Halley's Comet
Halley's Comet is the most famous comet of all. It swings past the Sun every 76 years, growing a long bright tail visible from Earth. People have been recording its visits for over 2,000 years, long before anyone realised it was always the same comet coming back. Its last visit was in 1986. Its next will be in 2061.
- How often?Every 76 yearsIt orbits the Sun once every 76 years
- Last seen1986Five spacecraft, including Giotto, took pictures
- Next visit2061You can probably see it from your back garden
- Nucleus size15 by 8 kmShaped a bit like a giant peanut
- Named afterEdmond HalleyEnglish astronomer, 1656 to 1742
- First record240 BCChinese astronomers saw it more than 2,200 years ago
How long is Halley compared to other famous comets?
Orbital period in years. Hale-Bopp's is mind-bendingly long.
Halley is the most famous of the "short-period" comets but is far from the shortest. Encke's comet, which orbits in just over three years, holds that title.
Who was Halley?
Edmond Halley was an English astronomer who lived from 1656 to 1742. In 1705 he was looking at the records of bright comets that had passed by Earth in the previous few centuries. He noticed that the comets seen in 1531, 1607 and 1682 all had very similar orbits. He realised they were not three different comets but the same comet returning every 76 years or so.
Halley used Isaac Newton's new theory of gravity to work out when the comet would come back next. He predicted it would return in 1758. Halley died in 1742 and never lived to see it, but the comet did appear right on schedule on Christmas night 1758. It has been named after him ever since.
What is Halley made of?
Halley's nucleus (the solid lump in the middle) is shaped like a giant peanut approx. 15 km long and 8 km wide. It is made of frozen water, frozen carbon dioxide, frozen methane and ammonia, mixed with dust and rocky bits. The surface is extremely dark, blacker than coal, because it is covered in carbon-rich material left behind as the lighter ices melt away on each visit to the Sun.
When Halley flies close to the Sun, the ices warm up and turn straight into gas. This gas (along with bits of dust) streams off into space and forms the comet's glowing tail. The tail can stretch for tens of millions of kilometres.
The 1986 visit
The 1986 visit was the first time we had spaceships to send out and meet a comet. Five different probes flew past Halley that year: two from the Soviet Union (Vega 1 and Vega 2), two from Japan (Sakigake and Suisei), and one from Europe (Giotto). Giotto got closest, flying within 600 km of the nucleus and sending back the first ever close-up pictures of a comet. Those pictures showed the dark peanut-shaped lump for the first time.
For people on Earth, 1986 was actually not a great year to see Halley with the naked eye. The comet was on the far side of the Sun from us when it was at its brightest. The 2061 return is expected to be much better.
When can we see it next?
Halley will be back at its closest to the Sun on 28 July 2061. After that, it will swing past Earth and should become bright enough to see clearly with the naked eye. The viewing conditions will be much better than 1986, so anyone born after approx. 2050 has a good chance of seeing one of the most famous sights in astronomy.
Deeper dive: Halley's orbit, Giotto, and ancient sightings
Halley's orbit is a long ellipse that stretches from just inside Venus's orbit (approx. 0.6 AU from the Sun) out to approx. 35 AU, beyond Neptune. The orbit is also retrograde (it goes around the Sun in the opposite direction to the planets) and inclined at 18 degrees to the plane of the planets. This unusual orbit means Halley is in a class of short-period comet called Halley-type comets rather than the more common Jupiter-family comets that come from the Kuiper Belt. Halley itself probably originated in the Oort Cloud and was captured into its current orbit by gravitational interactions with the outer planets billions of years ago.
The European Space Agency's Giotto mission of 1986 was a technical triumph. The 960 kg probe flew through the dust streaming off Halley at a relative speed of 68 km per second, taking 2,112 images of the nucleus before dust impacts damaged its camera and forced it to suspend operations. Giotto confirmed Fred Whipple's "dirty snowball" model of comets: a solid icy nucleus surrounded by a coma of gas and dust. The mission also discovered that the nucleus was darker than expected (only reflecting approx. 4% of sunlight, similar to charcoal) and that the gas being released contained organic molecules, supporting the idea that comets delivered some of the chemical building blocks of life to early Earth.
The ancient records of Halley make it the comet with the longest historical observation. The earliest probable sighting is from the Chinese Spring and Autumn Annals for 613 BC, with the first definite identified return being 240 BC. Babylonian clay tablets record the 164 BC and 87 BC apparitions in remarkable detail. The 12 AD visit was probably the inspiration for the Star of Bethlehem (although this is much disputed). Successive returns were also seen and recorded in 1066 (Bayeux Tapestry), 1301 (used by the Italian painter Giotto di Bondone as the model for the star in his Nativity painting, which is why ESA's probe was named Giotto), 1607 (observed by Kepler), 1682 (observed by Halley himself), and every visit since.
For more about comets in general, read what is a comet. For where comets like Halley come from, see the Oort Cloud.