ESA places the main asteroid belt between 2.1 and 3.3 astronomical units (AU) from the Sun. One AU is the average Earth-Sun distance, about 149.6 million km (93 million miles), so the belt's inner edge sits roughly 314 million km (195 million miles) out and its outer edge about 494 million km (307 million miles). That makes a band about 180 million km (112 million miles) wide, and ESA estimates all the material in it adds up to about 3 percent of the Moon's mass.
Movies often show the belt as a storm of tumbling rock that a ship must dodge. NASA describes something very different. This article covers where the belt is, why spacecraft cross it without trouble, what makes the gaps in it, and why Jupiter is the reason no planet grew there.
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Where the belt is and what is in it
The belt lies between the orbits of Mars and Jupiter. NASA's tour describes the region as filled with millions of small rocky bodies that come in all shapes, sizes and compositions. For the main classes of asteroid found there, see the guide to what asteroids are. The table collects the main numbers.
| Quantity | Value | Source and date |
|---|---|---|
| Distance from the Sun | 2.1 to 3.3 AU (314 to 494 million km) | ESA |
| Total mass | About 3% of the Moon's mass | ESA |
| Asteroids classed as main-belt | 1,396,236 | JPL Small-Body Database, November 11, 2026 |
| Largest member | Ceres, 939 km (583 miles) | JPL database; NASA |
| Ceres' share of belt mass | About 25% | NASA Ceres facts |
Ceres is the giant of the belt. NASA calls it the largest object there and says it has been classed as a dwarf planet since 2006. Estimates of how much of the belt Ceres holds vary by source. NASA's Ceres page gives about 25 percent, while another NASA tour page says about half. The spread shows that published shares differ by source and by date.
The count of 1,396,236 is a count of objects whose orbits are catalogued, not a count of everything that is there. Smaller asteroids are harder to detect, so the real total is higher, and the catalog grows as surveys continue.
Why it is mostly empty space
NASA's solar system tour is direct about it: despite the large numbers of asteroids, the main belt is still mostly empty space, and spacecraft can travel through it with almost no risk of collision. The reason is volume. The belt is a band about 180 million km (112 million miles) wide, and more than a million catalogued objects are spread through it. The bodies are small compared with the space between them.
A simple comparison shows the scale. Ceres, the largest member, spans 939 km (583 miles). Laid along the belt's 180-million-km width, it would cover only about five millionths of the distance. Most belt asteroids are far smaller than Ceres, so the proportion for a typical rock is smaller still.
The Planetary Society's overview adds another number: the belt today holds only small bodies of rock, ice and metal that together have less than 1 percent of Earth's mass.
Spacecraft have shown how routine a crossing is. Dawn, launched September 27, 2007, was the first probe to orbit an object in the main belt, according to JPL. A visit to a chosen asteroid still needs careful aiming, because the target has to be reached on purpose.
- September 27, 2007: Dawn launches.
- July 2011 to September 2012: Dawn orbits Vesta, the main-belt giant covered in the guide to Vesta.
- March 6, 2015: Dawn reaches Ceres and begins orbiting it.
- November 1, 2018: the Dawn mission ends.
The Kirkwood gaps
Plot how many asteroids sit at each orbit size and the chart shows dips. These are the Kirkwood gaps. JPL's Small-Body Database labels them 3:1, 5:2, 7:3 and 2:1, and says they are caused by mean-motion resonances between an asteroid and Jupiter.
A mean-motion resonance is a match between two orbital periods. In a 3:1 resonance, the asteroid circles the Sun three times while Jupiter circles once. The two bodies then meet in the same arrangement again and again, and Jupiter's small tugs repeat at the same spot in the asteroid's orbit. Repeated nudges add up, and over long times an asteroid in such an orbit is pushed onto a different path. The gaps are therefore dips in how many asteroids share an orbit size, not rings you could see in a photo. To read an orbit size and period yourself, see how orbital elements work.
Why no planet formed
The Planetary Society's history of the Solar System states that Jupiter did not allow planets to form in the belt. Dozens of Moon-sized and Mars-sized protoplanets once lay there, and Jupiter's gravity acted on them. They either smashed into others and broke apart or drifted out of the region. What remains is a thin collection of leftovers.
The same overview points out that Ceres, a dwarf planet under the tests laid out in the article on what a dwarf planet is, contains organics and water ice, so it probably formed farther out and moved inward later. Not every body in the belt, in other words, necessarily formed where it sits today.
Common mistakes and myths
Myth: the belt is a dense field you must dodge. NASA says spacecraft can cross it with almost no risk of collision.
Myth: the belt is the wreck of a destroyed planet. The total mass is about 3 percent of the Moon's by ESA's estimate. NASA states that all asteroids together weigh less than the Moon. That is far too little to be a lost planet. The explanation scientists give is a planet that never formed because of Jupiter.
Myth: every asteroid lives in the belt. Some orbit close to Earth, as covered in the article on near-Earth asteroids, and thousands share Jupiter's orbit, as in the story of Lucy and the Trojan asteroids.
Myth: the Kirkwood gaps are empty rings. They are dips in the number of asteroids at certain orbit sizes, not bands of empty space you could see from a spacecraft.
Myth: Ceres makes up half the belt, and that settles it. Published shares range from about 25 percent to about half, depending on the source and the date. Treat any single figure as an estimate.
Frequently asked questions
How far is the asteroid belt from Earth?
The belt lies 2.1 to 3.3 AU from the Sun, while Earth is 1 AU out. At the closest alignment, the inner edge is about 1.1 AU, or roughly 165 million km (102 million miles), from Earth.
Is the asteroid belt dangerous for spacecraft?
NASA says spacecraft can travel through the belt with almost no risk of collision. Probes such as Dawn have orbited objects inside it.
How many asteroids are in the belt?
JPL's database lists 1,396,236 main-belt asteroids with known orbits as of November 11, 2026. The true number is larger, since many small objects are not yet found.
Is Ceres an asteroid or a dwarf planet?
Both labels appear. NASA says Ceres was long considered an asteroid and was classified as a dwarf planet in 2006. It is the largest object in the main belt.
Sources
- Small objects in the Solar System. ESA. https://www.esa.int/media/small-objects-in-the-solar-system
- Solar System Tour: Asteroids. NASA Goddard Space Flight Center. https://science.gsfc.nasa.gov/690/solar-system-tour/Asteroids.html
- Ceres Facts. NASA Science. https://science.nasa.gov/dwarf-planets/ceres/facts/
- Kirkwood gaps (main-belt histogram). Small-Body Database, NASA/JPL. https://ssd.jpl.nasa.gov/diagrams/mb_hist.html
- Solar System History 101. The Planetary Society. https://www.planetary.org/articles/solar-system-history-101
- Dawn mission. NASA/JPL. https://www.jpl.nasa.gov/missions/dawn
- Small-Body Database Query API, main-belt count queried November 11, 2026. NASA/JPL. https://ssd-api.jpl.nasa.gov/sbdb_query.api?fields=full_name&sb-kind=a&sb-class=MBA&limit=1