Comets can be described as 'dirty snowballs'. They consist mainly of ice, metals, rock and gases.
Many reside at the edge of the Solar System, nearly one light-year from the Sun, in a giant spherical cloud called the Oort Cloud. This is the source of the long-period comets which pass through the Solar System. Occasionally comets are disturbed by a passing star or a collision with another comet from within the cloud. This results in the comet or its fragments making a journey towards the Sun and maybe the Earth.
Recently, several large icy bodies have been found beyond the orbit of Neptune. 1992QB1 and 1993FW were the first to be discovered and were initially thought to be asteroids. This supported the existence of another region of comets called the Kuiper Belt. Observations made by the Hubble Space Telescope in 1995 suggest that this region contains at least 200 million comets. This region is the origin of short-period comets which, unlike long-period comets, tend to stay in the plane of the Solar System.
The solid body of the comet is called the nucleus. It has an irregular shape and is only a few kilometres across. As it approaches the Sun the outer layers boil off forming a coma, which can measure thousands of kilometres across. The gas and dust in the coma are 'blown' back by the solar wind forming a long tail, that always points away from the Sun.
Most of the material that boils off the comet is left behind and forms a dusty band that encircles the Sun. If the Earth passes through this band as it orbits the Sun then a meteor shower occurs.
The best known comet is Halley's Comet. Comet Halley orbits the Sun every 76 years and measures 16 km x 7.5 km x 8 km. This comet was the first comet found to be periodic.
Two comets clearly visible in the Australian sky in recent years were Comet Hale-Bopp and Comet Hyakutake.
Comet Hale-Bopp was visible during the first half of 1997.
The Comet Hale-Bopp. The Sun is to the right of the comet. The solar wind pushes the comet's tail to the left.
Source: Fred Espenak, NASA GSFC
Hale-Bopp measures about 40 kilometres across and does a loop around the solar system every 4200 years. It was a bright comet that was discovered far from Earth, when it was still beyond the orbit of Jupiter.
Comet Hyakutake was discovered at the end of January 1996 by a Japanese amateur astronomer with very large binoculars.
The tail of the comet Hyakutake is composed of dust and gas driven off the icy comet nucleus by the Sun's heat and blown away by the solar wind.
Source: Jim Martin, Huntsville AL, Courtesy "Night of the Comet" (NASA / Ames)
Hyakutake appeared in Australian skies during the first week of May 1996. It continued to be visible for a few weeks until it faded beyond the limit of the naked eye and could only be seen with binoculars as a faint long smudge.