Halley's Comet, Comet Halley, or sometimes simply Halley, officially designated 1P/Halley, is the only known short-period comet that is consistently visible to the naked eye from Earth, appearing every 75–79 years. It last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061.
Halley's Comet on 8 March 1986
Orionid meteor originating from Halley's Comet streaking the sky below the Milky Way and to the right of Venus
The nucleus of Halley's Comet, imaged by the Giotto probe on 14 March 1986. The dark coloration of the nucleus can be observed, as well as the jets of dust and gas erupting from its surface.
Observation of Halley's Comet, recorded in cuneiform on a clay tablet between 22 and 28 September 164 BC, Babylon, Iraq. British Museum (BM 41462 Archived 19 April 2021 at the Wayback Machine)
A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma. These phenomena are due to the effects of solar radiation and the outstreaming solar wind plasma acting 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. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.
Comet Hale-Bopp
Structure of a comet
Nucleus of 103P/Hartley as imaged during a spacecraft flyby. The nucleus is about 2 km in length.
Comet 81P/Wild exhibits jets on light side and dark side, stark relief, and is dry.