Comet Hyakutake against a starfield. This comet, also known as 1996 B2, was one of the brightest comets of the 20th century. A comet is a ball of water ice and dust which has a highly elliptical orbit around the Sun. It spends most of its time in the outer solar system, only developing a tail when it approaches the Sun. The tail comprises gas and dust vaporised from the comet's surface. It always points away from the Sun due to the pressure of the solar wind. The tail may stretch for millions of kilometres. Photographed on the 19th April 1996.