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If you hear the word space rock, what comes to mind? Do you think about a chunk of meteorite you saw at a natural history museum or an asteroid floating in space? If you are thinking yes to either suggestion you would be right. The fact is that space rocks are basically the same as earth rocks except they have their origins outside of Earth. Like rock on earth they have subtypes depending on the processes the go through and their location. Knowing the type of rock will tell you a lot about its origins and composition.
The first thing to realize is that rocky celestial bodies are more common than you think. In fact there are probably thousands if not millions of starts that have rocky celestial objects orbiting them or falling towards planets or themselves. The reasons can be found in how stars are formed. All stars are formed from concentrated patches of the interstellar medium called nebulas. Nebulas are mixtures of gas and dust which under the influence of tidal forces group together in thicker concentrations. Over time gravity compresses the nebula further forming the beginnings of star as more mass is concentrated. A side effect in nebulas with heavier elements in their composition is the formation of space rocks in the forms of planetoids, asteroids, and meteorites. In fact just about any planet observed was formed by heavier elements in a given nebula compressing together under the influence of gravity.
Whatever is left over from the formation of planets is either caught up in the pull of the star and orbits or fall towards the closer influence of nearby planets’ gravity. So space rock that is too small to become a planet but large enough to maintain its orbit becomes asteroids. A good example is the asteroid belt of the inner solar system. However some rocks are even smaller. They also orbit the sun but can be pulled in by the gravity of a nearby planet and crash into the surface. These are called meteoroids and meteorites. A meteoroid will become a meteorite if survives impact on a planet.
We’ve also recorded an entire episode of Astronomy Cast all about the Asteroid Belt. Listen here, Episode 55: The Asteroid Belt.