A comet is a solar system body composed of small particles of ice and dust. Comets have high eccentric orbits around the Sun and as they approach the sun, some of their material is vaporized, forming a coma and a long tail. Comets range in size from a few hundred meters to tens of kilometers in diameter. Comets have varying orbital periods, which range from a few years to thousands of years. Those with a short period originate in a region known as the Kuiper belt and those with a long period are believed to originate in a hypothesized circular cloud known as the Oort cloud.
A comet´s nucleus may range from approximately 100 kilometers to up to 40 km. (24 miles) across. Comets are composed of small particles of rock, dust, ice and frozen gases, including methane, carbon monoxide, carbon dioxide and ammonia. Comets also contain a number of organic compounds, such as formaldehyde, ethanol, ethane, methanol hydrogen cyanide and amino acids. Scientist discovered glycine in a comet during NASA´s Stardust mission. It´s believed that some organic molecules found on Earth may have been created on comets and asteroids.
How does a comet tail form?
While a comet remains in the outer solar system, it remains frozen solid, but it can be knocked out of its orbit and sent into the inner solar system. As the comet approaches the Sun, it begins to get hotter. The dust on its surface heats up and the ice in its interior sublimates, changing into a gas without becoming a liquid. The gas and dust are carried away from the comet. The gas particles absorb solar radiation; while the dust scatters the light from the Sun. the effect of this is the formation of a tenuous envelope of gas and dust, surrounding the comet, known as a coma. The Sun´s radiation and the solar wind exert a force on the comet, producing an enormous tail of gas and dust that usually points away from the Sun.
Two tails on a comet
The particles of gas and dust each form distinct tails on a comet. The dust tail usually follows the trail of the comet, forming a curved and diffused tail. The ion tail always points away from the Sun regardless of the direction in which the comet is travelling. This occurs because the tail, which is made of ionized gases, is more affected by the solar wind than is the dust. A small tail known as the antitail, pointing in a different direction than the dust and ion tail can be seen occasionally. Astronomers have discovered that the antitail is the end of the dust tail, which seems to project away from the comet due to the viewing angle of an observer on Earth.
The observation of antitails helped astronomers discover the existence of the solar wind. While the coma is usually 50 km (31 miles) in diameter, a comet´s ion tail may be as large as one astronomical unit (150 million kilometers) or even more. The solid trail of debris left by comets originates meteor showers. This occurs when the path of the comet coincides with the path of the Earth. At this point meteor showers, which can be seen from the Earth, occur. According to nineplanets.org,since comets are brightest when they get closer to the Sun, they are normally visible at sunrise or sunset.