ma. Even though the coma can get to be very large, its size can actually decrease about the time it crosses the orbit of Mars. At this distance the particles that drift out from the sun act as a powerful wind which blows the gas and dust particles away from the nucleus and coma. This is the process, which makes the comets tail. The hydrogen cloud is very large at millions of kilometers in diameter. But it is only a very sparse body of neutral hydrogen. It was discovered from spectroscopy that was carried out by satellites in 1970. Hydrogen was discovered in comets Tago-Sato-Kosaka and Bennett. It is ionized hydrogen that forms the light that goes past the coma. The reason why the hydrogen cloud was not discovered for a long time is because it is not visible from Earth. Atomic hydrogen emits in the ultraviolet, but the ozone layer stops the waves from entering. The hydrogen cloud can only be observed from space, with satellites. The dust tail is usually up to 10 million kilometers long, and is composed of smoke-sized dust particles that come off the nucleus by escaping gases. The dust tail is also the most visible part of a comet to the naked eye. The tail has a potential to be long when it enters the orbit of Earth. The record for the longest tail is the length of the Great Comet of 1843; its tail extended more than 250 million kilometers. The ion tail, known as type I or plasma, is made up of ions. It can be up to 100 million kilometers long and 100,000 kilometers wide. The tail is straight and always is opposed to the direction of the Sun. The color of it, through a spectrum, is mostly blue. The reason why the tail is ionized is because of solar wind. Solar wind, which flows at about 400 kilometers per second, is filled with charged particles that are around the solar magnetic field. The gases in the tail are ionized by the process of photo ionization of the neutral molecules under the action of the solar ultraviolet radiation, or und...
