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Robert Tolson with the University of Maryland |
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Date: November 30, 2004
Time: 10:30pm
Location: NIA, Rm 404
Speaker: Robert Tolson with the University of Maryland
Subject:* "Aerobraking at Mars -Serendipitous Atmospheric Explorers"
Aerobraking, one method of using the atmosphere to beneficially modify a spacecraft orbit, has been used at Venus and Mars to change high eccentricity elliptical orbits into nearly circular orbits. To limit spacecraft heating, aerobraking takes place over hundreds of orbits by dipping into the upper atmosphere on each periapsis pass. On Mars missions, 300 and 800 passes were required to complete the aerobraking phase. The alternative to aerobraking is to perform a more traditional propulsive maneuver requiring fuel and resulting in a significant reduction in scientific payload. Based on the experience of the first aerobraking planetary mission, Venus Magellan, it was known that information about the upper atmosphere could be gleamed from data taken during aerobraking. However, nothing in the Magellan experience prepared flight operations for Mars Global Surveyor aerobraking. After only 7 aerobraking orbits, MGS encountered a near doubling of atmospheric density at the same altitude as previous passes. Forty orbits later, a regional dust storm in the southern hemisphere produced a doubling of density at high northern latitudes. It soon became clear that, in the winter polar regions, a 40% 1-sigma variability from orbit to orbit due to density variations with 20-100 km horizontal scales was typical. As aerobraking latitudes moved toward the equator, global stationary waves were encountered. While all these features excited atmospheric scientists, mission operations continued to sit on their edge of their seats because the spacecraft only had a 90% safety margin against density variations. Having over 800 passes of MGS data, many thought Mars had unveiled all of its atmospheric tricks and the next aerobraking mission, Mars Odyssey, was prepared. But the atmosphere had more tricks including a polar vortex and a surprisingly quiescent zone inside the vortex, until orbit 106.
This talk will first discuss spacecraft and mission design associated with aerobraking and present performance results for MGS and Odyssey. Second, many of the unanticipated atmospheric features will be presented along with how they impacted the mission, how they were handled during operations, and how they have added to our knowledge of the Martian upper atmosphere. Need and potential for interdisciplinary theoretical and experimental research will be discussed.
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