What is a comet? | NASA Space Square – NASA Science For Kids

Comets, like the comet ISON pictured here, are believed to contain material dating from when the Sun and planets were forming. They are like giant, frozen time capsules in our solar system. Credit: NASA / MSFC / Aaron Kingery

Where do comets come from?

Comets are mainly found in the solar system. Some exist in a large disc beyond Neptune’s orbit called the Kuiper Belt. We call these short period comets. They take less than 200 years to orbit the Sun.

Other comets live in the Oort Cloud, the sphere-shaped outer edge of the solar system that is about 50 times farther from the Sun than the Kuiper Belt. These are called long period comets because they take much longer to orbit the Sun. The comet with the longest known orbit takes more than 250,000 years to make a single revolution of the Sun!

A yellow circle, representing the Sun, is surrounded by concentric rings, representing the orbits of the planets and the Kuiper belt.  Around these rings is a bubble-shaped shape, representing the Oort cloud.

The Kuiper Belt is beyond the orbits of the planets in our solar system. The Oort Cloud is well beyond the Kuiper Belt. Credit: NASA / JPL-Caltech

What brings comets closer to Earth so that we can see them?

The gravity of a planet or a star can pull comets from their foci in the Kuiper Belt or the Oort Cloud. This tug can redirect a comet towards the Sun. The trajectories of these redirected comets look like long, stretched ovals.

As the comet is attracted faster and faster to the Sun, it rotates behind the Sun, then returns to where it came from. Some comets dive directly into the Sun, never to be seen again. When the comet is in the inner solar system, whether it enters or leaves, this is when we can see it in our sky.

An animation of a white point in an elliptical orbit making its way around the Sun.

This animation represents the 76-year elliptical orbit of Halley’s Comet (the white point) relative to the more circular orbits of the planets. Credit: NASA / JPL-Caltech

What are the parts of a comet?

At the heart of every comet is a solid, frozen nucleus called the core. This ball of dust and ice is typically less than 10 miles in diameter, roughly the size of a small town. When the comets are in the Kuiper Belt or the Oort Cloud, scientists think that’s about all there is to it – just frozen nuclei.

But when a comet approaches the Sun, it begins to heat up. Eventually, the ice begins to turn into gas. It can also cause jets of gas to shoot out of the comet, causing dust with it. The gas and dust create a huge hazy cloud around the core called the coma.

: A comet is represented by a blue circle with a yellow center and a white dot in the middle with a light blue tail pointing up and to the side and a light yellow tail pointing outward.

This diagram shows the anatomy of a comet. Credit: NASA / JPL-Caltech

Why do comets have tails?

As dust and gases move away from the nucleus, sunlight and particles from the Sun push them into a bright tail that stretches behind the comet for millions of miles.

When astronomers look closely, they discover that comets actually have two distinct tails. One looks white and is made of dust. This dust tail draw a wide, slightly curved path behind the comet. The other tail is bluish and is made up of electrically charged gas molecules, or ions. the ion tail always points directly away from the Sun.

A black curved line is dotted with small blue circles, representing the movement of the comet along its path.  Each blue circle has a yellow tail and a blue ribbon-shaped tail sticking out of it.  The blue tail is straight and moves away precisely from the Sun, while the yellow tail is curved and moves away from the Sun in a different direction.

A comet has two tails that lengthen as it approaches the Sun. The two tails are always directed away from the Sun. The ion tail (blue) always points directly away from the Sun, while the dust tail (yellow) moves away from the Sun in a slightly different direction than the ion tail. Credit: NASA / JPL-Caltech

How do we know comets?

People have been interested in comets for thousands of years. But it was not possible to get a good view of a comet nucleus from Earth as it is enveloped in the gas and dust of the coma. In recent years, however, several spacecraft have been fortunate enough to study comets up close.

NASA’s Stardust mission collected samples from Comet Wild 2 (pronounced as “Vilt two”) and brought them back to Earth. Scientists found that these particles were rich in hydrocarbons, which are chemicals that we think of as the “building blocks” of life.

Rosetta, a European Space Agency mission that had several NASA instruments on board, studied comet 67P Churyumov-Gerasimenko. Rosetta dropped a lander on the nucleus, then orbiting the comet for two years. Rosetta has also detected building blocks of life on this comet. And the images showed comet 67P to be a robust object with a lot of activity shaping its surface.

A black background with a rocky and rugged looking object illuminated from above.  The object has a larger egg-shaped part at the bottom and a smaller, rounder part at the top.  Weak jets of dust come out of the object.

Rosetta captured incredible images of comet 67P shaped like a rubber duck. Credit: ESA / Rosetta / NavCam – CC BY-SA IGO 3.0

Thanks to these missions and others like them, we now know much more about the structure of comets and the types of chemicals found on and around them. We even learned a little more about the formation of our solar system!

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