Comets

What is a comet?

A comet is a small icy body that travels around the Sun on a long, stretched-out orbit. Its solid core, the nucleus, is a mix of frozen water, frozen gases like carbon dioxide and carbon monoxide, dust and rocky grains. Astronomers sometimes call it a dirty snowball, though many comets turn out to be more dust than ice.

Comets are leftovers from the formation of the planets about 4.6 billion years ago. Because they spend most of their time in the deep cold far from the Sun, they have changed very little. That makes them time capsules for scientists who want to know what the early Solar System was made of.

Nucleus, coma and two tails

The nucleus is usually only a few miles across and very dark, reflecting only a few percent of the sunlight that hits it. Far from the Sun it is inactive and almost impossible to see. As it moves inward, sunlight warms the surface and ice turns straight into gas, a process called sublimation.

That escaping gas drags dust with it and forms the coma, a glowing, fuzzy cloud around the nucleus. The coma can become larger than a planet, even though the nucleus inside it is tiny.

Most comets grow two tails. The dust tail is made of small grains pushed gently by sunlight; it looks white or yellowish and often curves along the comet's path. The ion tail, or gas tail, is made of electrically charged gas swept straight back by the solar wind, the stream of particles flowing out of the Sun. It often glows blue. Both tails point away from the Sun, so a comet heading back out to space travels tail first.

Where comets come from

Short-period comets, which take less than 200 years to orbit the Sun, mostly come from the Kuiper Belt, a ring of icy bodies beyond Neptune. Gravity nudges from the giant planets can send some of them inward.

Long-period comets come from the Oort Cloud, a huge, mostly theoretical sphere of icy objects that may reach about 1.5 light-years from the Sun. No one has seen the Oort Cloud directly; its existence is inferred from the orbits of comets that arrive from all directions. A passing star or the pull of the Milky Way can loosen one of these bodies and send it on a journey of thousands or even millions of years toward the Sun.

Halley's Comet

Halley's Comet is the most famous periodic comet. It returns about every 76 years, and records of its visits go back more than 2,000 years. Edmond Halley worked out in 1705 that several historical comets were really the same object and predicted its return, which came true after his death.

Its last visit was in 1986, when a fleet of spacecraft, including ESA's Giotto, flew past it and photographed its dark, peanut-shaped nucleus about 9 miles long. Halley will next reach its closest point to the Sun in late July 2061, and it should be better placed for viewers than in 1986.

Each year, Earth passes through dust Halley left behind, which gives us two meteor showers: the Eta Aquariids in May and the Orionids in October.

Rosetta: landing on a comet

ESA's Rosetta mission reached comet 67P/Churyumov–Gerasimenko in 2014, after a ten-year trip. It was the first spacecraft to orbit a comet and follow it as it swung around the Sun.

In November 2014 its small lander, Philae, touched down on the nucleus. Philae bounced and came to rest in a shadowy spot, but it still sent back data. Rosetta found that 67P is shaped like a rubber duck about 2.5 miles long, is very porous, and holds organic molecules. It also found that the comet's water differs chemically from Earth's oceans, a clue that comets may not have supplied most of our water. The mission ended in 2016 with Rosetta gently crashing onto the comet.

How bright comets are spotted

Most comets are discovered by automated sky surveys that photograph the sky night after night and look for moving, fuzzy objects. Amateur astronomers still find some, especially comets close to the Sun in the twilight sky. A new comet is named after its discoverers or the survey that found it.

Predicting brightness is hard. A comet can fall apart, fade or suddenly flare. Only a handful each decade become easy naked-eye sights; Hale-Bopp in 1997 and Tsuchinshan-ATLAS in 2024 are recent examples.

How to see a comet yourself

Follow astronomy news so you know when a comet is expected to be bright and where it will be in the sky. Bright comets are often near the Sun, so look low in the west after sunset or low in the east before dawn.

Get away from city lights and let your eyes adjust to the dark for about 20 minutes. Binoculars are the best tool: they show the coma and the start of the tail far better than the eye alone. A long-exposure photo with a phone or camera on a tripod often reveals more tail than you can see. If the comet is close to the Sun, wait until the Sun has fully set and never sweep binoculars across the Sun.