Asteroid impact. Computer artwork of an asteroid colliding with the surface of the Moon. The crescent Earth is seen in the background, reflecting the light of the Sun. Asteroids are chunks of rock circling within the solar system, remnants from the time that the planets formed. They range in size from numerous, tiny dust particles to a few large objects, many kilometres wide. Perturbations in their orbits may bring them within the Earth's gravitational field. Most burn up in the Earth's atmosphere as meteors but some impact Earth's satellite, the Moon, and form the craters that make up the lunar landscape.