Dawn journal: seeing Vesta in a new light
Posted by Marc Rayman
2012/07/06 02:33 CDT
Dear Upside Dawn Readers,
Dawn is now seeing Vesta in a new light. Once again the probe is diligently mapping the ancient protoplanet it has been orbiting for nearly a year. Circling the alien world about twice a day, the ardent adventurer is observing the signatures of Vesta's tortured history, including the scars accumulated during more than 4.5 billion years in the main asteroid belt between Mars and Jupiter.
Having successfully completed its orbital raising maneuvers to ascend to its second high-altitude mapping orbit (HAMO2), Dawn looks down from about 680 kilometers (420 miles). This is the same height from which it mapped Vesta at the end of September and October 2011. The lifeless rocky landscape has not changed since then, but its appearance to the spacecraft's sensors has. The first high-altitude mapping orbit (HAMO1) was conducted shortly after southern hemisphere summer began on Vesta, so the sun was well south of the equator. That left the high northern latitudes in the deep darkness of winter night. With its slower progression around the sun than Earth, seasons on Vesta last correspondingly longer. Thanks to Dawn's capability to linger in orbit, rather than simply conduct a brief reconnaissance as it speeds by on its way to its next destination, the probe now can examine the surface with different lighting.
Much of the terrain that was hidden from the sun, and thus the camera, during HAMO1 is now illuminated. Even the scenery that was visible then is lit from a different angle now, so new observations will reveal many new details. In addition to the seasonal northward shift in the position of the sun, Dawn's orbit is oriented differently in HAMO2, as described last month, so that makes the opportunity for new insights and discoveries even greater.
The strategy for mapping Vesta is the same in HAMO2 now as it was in HAMO1. Dawn's orbital path takes it nearly over the north pole. (As we saw last month, the orbit does not go exactly over the poles but rather reaches to 86 degrees latitude. That slight difference is not important for this discussion.) During the ship's southward passage over the sunlit side, the camera and the visible and infrared mapping spectrometer (VIR) acquire their precious data. After passing (almost) above the south pole, Dawn sails north over the night side. Instead of pointing its sensors at the deep black of the ground below, the probe aims its main antenna to the extremely distant Earth and radios its findings to the exquisitely sensitive receivers of the Deep Space Network. The pattern repeats as the indefatigable spacecraft completes loop after loop after loop around the gigantic asteroid every 12.3 hours.
As Dawn revolves, Vesta rotates on its axis beneath it, turning once every 5.3 hours. Just as in HAMO1, mission planners artfully choreographed this celestial pas de deux so that over the course of 10 orbits, lasting just over five days, the camera would be able to view nearly all of the lit surface. A set of 10 orbits is known to Dawn team members (and to you, loyal readers) as a mapping cycle.
Until a few months ago, HAMO2 was planned to be four cycles. Thanks to the determination in April that Dawn could extend its residence at Vesta and still meet its 2015 appointment with dwarf planet Ceres, HAMO2 has been increased to six mapping cycles (plus even a little more, as we shall see below), promising a yet greater scientific return.
In cycle 1, which began on June 23, the camera was pointed at the surface directly underneath the spacecraft. The same view will be obtained in cycle 6. In cycles 2 through 5, images are acquired at other angles, providing different perspectives on the complex and dramatic landscape. Scientists combine the pictures to formulate topographical maps, revealing Vesta's full three-dimensional character from precipitous cliffs and towering peaks of enormous mountains to gently rolling plains and areas with mysterious ridges and grooves to vast troughs and craters punched deep into the crust. Knowing the elevations of the myriad features and the angles of slopes is essential to understanding the geological processes and forces that shaped this exotic mini-planet. In addition to the exceptional scientific value, the stereo imagery provides realistic, exciting views for anyone who wants to visualize this faraway world. If you have not traveled there yourself, be sure to visit the Image of the Day regularly and the video gallery occasionally to see what you and the rest of humankind had been missing during the two centuries of Vesta's appearance being only that of a faint, tiny blob in the night sky.
With 3-D movies and other familiar stereo pictures, only two angles are needed. That's sufficient to reproduce what our two eyes would perceive, but it does not tell the entire story. A left-right pair reveals nothing about the up-down dimension. Scientists chose the directions to point Dawn's camera that yield the best combinations of perspective and illumination to construct a complete contour map.
In cycle 2, the craft soars over the sunlit side with its camera pointed both ahead and to the left of the ground directly below. In cycle 3, the instrument will be targeted behind and slightly to the left. Cycle 4 will observe the surface farther back and to the right. Cycle 5 will look slightly ahead and to the right. Together these pictures will yield a fabulous sense of the detailed shape of Vesta, and combining them with the HAMO1 images will afford an extraordinarily comprehensive 3-D view.
The camera and VIR are mounted on the spacecraft so that they point in the same direction. During these six cycles, the direction is determined by what's needed for the topographic mapping, but VIR collects valuable spectra as well wherever it is aimed. A spectrum is a measure of the intensity of light at different wavelengths and is reminiscent of the rainbow you see when a glass prism or droplets of water separate white light into its constituent colors. The material on Vesta imprints its signature on the light it reflects from the sun, so VIR's measurements reveal the nature of the minerals. The sensor has already found that Vesta displays a highly varied composition, attesting to its complex geological history. VIR records light from ultraviolet through the entire visible range and into the infrared. Indeed, the instrument operates so far into the infrared that it can detect the meager heat emitted from the surface, thereby also functioning as a remote thermometer. Each VIR snapshot consists of the spectrum at 256 locations on the surface, providing a great richness of information.
Compared to the camera, VIR trades greater spectral coverage for smaller spatial coverage. VIR was the prime instrument in survey orbit, where it was high enough that even with its narrow view, it could observe most of the surface. At the lower altitude of HAMO1 and HAMO2, VIR cannot map all of Vesta in a single mapping cycle or even in six cycles. (And even with all the bonus data it collected during months of operation in the low-altitude mapping orbit (LAMO), the proximity to the surface allowed it to obtain excellent close-up views but only of small regions.) HAMO1 was so outstandingly productive that VIR did see much of the surface, and now the coverage is being increased significantly with HAMO2.
Because the mission has been going so well, mission planners decided to devote some extra time in HAMO2 to additional VIR measurements. From June 15 through 23, before the six mapping cycles commenced, VIR was the star of the celestial show again. Every orbit was dedicated exclusively to collecting as many spectra as could be transmitted to Earth. The telecommunications link that stretches across the solar system is very limited. By not splitting it between the camera's images and VIR's spectra, controllers could maximize the latter's coverage of Vesta.
Dawn's exceedingly productive exploration may make its accomplishments appear easy, but as with all such undertakings, the success is enabled by a group of people applying their collective expertise, discipline, creativity, and powerful drive to reveal the unknown. It is thanks to their extraordinary investment of time and energy that the distant probe is able to execute such an ambitious mission, unveiling an ancient world that previously had only been glimpsed from afar by telescopes.
When the previous log was unleashed upon readers of all dawnominations, Dawn was partway through its long spiral route from LAMO to HAMO2. (You can see the weekly progress in altitude by checking the May mission status reports.) Complex and challenging though it was, the flight went precisely as intended. Because maneuvering the spacecraft exactly to its targeted destination is so difficult, mission planners had scheduled a window to fine tune the orbit on June 9 and 10 after the main phase of ion thrusting was complete. This is very similar to the trajectory correction maneuvers planned before the swing past Mars. Nevertheless, upon carefully measuring the actual orbit following the end of thrusting on June 4, navigators determined that it was so good that no adjustments were needed.
Before the resumption of Vesta observations on June 15, engineers reversed some reconfigurations of the spacecraft they had made for operation at lower altitude. They also took advantage of the time to perform a routine verification of the health of the back-up camera, ensuring that it remained ready to take over if the primary camera encountered problems. Both instruments are in excellent condition.
As Dawn continues tirelessly to scrutinize Vesta and report its fascinating findings, the mission control team is putting the finishing touches on the plans for its departure. On July 25, the ship will begin climbing out of HAMO2, its sights set on Ceres. Just as during the approach phase, however, it will pause occasionally for some additional observations. As Vesta grows farther and smaller but sunlight touches more of the high northern latitudes, the instruments will take some parting shots. We will describe those plans in the next log. As we shall see, even as Dawn says goodbye to its companion of more than a year deep in the main asteroid belt, it will continue to discover new secrets to thrill and delight all the passionately curious and bold creatures who champion the eager explorer on its interplanetary voyage. Through this robot, they are transported far, far into space to behold sights and gain knowledge that otherwise would remain forever beyond their reach.
Dawn is 680 kilometers (420 miles) from Vesta. It is also 3.17 AU (474 million kilometers or 294 million miles) from Earth, or 1305 times as far as the moon and 3.12 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 53 minutes to make the round trip.
Dr. Marc D. Rayman
10:30 p.m. PDT June 30, 2012