What Are Asteroids? Types, Sizes and Where They Live

What an asteroid is

NASA describes asteroids as rocky, airless remnants of the early Solar System. "Airless" means they have no real atmosphere. NASA also notes that asteroids are sometimes called minor planets, a label that still appears in catalogs, where a body is given a number once its orbit is well known.

The line between an asteroid and a dwarf planet has moved over time. Ceres was long counted as an asteroid. NASA says scientists classified it as a dwarf planet in 2006, and it remains the largest object in the main belt. The rules behind that decision are laid out in the guide to what a dwarf planet is.

Comets are the other big family of small bodies. NASA's Center for Near-Earth Object Studies (CNEOS) explains that comets are mostly water ice with embedded dust and formed in the cold outer Solar System. Asteroids are mostly rocky and formed in the warmer inner Solar System, between Mars and Jupiter. A few objects blur that line, and the comparison of a comet and an asteroid covers the in-between cases.

Many asteroids are not solid boulders. JAXA, Japan's space agency, reports that both Itokawa and Ryugu turned out to be rubble piles: loose heaps of rock that were once part of a much larger body. The same summary says Bennu is a rubble pile too.

How big is an asteroid?

NASA puts the range between Vesta, about 530 km (329 miles) across, and bodies smaller than 10 m (33 feet). NASA calls Vesta (covered in detail in the guide to Vesta) the largest asteroid. Ceres is wider, but it is classed as a dwarf planet. The table shows effective diameters from JPL's Small-Body Database, which means one average width for an object that is really lumpy.

Sizes change as measurements improve. NASA lists Vesta at about 530 km, while the database gives 523 km, which is why numbers here are database values and are rounded. The drop from 939 km down to 330 m in this one table shows how wide the family is, and NASA's list reaches smaller still.

The C, S and M classes

Astronomers sort asteroids into classes by the light their surfaces reflect, which shifts with the minerals present. NASA's overview names three main groups:

• C-type: the most common. Probably clay and silicate rock, and dark in appearance.
• S-type: silicate materials and nickel-iron, often called "stony."
• M-type: metallic, made mostly of nickel-iron.
• Other letters: the database also uses B (Bennu and Pallas), V (Vesta) and P (Patroclus), among others, for more specific spectra.

The letters are shorthand, not a full description. JAXA's science summary says Bennu is a B-type asteroid that looks similar in observations to the C-type Ryugu.

Returned samples tie the letters to real rock. Grains brought back from the S-type Itokawa matched LL chondrite meteorites, which links an S-type asteroid to rocks that fall on Earth. That story is told in the Hayabusa and Hayabusa2 sample missions. Ryugu, a carbon-rich type, returned a sample with organic molecules, including amino acids. Vesta has its own class, V, and is linked to a whole family of meteorites.

Where asteroids live

The main belt. NASA says most asteroids orbit in the main belt between Mars and Jupiter. JPL's database holds 1,396,236 main-belt asteroids as of November 11, 2026. NASA estimates the belt contains between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mile), plus millions of smaller ones. Even so, the belt is mostly empty space, as the explanation of the asteroid belt shows.

Near-Earth space. Near-Earth asteroids are those whose orbits can bring them into Earth's neighborhood. NASA defines this by perihelion, the closest point to the Sun, which must be less than 1.3 astronomical units (AU). One AU is the average Earth-Sun distance, about 149.6 million km (93 million miles), so 1.3 AU is about 194 million km (121 million miles). JPL's database lists 43,025 near-Earth asteroids as of November 11, 2026. How they are found and followed is covered in the article on near-Earth asteroids.

Trojans. A Trojan asteroid shares an orbit with a larger planet without colliding with it, because it gathers around two stable spots called the L4 and L5 Lagrange points. Jupiter's Trojans are the most important group, with 16,472 known in JPL's database as of November 11, 2026. NASA also mentions Mars and Neptune Trojans and an Earth Trojan announced in 2011. A NASA spacecraft is now heading for the Jupiter swarms, as described in the Lucy mission guide.

Common mistakes and myths

Myth: the asteroid belt is a crowded field of rocks. NASA's solar system tour says the main belt is still mostly empty space and that spacecraft can pass through with almost no risk of collision.

Myth: asteroids are the pieces of a destroyed planet. NASA describes them as leftovers from the formation of the Solar System. Their combined mass is less than the Moon's, which is far too little to be the wreck of a planet.

Myth: every asteroid is a small rock. Vesta is about 530 km (329 miles) across by NASA's figure. The word covers everything from bodies under 10 m (33 feet) to worlds hundreds of kilometers wide.

Myth: Ceres is just another asteroid, or it is not one at all. It was long counted as an asteroid. Since 2006 it has been classed as a dwarf planet, and it still sits in the main belt.

Myth: all asteroids live in the belt. Many orbit near Earth, and thousands share Jupiter's orbit.

Frequently asked questions

How many asteroids are known?

JPL's Small-Body Database lists 1,580,112 asteroids as of November 11, 2026. The number rises as surveys find more, so check the database for a current count.

Are asteroids and minor planets the same thing?

NASA says asteroids are sometimes called minor planets. In catalogs the term minor planet is the formal umbrella, and numbered objects such as Ceres and Vesta appear in the same lists.

What is the largest asteroid?

NASA's asteroid pages name Vesta, at about 530 km (329 miles), as the largest. Ceres is bigger at about 939 km (583 miles) by JPL's figure, but it has been a dwarf planet since 2006.

Do all asteroids have the same history?

No. JAXA's science summary says Itokawa grains showed heating of up to 800 °C (1,470 °F), while Ryugu's sample showed no heating above 100 °C (212 °F) and held hydrated minerals. It also says Bennu hosted liquid water in its past.