How typical is Halley's Comet?

Abstract

Comet Halley spacecraft encounters and observations in 1985 to 1986 are compared to all c omets, both long and short-period. Halley's orbit is quite unusual for a short-period (SP) comet, characterized by a very small perihelion distance and a retrograde inclination. The comet is much brighter than most SP comets, and is even relatively bright for a long-period comet. Although Halley's orbit crosses the orbits of seven of the nine planets, it can currently make close approaches to only Venus, Earth, and Mars, planets likely too small to have played a major role in capturing Halley to a short-period orbit. This, and other evidence, suggests that Halley has been in its current orbit for many returns. A variety of dynamical paths for evolving Halley to its present orbit from either the inner or outer Oort clouds are possible, and thus only probabilistic arguments can be made as to the comet's past physical and dynamical history.