Want to stay on top of all the space news? Follow @universetoday on TwitterA comet tail does not appear on a comet until it approaches the Sun. As the Sun’s radiation heats up the dust and gases there are actually two tails formed. The dust forms the anititail and the gases form the true tail. In the outer solar system, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. As a comet approaches the inner solar system, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The force exerted on the coma by the Sun’s radiation pressure and solar wind cause an enormous tail to form, which points away from the sun.
The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet’s orbit so that it often forms a curved tail called the antitail. At the same time, the ion tail, made of gases, always points directly away from the Sun. This gas is more strongly affected by the solar wind than dust and follows magnetic field lines instead of an orbital trajectory. Parralax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.
Ion tails have been observed to extend 1 AU (150 million km) or more. The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of solar ultra-violet radiation acting on particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge which in turn gives rise to an induced magnetosphere around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. As the relative orbital speed of the comet and the solar wind becomes supersonic, a bow shock is formed upstream of the comet, in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions congregate and act to load the solar magnetic field with plasma, such that the field lines drape around the comet forming the ion tail.