Where Is the Asteroid Belt? Location, Size & Structure

The asteroid belt is one of the most recognisable features of our solar system, yet it is widely misunderstood. Far from the dense, ship-destroying obstacle course of science fiction, the real asteroid belt is an enormous region of mostly empty space, sprinkled with millions of rocky remnants from the solar system's formation.

Here is everything you need to know about where the asteroid belt is, how big it is, and what it contains.

Where Exactly Is the Asteroid Belt?

The asteroid belt occupies a broad, torus-shaped region between the orbits of Mars and Jupiter. Its inner edge begins at roughly 2.2 astronomical units (AU) from the Sun — about 329 million kilometres — just beyond the orbit of Mars at 1.52 AU. Its outer edge extends to approximately 3.2 AU from the Sun, or about 479 million kilometres, well inside Jupiter's orbit at 5.2 AU.

To put those distances in perspective, Earth orbits the Sun at 1 AU (150 million kilometres). The asteroid belt's inner boundary is about 1.2 AU beyond Earth, meaning the nearest asteroids in the main belt are roughly 180 million kilometres from our planet at closest approach. At its farthest, the outer edge of the belt lies over 2 AU — some 300 million kilometres — from Earth.

The belt is centred at roughly 2.7 AU from the Sun, which places the majority of its mass in a region that takes sunlight about 22 minutes to reach, compared to the 8 minutes it takes to reach Earth.

How Big Is the Asteroid Belt?

The asteroid belt spans about 1 AU in width — approximately 150 million kilometres from inner edge to outer edge. That is roughly the same distance as Earth to the Sun.

Despite this immense volume, the belt is startlingly empty. The total mass of all asteroids combined is estimated at just 3% of the Moon's mass, or about 4% of Ceres's mass spread across the entire belt. On average, individual asteroids are separated by about one million kilometres — more than twice the distance from the Earth to the Moon.

This extreme emptiness is why spacecraft routinely fly through the asteroid belt without incident. NASA's Pioneer 10, the first probe to traverse it in 1972, encountered no hazards at all. Since then, numerous missions including Voyager 1 and 2, Galileo, Cassini, New Horizons, and Juno have all passed through without any close calls.

If you could stand on one asteroid in the main belt, the nearest neighbouring asteroid would typically be invisible to the naked eye — it would be far too distant and too small to see.

Kirkwood Gaps: The Belt's Hidden Structure

The asteroid belt is not uniformly distributed. When astronomers plot asteroid orbits by distance from the Sun, they find distinct gaps — regions where very few asteroids orbit. These are called Kirkwood gaps, named after American astronomer Daniel Kirkwood, who first identified them in 1866.

Kirkwood gaps occur at distances where an asteroid's orbital period forms a simple ratio with Jupiter's orbital period — a phenomenon called orbital resonance. The most prominent gaps occur at:

• 2.06 AU — the 4:1 resonance (an asteroid here would orbit four times for every one Jupiter orbit)
• 2.50 AU — the 3:1 resonance
• 2.82 AU — the 5:2 resonance
• 2.95 AU — the 7:3 resonance
• 3.27 AU — the 2:1 resonance

At these resonant orbits, Jupiter's repeated gravitational tugs accumulate over millions of years, gradually increasing an asteroid's orbital eccentricity until it is flung out of the belt entirely. The result is a set of depopulated lanes running through the belt like gaps in the grooves of a vinyl record.

Between the gaps, asteroids cluster into distinct groups. The largest concentrations are the Flora, Nysa, Koronis, Eos, and Themis families, each named after their largest member and consisting of fragments from ancient collisions.

The Largest Objects in the Asteroid Belt

About 60% of the belt's total mass is concentrated in just four objects:

Ceres is by far the largest, with a diameter of about 950 kilometres — large enough to be classified as a dwarf planet. It alone accounts for roughly 39% of the entire belt's mass. NASA's Dawn spacecraft orbited Ceres from 2015 to 2018 and discovered bright salt deposits in Occator Crater, suggesting recent geological activity driven by a subsurface brine reservoir.

Vesta is the second largest at 525 kilometres across. It is the brightest asteroid visible from Earth and has a giant impact basin called Rheasilvia at its south pole — 505 kilometres wide and 19 kilometres deep, one of the largest impact structures in the solar system. Dawn also visited Vesta from 2011 to 2012.

Pallas measures about 513 kilometres in diameter. Its orbit is unusually tilted at 34.8 degrees to the ecliptic plane, making it difficult for spacecraft to visit. Its composition resembles carbonaceous chondrite meteorites.

Hygiea rounds out the top four at roughly 430 kilometres across. It is the largest C-type (carbonaceous) asteroid in the belt and recent observations suggest it is nearly spherical, raising questions about whether it too might qualify as a dwarf planet.

Beyond these four, over 1.1 million asteroids larger than 1 kilometre have been identified, with millions more smaller bodies estimated to exist.

Three Types of Asteroids

Asteroids in the main belt fall into three broad compositional categories:

C-type (carbonaceous) asteroids are the most common, making up about 75% of known asteroids. They are dark, carbon-rich bodies that reflect very little sunlight. C-type asteroids dominate the outer belt beyond 2.7 AU and are thought to be the most primitive objects in the solar system — largely unchanged since their formation 4.6 billion years ago. Their composition closely matches that of the Sun (minus hydrogen and helium).

S-type (silicate) asteroids account for about 17% of the belt population. They are brighter and composed primarily of iron and magnesium silicate minerals. S-type asteroids are concentrated in the inner belt, closer to 2.2 AU. Vesta is the most prominent S-type asteroid.

M-type (metallic) asteroids are relatively rare but scientifically fascinating. They are composed largely of iron and nickel, and many are thought to be the exposed cores of ancient planetesimals that were shattered by collisions. NASA's Psyche mission, launched in October 2023, is currently en route to 16 Psyche — a 226-kilometre M-type asteroid that may be the stripped metallic core of a proto-planet.

This compositional gradient across the belt — rocky in the inner regions, carbon-rich in the outer regions — reflects the temperature distribution of the early solar nebula, with volatile compounds surviving only at greater distances from the young Sun.

Why Is There an Asteroid Belt at All?

The asteroid belt exists because of Jupiter. During the solar system's first few million years, the region between Mars and Jupiter contained enough material to form a planet — perhaps one the size of Earth or larger. But Jupiter's enormous gravity prevented that from happening.

As Jupiter grew to its massive size, its gravitational influence stirred up the orbits of planetesimals in the belt region, increasing their velocities. Instead of gentle, constructive collisions that could build larger bodies, the impacts became violent and destructive, shattering growing protoplanets and grinding them down rather than building them up.

The Grand Tack hypothesis suggests that Jupiter's influence was even more dramatic. According to this model, Jupiter migrated inward toward the Sun early in the solar system's history, reaching as close as 1.5 AU (roughly Mars's current orbit) before Saturn's gravitational influence pulled it back outward. During this migration, Jupiter would have swept through the asteroid belt region twice, scattering and ejecting the vast majority of its original material.

The result is that the asteroid belt today contains less than 0.1% of the mass it once held. What remains is a sparse collection of survivors — fragments and remnants that found stable orbits in the gravitational gaps between Jupiter's influence zones.

3I/ATLAS and the Asteroid Belt

The asteroid belt takes on fresh relevance with 3I/ATLAS, the third confirmed interstellar object. This visitor from another star system entered our solar system on a hyperbolic trajectory tilted at 175 degrees to the ecliptic — essentially travelling in the opposite direction to the planets.

3I/ATLAS passed through the asteroid belt region during its inbound journey, reaching perihelion (closest approach to the Sun) on 29 October 2025 just inside the orbit of Mars. It is now heading back outward and will pass near Jupiter in March 2026 before leaving the solar system for good.

Because 3I/ATLAS crossed the belt at such a steep angle relative to the ecliptic plane, it spent only a brief time within the belt's vertical extent. But spectroscopic observations taken as it passed through this region have given astronomers valuable data on how its interstellar ices respond to the moderate solar heating found at asteroid belt distances — temperatures far gentler than the intense radiation it experienced near perihelion.

The contrast between our asteroid belt's native rocky bodies and this icy interstellar interloper highlights just how different the conditions are in other planetary systems. While our belt is dominated by dry, rocky material baked by billions of years of solar radiation, 3I/ATLAS carried pristine ices from a distant, colder environment.

Track 3I/ATLAS's trajectory through the solar system — including its path past the asteroid belt — on our interactive orbit viewer. For more on the solar system's structure, explore our science page and timeline.