CONCEPTS in Action
Comets and Meteoroids
Comets and meteoroids are two groups of objects that travel through the solar system. Sometimes they can be seen from Earth as spectacular sights in the night sky.
Comets are dirty snowballs—chunks of rock and ice. They originate in the Kuiper Belt and the Oort Cloud, both of which lie beyond the orbit of Pluto. If a comet comes close to the sun it vaporizes, producing clouds of gas and dust. These clouds appear as tails, which can be millions of kilometers long. If a comet is close enough to Earth, it can be seen as a bright object in the night sky. Meteoroids consist of rock or metal and they can be any size from tiny particles to huge boulders. Most meteoroids burn up as they enter Earth's atmosphere. They appear as streaks of light in the night sky, called meteors. Meteoroids that reach Earth's surface are called meteorites.
Comets
Most comets travel in highly elliptical orbits around the sun. Their orbital periods vary dramatically. Short-period comets, so called because they take less than 200 years to make one orbit, are the most predictable. Long-period comets may take thousands or millions of years to return to our sky.
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