Comet. Computer graphic of a comet flying towards Earth, with a blue-white tail stretching behind its head. A comet is a ball of ice and dust that orbits the Sun. As it passes near the Sun its surface evaporates, releasing a tail of dust and vapour that can extend for millions of kilometres. The tail always extends in the opposite direction from the Sun, whatever direction the comet is travelling in. Comets are thought to come from a cloud of billions of comets near the edge of the Solar System (the Oort Cloud). The gravitational effect of passing stars occasionally knocks a few out of the Oort Cloud, sending them on orbits that take them near the Sun, where they become visible.