A close look at Halley's comet

Halley's comet is not only the most famous and historically the most important of comets, but also one of the best-suited to space probe encounters. Halley's comet has an orbit that is sufficiently well known for a probe to be directed close enough to the nucleus to obtain good data. Moreover, the strong emission of gas and dust from the comet suggests that it retains much of its original icy and dusty components. In other words, the comet probably consists of pristine material, and so it reflects the conditions prevailing when the solar system was born. This paper is based primarily on the results obtained by the Giotto space probe. The Giotto mission made particularly significant contributions in determining the composition of the neutral and ionized gases in the coma. Undoubtedly the highlight of the Giotto voyage was the close approach of the spacecraft to the nucleus itself. Now the missions to Halley's comet have confirmed the essential features of the accepted model of cometary nuclei: the dirty snowball model. The dimensions of the nucleus have been estimated to be roughly 16-by-eight-by-eight kilometers. The surface area of the nucleus is approximately four times larger than had been thought. A further surprise is that the jets appear to be emitted from a relatively small fraction of the comet's total surface. 7 figs