A comet is an object that travels around the sun leaving a bright trail behind. For more than two thousand years, the return of
Halley's Comet has been observed and recorded on Earth every 76 years. Its 1986 visit, however, was the first time that humans took a close look at its nucleus. One spacecraft went within a few hundred kilometres of the nucleus. Two Soviet craft, Vega 1 and Vega 2, came within 10,000 km of the nucleus on March 6th and March 9th; and the European Space Agency's Giotto space probe passed within 600 km of Halley's Comet on March 14th. Pioneer Venus Orbiter
found that the cloud of gases and dust which make up the tail spread
over a region about 20,000,000 km across, 15 times larger than the Sun. Scientists also discovered that the comet was losing about ten metres o
f material from its surface every orbit, suggesting a lifetime of about only 1,000 orbits - in about 100,000 years it will disappear.
