Halley's Comet

1P/Halley (Halley's Comet)
A color image of comet Halley, shown flying to the left aligned flat against the sky
Halley's Comet on 8 March 1986
Discovered byPrehistoric observation;
Edmond Halley (recognition of periodicity)
Discovery date1758 (first predicted perihelion)
Orbital characteristics[2]
Epoch 17 February 1994 (2449400.5)
Aphelion35.082 AU
Perihelion0.586 AU
(last perihelion: 9 February 1986)
(next perihelion: 28 July 2061)[1]
17.834 AU
75.32 yr
Earth MOID0.0638 AU (9.54 million km)
Physical characteristics
Dimensions15 km × 8 km[3]
Mean diameter
11 km[2]
Mass2.2×1014 kg[4]
Mean density
0.6 g/cm3 (average)[5]
0.2–1.5 g/cm3 (est.)[6]
~0.002 km/s
2.2 d (52.8 h) (?)[7]
28.2 (in 2003)[9]

Halley's Comet or Comet Halley, officially designated 1P/Halley,[2] is a short-period comet visible from Earth every 75–76 years.[2][10][11][12] Halley last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061.[13]

Halley's returns to the inner Solar System have been observed and recorded by astronomers since at least 240 BC. Clear records of the comet's appearances were made by Chinese, Babylonian, and medieval European chroniclers, but, at those times, were not recognized as reappearances of the same object. The comet's periodicity was first determined in 1705 by English astronomer Edmond Halley, after whom it is now named.

During its 1986 apparition, Halley's Comet became the first comet to be observed in detail by spacecraft, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation.[14][15] These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple's "dirty snowball" model, which correctly predicted that Halley would be composed of a mixture of volatile ices—such as water, carbon dioxide, and ammonia—and dust. The missions also provided data that substantially reformed and reconfigured these ideas; for instance, it is now understood that the surface of Halley is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy.


Comet Halley is commonly pronounced i/, rhyming with valley, or i/, rhyming with daily.[16][17] Colin Ronan, one of Edmond Halley's biographers, preferred i/ (HAW-lee, similar to holly). Spellings of Halley's name during his lifetime included Hailey, Haley, Hayley, Halley, Hawley, and Hawly, so its contemporary pronunciation is uncertain, but contemporary individuals with this last name appear to prefer the version that rhymes with "valley".[18]