A strategy to rotate the Mars Observer orbit node line to advance the mapping schedule

The Mars Observer (MO) spacecraft was successfully launched on September 25, 1992 and will arrive at Mars on August 24, 1993. At Mars, the spacecraft will study the planet's surface, atmosphere, and gravitational and magnetic fields. In order to achieve these scientific objectives, MO will be placed in a 2 PM (descending node) sun-synchronous orbit. Upon arrival at Mars, however, the longitude of the descending node will be approximately 15 deg greater than the desired value. The baseline plan requires a 59 day `waiting' period for the correct solar orientation to occur. During this period, 28 days are required for scientific experimentation but the remaining 30.6 days potentially could be eliminated. The strategy developed in this study examined the possibility of using any `excess' Delta-V available at Mars arrival to rotate the node line to the desired value and thus allow mapping to begin earlier. A preliminary analysis completed prior to launch is described that examined the entire launch period including the required Delta-V to perform the needed nodal rotation. A more detailed study performed after launch is also summarized.