Comets

                             Comets

Comets are formed from huge cloud of dust and frozen gases which travel round the Sun through a distinct route. When a comet comes near the Sun, its surface vapourises to form a huge sphere of dust and gas around a nucleus called coma, that shines brightly. A long tail of gas and dust is also formed. The tail lengthens to about 160 million kilometres. The most clearly observed comet is the Halley's comet which was recognised by the English astronomer Edmond Halley in 1682. It appeared last time in 1986 and supposed to reappear after every 76 years.

                   Halley's Comet

Halley's comet passed by the Earth during March-April 1986. Two Soviet probes Vega 1 and Vega 2, and a European satellite Giotto observed the passage of the comet and collected many details about the interior structure and functions. These probes found that the comet has an oblong nucleus of about 15 kilometres long and 8 kilometres broad. Rock dust particles and ice form the comet. A black crust having temperature of about 170°F surrounds the comet. While the comet rotates on its axis, the side which faces the Sun pushes out dust and gases through the crust.