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What Is a Comet? Anatomy of a Dirty Snowball

The parts of a comet, from the frozen nucleus to the coma and two tails, and how close-up visits changed the old snowball picture.

CometsBy the duorolt.com editorial teamPublished Facts checked 7 min read

At its brightest in the spring of 1997, Comet Hale-Bopp was wrapped in a cloud of hydrogen about 100 million km (62 million miles) wide, roughly 70 times the width of the Sun. ESA's SOHO spacecraft mapped that cloud in ultraviolet light, which Earth's atmosphere blocks. The solid core at the center of it was far smaller, with estimates ranging from 40 to 60 km (25 to 37 miles) across.

A comet is a small body of ice, rock and dust that orbits the Sun. NASA describes comets as cosmic snowballs of frozen gases, rock and dust. Most spend nearly all of their orbit dark, cold and out of sight. Only when one swings in toward the Sun does it grow a glowing head and, sometimes, a tail. As of November 2026, ESA's comet catalog lists 4,088 known comets, and NASA says there are likely billions orbiting the Sun.

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The four parts of a comet

An active comet can show up to four parts. Not every comet shows all of them, and one of them cannot be seen from the ground at all.

Part What it is Scale
Nucleus The solid core of ice, dust and rock Often a few kilometers across
Coma The cloud of gas and dust around the nucleus Up to hundreds of thousands of km
Hydrogen envelope A cloud of hydrogen atoms around the coma Hale-Bopp's was about 100 million km
Tails A dust tail and an ion (charged gas) tail Millions of km

The nucleus is the solid part. NASA says it is often a few miles across. Comet Borrelly's nucleus is about 8 km (5 miles) long. Comet 67P, the target of ESA's Rosetta mission, is made of two joined lobes, and the larger one measures about 4.1 km (2.5 miles) in its longest direction. From Earth the nucleus is almost never visible, because it is small and dim.

The coma forms when the nucleus warms. Gas and dust leak out and spread into a fuzzy cloud. The coma and nucleus together make up the comet's head. NASA says a coma can reach hundreds of thousands of kilometers, and that a comet's glowing head can be larger than most planets even though the frozen nucleus is only the size of a small town.

The hydrogen envelope comes from water. Sunlight breaks water vapor apart, and the freed hydrogen atoms glow in ultraviolet light. Earth's atmosphere absorbs that light, so only spacecraft can see the envelope.

The tails are the part most people picture. There are two kinds, made of different material and pushed by different forces. The full story is in the guide to ion, dust and anti-tails.

What comets are made of

Comets are leftovers from the birth of the Solar System about 4.6 billion years ago. NASA describes them as mostly ice coated with dark organic material. Organic here means carbon-based. It does not mean alive. The ice is mainly water ice, mixed with frozen gases and dust.

Spacecraft have added detail. ESA's Rosetta mission studied Comet 67P/Churyumov-Gerasimenko from orbit for about two years. It found that the comet's water is quite different from Earth's water, and that the comet holds molecular oxygen, molecular nitrogen and noble gases. ESA points out that nitrogen and oxygen can only be trapped in ice at very low temperatures. That suggests 67P formed in a very cold region far from the Sun. The Rosetta mission story covers how those results were gathered.

The old "dirty snowball" label is still useful, but the balance is off. At 67P, ESA measured about four times more dust than gas by mass, and a porosity of 70 to 80 percent. Porosity is the share of a body that is empty space. A better picture is a crumbly, dark mix of ice and dust with a lot of gaps inside.

Why a comet brightens near the Sun

Far from the Sun, a comet is a frozen, dark lump. As it moves closer, sunlight warms the ices and they turn straight into gas. The gas streams away and carries dust with it. That is the coma, and it grows as the comet gets closer.

The amounts are large. Hale-Bopp's water vapor outflow peaked at almost 50 million metric tons per day (about 55 million US tons), according to ESA's analysis of the SOHO data. Each trip past the Sun costs a comet some of its ice. NASA notes that comets lose volatiles, the easily vaporized ices, on every visit, so they are short-lived on cosmic timescales.

The glow has two sources. Dust reflects sunlight. Gas in the coma absorbs ultraviolet light and re-emits it, which is called fluorescence. A comet is bright only because the Sun is shining on it. It makes no light of its own.

Worn-out comets may look like something else. NASA says some asteroids may be extinct comet nuclei, bodies that ran out of ice. That overlap is the subject of the comparison of comets and asteroids.

Short-period and long-period comets

Astronomers sort comets by how long one orbit takes. NASA says short-period comets circle the Sun in less than 200 years. Long-period comets can take as long as 30 million years.

Many short-period comets are thought to come from the Kuiper Belt, a doughnut-shaped zone of icy bodies beyond Neptune. Halley's Comet is the best-known short-period comet. Long-period comets are thought to arrive from the far larger Oort Cloud, a shell of icy bodies that NASA places about 100,000 astronomical units from the Sun. One astronomical unit (AU) is the average Earth-Sun distance, about 150 million km (93 million miles).

Real orbits cover a wide range. NASA lists Hale-Bopp at about 2,534 years per orbit. The Kuiper Belt article explains why the short-period group is easier to predict and to find again.

Common mistakes and myths

"The tail trails behind the comet like smoke." Tails point away from the Sun, not away from the direction of travel. A comet leaving the Sun moves tail-first. NASA describes the tail as stretching away from the Sun for millions of miles.

"A comet is a clean ball of ice." The surface is dark. NASA's description is ice coated with dark organic material, and Rosetta's measurements show far more dust than gas.

"A comet flashes across the sky like a shooting star." A shooting star is a meteor, a tiny grain of rock burning up in Earth's air. A comet is a large body far away and moves slowly against the sky. NASA says Hale-Bopp was visible to the naked eye for 18 months in 1996 and 1997. Comets do leave behind dust that causes meteor showers, a link explained in the guide to how meteor showers work.

"Comets are on fire." Nothing burns. A comet is very cold. Its ice turns to gas, and sunlight makes the gas and dust shine.

Frequently asked questions

How big is a comet?

The solid nucleus is usually a few kilometers across. Borrelly is about 8 km (5 miles) long. Hale-Bopp was far larger, with estimates from about 40 km to 60 km (25 to 37 miles).

How many comets are known?

ESA's comet catalog lists 4,088 as of its November 10, 2026 update. NASA says there are likely billions in total, because most are too faint and far away to find.

Is a comet an asteroid?

No, but the line is blurry. Asteroids are mostly rock and metal. Comets are rich in ice and form tails when warmed. A few objects show traits of both.

Can a comet be seen without a telescope?

Sometimes, but only a few comets each decade get bright enough. Binoculars show many more. Never point binoculars or a telescope at the Sun without a proper solar filter made for that purpose. The guide to seeing a comet covers how to plan.

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