The most famous of all periodic comets. It travels in an elongated elliptical orbit around the Sun, returning to the Earth's vicinity in the inner solar system every 76 years. From historical records it has been shown that Halley's Comet has been observed for over 2,200 years.
Edmond Halley (1656-1742), in whose honour the comet is named, was not the comet's discoverer but the first person to realize the connection between the comet he saw in 1682 and certain other recorded appearances of comets separated by intervals of 76 years. He calculated the orbits of a number of comets, using Isaac Newton's newly published theory of gravitation. Noticing the similarity between the orbits of the comets seen in 1531, 1607 and 1682, he went on to predict a return in1759, which was duly observed after his death.
The orbit of Halley's Comet has a perihelion distance of 0.59 AU, between the orbits of Mercury and Venus. At its most distant, it travels beyond the orbit of Neptune. The orbit is inclined to the main plane of the solar system at an angle of 162í and the comet travels around its orbit in the direction opposite to the motion of the planets.
The return of 1986 was very unfavourable for observation from Earth but several countries launched space probes to investigate the comet, with considerable success. The closest approach was made by the European Giotto probe, which passed within 605 kilometres (375 miles) of the nucleus on 14 March 1986. The Soviet probes Vega 1 and Vega 2 surveyed the nucleus from distances of 8,890 and 8,030 kilometres (5,550 and 5,020 miles) on 6 and 9 March 1986, and information they gathered was used for last-minute corrections to the course of Giotto. Two small Japanese probes were also deployed.
The results demonstrated conclusively the existence of a solid nucleus, probably made of ice and dust. It has an irregular elongated shape, reminiscent of a potato, measuring 16x8 kilometres (9x5 miles). It is also dark, reflecting only 4 per cent of incident sunlight. The nucleus rotates slowly - once in 7.1 days, with a 3.7-day precession. On the side facing the Sun, temperatures as high as 350 K were measured, enough to melt ice, and escaping jets of material were observed.
Two meteor showers, the Eta Aquarids and the Orionids, are associated with Halley's Comet.
