All About Comets

Where do comets come from?

Diagram showing solar system, with Kuiper Belt and Oort Cloud.

Short-period comets come from the Kuiper Belt and long-period comets come from the Oort Cloud.

Most comets come from the Kuiper Belt, a region beyond the orbit of Neptune. Comets from this neighborhood usually take 200 years or less to make one orbit around the sun. These are called short-period comets.

Comets also come from their other hangout, the Oort Cloud, a far-far-distant cloud of comets that surrounds the solar system. Scientists think there could be about a trillion comets orbiting way out there. One trip around the sun could take one of these comets 30 million years! They are called long-period comets.

What brings comets near Earth?

Diagram showing elongated orbit of a comet, and how coma forms as it approaches sun and tail always points away from sun.

A comet's "hairy" coma and tails develop as the comet approaches the sun.

Sometimes the gravitational pull of a passing star stirs up comets in the Oort Cloud. Some might get sent flying into the inner solar system.

Sometimes the gravitational pull of a planet can disturb comets in the Kuiper Belt and fling one headlong toward the sun.

The sun's gravitational pull takes over, shaping the comet's path into a lop-sided orbit. The comet is pulled faster and faster toward the sun, it swings around close to the back-side, then it heads out again to more or less where it came from. Some comets dive right into the sun, never to be seen again. When the comet is in the inner solar system, either coming or going, that's when we may see it in our skies.

What makes comets look fuzzy?

Nucleus of Halley's comet elooks very fuzzy.

Halley's Comet, as seen by the European Space Agency's Giotto Mission

When they are at home in the Oort Cloud or Kuiper Belt, comets are just dull, dark chunks of ice, dust, and rock. In this state, they may not be much different from asteroids. But as comets get closer to the sun and begin to warm up, some of their materials start to boil off. This material forms a cloud around the nucleus. The cloud is called the coma and may be hundreds of thousands of miles across.

What do comets look like up close?

Comet Hartley-2.

Comet Hartley-2, as seen by the EPOXI spacecraft from 435 mi (700 km).

The nucleus, or solid part, of a comet is usually less than about 6 miles across, but may be as big as about 25 miles across. Recent space missions have given us some close-up views, so we don't have to guess what they look like anymore.

The Deep Space 1 mission found rugged terrain, smooth rolling plains, deep fractures and very, very dark material on Comet Borrelly in 2001. A few years later, space mission Deep Impact flew very close to Comet Tempel 1. That comet also appeared very black on the outside—as black as the charcoal briquettes for your barbeque.

What's inside a comet?

Comet Tempel-1, as it is struck by Deep Impact Impactor.

Comet Tempel-1 as it was hit by the impactor sent into its path by the Deep Impact spacecraft.

Comets seem to contain a lot of ice, some rocks and dust, and some gas. Deep Impact crashed a "smart impactor" into Comet Tempel 1 and studied the debris that spewed out. It found that the surface of the comet is very fragile and weak. Inside it is spongy, with lots of holes. It has ice beneath its surface. It contains material from outer, middle, and inner parts of the solar system. Other comets may be different.

Why do comets have tails?

Comet Hale-Bopp

Comet Hale-Bopp with its white dust tail and bright blue ion tail visible from Earth with the unaided eye.

Comet tails appear as the comet approaches the sun and can grow to be millions of miles long. The particles in the solar wind push the small dust particles in the coma into a long curved path. This tail is known as the dust tail. Another tail, the ion tail, is made of electrically charged molecules of gas. The ion tail points directly away from the sun.

article last updated February 11, 2021
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