What Is a Comet?
A comet is a giant snowball made of frozen water, dust and rocky bits. For most of its life it lives in the freezing edges of the Solar System and you would never see it. But when a comet swings in close to the Sun, its ice starts to melt. The melting ice and dust stream out into space and catch the sunlight, giving the comet a glowing tail that can stretch for millions of kilometres.
- What is it?A space snowballMade of ice, dust and rocky bits
- How big is the core?Usually 1 to 50 kmSmall compared to a moon or asteroid
- Where do they come from?The Oort CloudA huge bubble of comets way beyond Pluto
- How long is the tail?Up to 150 million kmAbout the distance from the Earth to the Sun
- Famous exampleHalley's CometVisits us every 76 years
- Next big visitorHalley in 2061You can probably see it from your back garden
How long are comet tails?
Length when at their brightest, in millions of km.
Hale-Bopp's tail when it visited in 1997 stretched approx. 150 million km, which is the same as the distance from the Earth to the Sun. Hyakutake's gas tail was even longer.
What is a comet made of?
A comet has a solid centre called the nucleus, made of frozen water, frozen carbon dioxide ("dry ice"), other ices, and dust. The nucleus is usually quite small (a few kilometres across) and very dark, blacker than a lump of coal.
When the comet flies close to the Sun, the ices start to turn straight from solid to gas in a process called sublimation. This creates a fuzzy glowing cloud around the nucleus called the coma, sometimes as wide as a planet. Solar wind and sunlight push some of this gas and dust out behind the comet, forming the tail. Comets always have two tails when they get close to the Sun: a dust tail that curves, and a straight blue gas tail.
Where do comets come from?
There are two homes. Short-period comets (the ones that visit every few decades or centuries) come from the Kuiper Belt, a ring of icy objects just past Neptune. Long-period comets (which take thousands or millions of years to orbit the Sun) come from much further out, from a giant bubble of icy bodies called the Oort Cloud that surrounds the whole Solar System.
A comet leaves its home only when its orbit is disturbed, usually by the gravity of a passing planet or star. It then falls into the inner Solar System, swings around the Sun, and either heads back out or stays in a new shorter orbit.
Famous comets
Halley's Comet is the most famous of all. It comes back every 76 years and was last seen in 1986. People have been recording its visits for over 2,000 years.
Comet Hale-Bopp appeared in 1997 and was so bright you could see it for 18 months without a telescope. Its next visit will be in around 4380 AD.
Comet Shoemaker-Levy 9 smashed into Jupiter in 1994. The impacts left dark scars on the giant planet that lasted for months.
Deeper dive: comet composition, missions and the dirty-snowball model
The modern picture of comets goes back to American astronomer Fred Whipple's 1950 "dirty snowball" model, which proposed that comets are loose mixtures of water ice and dusty rocky material. This was confirmed when the European Space Agency's Giotto probe flew through Halley's coma in 1986 and photographed the dark, peanut-shaped nucleus directly. The Rosetta mission to comet 67P/Churyumov-Gerasimenko (2014 to 2016) gave us our most detailed look ever, including landing the Philae probe on the surface.
Rosetta found that 67P's ice is enriched in deuterium (a heavier form of hydrogen) compared to Earth's oceans, suggesting that not all of Earth's water came from comets like this one. Comets do, however, contain a rich mix of organic compounds, including the amino-acid glycine, supporting the idea that comet impacts in the first billion years of Earth helped seed the chemistry that would later become life.
Comets are classified by their orbital period. Short-period comets orbit in under 200 years and mostly come from the Kuiper Belt. Long-period comets take longer than 200 years and come from the Oort Cloud. Both types ultimately formed in the outer Solar System, where it was cold enough for water, methane and ammonia ice to condense. Some have been ejected entirely, becoming free-floating interstellar comets like ʻOumuamua (2017) and 2I/Borisov (2019), the first two confirmed visitors from other star systems.
For the comet that returns every 76 years, read about Halley's Comet. For where most short-period comets come from, see the Kuiper Belt. For the long-period comet home, see the Oort Cloud.