Comets are cosmic snowballs of frozen gases, rock and dust that orbit the Sun. When frozen, they are the size of a small town. When a comet's orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of miles. There are likely billions of comets orbiting our Sun in the Kuiper Belt and even more distant Oort Cloud.
The current number of known comets is:
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Key Science Targets
NASA Space Place: All About Comets ›
Comets orbit the Sun just like planets and asteroids do, except a comet usually has a very elongated orbit.
As the comet gets closer to the Sun, some of the ice starts to melt and boil off, along with particles of dust. These particles and gases make a cloud around the nucleus, called a coma.
The coma is lit by the Sun. The sunlight also pushes this material into the beautiful brightly lit tail of the comet.
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