The most famous comet
Halley's Comet
It is the comet everyone knows by name: it returns to the Sun every 75–76 years and its appearances have been recorded for more than two thousand years. In 1986 a fleet of spacecraft went out to meet it and Europe's Giotto took the first pictures of its nucleus: a dark block 15 km long, with gas and dust escaping in jets. It will be back in the summer of 2061.
Perihelion passage of 9 February 1986, from the 3D simulation
- Nucleus
- 15.3 × 7.2 × 7.2 km
- Orbital period
- 75.6 years
- Perihelion
- 0.59 AU
- Aphelion
- 35.2 AU
- Last perihelion
- 9 February 1986
- Next perihelion
- 28 July 2061
Nucleus and tail
Halley's nucleus measures 15.3 × 7.2 × 7.2 km: it is shaped like an elongated potato and is one of the darkest objects in the Solar System, reflecting between 3 and 4% of the light it receives, less than coal. It is a porous mix of water ice, dust and carbon compounds, and it spins slowly, in about 2.2 days, with the irregular motion of a wobbling top.
Far from the Sun Halley is just that: a dark, inert rock. As it approaches, its ice begins to sublimate, turning straight from solid to gas, and drags dust along with it. A coma forms around the nucleus, a cloud more than a hundred thousand kilometres wide, and two tails stretch out: the ion tail, blue and straight, pushed by the solar wind directly away from the Sun, and the dust tail, yellowish and curved, which lags behind along the orbit.
Orbit
Halley follows a very elongated ellipse: at perihelion it passes 0.59 AU from the Sun, between the orbits of Mercury and Venus, and at aphelion it reaches 35 AU, beyond Neptune. One lap takes 75.6 years on average, but the period changes at every return, between 74 and 79 years, because the giant planets deflect its path and its gas jets act like small rockets.
Its orbit is tilted 162° to the ecliptic: this means Halley travels around the Sun in the opposite direction to the planets. That is why it crosses Earth's path at very high speed, and the dust it sheds along the way gives rise to two meteor showers, the Eta Aquariids in May and the Orionids in October. It reached aphelion in December 2023 and is now heading back towards the Sun.
Discovery and exploration
No one discovered Halley's Comet: it is visible to the naked eye, and Chinese chronicles record its passage as early as 240 BC. It was Edmond Halley who, in 1705, realised that the comets seen in 1531, 1607 and 1682 were one and the same object, and predicted its return for 1758. The comet reappeared on Christmas night of that year, nearly seventeen years after the astronomer's death: the first proof that comets, too, obey Newton's gravity.
At its 1986 return a fleet of five spacecraft went out to meet it: two Soviet Vegas, two Japanese probes and Europe's Giotto, which on 14 March passed 596 km from the nucleus and took the first close-up images ever obtained of a comet. From Earth that passage was unimpressive, because the comet stayed far away and on the opposite side of the Sun. The 2061 return should be better: Halley will be on the same side of the Sun as Earth and will appear much brighter.
Frequently asked questions
When will Halley's Comet return?
The next perihelion is expected on 28 July 2061. The comet will be visible to the naked eye for a few weeks around that date, under better conditions than in 1986. Anyone born after that year will be seeing it for the first time; the following passage will be in 2134.
Why does Halley's Comet return every 76 years?
Because that is the time it takes to travel its orbit, an ellipse stretching from 0.59 to 35 AU from the Sun. Like any orbiting body it moves fast near the Sun and very slowly far away, where it spends almost all of its life. The period is not fixed: it varies between 74 and 79 years because of the pull of the giant planets.
Has Halley's Comet ever been seen up close?
Yes. In March 1986 Europe's Giotto spacecraft passed 596 km from the nucleus and photographed it: a dark, elongated body about 15 km long, with jets of gas and dust on its sunlit side. Four other spacecraft, Soviet and Japanese, studied it from farther away in the same days.
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