What is an asteroid?
An asteroid is a small, mostly rocky or metallic body orbiting the Sun.
How it works and key principles
Origins and orbits: Many asteroids occupy the main belt between Mars and Jupiter; some have orbits that approach Earth. Their sizes and compositions vary. They preserve clues to material left from planet formation, and not every asteroid is a single solid rock.
Evidence, examples and limits
Tracking and risk: Repeated telescope measurements constrain an asteroid’s orbit. A close approach does not itself mean a collision; uncertainty and future encounters must be evaluated. Spacecraft investigate surfaces, densities and the composition of returned samples.
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