Asteroids are rocky leftovers from the early solar system, mostly orbiting between Mars and Jupiter.

The longer version

Leftover planetesimals still circle the Sun because Jupiter's gravity stirred that region and prevented the material from ever assembling into a planet. The main belt spans a broad zone between the orbits of Mars and Jupiter. Collisions between them produce fragments and dust, and some pieces are nudged onto paths that cross planetary orbits.

Most asteroids are irregular in shape, too small for their gravity to pull them into spheres, and their surfaces are covered with rubble and craters. Their composition varies from carbon rich dark material to metal rich bodies, preserving clues about conditions in the early solar system.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.