How Do We Know What Asteroids Are Made Of?

Reading sunlight: spectra, color and brightness

Sunlight bounces off an asteroid, and the surface absorbs some colors more than others. A reflectance spectrum records which wavelengths are absorbed. NASA explains that such spectra reveal what a surface is made of, and they can also show whether ice is amorphous, crystalline or irradiated, which hints at the temperature history. Telescopes such as Webb collect them, as described in the guide to the telescopes that find and study small bodies.

Astronomers sort spectra into types. NASA describes three main classes. C-type asteroids are the most common. They look dark and are thought to be made of clay and silicate rock. S-type asteroids are stony, made of silicate materials and nickel-iron. M-type asteroids are metallic, made of nickel-iron. NASA says the differences relate to how far from the Sun each asteroid formed. Some asteroids were also heated after forming and partly melted, with iron sinking to the center and basalt, a volcanic rock, rising to the surface.

JPL's database records two numbers that come from this kind of work: a spectral type and an albedo, which is the fraction of light a surface reflects. The values below were returned on November 11, 2026.

In the table, n/a marks a value that was not retrieved for this article, and density is in grams per cubic centimeter. Water has a density of 1.0, which is about 62 pounds per cubic foot.

A spectrum has one big limit: it reads only the outer surface. NASA's Dawn team reports that hydrated materials in young craters on Vesta likely came from outside Vesta. A surface can carry material that does not belong to the body underneath.

Weighing a body: density

Density is mass divided by volume. Mass comes from gravity: the pull on a passing spacecraft, or on a moon, shows how much material is inside. Dawn measured Vesta's gravity to determine its interior structure. The data are consistent with an iron core about 225 km (140 miles) across, and NASA says the evidence points to widespread, perhaps global, melting early in Vesta's history.

Low densities carry a different message. Bennu and Ryugu are only about 19 percent denser than water. Mathilde, at 1.3, and Itokawa, at 1.9, are also well below Vesta. Vesta's rock is nearly three times denser than Bennu. A low density means lots of empty space inside, either in pores in the rock or in gaps between pieces. NASA describes Itokawa as a rubble pile, a loose collection of rocks held together by gravity. For Bennu, NASA reports a March 2026 paper showing that its boulders are porous and riddled with networks of cracks.

Shape matters here too, because volume needs a size and a shape. Shadows cast during a stellar occultation can supply an outline, as the guide to stellar occultations explains.

Bouncing radio waves: radar

Radar sends a radio beam at an asteroid and listens to the echo. JPL says that depending on size and distance, radar can image the surface in fine detail and measure size, shape, spin rate and whether the body has moons. In a 2021 release, JPL described radar images of asteroid 2016 AJ193 that showed ridges, hills, flat areas and possible boulders.

Radar therefore tells how an asteroid looks and moves. Mineral makeup comes from the other methods. Combined with a mass from gravity, a radar shape gives density.

Matching asteroids to meteorites

Meteorites are rocks from space that landed on Earth, and they can be tested in a lab. If an asteroid's remote measurements match a meteorite type, scientists gain a way to read the asteroid from a rock they can hold.

Vesta is the clearest case. NASA says Dawn confirmed Vesta as the parent body of the howardite, eucrite and diogenite meteorites, known together as HED, based on confident matches between lab measurements and Dawn data. The page estimates that about 6 percent of the meteorites found on Earth came from impacts that threw debris off Vesta. The meteorite groups themselves are described in the guide to types of meteorites.

Itokawa gave a second match. NASA says the dust returned from Itokawa supports the idea that S-type asteroids are the source of ordinary chondrites, the most common kind of meteorite found on Earth. Many particles appear to have cooled slowly after forming deep inside a rocky object at about 800 °C (1,470 °F). That suggests a parent body more than 20 km (12 miles) across, while Itokawa's longest dimension is only about 0.5 km (0.3 miles). Itokawa is probably a fragment of something much larger that was shattered by a collision.

Bringing material home

Sample return gives the final check. Japan's Hayabusa touched down twice on Itokawa and, NASA notes, collected a small amount of dust even though the collection mechanism failed. The capsule reached Earth on June 13, 2010.

NASA's OSIRIS-REx collected dust and pebbles from Bennu on October 20, 2020, and its capsule landed in Utah on September 24, 2023. On February 15, 2024, NASA announced that the sample weighed 121.6 grams (4.29 ounces), the largest asteroid sample ever collected in space. Findings so far include phosphates, reported in June 2024. Early studies in January 2025 found molecules key to life on Earth plus evidence of past saltwater. In December 2025, papers reported sugars essential to life, a mysterious ancient gum-like material and presolar grains, which formed before the Sun did. The mission's goal is to help determine whether asteroids that struck early Earth delivered water and other ingredients for life.

The timeline of missions to small bodies lists the other samples and when they came home. For anyone curious about the numbers behind the table above, the walkthrough of the JPL Small-Body Database shows where to find each field.

Common mistakes and myths

"Asteroids are solid rock." Several are loose rubble. Itokawa is described as a rubble pile, and Bennu, Ryugu and Mathilde have densities only about 20 to 30 percent above that of water.

"All asteroids are made of the same stuff." There are at least three broad classes, and Vesta melted and separated into layers.

"A spectrum tells you the whole composition." It reads the surface. Material from elsewhere can show up in it, as Dawn reported on Vesta.

"A returned sample is the whole story." A 121.6-gram sample comes from one place on one asteroid. Scientists still compare it with the remote data from the whole body.

"Meteorites and asteroids are unrelated." Vesta and the HED meteorites show a direct link, and Itokawa points to a link between S-types and ordinary chondrites.

Frequently asked questions

What does an asteroid spectral type mean?

It is a label for the pattern of colors an asteroid reflects. JPL's database lists types such as S for Eros, V for Vesta and B for Bennu.

Why is Bennu's density so low?

A low density means a lot of empty space inside. NASA reports that Bennu's boulders are porous and cracked, which is part of the explanation.

How much of Bennu did OSIRIS-REx bring back?

NASA announced a bulk sample mass of 121.6 grams (4.29 ounces) on February 15, 2024.

Which meteorites come from Vesta?

The HED meteorites: howardites, eucrites and diogenites. NASA says about 6 percent of meteorites found on Earth are linked to Vesta.