HALLEY'S COMET

first comet whose return was predicted, demonstrating that at least some comets are members of the solar system. In 1705 the English astronomer Edmond Halley published a work that included his calculations showing that comets observed in 1531, 1607, and 1682 were really one comet and predicting that comet's return in 1758. The comet was sighted late in 1758, passed perihelion in March 1759, and was named in Halley's honour. Later calculations identify it with the large, bright comet seen during the Norman Conquest of England (1066) and shown in the Bayeux Tapestry of that time and with other comet sightings at intervals of about 76 years, from 240 BC. During the comet's approach in 1910 the Earth probably passed through part of its tail, which was millions of kilometres in length. As predicted, the comet, moving at a speed of more than 80,000 mph (more than 128,000 kph), again passed by the Earth and Sun in NovemberDecember 1985 and MarchApril 1986. The comet's passage was observed by two Soviet spacecraft (Vega 1 and Vega 2) and by a European probe (Giotto). Data collected by these craft indicate that Halley's Comet has an oblong nucleus with dimensions of 15 by 8 km (about 9 by 5 miles). The comet is composed of ice and rock dust covered by a porous black crust. The temperature of the crust was measured at about 170 F (about 80 C, or 350 K), an unexpectedly high temperature attributable to surface absorption of solar radiation. As the comet rotates on its axis, the sunward side vents dust and gases through the crust. The Orionid and Eta Aquarid meteor showers seem to travel in the orbit of Halley's Comet.