Any mission to Halley should be designed to maximize the science in terms of the objectives of the comet exploration program set down by the NASA Comet Science Working Group and the Space Science Board (Report of the Comet Science Working Group, NASA TM 80543,1979;Strategy for the Exploration of Primitive Solar System Bodies—Asteroids, Comets, and Meteoroids: 1980–1990, Committee on Planetary and Lunar Exploration, Space Science Board, Washington, DC, 1980). In order of priority, these objectives are: (1) to determine the chemical nature and physical structure of comet nuclei and to characterize the changes that occur as functions of time and orbital posit