DEEP IMPACT GROUND NAVIGATION MANEUVER DESIGN AND PERFORMANCE

摘要

Deep Impact, consisting of an Impactor and a Flyby spacecraft, successfully completed its mission goals on July 4, 2005 when the Impactor collided with the comet 9P/Tempel 1 and the Flyby spacecraft transmitted hundreds of pictures of the comet ejecta during and after the collision. The objective of the ground navigation maneuver strategy was to release the Impactor within 5 km of the estimated center of Tempel 1 at its release time (24 hrs prior to impact) and to retarget the Flyby spacecraft to a flyby altitude of 500 km with 850 sec delay in comet approach relative to the actual impact. In-flight changes in the comet ephemeris improvement plan due to unavailability of images from the High Resolution Instrument and frequent changes in the maneuver scheduling significantly influenced the maneuver design strategy. In addition, the maneuver design had to account for several geometric pointing constraints that restricted the allowable thrusting attitudes. These restrictions posed a great challenge to the maneuver design task. A robust, adaptive, and flexible maneuver sequence was designed and implemented to realize performance better than the mission requirements. The trajectory reconstruction, after the impact, revealed that the achieved impact miss distance by ground navigation was about 2.4 km. The 500 km flyby target altitude achieved, by the Flyby spacecraft, was within ～3 km while desired observing time matched the 850 sec within ～1 sec.