Sungrazing comet. Artist's impression of a comet flying near the Sun, with a double tail stretching behind its head. A comet is a ball of ice and dust which has a long, eccentric orbit around the Sun; those that pass close to the Sun are called 'sungrazing comets'. As the comet passes near the Sun its surface evaporates, releasing a tail of dust and vapour that can extend for millions of kilometres. The solar wind, a stream of charged particles from the Sun, pushes the tail so that it always extends away from the Sun regardless of the comet's direction of travel. The dust sometimes forms a second tail, pushed by radiation pressure.