Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL
Dawn Journal | June 30

by Marc Rayman

 

Dear Mastodawns,

Deep in the main asteroid belt, between Mars and Jupiter, far from Earth, far from the sun, far now even from the giant protoplanet Vesta that it orbited for 14 months, Dawn flies with its sights set on dwarf planet Ceres. Using the uniquely efficient, whisper-like thrust of its remarkable ion propulsion system, the interplanetary adventurer is making good progress toward its rendezvous with the uncharted, alien world in about nine months.

Dawn’s ambitious mission of exploration will require it to carry out a complex plan at Ceres. In December, we had a preview of the “approach phase,” and in January, we saw how the high velocity beam of xenon ions will let the ship slip smoothly into Ceres’s gravitational embrace. We followed that with a description in February of the first of four orbital phases (with the delightfully irreverent name RC3), in which the probe will scrutinize the exotic landscape from an altitude of 8,400 miles (13,500 kilometers). We saw in April how the spacecraft will take advantage of the extraordinary maneuverability of ion propulsion to spiral from one observation orbit to another, each one lower than the one before, and each one affording a more detailed view of the exotic world of rock and ice. The second orbit, at an altitude of about 2,730 miles (4,400 kilometers), known to insiders (like you, faithful reader) as “survey orbit,” was the topic of our preview in May. This month, we will have an overview of the plan for the third and penultimate orbital phase, the “high altitude mapping orbit” (HAMO).

(The origins of the names of the phases are based on ancient ideas, and the reasons are, or should be, lost in the mists of time. Readers should avoid trying to infer anything at all meaningful in the designations. After some careful consideration, your correspondent chose to use the same names the Dawn team uses rather than create more helpful descriptors for the purposes of these logs. What is important is not what the different orbits are called but rather what amazing new discoveries each one enables.)

It will take Dawn almost six weeks to descend to HAMO, where it will be 910 miles (1,470 kilometers) high, or three times closer to the mysterious surface than in survey orbit. As we have seen before, a lower orbit, whether around Ceres, Earth, the sun, or the Milky Way galaxy, means greater orbital velocity to balance the stronger gravitational grip. In HAMO, the spacecraft will complete each loop around Ceres in 19 hours, only one quarter of the time it will take in survey orbit.

Sprial to Hamo

Dawn’s spiral descent from survey orbit to the high altitude mapping orbit. The trajectory progresses from blue to red over the course of the six weeks. The red dashed segments are where the spacecraft is not thrusting with its ion propulsion system (as explained in April). Credit: NASA/JPL

In formulating the HAMO plans, Dawn’s human colleagues (most of whom reside much, much closer to Earth than the spacecraft does) have taken advantage of their tremendous successes with HAMO1 and HAMO2 at Vesta. We will see below, however, there is one particularly interesting difference.

As in all observation phases at Ceres (and Vesta), Dawn’s orbital path will take it from pole to pole and back. It will fly over the sunlit side as it travels from north to south and then above the side in the deep darkness of night on the northward segment of each orbit. This polar orbit ensures a view of all latitudes. As Ceres pirouettes on its axis, it presents all longitudes to the orbiting observer. The mission planners have choreographed the celestial pas de deux so that in a dozen revolutions, Dawn’s camera can map nearly the entire surface.

Rather than mapping once, however, the spacecraft will map Ceres up to six times. One of Dawn’s many objectives is to develop a topographical map, revealing the detailed contours of the terrain, such as the depths of craters, the heights of mountains, and the slopes and variations of plains. To do so, it will follow the same strategy employed so successfully at Vesta, by taking pictures at different angles, much like stereo imaging. The spacecraft will make its first HAMO map by aiming its camera straight down, photographing the ground directly beneath it. Then it will map the surface again with the camera pointed in a slightly different direction, and it will repeat this for a total of six maps, or six mapping “cycles.” With views from up to six different perspectives, the landscape will pop from flat images into its full three dimensionality. (As with all the plans, engineers recognize that complex and challenging operations in the forbidding, unforgiving depths of space do not always go as intended. So they plan to collect more data than they need. If some of the images, or even entire maps, are not acquired, there should still be plenty of pictures to use in revealing the topography.)

In addition to acquiring the photos, Dawn will make other measurements in HAMO. During some of the cycles, the camera will use its color filters to glean more about the nature of the surface. The visible and infrared mapping spectrometer will collect spectra to help scientists determine the composition of the surface, its temperature, and other properties.

Exquisitely accurate radio tracking of the spacecraft in its orbit, as indicated by the Doppler shift (the change in frequency, or pitch, as the craft moves toward or away from Earth) and by the time it takes radio signals to make the round trip from Earth, allows navigators to determine the strength of the gravitational tugging. That can be translated into not only the mass of Ceres but also how the mass is distributed in its interior. In August, when we look ahead to the fourth and final science phase of the Ceres mission, the low altitude mapping orbit, we will explain this in greater detail.

Although still too high for anything but the weakest indication of radiation from Ceres, the gamma ray and neutron detector will measure the radiation environment in HAMO. This will yield a useful reference for the stronger signals it will detect when it is closer.

Artist's concept of Dawn in HAMO at Ceres

Artist’s concept of Dawn in its high altitude mapping orbit at dwarf planet Ceres. Credit: NASA/JPL

There is a noteworthy difference between how Dawn will operate in HAMO and how it operated in HAMO1 and HAMO2 at Vesta and even how it will operate in survey orbit at Ceres. In those other orbits, whenever the spacecraft flies above the hemisphere in sunlight, it keeps its sensors pointed at the surface, and whenever it is over the night side, it points its main antenna to Earth. At Vesta, where each HAMO revolution took just over 12 hours, this meant that about every six hours, it had to execute a turn. Were it to follow the same plan at Ceres with a 19-hour HAMO period, when it passed over the north pole, it would begin aiming its scientific instruments at the dwarf planet. When it reached the south pole 9.5 hours later, it would rotate to point its antenna to Earth. Another 9.5 hours after that, when it reached the north pole again, it would pivot to bring the alien terrain back into its sights.

If the robot had its full complement of functioning reaction wheels, that is what it would do in HAMO. Reaction wheels are similar to gyroscopes, and by electrically changing the speed at which they spin, the probe can turn or stabilize itself. The mission was designed to use three reaction wheels, so the ship was outfitted with four. Two are no longer operable. While such a loss could be devastating for some spacecraft, the Dawn flight team has devised highly innovative solutions to accomplish all of the original, ambitious objectives, regardless of the condition of any of the wheels, even the two that are (currently) still healthy. Key to Dawn’s success will be conserving hydrazine, the conventional rocket fuel that it can use to accomplish turns. Dawn’s controllers are taking care with every soupçon of the precious propellant, stretching the supply to allow the mission to complete its bold plans. When the hydrazine is exhausted, Dawn’s expedition will conclude.

Turning so often in HAMO, keeping up with the frequent transitions between flying over the illuminated surface and the surface in the darkness of night, would be unaffordable without reaction wheels, a profligate use of the irreplaceable hydrazine. Instead, it is significantly more efficient to turn less often, allowing the spacecraft sometimes to wait patiently for half an orbit as its instruments stare at the undetectably dark land beneath it and sometimes to maintain its antenna pointing at Earth, even when it is passing over features it otherwise could see. It will see them on other loops however. With this strategy, Ceres can be mapped extensively in HAMO without consuming an excessive amount of hydrazine.

In each mapping cycle, Dawn will make two and a half or three and a half revolutions peering at Ceres, storing images and other valuable data onboard. (The specific duration varies from one cycle to another.) Then, with its memory full, it will turn so it can beam some of its precious findings to distant Earth while it is on the night side of Ceres. That will not be long enough to completely empty the memory but will be sufficient to make room for more data, so after half an orbit, it will turn back to resume its observations. It will follow this pattern for one full cycle, with the dozen passages over the day side providing enough opportunities to complete one map. Then it will devote two and a half revolutions, or two full days, to transmitting the rest of its scientific treasures for the benefit of all those on Earth who ever look to the sky with wonder.

So over the course of 14 complete circuits around Ceres in 11 days, the spacecraft will turn only six or eight times. Ever the responsible conservationists, the team developed all the details of this plan to acquire as much data as possible with the minimum expenditure of hydrazine.

Survey_HAMO

This figure shows Dawn’s survey orbit (at 2,730 miles, or 4,400 kilometers) and the high altitude mapping orbit (at 910 miles, or 1,470 kilometers) at the same scale as the size of Ceres. Credit: NASA/JPL

It will take more than two months to carry out all the HAMO activities, with the spacecraft making more than 80 orbital loops. This continues the trend in which the explorer will spend more time in each successive orbital phase than in the ones before. It will complete its assignment in survey orbit in 22 days, during which it will circle Ceres seven times. As we will see in August, the final orbital phase will last even longer than HAMO and include many more revolutions.

Each phase of the mission at Ceres will reveal exciting new insights into a relict from the dawn of the solar system. That same solar system’s complex ballet happens to be playing out now in a way that affords terrestrial observers a nice view of Ceres, Vesta, Mars and the moon. (It also affords Cerean observers a nice view of Vesta, Mars, the moon, and Earth, but that will be described in more detail in the special Cerean local edition of this log.) We wrote in March about the alignment and provided a chart you can still use to locate Vesta and Ceres with a small telescope or even good binoculars. On July 5, Ceres and Vesta will appear to be separated by only one third the diameter of the full moon, even as these distant worlds are 0.57 AU (52 million miles, or 85 million kilometers) from each other. In Earth’s skies, Mars and the moon (both of which are closer to Earth) will not be far away, all of them in Virgo.

Although even the most powerful telescopes are quite insufficient to show it, when we turn our mind’s eye to the sky, with its greater visual acuity, we can discern one more object in this lovely arrangement of gleaming celestial jewels set against the backdrop of the incomparable blackness of the universe. A probe from Earth, a robotic ambassador to the cosmos, on a long and daring expedition, is in transit from Vesta to Ceres. Even as those terrestrial observers enjoy the view, Dawn is patiently making its way through the interplanetary void to a world that has been glimpsed only from afar for more than two centuries. Soon it will undertake a new phase of its extraordinary mission, promising exciting new knowledge and surprising new insights. Engaged in one of humankind’s grand adventures, we extend the best we have within ourselves to reach far, far beyond our humble home.

Dawn is 5.6 million miles (9.0 million kilometers) from Ceres. It is also 2.24 AU (208 million miles, or 335 million kilometers) from Earth, or 825 times as far as the moon and 2.20 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 37 minutes to make the round trip.

Dr. Marc D. Rayman
7:00 p.m. PDT June 30, 2014

All Dawn Journal entries

 


18 Responses to “Dawn Journal | June 30”

  1. Ben says:

    When will Dawn take its first candid shot of Ceres? Is there a web-available archive of all the pics taken by Dawn?

    Rgds,
    Ben

    • Marc Rayman says:

      Hi Ben,

      Dawn will start taking pictures of Ceres during the approach phase, which I described in some detail in December. The exact dates depend on details of the progress of ion thrusting, but the first pictures will be in late January 2015 or, more likely, February. (I don’t believe the first pictures will be either more or less candid than the later ones.)

      All of Dawn’s data are made available through the Planetary Data System. The pictures are also available on the web here. Follow the link to Data, then Framing Camera (FC) Images, then have fun.

      Thank you for your interest!

      Marc

  2. “… has been glimpsed only from afar for more than two centuries …”. Yes, and using probes with technical “eyes” instead of human eyes will safeguard against what happened when Galileo spotted “canals” on Mars. If we could call it peer review – his contemporary astronomer colleagues “saw” them too. With robotic probes we can now collect data that humankind can revisit in coming centuries (when no doubt they will be able to go such places in person) and compare notes that have not been tainted by phantasy or wishful thinking.

    • Dawn Education and Public Outreach says:

      Thanks for your comment.

      As a note on the history of “canals” on Mars, Galileo did not claim to see them. The Italian astronomer, Giovanni Schiaparelli, claimed to see something in 1877 that was poorly translated into English as “canals.” It wasn’t long before most astronomers recognized them to be an illusion, though the concept remained popular in the imaginations of some members of general public into the 1900’s.

      Best,
      Whitney, Dawn Education and Public Outreach

  3. Mark Gaponoff says:

    Marc,
    In regards to the question below about the ultimate disposition of the Dawn Spacecraft, I was wondering how long the final parking orbit would last in the face of possible gravity anomalies in Ceres? To rephrase, what order of gravity anomaly would destabilize the orbit over short geologic time?
    (I guess I could answer that myself, but my skills are pretty rusty…)
    Oh yes, and thanks for continuing the Dawn Journals…
    –Mark

    • Marc Rayman says:

      Hi Mark,

      Thank you for your question. It may be more challenging to answer than you might think, even with skills that are sharp.

      For other readers, the “order” of the gravity field is a partial description of how irregular it is, reflecting variations in the distribution of mass within Ceres. One can think of this as a mathematical description of how rough the gravitational waters are that Dawn will sail.

      Dawn’s “planetary protection” requirement is that the spacecraft must not impact Ceres for at least 20 years, even assuming the most pernicious possible gravity field. “Possible” is an important qualifier. We needn’t be concerned about gravity fields that are unrealistic.

      Of course, we do not know what the actual gravity field is, but we can establish good bounds on it by combining Hubble Space Telescope observations of the shape of Ceres with other data and some compelling geophysical lines of reasoning. (To address your rephrased question, we include terms in the gravity field to degree and order six.) To investigate the effects of all these possible fields, we have performed extensive studies known to experts as Monte Carlo analyses. In essence, we have analyzed 75 thousand versions of the Ceres gravity field, and then we computed the effect on Dawn’s orbital lifetime in tens of thousands of different circumstances. With all of that, we have never found even one circumstance in which Dawn impacts Ceres within 50 years, which is how long our most detailed studies ran. In fact, the spacecraft never even comes close to the surface. (We did investigate a smaller number of cases for even longer intervals, and all indications are that the orbit persists for much longer.) We can be confident Dawn will remain in orbit for a very long time after its mission is complete.

      This result should not be surprising. After all, one of the criteria we used in selecting the final orbit was that we could ensure that the mysterious surface of the rocky and icy dwarf planet would not be disturbed by the terrestrial materials in Dawn.

      I hope this answers your question.

      Marc

  4. Hello Marc.
    Please forgive me for not having read your other journal entries yet. It is possible you have already answered some of my questions there already. First up, many thanks for this journal. I have been a big fan of Ceres ever since I read John Wyndham’s Exiles on Asperus which introduced me to the whole concept of asteroids when I was just a kid. It fascinated me that here was a planetoid that I haddn’t been taught about in school at all. Since then I’ve devoured everything about Ceres.
    My first question is, where can I find the most detailed images of Ceres? I’ve seen some on the Wikipedia article which seems to be about the most detailed I’ve seen. I am curious about the surface features.
    Second, I seem to recall reading that there was an origional plan to extend Dawn’s mission to a third target. Was that the case? Of course that was in the mass media, so I don’t know how accurate that is.
    Thanks again for the journal. I shall go read your past entries.

    • Marc Rayman says:

      Dear Romana,

      Thank you for your message. Many people first learned about asteroids in science fiction. To me, one of the many rewards of a mission like this is being able to go beyond the fiction to reality, which can be far more exciting. As you probably know, Ceres is entirely unlike the other asteroids.

      I will write more about Ceres in my December 2014 Dawn Journal, but as I mentioned in September 2012, this dwarf planet is truly a colossus. Although the population of the main asteroid belt numbers in the millions, this one object contains about one third of the total mass. Vesta, by the way, is the second most massive resident, so Dawn has single-handedly examined about eight percent of the mass of the entire asteroid belt, and by the time it finishes at Ceres, it will have explored around 40 percent.

      The best views of Ceres have been captured by the Hubble Space Telescope, and you can see them in my December 2013 and February 2014 Dawn Journals. They do not reveal much about the surface however. Each pixel is 19 miles (30 kilometers). In February 2015, during Dawn’s approach to Ceres, the spacecraft will provide us with better pictures, and the view will only improve after that.

      We have never made plans to go to a third target. I too have read in the mass media that we might go somewhere after Ceres, but that was a misunderstanding. In brief, when Dawn completes its primary mission, it will have enough xenon propellant left that (if xenon were the sole consideration) it could use its ion propulsion system to break out of Ceres orbit and go elsewhere. That has never been our intent. The first choice for a follow-on assignment for this explorer would be to conduct an extended mission right there, as there would still be interesting observations to make of this fascinating alien world. More to the point, however, after the loss of two reaction wheels, the spacecraft’s lifetime will be limited by hydrazine, not xenon. It is quite remarkable that it is even possible to complete the originally planned mission under such circumstances, but thanks to the flight team, it is. When the hydrazine is expended, the mission will conclude. I wrote a little more about the end of the mission in response to a question posted on my February Dawn Journal.

      Thanks again for your message. I’m grateful you’re going to read my past entries. I have made an effort to capture the drama, the complexity, the excitement, the extraordinary challenges, the amazing solutions, the inspiration, and the sheer awesomeness of this ambitious interplanetary adventure, and I believe we can all share in that. Welcome aboard!

      Marc

      • Thanks again Marc.
        On the level of cool things, this rates among the coolest! Some days I wish I had done my uni degree in astronomy rather than IT. Driving a space probe is far more exciting than driving a computer or even my phenomenally exciting 4 wheel drive van.
        I’ve been reading through your journals. How cool was it that you were dancing when DAWN approached Vesta. I’ve been looking at the pictures too. I learn visually so pictures really pique my interest, even if they are just orbital diagrams. Oh wow, February 2015, that’s so cool. Better quality than the existing hubble images. Ohh, this is pretty exciting stuff I have to admit. I’m sure to most people it would seem boring, but as I understand it, the total surface area of Ceres is greater than that of the USA’s lower 48 states. This is going to be like seeing and exploring a whole new country, when it is really a whole new world. Very cool stuff.

        • Oh, and I thought I should mention, my last name is an English translation of an Aboriginal one. I’m from the Watharong Tribe. We’ve been doing astronomy for a bit longer than NASA, but we don’t have as nice up to date instruments as you. Our main instrument is a bit older; http://www2.astronomicalheritage.net/index.php/show-entity?identity=15&idsubentity=1 but still pretty cool never the less.

        • Marc Rayman says:

          I greatly appreciate your enthusiasm and interest, Romana. And I agree with you about how cool it is. You don’t have to be working on the Dawn project to be involved. Regardless of your field, as long as you are curious about the universe, as long as you appreciate the grandeur of ambitious interplanetary adventures, as long as you yearn to know more about the cosmos, you are a part of the mission and you can share in the thrill of humankind’s exploration of uncharted worlds.

          As for my dancing when Dawn entered orbit around Vesta, I would say that that illustrates that it doesn’t have to be tense to be exciting!

          Your comment about the better quality views than Hubble makes me wonder if the captions were not clear. Please note that the four photos of Ceres in the February 2015 Dawn Journal were taken by the Hubble Space Telescope. The other view is an artist’s concept, so we don’t have better views yet. By the time Dawn reaches its final phase, however, our pictures will be more than 800 times sharper! (I will write about that in August.)

          Ceres certainly is huge. It is the largest body between the sun and Pluto that a spacecraft has not yet visited (and, like Pluto, it used to be classified as a planet), but it’s not as large as you may have read somewhere. It has 38% of the total surface area of the 48 states, which is still enormous. As I mentioned in my first Dawn Journal, Vesta and Ceres often are compared to US states on the basis of width, which I consider to be slightly misleading. States are mostly flat, but our destinations are three dimensional, so they have far more area. Ceres has 37% of the area of your home country of Australia. If you think about how vast and varied the geology and geography of our countries are, it is easy to imagine that there is a great deal of opportunity for a huge range of diversity on this alien world. There is much there just waiting to be discovered.

          Once again, I’m glad you are going to be part of the exciting journey ahead!

          Marc

          • Thank you again for such polite, concise and informative replies Marc. You have already vastly improved my knowledge of Ceres and I thought I knew a lot about it.
            Much appreciated.

          • Oh, and yes, I realised the artists image was that, rather than a Hubble image. As I understand it, this enhanced colour image on Wikipedia is the best we have so far?
            http://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29#mediaviewer/File:Ceres_optimized.jpg
            Even this small images shows a rather beautiful and mysterious dwarf planet.
            I must see if I can find the proposed surface feature naming conventions for Ceres. I am sure there will be some rather large features to give names to. After all, 37% of the size of Australia is still rather large. I expect there will be the usual impact craters, but I wonder what other surprises there will be.
            I expect this is a bit like the explorers of the larger cave systems in the world like Mammoth Cave. People know it is there, they know it is a cave, they know it will have cave like features, but until they get close and light the way, the beauty and uniqueness will not be known. I hope my second favourite place in the solar system has some wonderful surprises for us.
            Less than a year before Dawn gets there! I sure hope I can hang out that long.
            Thanks again massively for your amazing journal and the incredible enthusiasm you have for this mission. And thanks also to NASA and the US government for giving us the wonderful tool which is Dawn. Ahh, I wish my country still had a space program.

          • Marc Rayman says:

            The image at Wikipedia is from the same set of Hubble Space Telescope observations as the pictures shown in the December 2013 and February 2014 Dawn Journals. They are all equally good (although processed somewhat differently for presentation purposes), and together they are the best we have. But not for much longer!

            The Dawn project is working with the International Astronomical Union to finalize the naming conventions for Ceres, so you won’t have long to wait to find out what the system is. You might be interested to know that on Vesta, the first world Dawn explored, we name craters for Vestal Virgins and other famous Roman women. Other features are named for towns and festivals associated with the Vestals. You can see a list of all the features named (so far) on Vesta here, including the meaning of the name. They are also shown on this map. More will be named as scientists continue analyzing the wealth of data Dawn acquired.

            Australia has a long and interesting history in exploring and utilizing space, and it plays a very important role in the Dawn mission. One of the three complexes for the Deep Space Network is near Canberra, and we regularly use the 112-foot (32-meter) and 230-foot (70-meter) antennas there, along with their extremely sophisticated electronics, to communicate with the distant spacecraft.

            Finally, I would reiterate that these ambitious space expeditions are undertakings of humankind. We, as a species, are reaching out into the cosmos. I believe that that transcends national borders.

            Thanks again for your enthusiastic interest and support, Romana. With a little more patience (and a great deal of work), exciting rewards lie ahead.

            Marc

  5. Henk Bloete says:

    Dear Marc,

    The mission is designed to use 3 reaction wheels, while only 2 seem to be operational.
    Can you redisign the mission to use 2 wheels only, or, at least, to use the 2 wheels
    to further reduce Dawn’s consumption of precious hydrazine?

    By the way, my compliments for your monthly texts, which are always informative and a
    pleasure to read.

    Henk.

    • Marc Rayman says:

      Dear Henk,

      We had the same idea as you: even though we designed the mission to use three reaction wheels, we recognized that we could save some hydrazine by using two. I have written more about this in previous Dawn Journals, with November 2013 perhaps providing the best overview of how we have dealt with the loss of the two wheels. Here is a summary:

      We did indeed develop that two-wheel capability and install the new software on the spacecraft shortly before we began Vesta operations. This “hybrid” control system does reduce the expenditure of hydrazine compared to using no wheels at all, but it will yield the largest savings when Dawn is in its lowest altitude orbit at Ceres (which I will describe in my August Dawn Journal). Given that two of the ship’s four wheels have run into difficulty, we cannot be confident in the lifetime of the remaining two. Therefore, we are preserving them for that final phase of Ceres, where they will provide the greatest benefit. Because we have devised so many other effective methods of conserving hydrazine, we can afford to wait until then to use them. That is, we can stretch our hydrazine supply so that Dawn can accomplish all of its other objectives and still have confidence we can reach that final orbit and conduct our desired observations there. In fact, even if the wheels do not operate when we get there, we should have enough hydrazine to complete our plans. If the wheels do function, it would be a bonus, and the hydrazine supply would last even longer.

      I should point out that it is not readily apparent that this would even be possible. It is to the great credit of the Dawn flight team that they have been able to overcome such an extraordinary challenge. Regardless of the condition of the wheels, we have the capability to leave Vesta orbit, fly the spacecraft for 2.5 years to Ceres, and then carry out the complex exploration of that dwarf planet. That was not envisioned before the first wheel faltered.

      I hope this, plus the details in my prior Dawn Journals, answers your question.

      And thank you for your nice feedback on my writing! It is always a pleasure to know that it is well received, and I’m glad you find my Dawn Journals a helpful way to be engaged in this exciting adventure.

      Marc

  6. Pierre ARPIN says:

    Do you plan to crash DAWN on Ceres when the mission will be over ?

    • Marc Rayman says:

      Hi Pierre,

      Thank you for your question.

      I addressed a similar question posted on my February Dawn Journal, and I hope you will find the complete answer there interesting. In brief, however, to protect the alien world from any terrestrial contamination, we will not allow Dawn to reach the surface. The mission will end with the spacecraft in orbit around the dwarf planet it will have explored. Long after you and I and everyone else involved in the mission (whether directly or by virtue of sharing in the excitement and the wonder of such a bold undertaking) are gone, Dawn will remain a celestial monument representing human curiosity, creativity, ingenuity, and passion for adventure and knowledge.

      Marc

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