the right direction, and how do you do that if the right direction is constantly changing?

    To me this is fascinating stuff. Just as sailing ships were once the peak of human technology, and then steam trains took over that lead, followed eventually by aircraft, spacecraft now are the very essence of the start of the art of human technology. We pour or combined wisdom, know-how and ingenuity into these little hunks of metal, ceramics and plastic and they go out there and become our version of Carter at Tutankhamen’s tomb, where they see “Wonderful things” We’re living in an age of wonders. I can’t wait to see the images from Ceres close up in March next year.

    Thanks again for keeping us informed, and my best wishes to you Marc and the team for a safe, enjoyable and productive festive season.

    • Whitney says:

      Dear Romana,

      Marc and the team are busy finishing Dawn’s first approach phase commands, and he regrets that he just doesn’t have the time to reply right now. We do love your enthusiasm and the questions you raise; as a physicist, the topics you touch on are of the greatest interest to Marc.

      For now, know that to detect a moon of Ceres with a telescope, you don’t have to resolve it. That is, it doesn’t have to be as large as a pixel. It just has to reflect enough light that it shows up. Marc has referred to Dawn’s search for moons around Ceres in December 2013 and October 2014 journals. You may have confused navigating with orienting (easy to do). Marc touches on this in quite a few blogs, including June 23, 2011 and Jan. 27, 2012 (with the paragraph beginning “Far away, traveling”). He has also written about a kind of interplanetary navigation here.

      Finally, have a splendid holiday season with just the amount of sparkle you hope for. We appreciate, as ever, your interest in the Dawn mission.

      Dawn Education and Communications

      • Romana Starfield says:

        Thanks again. I also read the September 17 2006 journal entry which helped me understand this better. It goes into great detail about the sub systems.

        Merry Christmas all.

    • Romana , if a moon exists ( IF) in orbit around Ceres , is about or Less than 1km if have The same albedo ( According Allyson Bieryla search) Marc Rayman you can confirm ?
      In this moment dawn is Approximately 2 lunar distance to Ceres ; If at 3 lunardistance was 9 pixels in dimension , now I Think 16-18 pixels , right ? : – )
      I am excited … impatient ! : – )

      • Marc Rayman says:

        Daniele and Romana,

        Daniele is correct that for a moon of Ceres to have escaped detection from the searches conducted so far, it probably would have to be smaller than about one mile (one to two kilometers) in diameter.

        Dawn is getting closer to Ceres every day! Today they are separated by slightly less than twice the Earth-moon distance. Ceres would be about 14 pixels across in Dawn’s camera. When we next observe the dwarf planet on Jan. 13, for the first “optical navigation” pictures, it will be about 27 pixels across. (The exact number of pixels is slightly different from what I wrote in October because of minor details in the trajectory and the time on Jan. 13 that we will actually take the pictures.)

        You don’t have to wait much longer, and the excitement we all share will soon be rewarded with fantastic and ever-improving views!


  5. Mark says:

    Thanks for your great blog. It seems everyone is getting excited now. Anyway, Is there any commonality between Kepler’s reaction wheels and Dawn’s? Is there a new reaction wheel technology that is more reliable for future missions?


    • Marc Rayman says:

      I’m glad you like my blogs, Mark. Thank you.

      The reaction wheels on Dawn and Kepler are indeed similar. Engineers now consider other reaction wheel designs to be reliable.

      Thanks to the creativity and ingenuity of the Dawn flight team, we are no longer dependent on reaction wheels to conduct our bold mission at Ceres. Nevertheless, there are still many great challenges ahead as we travel to and orbit this unexplored world. I share everyone’s excitement at the prospect of discovering many of the secrets of an exotic, mysterious dwarf planet. I hope you do too!


  6. HI, LiKe Ramona starfield [see below] I wanted to ask of prospective names Ceres characteristics: I thought that its small moon, if it exists, can be called ‘proserpina’ (the daughter of Ceres and Jupiter, abducted from pluto); its possible fissure, pit or Fossae can be called ”Caereris Mundus”(A fossa that was open 3 times in a year, to join the living and the dead kingdom)..
    2.One last question. Because in “vie of Ceres from dawn” ceres remains in the same dimension? Should not it grow? Ceres now is more bright to Venus.
    Many Thanks,
    Daniele – Italy

    • Marc Rayman says:

      Hi Daniele,

      Your suggestions for names are very good! Should a moon be found, I am sure many names will be considered by the team members who discover it, and the final decision will rest with the International Astronomical Union. I agree that Proserpina would be a wonderful choice. Because there is an asteroid with that name, I believe other mythological sources would need to be considered. You should be proud to know that you came up with a name that has been approved as a very appropriate designation for a resident of the main asteroid belt.

      You are quite correct that Ceres becomes brighter and larger as Dawn gets closer. As I have explained, Dawn will not turn to observe its destination frequently, because we need to conserve hydrazine. Nevertheless, if the probe peered at it today (Dec. 16), Ceres would be nearly twice as bright as it was when we took the pictures on Dec. 1. (It would be about visual magnitude -4.3.) The ever-growing target would appear to Dawn’s camera to be 12 pixels across, rather than the nine pixels it was on Dec. 1.

      Thank you for your comments and your continuing interest!


      • Hello Mark, I probably would not explain well, for my bad English.
        I know that Dawn will not turn to observe its destination frequently, because you need to conserve hydrazine.
        But I meant the image of computer navigation:View of Ceres from Dawn:
        Should be more ‘big in this picture Now;right?
        But it appears smallest of each star..

        I hope in International Astronomical Union, and I hope a Fossae will be named “Caerae Mundus” and in a crater of Name “Palermo”, the sicilian city where Giuseppe Piazzi discovered Ceres, and the city of my Mother :-)
        You can not imagine that joy for me image of December 3 !!
        Thanks !!

        • Marc Rayman says:

          Hi Daniele,

          I apologize for misunderstanding your question.

          The trajectory simulator we use for creating the visualization for outreach was intended principally to show the apparent geometry, or the relative positions of all the objects. It does not include calculations for how bright Ceres would be. In addition, because it displays a wide field of view (much larger than the camera could see), Ceres still appears only to be a point. You may rest assured that when Dawn is closer, the simulator will show the size of Ceres.

          Thank you again for being so interested!


  7. Romana Starfield says:

    Hi Marc.

    Thanks again for keeping us informed with the mission progress. I was wondering if the naming convention /standard had been finalised for the Ceres landmarks yet? I seem to recall you mentioning this in the past during an answer to one of my questions, but writing it was proposed.
    Many kind regards,

    • Marc Rayman says:

      Hi Romana,

      I’m glad you’re continuing to stay up to date with the mission!

      Yes, in June I mentioned that the Dawn project was working with the International Astronomical Union to finalize the naming convention for Ceres. The plan is that craters will be named for gods and goddesses of agriculture and vegetation from world mythology. Other features will be named for agricultural festivals.

      And we don’t have long to wait until some of these features start coming into view!


  8. Jan says:

    Would it be possible to rescue the mission if there is a safe mode during the conjunction and Dawn misses a week of thrusting again ?

    • Marc Rayman says:


      For others, “conjunction” is when Dawn appears to be close to the sun (even though it is far beyond it).

      Over the course of its more than seven years of spaceflight, Dawn has performed extremely well. This is the fifth solar conjunction during its deep-space adventure. (I’ve written a little bit about the other conjunctions in Dec. 2008, Oct. 2010, March 2012, and Aug. 2013.) While safe mode is always possible, the ship has proven itself to be quite reliable.

      Nevertheless, if the spacecraft encounters some difficulty during conjunction, it will wait safely and patiently in safe mode until we resume reliable communications. Thanks to the amazing flexibility provided by ion propulsion, however, that ultimately would mean no more than a delay in our exploration of Ceres. Unlike missions with conventional chemical propulsion, Dawn does not have a narrow window during which it must complete a critical burn in order to get into orbit. We eschew such tense, nail-biting events and instead have long periods of gentle, gradual maneuvering. This is still a very ambitious and very challenging mission, but to me, the great drama is in the thrill of discovery as we explore uncharted, alien worlds and extend humankind’s reach to new and exciting destinations far from Earth.

      As I wrote last month, even with the interruption in thrust in September, reaching orbit around Ceres was never in doubt. So if the spacecraft misses some thrusting again, we will restore it to normal operation again and use that remarkable ion engine to take us on a new path to the same orbital destination.

      Thank you for your interest!


  9. Alex says:

    Marc, I’ve been coming back to your blog for years (since 2009, I believe). Thank you for writing it, and for working on this amazing project. I am sure you wrote it somewhere, but I could not find it. What would be the total distance travelled by Dawn when it achieves its final orbit of Ceres?
    Thank you.

    • Marc Rayman says:

      I’m glad you’re a regular reader, Alex!

      When Dawn enters orbit around Ceres in March, it will have traveled about 3.1 billion miles (4.9 billion kilometers) since launch in 2007. Of course, the spacecraft will continue moving around the sun along with Ceres (that’s the point of being in orbit), so by the time it arrives in its final orbit, which I described in August, they will have traveled together for another 270 million miles (430 million kilometers).

      I haven’t written that before, but as I have explained, you and I, Alex (although I haven’t specifically mentioned either of us by name), and everyone else on or near Earth travel around the solar system much faster. Under the tighter gravitational grip of the sun, Earth needs to zip around in orbit more quickly than distant Ceres. (Similarly, the moon, in a very high altitude orbit around Earth, progresses at a much more relaxed pace than satellites very close to Earth, including the International Space Station.) During the time between Dawn’s launch and its arrival at Ceres, our planet has taken us 4.3 billion miles (7.0 billion kilometers).

      In fact, since you started reading my blogs in 2009, you have traveled farther than Dawn has since its launch in 2007. (I’m assuming here you haven’t strayed very far from Earth.)

      If you would like to read more about why Dawn has really used its ion propulsion system to slow down, not speed up, my February 2013 Dawn Journal might be helpful. In September, as every year in celebration of the anniversary of launch, I gave some other fun facts about how far Earth, Vesta, Ceres, and Dawn had traveled since launch.

      I hope this is not only helpful but interesting.


  10. Steve Gribble (dawn-o-phile) says:

    Thanks for your wonderful explanations! The new trajectory wizardry is amazing compared to the original plan. I marvel at the simulation tools and creativity the team must be using to come up with such. I imagine it is quite a marriage of the two.

    I have two questions. Compared to the original plan, is the current plan more or less fuel-efficient, or does it come down pretty much to the same thing, given that the gravitational potential energy and kinetic energy of the final orbit are identical?

    My second question relates to your description of the use of the ion thrusters to stabilize the craft for imaging. Can the ion thrusters play a role to help the hydrazine thrusters, in particular during the orbital science phase in the event of the loss of another reaction wheel?

    Thanks in advance for your time.

    • Marc Rayman says:

      Thank you for your message, Steve. You’re quite right about the combination of tools and creativity. The software is both complex and powerful. And using it to generate flyable trajectories that satisfy our myriad constraints requires a tremendous amount of experience and technical insight. Like many aspects of flying an ambitious interplanetary mission, it requires science and art. Fortunately, JPL’s trajectory experts are the best in the world. Dawn is in good hands.

      Let me address your questions in reverse order. The ion thrusters not only can but actually do play an indispensable role in helping the hydrazine thrusters. I have referred a number of times (including as recently as April) to using the ion thrusters to control the orientation. Given the loss of two of our four reaction wheels, the hydrazine savings from operating the ion engine is one of the essential elements of our plan to complete the mission. This too is something we learned on Deep Space 1, which also ran low on hydrazine in its extended mission (but for an entirely different reason). On that mission, we did what you suggest: we operated the ion thrust even when we didn’t need it to adjust the trajectory, but we did need it to conserve hydrazine. I called that kind of thrusting “impulse power,” a term any Star Trek fan will recognize. I wrote about it several times on DS1 after introducing it here. Just as Dawn’s expedition to Ceres depends on ion thrusting to save hydrazine, DS1 would not have been able to reach comet Borrelly for its spectacular adventure there had we not done that.

      This trick would not work as well at Ceres, however, because even impulse power would change the orbit more than we want. We could manage it, but doing so would be complicated. It turns out not to be necessary anyway. We were so successful in our hydrazine conservation campaign that we do not have to contend with this additional complexity. We anticipate accomplishing all of the original mission objectives regardless of the health of any of the reaction wheels. I call this my “zero reaction wheel plan.” Any remaining wheel lifetime will be a bonus.

      To your first question, we actually do not have to be concerned about xenon efficiency at this point. We launched with 937 pounds of the propellant, and by the end of the mission, we will have expended around 875 pounds. The ion thrusting has been extremely smooth. We have enough to spare that we needn’t worry about it. As I’ve explained before, the mission lifetime is most likely to be limited by hydrazine. The new approach does consume a little bit more xenon between capture and the start of RC3 than the original plan did. One way to see that is to consider the figure above that compares the two trajectories. The new one requires more thrust time. That additional thrusting, however, will consume less than 3 pounds of xenon.

      You might be right. Even to me, sometimes it seems almost like wizardry!

      I hope this answers your questions. Thank you for your interest!

  11. Chris S says:

    Just watched your von Karman talk-

    It was amazing! People have called you a PowerPoint Legend, and I agree! But the PowerPoint was a backdrop to a fantastic presentation. I sent the live link to a friend as an introduction to the mission, and she enjoyed it immensely. We really couldn’t overstate how entertaining and informative it was. Science communication truly is an art.

    Thanks for being you! It is very inspiring. It seems like you have one of the best jobs in the world – so if you ever want to take a vacation, I can drive over from DC and cover for you for a few months.

    • Marc Rayman says:

      Thank you very much, Chris. I greatly appreciate your enthusiastic review! For those who don’t know, my “von Karman talk” is an overview of the Dawn mission, and loyal readers from throughout the cosmos may consider watching the archived version. I believe it will be posted here sometime on Dec. 5.

      I’m amused that I’m considered a PowerPoint legend. PowerPoint is simply one tool for communicating. As with my Dawn Journals, I want to communicate all that’s exciting about this mission, and I’m glad you found it effective.

      Thanks for your offer to take care of my job for a while, and if you do, don’t forget to water my plant too. But instead of taking a vacation, I’m planning to go to Ceres, and you’re invited to come along with me!

      Thank you again.


  12. Luca G says:

    Dear Marc,

    thank you for this. I only discovered your Journal today, and spent the last few hours reading though all your entries!!

    Really congratulations on the truly amazing job you do: beside being Mission Director – surely not a mean feat – you also manage write in a great, informative and funny way. I especially like the frequent back-links to previous entries.

    Fantastic, truly inspiring job!
    Congratulations to all your JPL and NASA colleagues as well.

    Best regards,
    Luca G.

    • Marc Rayman says:

      Dear Luca,

      Thank you so much for your kind message! It really pleases me that you enjoy my Dawn Journals. As I explained in my blog about these blogs, I know I am very fortunate to work on this incredibly cool mission, and I want to help others, like you, share in the thrill of exploring the cosmos. It is very gratifying to know my efforts are appreciated.

      If you’ve read through all of my Dawn Journals in a few hours, you read a lot faster than I could. Nevertheless, I hope you gained a sense of what a remarkable adventure this has been. I also hope you’ll join us for the rest of it. We are now on the threshold of a mysterious alien world, and along with everyone else who ever looks to the sky in wonder, we are soon going to have the extraordinary exhilaration of discovery and of new knowledge.

      Excitement, great surprises, perhaps even some real shocks, formidable challenges, probably some disappointments, and whatever else lies ahead, we’re all in this together. Welcome aboard.


      P.S. Here is another link you might enjoy following back.

      • Luca G says:

        Dear Marc,

        many thanks for taking the time to reply personally.

        Well, let’s day I read through MOST of your Dawn Journals in a few hours! :-)

        I graduated in aerospace engineering, but the twists of life led me to do work in a completely different research sector. It is only the recent success of the Rosetta mission that suddenly rekindled my interest in space. So I started reading more and more avidly and quickly discovered the many excellent websites that JPL and NASA maintains. The Mars rovers, Cassini, Venus Express, Messenger, New Horizons, and of course Dawn: truly truly awe-inspiring feats!

        So yes… it is only thanks to amazing people like you that I can still enjoy the thrill of exploring the cosmos. Believe me, your efforts are truly appreciated!

        I’ll be eagerly awaiting arrival at Ceres!

        Best regards
        Luca G.

  13. Marco says:

    Hi Marc, great update!
    When will we see the first NavCam image of Ceres?
    Marco di Lorenzo

    • Marc Rayman says:

      Hi Marco,

      We will be releasing our approach images of Ceres shortly after we obtain them. You will be able to see the picture acquired by the spacecraft last night very soon.

      As a small matter of possible interest, Dawn does not have a NavCam (navigation camera). Some spacecraft do carry a separate camera for navigation, but in order to keep the cost down, Dawn does not. And as I explained in October, we start acquiring images for navigation purposes in January. The pictures we acquired this week are for calibration of the camera for science purposes. They are not needed for navigation.

      Regardless of the purpose, I’m sure we agree that all of Dawn’s views of Ceres will be exciting and tantalizing. We are on the threshold of a new world!


  14. Oliver says:

    That new trajectory diagram is mind boggling. I cannot imagine how one could even begin to solve a problem like that.

    Looking forward to next year’s images.

    • Marc Rayman says:

      Hi Oliver,

      It is rocket science after all!

      To me, this is a wonderful illustration of the power of human creativity and ingenuity. Conducting such a complex and ambitious mission requires an experienced team with great expertise in many areas and unwavering diligence. And the result is incredibly cool, isn’t it?

      Thank you for your interest.


    • eSpace says:

      Oliver, I agree with you; it is amazing what these folks are able to do! For another great example of extreme flight operations go over to the Rosetta Blog and take a look at how they put Rosetta into orbit around Comet 67P and subsequent flight maneuvers.