Comet Giacobini–Zinner
The parent of the October Draconids, and the first comet ever visited by a spacecraft.
Key facts
- Designation
- 21P/Giacobini–Zinner
- Type
- Jupiter-family comet
- Orbital period
- about 6.5 years
- Closest to the Sun
- about 1.01 AU (about 151 million km), just outside Earth's orbit
- Farthest from the Sun
- about 6 AU, beyond Jupiter
- Tilt of orbit
- about 32°
- Nucleus
- about 2 km across
- Discovered
- by Michel Giacobini at Nice on 20 December 1900; recovered by Ernst Zinner at Bamberg on 23 October 1913
- Returns
- seen at most returns; perihelion was in September 2018 and again in 2025
Orbit
Giacobini–Zinner is a Jupiter-family comet: a short-period comet whose orbit is controlled by Jupiter's gravity. It goes round the Sun every six and a half years, coming in to about Earth's distance and going out beyond Jupiter. Because it travels the same way as the planets and Earth catches up with its dust from behind, Draconid meteors are slow, about 20 km/s, among the gentlest meteors of the year.
Size and nature
The nucleus is about 2 km across. Studies of its gas show it is poor in carbon-chain molecules compared with most comets, one of the first signs that comets do not all share the same recipe. Its dust is fragile, which is why Draconids break up high in the atmosphere.
History
Michel Giacobini found it from Nice in December 1900, and Ernst Zinner, an astronomer at Bamberg, picked it up again in 1913, so it carries both names. When Earth passed close behind the comet in October 1933, observers in Europe saw a meteor storm of several thousand an hour. Another came in October 1946, when the comet had passed only about two weeks earlier; it was one of the first showers studied by radar.
Meteor showers
The Draconids are seen in early October, best in the evening, because the radiant in Draco is high after dusk. Most years bring only a few meteors, but outbursts occurred in 1985, 1998, 2011 and 2018, when Earth crossed fresh trails.
Missions
On 11 September 1985 the International Cometary Explorer, a solar-wind probe redirected by a close pass of the Moon, flew through the comet's tail about 7,800 km behind the nucleus. It measured the gas and magnetic fields six months before the Halley fleet arrived.
Its meteor showers
- The Draconids peak around 8–9 October; about usually under 10, but capable of rare storms an hour at best; 20 km/s
Space missions
- 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 |
|---|---|
| 20 December 1900 | Michel Giacobini discovered the comet now called Giacobini–Zinner. |
| 23 October 1913 | Ernst Zinner recovered Comet Giacobini–Zinner. |
| 9 October 1933 | A Draconid meteor storm from Comet Giacobini–Zinner was seen over Europe. |
| 10 October 1946 | A second great Draconid storm, one of the first showers measured by radar. |
Did you know?
The astronomer Ernst Zinner who recovered this comet lived from 1886 to 1970; he is not the cosmochemist of the same name.
The International Cometary Explorer was never designed to visit a comet; it was a solar-wind monitor given a new job.
Draconid meteors are so slow they can look as if they drift across the sky.