Comet Hale-Bopp (lower centre), with the branches of a silhouetted tree. Hale-Bopp was one of the brightest comets of the 20th century, and was seen for much of early 1997. A comet is a lump of ice and rock whose orbit brings it close enough to the Sun for its icy core to melt and boil into space. The Sun's solar wind and radiation blows the resulting ions and dust into tails that can be millions of kilometres long. The ion tail glows with its own light, while the dust tail (seen here) reflects sunlight. Also known as C/1995 O1, Comet Hale-Bopp was closest to the Sun in March 1997. It is a long-period comet: it will be around 4000 years before it reappears in the inner solar system.