40. Comet Halley Nucleus
Comets dominate the realm of the solar system beyond Neptune and Pluto. Comets are perturbed from their long orbits around the Sun by passing stars and brought into the inner solar system where some become visible from Earth. Probably the best-known comet is named after Sir Edmund Halley, who recognized it to have a periodic orbit and successfully predicted when it would be seen again. Comet Halley returned most recently in 1986, at which time several nations (with the notable exception of the United States) sent spacecraft to study it. The European Space Agency obtained this image of the nucleus of Comet Halley as the Giotto spacecraft approached to within 300 kilometers (200 miles) of the nucleus. The dark, irregularly shaped object (left of center), from which bright jets of dust and gas are escaping, is the nucleus of the comet. The nucleus is only 15 × 10 kilometers (9 × 6 miles) in size with an extremely dark surface (darker than a black-topped road). The dark surface results from the accumulation of solid material on the nucleus. Ice, which makes up the bulk of the nucleus, is vaporized by sunlight. Both dust and gas contribute to the formation of the diffuse tail that extends millions of kilometers away from the nucleus, pointed away from the Sun.
Image obtained on March 14, 1986, with the Halley Multicolour Camera on the European Space Agency spacecraft Giotto, courtesy of Dr. Horst U. Keller (copyright 1986 by the Max Planck Institut für Aeronomie).