Asteroids
Rocky and metallic remnants from the early Solar System.
Overview
Asteroids are rocky or metallic small bodies, most orbiting the Sun in the main belt between Mars and Jupiter. They are leftover material from the formation of the Solar System that never accreted into a planet — and their composition preserves a record of conditions at that time.
- The main belt did not fail to form a planet because of an explosion; Jupiter's gravity prevented accretion.
- Total belt mass is small — a few percent of the Moon's mass — and the belt is overwhelmingly empty space.
- Spectral classes broadly track composition and correlate with distance from the Sun.
- Near-Earth asteroids are catalogued and tracked systematically; known objects have no significant impact probability this century.
Why there is a belt and not a planet
The main belt lies roughly between 2.1 and 3.3 astronomical units from the Sun. Jupiter's gravitational influence stirred relative velocities in this region so much that colliding planetesimals fragmented rather than merged, preventing accretion into a planet.
The belt is far emptier than illustrations suggest. Its total mass is only a few percent of the Moon's, roughly a third of it in Ceres alone, and typical separations between objects are millions of kilometres — every spacecraft that has crossed it has done so without incident.
Composition and spectral classes
- C-type (carbonaceous) asteroids are dark and carbon-rich, dominate the outer belt, and are the most common class. Carbonaceous chondrite meteorites are their probable fragments.
- S-type (silicaceous) asteroids are stony, brighter, and more common in the inner belt.
- M-type asteroids show metallic signatures and may include exposed cores of differentiated bodies that were subsequently disrupted.
- The gradient with distance reflects formation temperature — volatile-rich material survived further from the Sun — and preserves information about the early solar nebula.
Families, groups, and resonances
Asteroids sharing similar orbital elements form families, understood as fragments of a single disrupted parent body. Identifying families lets astronomers reconstruct collisions that happened hundreds of millions of years ago.
Orbital resonances with Jupiter produce the Kirkwood gaps — distances where an asteroid's orbital period would be a simple fraction of Jupiter's, and repeated gravitational tugs destabilise the orbit. Resonances also feed material inward, which is one route by which main-belt fragments become near-Earth objects. Trojan asteroids, by contrast, are stably trapped near Jupiter's Lagrange points, leading and trailing the planet.
Near-Earth objects and planetary defence
Near-Earth asteroids have orbits bringing them within 1.3 astronomical units of the Sun. Systematic surveys have catalogued the large majority of the kilometre-scale population, and no known object poses a significant impact risk within the coming century. Smaller objects remain substantially incomplete in the catalogue, and small impacts do occur — the Chelyabinsk event in 2013 involved an object roughly twenty metres across that was not detected in advance.
The DART mission demonstrated in 2022 that a spacecraft impact can measurably change an asteroid's orbital period, shortening Dimorphos's orbit around Didymos. That is the first practical test of a deflection technique, though it applied to a small moonlet in a binary system rather than to an Earth-threatening object.
What visits have shown
Spacecraft have transformed asteroids from points of light into geological objects. NEAR Shoemaker orbited and landed on Eros; Dawn orbited both Vesta and Ceres; Hayabusa and Hayabusa2 returned samples from Itokawa and Ryugu; OSIRIS-REx returned material from Bennu.
The sample returns matter disproportionately. Laboratory analysis of pristine, uncontaminated material provides compositional and isotopic detail that no remote measurement can match, and both Ryugu and Bennu samples contain organic compounds and hydrated minerals relevant to how volatiles were delivered to the early inner Solar System.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- Was the asteroid belt once a planet?
- No. The belt's total mass is only a few percent of the Moon's — far too little to have ever been a planet. Jupiter's gravity stirred the region so that colliding bodies fragmented rather than accreting, so a planet never formed there in the first place.
- Could an asteroid hit Earth?
- Small ones do regularly, mostly burning up in the atmosphere. For large objects, surveys have catalogued most of the kilometre-scale near-Earth population and none has a significant impact probability in the coming century. Smaller objects capable of regional damage are less completely catalogued, which is why survey completion and the DART deflection test are both active priorities.
- What is the difference between an asteroid and a comet?
- Composition and behaviour. Comets contain enough volatile ices to develop a coma and tail when heated by the Sun; asteroids are predominantly rock and metal and do not. The boundary is not perfectly sharp — some objects show intermittent activity, and a few appear to be transitional — but the distinction is meaningful for most bodies.
- Why send spacecraft to asteroids?
- Because they preserve early Solar System material that planetary bodies have destroyed through heating and geological processing. Returned samples from Ryugu and Bennu allow laboratory analysis of pristine material, giving compositional and isotopic detail no remote observation can match — including on how water and organic compounds were delivered to the inner Solar System.