The Topography (and Ephemeris) of Phobos from MOLA Ranging
The MGS spacecraft experienced four close encounters with Phobos in the late summer of 1998. The last (and closest) of these, on September 12, 1998, had an encounter distance of 265 km, well within the maximum MOLA range of 780 km. The apparent motion of Phobos at encounter was 0.7 deg/sec (well in excess of the maximum MGS roll rate of 0.37 sec), which would have resulted in about 6 seconds on target for a fixed spacecraft orientation, or about 10 seconds using a spacecraft roll to partially compensate for the motion. A scheme was devised to maximize the ranging time on Phobos by overtaking the trailing limb with the MGS slew while at a distance (roughly 530 km) such that the apparent motion of Phobos was still less than 0.3 deg/sec. As the track crossed Phobos and the distance decreased, the increase in the apparent motion slowed and eventually reversed the track before the entire disk was traversed. The track then re-crossed the trailing limb at a range of about 350 km. This operation resulted in the first successful active spacecraft ranging to a small body, with nearly 70 seconds of time on target and 627 valid ranging measurements along two nearly coincident, but slightly offset tracks. These tracks cross the Mars-facing hemisphere from SE to NW, covering a length of about 120 deg of arc. At these ranges the laser footprint varied in size from 130 to 200 m and the footprint spacing ranged from less than 10 m near the reversal point to a few hundred meters near the limb. Successful returns were obtained at emission angles up to 80 deg.