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Marc Rayman

Dawn Journal: HAMO at Ceres

Posted by Marc Rayman

01-07-2014 11:34 CDT

Topics: mission status, Dawn

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.

Dawn’s spiral descent from survey orbit to the high altitude mapping orbit

NASA / JPL-Caltech

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).

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.

Dawn at Ceres

NASA / JPL-Caltech

Dawn at Ceres
Artist’s concept of Dawn in its high altitude mapping orbit at dwarf planet Ceres.

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.

Dawn Survey vs. HAMO

NASA / JPL-Caltech

Dawn Survey vs. 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.

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

 
See other posts from July 2014

 

Or read more blog entries about: mission status, Dawn

Comments:

Anonymous: 07/02/2014 12:52 CDT

Great article! After the recent success of the ISEE-3, it got me thinking, how much xenon will be left after fully observing Ceres and completing its primary mission? Are there any plans to send Dawn to other locations, maybe Jupiter and its moons(though that raises the question of if Dawn would get enough light from the sun to power itself...) or possibly send it to another asteroid such as Hygiea?

Marc Rayman: 07/02/2014 01:33 CDT

When Dawn completes its primary mission, it will have about 30 kg of xenon. Because the ion propulsion is so efficient, that represents a great deal of maneuvering capability. It would be sufficient to break out of Ceres orbit and go elsewhere, but contrary to comments I see occasionally on the web, we have never had plans to do so. 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 at the dwarf planet. 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 blog at the Dawn website.

Nick Morrish: 07/02/2014 01:55 CDT

Thanks Marc! I knew about Keplers reaction wheel giving out, but I didn't know Dawn's had had two wheels give out! That's really neat that they've been using the hydrazine to stabilize it, Go Nasa! Makes sense that we shouldn't count our chickens before they've hatched, especially since we haven't even reached Ceres yet. Thanks again for the response!

Marc Rayman: 07/02/2014 03:16 CDT

You're very welcome, Nick. I appreciate your interest! Dawn experienced its first reaction wheel difficulty in 2010, a year before we reached Vesta, and a second one faltered in 2012, as we were climbing away from the giant protoplanet. We did not launch with the intent to use hydrazine to stabilize the spacecraft, and the flight team did a superb job devising creative ways to extend the remaining supply to allow us to travel to and explore Ceres. If you're interested, I summarized the reaction wheel and hydrazine story in a reply to Henk on my June Dawn Journal at the Dawn website (http://dawn.jpl.nasa.gov), and I include a link there for still more on how we have accomplished this.

Bob Ware: 07/04/2014 04:05 CDT

Thanks for another wonderful article! Would there be any benefit, if possible from an engineering position, to leave Dawn as a radio beacon so we can watch Ceres orbital characteristics over time, post mission or are the end of mission guidelines firmly in place?

Marc Rayman: 07/04/2014 07:40 CDT

That's a nifty idea, Bob. I'm not sure of an engineering benefit, but the scientific benefit of continuing to refine its orbit might be of some value. However, if an extended mission is conducted, we will operate Dawn in its full science mode, returning data on all investigations. And once the hydrazine is expended, the spacecraft will cease operating. In brief, it will neither be able to keep its solar arrays pointed at the sun (the radio transmitter takes a fair amount of power) nor even one of its low gain antennas pointed near Earth. Even if there were a way to keep the spacecraft operating, without some attitude control, the intermittent contact might make it difficult to disentangle spacecraft orbit perturbations and attitude motion from Ceres' own heliocentric orbital variations. I expect that well before Dawn has completed its primary mission, we will have measured Ceres' orbit so well that we would not be able to improve upon it with a little more tracking of the radio signal.

Bob Ware: 07/05/2014 09:15 CDT

Thanks for the response Dr. Rayman. I understand what you are saying and it would have been nice if "a few drops", so to speak, we left over so that some University or College space science class could practice tracking and learning about orbital perturbations from a hands on situation, post mission, when the team no longer needs the spacecraft. That's where I was thinking. The recent ISEE recovery gave me the idea. But I do like the finish you all are doing... right to the last drop! : )

Marc Rayman: 07/07/2014 11:59 CDT

I love the ISEE-3 recovery work, and your sentiment is well taken, Bob. The challenges in tracking (much less actually controlling) Dawn and getting anything useful out of it would be extraordinary for students. There are too many reasons to explain here, but two of them are that the spacecraft is very, very far from Earth, and the hydrazine gauge will be constantly dropping. While there are many factors for NASA to weigh in deciding on an extended mission, I think the best use of this deep-space explorer's very limited lifetime after the primary mission would be to keep it operating productively, squeezing as much out of Ceres as we can, reaping new rewards until, as you put it, the last drop.

Bob Ware: 07/07/2014 08:49 CDT

I agree with the NASA guidelines on flight and I'm not pushing for change so I hope no one starts a public drive for that. It was just a nice wish for students. Thanks again for the great articles! Dawn is a fantastic mission! : )

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