Cometary ephemerides for spacecraft flyby missions

The determination of cometary ephemerides is considered with reference to proposed missions to Comet Halley. It is noted that the ability of spacecraft without onboard navigation capability to fly close to target comets is limited chiefly by the comet's ephemeris uncertainty. The situation is complicated by nongravitational forces acting on a comet's nucleus and the paucity of observers currently making astrometric observations of comets. The nongravitational forces affecting Comet Halley are consistent with the rocket effect of an outgassing water ice nucleus; the nucleus is apparently rotating in a direct sense about a stable spin axis. It is emphasized that accurate Comet Halley ephemerides for close spacecraft flybys will require continued efforts to refine the existing nongravitational force model. In addition, the flyby mission to Comet Halley will require a well-organized network of astrometric observers. For this purpose, an Astrometry Network is being set up within NASA's International Halley Watch program.