Marc Rayman • Sep 02, 2011
Dawn Journal: Spiraling Down from Survey Orbit
Dawn has completed the first phase of its exploration of Vesta with tremendous success, and the peripatetic adventurer is now in powered flight again, on its way to a new location from which to scrutinize its subject. Meanwhile, scientists are deeply engaged in analyzing the magnificent views the stalwart surveyor has transmitted to Earth.
Most of August was devoted to survey orbit. At an altitude of about 2,700 kilometers (1,700 miles), the ship sailed slowly around the world beneath it, completing a loop every 69 hours. Vesta rotates faster, turning once on its axis each 5 hours, 20 minutes. As we saw in the previous log, the survey orbit phase of the mission consisted of seven revolutions around Vesta, providing ample opportunities to acquire the rich bounty of data that scientists yearned for.
As Dawn follows its course, it passes over the north pole, then heads south on the day side of Vesta. On each orbit, it trained its sensors on the illuminated surface and filled its memory with the spectacular sights. On the other half of its orbit, gliding high above the dark landscape, it radioed its findings to distant Earth.
As we discussed last year, Vesta has seasons, just as your planet probably does. For readers on Earth, for example, it is summer in the northern hemisphere, and a region around the south pole is in constant darkness. On Vesta right now, the southern hemisphere is facing the sun, so everywhere between about 52 degrees north latitude and the north pole is in a long night. That ten percent of the surface is presently impossible to see. Because Dawn will stay in orbit around Vesta as together they travel around the sun, in 2012 it will be able to see some of this hidden scenery as the seasons advance.
The campaign of acquiring data in survey orbit was very complex. On the second, fourth, fifth, and sixth loops, the strategy included collecting more than Dawn's memory could accommodate in the half of an orbit in which it was over sunlit terrain. Therefore, during those orbits, mission planners incorporated instructions to turn away from looking at Vesta to allow the spacecraft to point its main antenna to Earth for five to six hours. That provided time to transmit enough of its precious findings to make room for still more during the rest of the passage over the day side.
On the first and third revolutions, the computer in the visible and infrared mapping spectrometer (VIR) encountered an unexpected condition, so it stopped collecting data. When the spacecraft was next on the night side, controllers reconfigured the instrument so it could resume normal operation for the subsequent lap. Engineers and scientists from Italy who developed the complex device and from JPL are working closely together to establish the underlying cause. They have taken advantage of the extended periods in each orbit when the main antenna is pointing to Earth to run diagnostic tests on the unit. All indications are that it is healthy, and evidence points strongly to the glitches being related to some detail of the mode in which VIR collects and processes data. The team is confident that once they understand the behavior, they will be able to formulate plans to operate the spectrometer in ways that avoid triggering it.
Thanks to the strategy to perform more observations than needed, even with the interruptions, VIR accumulated a fantastic wealth of information. The principal scientific objective of survey orbit was to collect 5,000 sets of spectra or "frames." A spectrum is the intensity of light at different wavelengths, and each frame consists of visible and infrared spectra at 256 locations on Vesta's complex and mysterious surface. By the end of survey orbit, Dawn had obtained well in excess of 13,000 frames, or more than three million spectra. Acquiring more than one spectrum of the same location is valuable, as different angles of incident or reflected sunlight allow scientists to gain greater insight into the mineralogical composition and properties of the material. With an initial plan of observing 52 percent of the surface with VIR from survey orbit, the team is elated now to have spectra from about 63 percent.
The science camera has similarly overachieved. The intent was to photograph 60 percent of Vesta, but the entire 90 percent not in the darkness of northern winter has been captured at least five times. With pictures taken from multiple angles, stereo views can be constructed; and images at different times allow features to be observed under varied lighting conditions. All of the camera's color filters were used, providing coverage in the near infrared and visible. Until recently, Vesta was known as little more than a smudge of light, but now scientists have more than 2,800 photos from Dawn's survey.
A selection of stunning scenes of the latest world to come into the realm of humankind's knowledge is here. As scientists pore through the treasure trove, they will continue to add their favorite views to that site.
This mission has already revealed far more about Vesta than a flyby mission could. While much more data will be obtained during the rest of Dawn's residence there, the six gigabytes from VIR and the three gigabytes from the camera so far are enough to keep researchers busy (and extremely happy!) for a very long time as they tease out the nature of this alien world.
Even before the outstandingly successful survey orbit had begun, navigators were starting to plan the flight to the next science orbit. Throughout Dawn's approach and survey orbit, they have been refining measurements of Vesta's mass and therefore its gravitational strength. The closer Dawn has come, the better they have been able to detect variations in the gravity field that are due to the uneven distribution of mass within the protoplanet. With their improved charts of the waters around Vesta, they plotted the ship's course, and it is now under sail. Thrusting with the ion propulsion system began on August 31 at 4:05 p.m. PDT, and this trip to the high altitude mapping orbit will take a month.
In survey orbit, Dawn was 2,700 kilometers (1,700 miles) above Vesta. Its next orbital target lies at an altitude of about 680 kilometers (420 miles). The separation between them may seem relatively small, but maneuvering in orbit requires far more work than may be evident simply from the distance. In addition, Dawn is doing even more than flying down to a lower altitude. Each of the observation orbits at Vesta is designed to optimize a set of scientific investigations. Scientists want to shift the plane of Dawn's orbit in going from survey to HAMO in order to change the illumination presented to the sensors.
To visualize the nature of the shift, picture the orbit as a ring around the world, going over both poles and crossing the equator at right angles. Many globes are supported within a ring like this. Now for this explanation, we have the permission of the residents to ignore Vesta's rotation, so the ring is like a circle of constant longitude hovering in space. For the purpose of illustration, let's say survey orbit is at a longitude of 15 degrees. (The distant sun would be at a longitude of 0 degrees. Shining south of the equator at a latitude of 27 degrees, the star is more than 2.25 times farther from Vesta than it is from Earth.) HAMO is not only four times closer to the surface than survey orbit but also rotated so it is at 30 degrees longitude. Of course, Vesta will continue to turn on its axis, but with the plane of Dawn's orbit changed, the angle of sunlight falling on the surface below will be different. (Once again, "longitude" is used here only to illustrate the relative orientation of the orbit planes; it is not intended to describe a relationship to specific coordinates on the ancient, battered, rocky world.)
To travel from survey to HAMO, Dawn will have to accomplish the equivalent of a change in speed of around 65 meters per second (145 mph). Compared to the 6.8 kilometers per second (15,200 mph) it achieved in its trek from Earth to Vesta, this is very modest indeed. Nevertheless, as the spacecraft spirals lower, the flight plan is much more complex than it was during the interplanetary flight. The outcome will be described in the next log.
Even as Dawn ventures closer to the giant that holds it in orbit, the splendid results of its first detailed survey of Vesta will continue to dazzle and excite us. The images and other data beamed to Earth are filled not only with scientific value but also with the exhilaration of discovery and the thrill of exploration. The drama upon beholding some of humankind's first views of an alien world is something everyone can experience. After all, it is the collective passion for extending our reach beyond the confines of our terrestrial neighborhood and the shared hunger for knowledge of the cosmos that enabled an emissary from Earth to take up residence far from home, deep in the asteroid belt. There, our species' yearning for noble adventures is now being fulfilled.
Dawn is 2,600 kilometers (1,600 miles) from Vesta. It is also 1.35 AU (202 million kilometers or 125 million miles) from Earth, or 555 times as far as the moon and 1.34 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 22 minutes to make the round trip.
Dr. Marc D. Rayman
10:00 a.m. PDT September 1, 2011
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