Comet. Computer illustration of a comet near Earth and the Moon. Two separate tails are seen here. The dust tail (yellow) consists of grains of dust pushed away from the comet's head by the radiation of sunlight. The ion tail (blue/grey) is made up of gases ionised by the effects of the solar wind. Both tails always point away from the Sun. Comets have a nucleus of ice and dust. As they approach the Sun their surface evaporates, releasing a tail millions of kilometres long. Their orbits around the Sun are highly elliptical, so that they spend most of their time as a solid, tail-less lump in the cold and distant reaches of the solar system.