Comet Hale-Bopp was one of the brightest comets of the twentieth century, and has two characteristic tails of gas and dust. It was discovered in July 1995 by two US astronomers, Alan Hale and Thomas Joel Bopp. As a comet approaches the Sun, it warms, vaporizing water-ice and frozen gases and releasing dusty grains to form a coma - a huge envelope of gas and dust around the solid nucleus - which is swept away by the solar wind into a tail. The tail seen here glows blue in the Sun's ultraviolet radiation and is 150 kilometers long. The white tail consists of tiny dust particles, pushed away from the coma by radiation pressure from the Sun. Hale-Bopp will return to the vicinity of the Earth in 2,400 years.