The appearance of a comet is called an apparition.Ĭomets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Comets have been observed and recorded since ancient times by many cultures and religions.Ĭomets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. If sufficiently bright, a comet may be seen from Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit.

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. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. This produces a visible atmosphere or coma, and sometimes also a tail. Bottom: Hale–Bopp seen from Earth in 1997, and C/2011 W3 (Lovejoy) imaged from Earth orbitĪ comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing.Middle: 17P/Holmes and its blue ionized tail, and 81P/Wild (Wild 2) visited by Stardust.Top: 9P/Tempel (impactor collision: Deep Impact), and 67P/Churyumov–Gerasimenko ( Rosetta).

Comet moth