Mars global surveyor: Mapping orbit evolution and control throughout one Mars year

This paper presents the evolution and control of Mars Global Surveyor's (MGS) mapping orbit throughout one Mars year of the primary mapping mission. Starting on 3/9/99, after the completion of the second phase of aerobraking, this phase had a duration of 695 days or 8,505 orbits and ended on 2/1/01. At present, the project is in an extended mission which shall end on 4/22/02. The MGS orbit is the first short period, low altitude, nearly circular and polar, frozen-orbit and sun-synchronous orbiter of Mars. The two major effects influencing the orbit evolution are: a) small, frequent and nearly instantaneous velocity perturbations induced by the spacecraft's angular momentum desaturations (AMD) and b) the gravity field of Mars. The accumulated effect of AMDs has had a significant effect on the semi-major axis and, therefore, on the ground track walk and the uniform distribution of the ground tracks. In addition, predicting the influence of AMDs directly impacts the accuracy of the predicted time of descending equator crossings, Tdeqx, which is necessary for precise image targeting. Current plans for MGS are: a) continue science observations at least until 4/22/02, b) assist Mars Odyssey during aerobraking as an early warning system for the detection of dust storms, c) obtain high resolution images of potential landing sites for the Mars Exploration Rover Mission (MER) and d) to act as a relay satellite for telemetry transmission during the descent phase of the two MER rovers.