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Quantitative descriptions of environments near the nucleus of comet P/Halley have been developed to support spacecraft and mission design for the flyby encounters in March, 1986. To summarize these models as they exist just before the encounters, we review the relevant data from prior Halley apparitions and from recent cometary research. Orbital elements, visual magnitudes, and parameter values and analysis for the nucleus, gas and dust are combined to predict Halley's position, production rates, gas and dust distributions, and electromagnetic radiation field for the current perihelion passage. The predicted numerical results have been useful for estimating likely spacecraft effects, such as impact damage and attitude perturbation. Sample applications are cited, including design of a dust shield for spacecraft structure, and threshold and dynamic range selection for flight experiments. We expect that the comet's activity may be more irregular than these smoothly varying models predict, and that comparison with the flyby data will be instructive.

Six interplanetary spacecraft, three earth orbital experiments, and one spacecraft orbiting Venus will observe comets Halley and Giacobini-Zinner in 1985 - 86. At comet Halley, attempts will be made to image the nucleus, remote sensing will be made by spectrometers in wavelength ranges from the IR to the UV, and in-situ observations will be made with neutral, ion and dust mass spectrometers. Plasma measurements will be made at both comets and at comet Halley the upstream solar wind flux will be simultaneously monitored by nearby spacecraft. In the post-Halley era, there are several missions being planned for the continued exploration of our solar system's most primitive bodies - comets and asteroids.

The orbital motion of Comet Wolf is investigated over the interval 1884-1976. In order to represent successfully the observations prior to 1925, nongravitational terms are required in the equations of motion. Mter a close approach to Jupiter in 1922 increased the perihelion distance of Comet Wolf from 1.6 to 2.4 astronomical units, the nongravitational effects were found to be negligible. The motion of Comet Barnard 3 was investigated and no evidence was found to indicate a common origin with Comet Wolf. Key words: comets - celestial mechanics

Nongravitational terms are required in the equations of motion ff the observations of Comet Kopff are to be successfiilly represented over the 1906-70 interval. A secular decrease in the deceleration of the comet's motion is followed by a secular increase in the acceleration of the motion. The results are not easily explained in terms of intrinsic changes in the icyconglomerate model of the cometary nucleus. The orbital behavior is compatible with the assumption that the rotation axis of the cometary nucleus passed tllrough the orbit plane prior to midcentury. Key words: comets - celestial mechanics L Intmduction