Comet vs. Asteroid: How to Tell Them Apart

The main differences

NASA describes an asteroid as a small, rocky object and a comet as an object made of ice and dust. Its summary calls both chunks of rock, ice and metal left over from the formation of the Solar System. The real difference is one of proportions and behavior.

NASA says there are far, far fewer comets than asteroids. Many comets are too faint and far away to find, so the known list understates how many exist.

Asteroids are not all alike either. NASA lists three main classes. C-type asteroids are the most common, likely made of clay and silicate rock, and they are dark. S-type asteroids are silicate rock mixed with nickel-iron. M-type asteroids are mostly metal, again nickel-iron. All three classes describe rocky or metallic bodies. Comets form a separate, icier family, and the in-between cases below are the exceptions.

Why ice changes the behavior

The key difference is what happens near the Sun. An asteroid stays inert. A comet's ice turns to gas as it warms, and the gas carries dust into a cloud called a coma. NASA's description is that near the Sun a comet's ice and dust vaporize, so in a telescope it looks fuzzy or grows a tail. The anatomy of a comet shows how the coma and tails form, and the guide to comet tails covers why the tails point away from the Sun.

The Planetary Society makes the same point about formation. Comets are similar to asteroids, but icier, because they formed farther from the Sun. Asteroids, by comparison, formed closer to the Sun.

Where each one lives

Most asteroids orbit the Sun in the main belt between Mars and Jupiter. NASA also lists Trojans, which share a planet's orbit, and near-Earth asteroids, which come close to Earth. The overview of asteroids and their types covers those groups.

Comets spend most of their orbits far out, beyond Jupiter and Neptune, according to the Planetary Society. They are thought to come from far-off reservoirs such as the Kuiper Belt and the Oort Cloud. The Kuiper Belt article and the Oort Cloud article describe them. Because a comet spends most of its orbit in the cold, it is quiet for most of its life and active only near the Sun.

How astronomers decide

No single test settles every case, so astronomers combine clues. Appearance is the first. A coma or tail points to a comet, while a plain point of light points to an asteroid. Orbit is the second. A path that spends most of its time beyond Jupiter fits a comet, and a path inside the main belt fits an asteroid.

Motion is the third clue, and it is the one that exposed 1998 SH2. NASA's team first tried to bounce radar off the object during its 2025 close pass, using the Deep Space Network, based on its predicted position. The object did not appear where expected. Careful position measurements and a review of every observation since 1998 then showed small pushes that gravity could not explain. Gas venting from a warming comet fits that kind of push.

A spacecraft visit removes the doubt for one object, as the story of Rosetta at Comet 67P shows, but only a small fraction of bodies have been visited.

The in-between cases

Real objects do not always follow the rule. Astronomers have found several kinds that sit between the groups, and the dates below show how the list has grown.

• 1996: The first main-belt comet, 133P/Elst-Pizarro, was found. It orbits in the asteroid belt but shows comet-like activity.
• 2005: Two more were found, and astronomers recognized main-belt comets as a distinct class.
• 2010: A survey spotted an asteroid-belt object, P/2010 A2, trailing a comet-like tail.
• 2013: Hubble photographed P/2013 P5, an asteroid with six dust tails.
• 2016: The first "dark comet" was identified.
• 2026: NASA announced that 1998 SH2 is a comet.

Active asteroids are objects on asteroid-like orbits that show a coma or tail. The cause matters. Astronomer Henry Hsieh has described main-belt comets as objects that show comet-like activity, probably from sublimating ice, while orbiting in the asteroid belt. Sublimation means turning from solid to gas without melting. Other active asteroids shed dust for different reasons. P/2013 P5 was probably spun up by sunlight until its surface began shedding dust. NASA says it is likely made of metamorphic rock that cannot hold ice.

P/2010 A2 may be the debris from a collision. NASA reported that Hubble found an X-shaped pattern at its head, and astronomers suspected a small rock hit a larger one. The evidence was strong but not conclusive, and a spin-up explanation was still possible.

Dark comets show a push without a visible tail. NASA says dark comets have trajectory changes that gravity cannot explain, but no visible coma, tail or outgassing. About a dozen have been identified since the first one in 2016. 1998 SH2 showed the same kind of unexplained push, and then its faint tail was imaged. NASA's hypothesis is a small thrust from gas venting as the Sun heats ice in the object, and it says many larger dark comets with similar orbits could turn out to be regular comets.

A dormant comet is one that shows no activity at the moment. Without a visible coma or tail, it can pass for an asteroid until careful tracking of its motion gives it away.

Common mistakes and myths

"Comets are ice and asteroids are rock, full stop." NASA's summary says both groups hold rock, ice and metal. They differ in proportions.

"Anything with a tail is a comet." A tail means an object is losing material. Collisions and fast spin can do that too, as P/2013 P5 and P/2010 A2 show.

"Asteroids all stay in the belt." Many do, but near-Earth asteroids and Trojans live elsewhere, as the asteroid overview explains.

"The labels are settled forever." They are not. When astronomers find new evidence, objects change classes, as 1998 SH2 did in 2026.

"A comet always looks like a comet." Far from the Sun, a comet shows no coma or tail. Only warmth makes the show.

Frequently asked questions

Can an asteroid turn into a comet?

An object does not change its nature, but its label can change. A body that looked inactive may be found to vent gas after all. NASA's study of 1998 SH2 is a recent case.

Which is more common?

Asteroids. NASA says there are far, far fewer comets than asteroids.

How can a telescope user tell them apart?

NASA says asteroids look like points of light, while comets look fuzzy or have a tail. A faint comet can still look like a point, so astronomers also measure orbits and motion. Never point binoculars or a telescope at the Sun without a proper solar filter made for that purpose.

Are comets and asteroids related?

Yes. NASA says both are chunks of rock, ice and metal left over from the formation of the Solar System.