Halley's Comet is a comet that comes back to the inner Solar System about every 76 years. Its solid core, called the nucleus, is an irregular lump about 15 kilometers (9.3 miles) long and 8 kilometers (5 miles) wide. Because it is not round, NASA also gives it a single overall diameter of about 11 kilometers (6.8 miles), which is the size shown here.

Size

Near the Sun, the nucleus gives off gas and dust, which surround it in a cloud called the coma. The sizes given here are for the nucleus alone.

In March 1986, a fleet of spacecraft met the comet. The European Space Agency's Giotto flew closest, passing within 596 kilometers of the nucleus on March 14. Its pictures revealed the size and shape of the nucleus. Measurements published in 1987 give it as 14.9 by 8.2 kilometers.

The surface is very dark. It reflects only about 3 to 4 percent of the sunlight that falls on it, which makes the nucleus one of the darkest objects in the Solar System. Giotto also saw bright jets of gas and dust shooting out of the nucleus.

Size comparisons

• The nucleus is about as long as Deimos, the smaller moon of Mars, which measures about 16 by 12 by 10 kilometers, but the nucleus is much slimmer.
• Stood on end in the Challenger Deep, the deepest known point of the Mariana Trench at 10,935 meters (35,876 feet), the nucleus would stick out about 4 kilometers (13,000 feet) above the waves.
• Its length is a little more than a third of a marathon, which is 42.195 kilometers (26.2 miles).
• If the Sun were a basketball, Halley's orbit would swing from 15 meters (50 feet) from the ball out to about 910 meters (3,000 feet).

An orbit of about 76 years

Halley's orbit is a long, narrow loop. At its closest to the Sun it passes inside the orbit of Venus, and at its farthest it goes beyond the orbit of Neptune. It travels around the Sun in the opposite direction to Earth.

It last passed closest to the Sun on February 9, 1986, at 87.8 million kilometers. Before that pass, it was at its farthest in 1948, 5.27 billion kilometers away. It will return in 2061. The time between returns has ranged from about 74 to 79 years, because the pull of the planets changes its path.

Each time it passes the Sun, the nucleus loses an estimated 1 to 3 meters (3 to 10 feet) from its surface. Debris left along its path causes two weak meteor showers each year: the Eta Aquarids in May and the Orionids in October.

How it got its name

Until the time of the English astronomer Edmond Halley, comets were thought to pass through the Solar System only once. In 1705, Halley used Isaac Newton's laws of gravity and motion to work out the orbits of several comets. He found that comets seen in 1531, 1607 and 1682 had very similar orbits, and said they were one comet returning. He predicted that it would come back in 1758.

Halley died in 1742, but the comet was seen again on December 25, 1758, by Johann Georg Palitzsch. It was the first comet known to return, and it was named in Halley's honor. Records of its visits go back more than 2,000 years, and it appears in the Bayeux Tapestry, which tells the story of the Battle of Hastings in 1066.

Sources

• 1P/Halley. NASA Science, 2024.
• Small-Body Database Lookup: 1P/Halley. NASA Jet Propulsion Laboratory, 2026.
• Giotto overview. European Space Agency.
• Mars Fact Sheet. NASA, 2025.
• How deep is the ocean?. NOAA National Ocean Service, 2024.
• Marathon. World Athletics.
• Sun Fact Sheet. NASA, 2024.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.