Dawn Journal: Maneuvering Around Ceres
Posted by Marc Rayman
03-03-2014 6:00 CST
Dear Ardawnt Readers,
Continuing its daring mission to explore some of the last uncharted worlds in the inner solar system, Dawn remains on course and on schedule for its rendezvous with dwarf planet Ceres next year. Silently and patiently streaking through the main asteroid belt between Mars and Jupiter, the ardent adventurer is gradually reshaping its orbit around the sun with its uniquely efficient ion propulsion system. Vesta, the giant protoplanet it unveiled during its spectacular expedition there in 2011-2012, grows ever more distant.
In December and January, we saw Dawn's plans for the "approach phase" to Ceres and how it will slip gracefully into orbit under the gentle control of its ion engine. Entering orbit, gratifying and historic though it will be, is only a means to an end. The reason for orbiting its destinations is to have all the time needed to use its suite of sophisticated sensors to scrutinize these alien worlds.
As at Vesta, Dawn will take advantage of the extraordinary capability of its ion propulsion system to maneuver extensively in orbit at Ceres. During the course of its long mission there, it will fly to four successively lower orbital altitudes, each chosen to optimize certain investigations. (The probe occupied six different orbits at Vesta, where two of them followed the lowest altitude. As the spacecraft will not leave Ceres, there is no value in ascending from its fourth and lowest orbit.) All of the plans for exploring Ceres have been developed to discover as much as possible about this mysterious dwarf planet while husbanding the precious hydrazine propellant, ensuring that Dawn will complete its ambitious mission there regardless of the health of its reaction wheels.
All of its orbits at Ceres will be circular and polar, meaning the spacecraft will pass over the north pole and the south pole, so all latitudes will come within view. Thanks to Ceres's own rotation, all longitudes will be presented to the orbiting observer. To visualize this, think of (or even look at) a common globe of Earth. A ring encircling it represents Dawn's orbital path. If the ring is only over the equator, the spacecraft cannot attain good views of the high northern and southern latitudes. If, instead, the ring goes over both poles, then the combined motion of the globe spinning on its axis and the craft moving along the ring provides an opportunity for complete coverage.
Dawn will orbit in the same direction it did at Vesta, traveling from north to south over the side illuminated by the distant sun. After flying over the south pole, it will head north, the lands directly beneath it in the dark of night. (The orbit geometry is designed to ensure that the spacecraft always remains in sunlight, regardless of the lighting at the surface.) When it travels over the north pole, the terrain below will come into daylight and the ship will sail south again.
Dawn's first orbital phase is distinguished not only by providing the first opportunity to conduct intensive observations of Ceres but also by having the least appealing designation of any of the Ceres phases. It is known as RC3, or the third "rotation characterization" of the Ceres mission. (RC1 and RC2 will occur during the approach phase, as described in December.) What RC3 lacks in the catchiness of its name it more than makes up for in the incredible coolness of what it will accomplish.
During RC3 in April 2015, Dawn will have its first opportunity for a global characterization of its new residence in the asteroid belt. It will take many pictures and record visible and infrared spectra of the surface, which will help scientists determine its composition. In addition to learning about the appearance and makeup of Ceres, these observations will allow scientists to establish exactly where Ceres's pole points. The axis Earth rotates around, for example, happens to point very near a star that has been correspondingly named Polaris, or the North Star. [Note to editors of local editions: You may change the preceding sentence to describe wherever the axis of your planet points.] We know only roughly where Ceres's pole is from our telescopic studies, but Dawn's measurements in RC3 will yield a much more accurate result. Also, as the spacecraft circles the behemoth, navigators will measure the strength of Ceres's gravitational attraction and hence its overall mass.
RC3 will be at an orbital altitude of about 8,400 miles (13,500 kilometers). From there, the dwarf planet will appear eight times larger than the moon as viewed from Earth, or about the size of a soccer ball seen from 10 feet (3.1 meters). At that distance, Dawn will be able to capture the entire disk of Ceres in its pictures. The explorer's camera, designed for mapping unfamiliar extraterrestrial landscapes from orbit, will see details more than 20 times finer than we have now from the Hubble Space Telescope.
Although all instruments will be operated in RC3, the gamma ray and neutron detector (GRaND) will not be able to detect the faint nuclear emissions from Ceres when it is this far away. Rather, it will measure cosmic radiation. In August we will learn more about how GRaND will measure Ceres's atomic composition when it is closer.
It will take about 15 days to complete a single orbital revolution at this altitude. Meanwhile, Ceres turns once on its axis (one Cerean day) in just over nine hours (more than two and a half times faster than Earth). Dawn's leisurely pace compared to the spinning world beneath it presents a very convenient way to map it. It is almost as if the probe hovers in place, progressing only through a short arc of its orbit as Ceres pirouettes helpfully before it.
When Dawn is on the lit side of Ceres over a latitude of about 43 degrees north, it will point its scientific instruments at the unknown, exotic surface. As Ceres completes one full rotation, the robot will fill its data buffers with as much as they can hold, storing images and spectra. By then, most of the northern hemisphere will have presented itself, and Dawn will have traveled to about 34 degrees north latitude. The spacecraft will then aim its main antenna to Earth and beam its prized findings back for all those who long to know more about the mysteries of the solar system. When Dawn is between 3 degrees north and 6 degrees south latitude, it will perform the same routine, acquiring more photos and spectra as Ceres turns to reveal its equatorial regions. To gain a thorough view of the southern latitudes, it will follow the same strategy as it orbits from 34 degrees south to 43 degrees south.
When Dawn goes over to the dark side, it will still have important measurements to make (as long as Darth Vader does not interfere). While the ground immediately beneath it will be in darkness, part of the limb will be illuminated, displaying a lovely crescent against the blackness of space. Both in the southern hemisphere and in the northern, the spacecraft will collect more pictures and spectra from this unique perspective. Dawn's orbital dance has been carefully choreographed to ensure the sensitive instruments are not pointed too close to the sun.
Although it is not the primary objective of the measurements, team members are working to determine whether observations from the vantage point of the night side of RC3 might shed more light on the recent fascinating detection of water vapor around Ceres by the Herschel Space Observatory. Whether the water is lofted into space by ice sublimating on the surface or by geysers or cryovolcanoes ("cold volcanoes," which may be active on this small, frigid world of rock and ice far from the sun) is not yet known. Scientists do not even know whether any water vapor will still be there when Dawn is. Even if it is not, it may be that signs of water will be evident on the surface from other measurements. We will discuss this intriguing possibility more in the December 2014 log.
Dawn's controllers will take advantage of the flexibility afforded by ion propulsion to guide the spacecraft into whatever part of the RC3 orbit turns out to be most efficient, based on details of the trajectory as it closes in on Ceres. So, for example, if it spirals down to RC3 over the unlit side, its observations of the day hemisphere will first be in the north, then the equator, then the south. But if it arrives in RC3 over the low northern latitudes on the side lit by the sun, it will begin its observations over the equator and then continue in the south. After it flies north over the other side and then returns to the half of Ceres that is in daylight, it will be ready to conclude RC3 by collecting its northern hemisphere data. The flight team has formulated the plan so that the activities can be executed in whatever order is most natural. The schedule will be finalized during the approach phase, and readers may rest assured that the answer will be presented in these logs.
If all goes according to plan, which is never assured when undertaking challenging tasks in a forbidding, distant, alien environment that has never even been visited by a flyby spacecraft for an initial reconnaissance, Dawn will collect in excess of 1,000 pictures and several million spectra in RC3. After that rich bounty is securely on Earth, it will resume ion thrusting to lower its altitude to the next orbit. We will discuss the spiral descent in April and that second observation phase in May.
Dawn's first inspection of Ceres in RC3 promises both to provide tremendous advancements in our knowledge and whet our appetites for its subsequent examinations. The most massive resident of the main asteroid belt was also the first one to be discovered. Yet for the more than two centuries since then, our glimpses from afar have shown little more than a fuzzy round dot. That distant orb, shining among the stars, has intrigued us for so long. When finally its invitation for an ambassador from Earth is answered next year, the secrets it has held since the dawn of the solar system will begin to be revealed. The rewards for the long and challenging journey will be new insights, new understanding, and new fuel for the fires that burn within everyone who feels the passion to explore.
Dawn is 14 million miles (22 million kilometers) from Ceres. It is also 1.76 AU (163 million miles, or 263 million kilometers) from Earth, or 725 times as far as the moon and 1.77 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 29 minutes to make the round trip.
Dr. Marc D. Rayman
3:00 p.m. PST February 28, 2014