Halley’s, << HAL eez, >> Comet is a comet that appears in the sky about every 76 years. Halley’s Comet has become the best-known comet because it returns more often than most comets and can usually be seen without the aid of a telescope.
The nucleus (core) of Halley’s Comet is an irregularly shaped ball of frozen material, dust, and rock. It measures about 9 miles by 5 miles by 5 miles (15 kilometers by 8 kilometers by 8 kilometers). Most of the frozen material is water ice, but the comet also contains small amounts of frozen ammonia, carbon monoxide, carbon dioxide, and methane. The dusty surface of the nucleus appears black.
Halley’s Comet orbits the sun in a long, oval path that brings it close to the sun about every 76 years. As the comet approaches the sun, solar energy vaporizes some of the ice in the nucleus, spewing dust and gas into space. These particles form the comet’s tail and a cloud around the nucleus called the coma. The glowing coma and tail can easily be seen from Earth.
Halley’s Comet was named for the English astronomer Edmond Halley, who studied the paths of comets in the late 1600’s. Before Halley, most people believed comets did not travel in regular paths. Halley demonstrated that comets, like planets, orbit the sun. He observed that the paths of comets spotted in 1531 and 1607 matched the path of a comet seen in 1682. Halley concluded that all three of these comets were actually a single comet traveling in an orbit around the sun. He predicted that the comet would reappear in 1758 and about every 76 years thereafter. The comet returned in 1758 and became known as Halley’s Comet.
On Oct. 16, 1982, astronomers at the Palomar Observatory in California first observed Halley’s Comet during its most recent return. They photographed the comet when it was about 1 billion miles (1.6 billion kilometers) from the sun. The comet came closest to the sun on Feb. 9, 1986. In March of that year, five robotic spacecraft approached the comet’s nucleus and collected information about its composition and size. The European Space Agency’s Giotto spacecraft made the closest approach, coming within about 370 miles (600 kilometers) of the comet’s nucleus.
Chinese astronomers first recorded seeing Halley’s Comet around 240 B.C. Since then, the comet has been sighted each time it has returned. Astronomers expect the comet’s next return in 2061.
See also Comet ; Halley, Edmond ; Space exploration (Probes to comets) .
