Short-period comets complete their orbits in less than 200 years while long-period comets may take hundreds of thousands of years to complete an orbit. A. The size of a comet's nucleus is determined by two solar forces, radiation pressure and solar wind. B. Comets accelerate as they move away from the Sun in part because they lose material from the coma each time they pass near the Sun. C. Comets are at times sent into the inner solar system as a result of a collision or a gravitational disturbance. D. Although a comet's coma and tail may be quite large, these features do not develop until a comet is close enough to the Sun for solar energy to vaporize its frozen gases. E. While short-period comets have circular orbits that lie close to the plane of the planets, long-period comets have more elliptical orbits that form a spherical shell around the solar system. F. Although studies of Halley's Comet and other short-period comets provide some clues, astronomers do not yet fully understand how and when comets formed.