Objects · Solar System scale
Asteroid
A small rocky body orbiting the Sun. Most are found in the asteroid belt between Mars and Jupiter. Ranges from pebble-sized to hundreds of kilometres across. Leftover material from the formation of the Solar System.
Notable example
Ceres, the largest asteroid at 940 km diameter, is now classified as a dwarf planet — the only one in the inner Solar System.
Source: NASA Solar System Exploration
Asteroids and planetary defence
Asteroids are remnants of the Solar System's formation 4.6 billion years ago. The gravitational influence of Jupiter prevented the material in the asteroid belt from accreting into a planet, leaving it as a fossil record of early solar system chemistry. Sample-return missions to Ryugu (Hayabusa2) and Bennu (OSIRIS-REx) found organic compounds and hydrated minerals — evidence that asteroid impacts may have delivered water and organic building blocks to the early Earth.
For the full story on how asteroids, comets, and meteors relate to each other, see our guide to asteroids, comets and meteors.
Frequently asked questions
- These three terms describe the same type of rocky material at different sizes and locations. An asteroid is a rocky body in space — generally over 1 metre across — orbiting the Sun. A meteoroid is a smaller fragment (millimetres to metres) in space. When a meteoroid enters Earth's atmosphere and burns up, the glowing trail is a meteor (or "shooting star"). If a piece survives to reach Earth's surface, it is a meteorite. Our guide to asteroids, comets and meteors explains the full taxonomy.
- The main asteroid belt lies between Mars and Jupiter, roughly 2.2 to 3.2 AU from the Sun. It contains millions of asteroids but is far less dense than often depicted in films — spacecraft pass through it routinely without collisions. The total mass of the asteroid belt is about 4% of the Moon's mass. The largest objects are Ceres (diameter 940 km, now classified as a dwarf planet) and Vesta (530 km), Pallas (511 km), and Hygiea (430 km). Jupiter's gravity prevents these objects from accreting into a planet. NASA's asteroid belt overview has detailed composition data.
- Most asteroids pose no near-term threat, but impacts are a genuine long-term risk. Near-Earth Asteroids (NEAs) have orbits that cross or approach Earth's path. NASA's Planetary Defense Coordination Office tracks over 2,300 "potentially hazardous asteroids" (PHAs) — those over 140 m across that approach within 7.5 million km of Earth's orbit. In 2022, NASA's DART mission successfully changed the orbit of asteroid Dimorphos by crashing a spacecraft into it — the first planetary defence demonstration. No known asteroid poses a significant impact risk this century.
- Asteroids come in several compositional types. C-type (carbonaceous) are the most common — dark, carbon-rich, and similar to primitive early solar system material. S-type (silicaceous) are brighter and contain silicate minerals and metals — the source of most stony meteorites. M-type are metallic (iron-nickel) — the remnant cores of differentiated bodies that were shattered by collisions. The Hayabusa2 mission returned samples from C-type asteroid Ryugu in 2020; NASA's OSIRIS-REx returned samples from Bennu in 2023. Both found organic compounds and water-bearing minerals — hinting that asteroids delivered ingredients for life to early Earth.
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