Dawn Journal: HAMO at Ceres
Posted by Marc Rayman
01-07-2014 11:34 CDT
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.
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.
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.
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