Below are 7 things to know about asteroids, comets, meteors and other space rocks from NASA.
1) Asteroids are rocky fragments left over from the formation of the solar system about 4.6 billion years ago. Most asteroids orbit the sun in a belt between Mars and Jupiter. Scientists think there are probably millions of asteroids, ranging widely in size from hundreds of kilometers across to less than one kilometer (a little more than one-half mile) wide.
2) Comets are relatively small, fragile, irregularly shaped bodies and, like asteroids, they are left over from the solar system formation process. Comets, however, are icy dirtballs that form in the outer solar system. The icy surface is embedded with dust, grit and particles from space. Scientists believe that impacts from comets played a role in the evolution of Earth billions of years ago. One theory suggests that comets brought some of the water and a variety of organic molecules to the early Earth.
3) If a comet or asteroid's approach brings it to within 1.3 astronomical units of the sun, we call it a near-Earth object. [One astronomical unit is close to the mean distance between the sun and Earth ? approximately 150 million kilometers (about 93 million miles).]
4) Meteoroids are small particles and fragments from asteroids that collide with each other and break up into smaller fragments, or comets shedding dust.
5) When one approaches Earth, it burns up as it goes through Earth's atmosphere. Thus a meteor, or shooting star, is formed.
6) Fireballs are larger meteoroids, roughly ranging in size anywhere from a basketball to a Volkswagen. They also make very impressive sky displays as they break into fragments and burn up in their passage through Earth's atmosphere.
7) Some meteoroids survive passage through Earth's atmosphere and hit the ground. These are called meteorites.
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