Comets are icy objects that release gas and dust as they orbit the Sun. The solid part of a comet is called the nucleus and is mainly made of frozen water, dust and sometimes other frozen substances such as ammonia.
Solar radiation heats the nucleus and gives it an atmosphere of gas and dust called the coma. A comet's distinctive tail is caused by solar radiation and a stream of charged particles that constantly jets away from the Sun called the solar wind.
It is thought that comets are material leftover from the formation of the outer planets, although another theory is that many formed outside our solar system.
Photo: Comet Holmes taken by the Spitzer Space Telescope (NASA)
Leftover building material orbits the Sun.
Patrick Moore talks about the amazing light show that takes place every August.
Patrick Moore talks to guest Dr John Mason about the Perseid meteor shower that takes place every August when the Earth passes through the dust and gas trail left by Comet Swift-Tuttle. In 2007 when this interview took place visibility was very good but cloud cover and the Moon can get in the way.
The comet's impact shows planet building in action.
The comet Shoemaker-Levy 9 struck Jupiter on 16 July 1994. Clearly visible through telescopes on Earth, the impact was the first time a collision between two objects in the Solar System had been seen. Researchers believe that the planets formed through a series of such collisions. This clip features commentary by Dr David Levy, co-discoverer of Shoemaker-Levy 9.
Volcanoes and comets bring water to the Earth.
Dr Iain Stewart explains the theory that steam from volcanoes and water from comets filled the Earth's oceans.
The Vega probes give astronomers a close-up look at the comet.
In 1986 the Russian Vega probes captured close-up images of Halley's comet in preparation for the European Giotto probe.
The Earth makes a dangerous journey through the galaxy.
The Earth makes a dangerous journey through our galaxy, the Milky Way, periodically passing through areas of dense stars. The influence of other stars' gravity sends comets in the outer Solar System hurtling towards Earth.
A comet is an icy small Solar System body (SSSB) that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust and small rocky particles. Comets have been observed since ancient times.
Comets have a wide range of orbital periods, ranging from a few years to hundreds of thousands of years. Short-period comets originate in the Kuiper belt, or its associated scattered disc, which lie beyond the orbit of Neptune. Longer-period comets are thought to originate in the Oort cloud, a hypothesized spherical cloud of icy bodies in the outer Solar System. Long-period comets plunge towards the Sun from the Oort cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars. Rare hyperbolic comets pass once through the inner Solar System before being thrown out into interstellar space along hyperbolic trajectories. Exocomets, comets beyond our solar system, have also been detected and may be common in the Milky Way Galaxy.
Comets are distinguished from asteroids by the presence of a coma or a tail. However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets (see asteroid terminology).
As of January 2011[update] there are a reported 4,185 known comets of which about 1,500 are Kreutz Sungrazers and about 484 are short-period. This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer Solar System may number one trillion. The number visible to the naked eye averages roughly one per year, though many of these are faint and unspectacular. Particularly bright or notable examples are called "Great Comets".