Halley's Comet
The most famous comet of all, and the parent of two meteor showers: the Eta Aquariids in May and the Orionids in October.
Key facts
- Designation
- 1P/Halley
- Type
- Halley-type comet
- Orbital period
- about 76 years (it varies between about 74 and 79)
- Closest to the Sun
- 0.59 AU (about 88 million km), inside the orbit of Venus
- Farthest from the Sun
- about 35 AU, beyond Neptune
- Tilt of orbit
- 162° (it travels around the Sun backwards, against the planets)
- Nucleus
- about 15 × 8 km, potato-shaped and as dark as coal
- Discovered
- recorded since at least 240 BC; recognised as periodic by Edmond Halley in 1705
- Returns
- last at perihelion 9 February 1986; next 28 July 2061
Orbit
Halley's Comet swings around the Sun every 76 years or so on a long, stretched orbit that reaches from inside the orbit of Venus to beyond Neptune. The orbit is tilted so far that the comet actually goes round the Sun in the opposite direction to the planets. That retrograde path is why its dust strikes Earth almost head-on, at about 66 km/s, among the fastest meteors of the year.
Size and nature
The nucleus is a single lump of ice and dust about 15 km long and 8 km wide. When the Giotto spacecraft photographed it in 1986, it was the first comet nucleus ever seen. Its surface reflects only about 4 per cent of the sunlight that falls on it, darker than coal, because a crust of dust and carbon-rich compounds covers the ice. Jets of gas burst out from a few active patches on the sunlit side.
History
Astronomers in China recorded the comet in 240 BC, and it appears on the Bayeux Tapestry before the Norman Conquest of 1066. In 1705 Edmond Halley used Newton's new laws of gravity to show that the comets of 1531, 1607 and 1682 were one object, and predicted its return. It was recovered on Christmas night in 1758, years after Halley's death, the first proof that a comet could come back. In 1910 Earth passed through its tail without harm.
Meteor showers
Each pass leaves a little more dust strung along the orbit. Earth crosses this stream twice a year, once as the comet's dust approaches the Sun and once as it leaves. The May crossing gives the Eta Aquariids, best seen from the southern hemisphere before dawn; the October crossing gives the Orionids. The grains you see burning up were shed hundreds or thousands of years ago.
Missions
In March 1986 an international fleet met the comet: two Soviet Vega probes, two Japanese craft, Sakigake and Suisei, and the European Giotto, which passed just 600 km from the nucleus. The Vega craft helped steer Giotto in. Their dust analysers found grains rich in carbon, hydrogen, oxygen and nitrogen, the first direct sample of what comets are made of.
Its meteor showers
- The Eta Aquariids peak around 5–6 May; about 50 (varies from year to year) an hour at best; 66 km/s
- The Orionids peak around 21–22 October; about 20 an hour at best; 66 km/s
Space missions
- Giotto Europe's first deep-space mission flew within 600 kilometres of Halley's Comet.
- Vega 1 and 2 Twin spacecraft that dropped balloons into Venus's clouds, then took the first pictures of a comet's nucleus.
- Sakigake and Suisei Japan's first interplanetary spacecraft, sent to meet Halley's Comet.
- International Cometary Explorer The first spacecraft to meet a comet, flying through the tail of the parent of the Draconid meteors.
Key dates
| Date | What happened |
|---|---|
| 25 December 1758 | Johann Palitzsch recovered Halley's Comet, proving Halley's prediction of its return. |
| May 1910 | Halley's Comet passed between Earth and the Sun, and Earth moved through its tail. |
| 9 February 1986 | Halley's Comet passed closest to the Sun. |
| 14 March 1986 | Giotto flew about 600 km from the nucleus of Halley's Comet. |
| 28 July 2061 | Halley's Comet is due back at perihelion. |
Did you know?
Mark Twain was born in 1835, just after Halley's Comet passed, and predicted he would go out with it. He died in April 1910, the day after its closest approach to the Sun.
Every pass costs the comet a surface layer a few metres deep; it will last many thousands of years yet.
Its farthest point from the Sun was reached in December 2023. It is now on its way back.