Comets: what they are and how to see one

A comet is a frozen body of ice, rock and dust that spends most of its time far from the Sun.^[1] When its orbit brings it closer, the Sun's heat turns some of its ice to gas, and the comet grows a glowing head and a tail that can stretch for millions of miles, always pointing away from the Sun.^[1] Comets are leftovers from the formation of the solar system about 4.6 billion years ago, which is why scientists study them for clues to how the planets formed.^[2]

Nucleus, coma and tails

The solid part of a comet, the nucleus, is often only a few miles across, a mix of ice, frozen gases and embedded dust.^[2] As the comet warms near the Sun, it develops a coma, a thin atmosphere of gas and dust around the nucleus that can extend for hundreds of thousands of kilometers.^[2] Sunlight and the solar wind push this material away from the Sun into two tails: a dust tail and an ion tail of gas.^[2]

Comets are more fragile than they look. Passing close to the Sun can break a comet apart: comet ISON did not survive its pass in 2013.^[3]

Where comets come from

Comets are grouped by how long they take to go around the Sun.

• Short-period comets take less than 200 years and come mostly from the Kuiper Belt, a disc of icy bodies beyond Neptune; because they have passed before, their returns can often be predicted.^[2]
• [Jupiter-family comets](term:jupiter-family-comet) are the shortest-period group, under 20 years, and [Halley-type comets](term:halley-type-comet) take between 20 and 200 years.^[4] Comet 67P/Churyumov-Gerasimenko, which ESA's Rosetta orbited, goes around the Sun about every 6.5 years.^[5] Halley's Comet returns about every 76 years and will next be back in 2061.^[6]
• [Long-period comets](term:long-period-comet) come from the Oort Cloud, about 100,000 astronomical units from the Sun, and can take as long as 30 million years to complete one orbit.^[2] Their arrivals are much less predictable.^[2]

How comets are named

Comets are generally named after whoever discovers them, a person or a survey, which is why so many carry names such as LINEAR, SOHO or ATLAS.^[2] The name follows a designation set by the International Astronomical Union. A periodic comet, one that takes less than 200 years or has been seen at more than one close pass by the Sun, carries the prefix P/; other comets carry C/.^[7] Once a periodic comet has been seen to return, it receives a number, as in 1P/Halley.^[7] A comet first seen in 2023 by two observatories, for example, became C/2023 A3 Tsuchinshan-ATLAS.^[3]

How to see one

Comet brightness is given in magnitude, where a lower number means brighter. Under perfect skies the eye can reach about magnitude 6.5, but such conditions are rare, and city lights and a bright Moon make faint objects disappear.^[8]

• Know where to look. Check the comet's position for the night you plan to look and find it by the bright stars and constellations around it, from a dark spot just after full nightfall.^[3]
• Get away from lights. Find a dark place with a low, clear horizon in the comet's direction.^[3]
• Use binoculars. Binoculars are an excellent first instrument for skywatching, easy to use and more versatile than most telescopes.^[9]
• Expect surprises. Scientists thought Tsuchinshan-ATLAS might break up when it passed the Sun in September 2024; it survived, while ISON in 2013 did not.^[3] Truly brilliant comets are rare: in the past 300 years, only nine have been bright enough to see in daylight, the last being Comet West in 1976 and, under ideal conditions, Hale-Bopp in 1997.^[3]

Comets that come from beyond the solar system are covered in Interstellar visitors. The dust comets leave along their orbits is what Earth meets in a meteor shower; see How to watch a meteor shower.