How do we know what asteroids are made out of?

Asteroids are some of the oldest objects in the Solar System: leftovers from the chaotic time when planets were assembling from dust and rock. They’re time capsules, preserving clues about what the early Solar System was like, and, ultimately, what the building blocks of planets are.

Knowing what an asteroid is made of also matters for very practical reasons. If an asteroid were ever on a collision course with Earth, its composition would affect how dangerous it is, how it breaks up in the atmosphere, and how we might successfully nudge it away. This area of research is called planetary defence.

Understanding the make-up of asteroids also matters for the future of exploration: some asteroids may contain metals, minerals, and even water – potentially useful resources. But how can we tell what asteroids are made of when most of them are millions of kilometres away?

Asteroid ‘fingerprints’

One of the most powerful techniques is spectroscopy, the science of splitting light into components and measuring what wavelengths are absorbed or reflected. Minerals interact with light in characteristic ways, leaving subtle dips and slopes in a spectrum. In effect, an asteroid’s surface leaves behind a chemical fingerprint in sunlight.

These fingerprints let us place asteroids into broad families. One of the most common groups near Earth is the S-complex, a class of relatively reflective asteroids often associated with silicate minerals such as olivine and pyroxene. For decades, researchers suspected that S-complex asteroids were linked to a particular category of meteorites that frequently fall to Earth: the ordinary chondrites.

A phenomenal example of how well this can work came from Japan’s sample-return mission Hayabusa, which visited the near-Earth asteroid (25143) Itokawa. Hayabusa reached the asteroid in September 2005. From its reflected light, Itokawa was inferred to be an S-complex asteroid, and spectroscopic comparisons suggested it should resemble ordinary chondrites, particularly the LL subgroup.

Hayabusa returned tiny grains of asteroid regolith to Earth, and laboratory analyses showed the mineralogy and mineral chemistry were identical to LL chondrites. In other words, the remote spectral prediction matched the physical reality of the samples.

Then Dart arrived — and raised the stakes. In September 2022, Nasa deliberately slammed a spacecraft into the small moonlet Dimorphos, which orbits the larger asteroid Didymos, in the Dart (Double Asteroid Redirection Test) mission.

The goal wasn’t to destroy the asteroid; it was to test whether a kinetic impact could measurably change its orbit. Didymos has been observed extensively with spectroscopy and is classified as an S-complex and inferred to have a LL chondrite composition.

But is there a possibility we could we be misreading the make up of some space rocks? A 2026 paper argues that another meteorite group, brachinites, can show spectral properties that overlap with S-complex asteroids. One sample (NWA 14635) even shows spectroscopic band parameters similar to Didymos.

This is a big deal, because it means there may not be a neat one-to-one mapping between asteroid types and meteorite types. Asteroids are the left over building blocks of planets in our Solar System, often termed “space rocks”. Meteorites are space rocks that have survived the journey through a planet’s atmosphere, reaching the surface.

For planetary defence, this distinction matters. A chondritic “rubble pile”, composed of loosely bound rocks, and a more strongly processed, coherent igneous body (which would cover the brachinites) might respond differently when hit.

An ordinary chondrite-like surface might absorb energy like a “cosmic beanbag”, while a more magmatic surface might behave more like brittle rock. If we want to predict what happens when we try to deflect an asteroid, we need to know what its surface resembles.

This is exactly why the European Space Agency’s Hera mission is so exciting. Hera isn’t repeating Dart; it’s doing the follow-up crime scene investigation. Hera launched in October 2024 and is now on its way to the Didymos system, with arrival planned for late 2026. Once there, it will map both asteroids in detail.

Hera also comes with two small satellites known as cubesats: Juventas and Milani. Milani will help study the surface composition. This will give insights into not just what Dimorphos looks like from a distance, but what it’s made of, how it’s structured, and how it responded to Dart’s impact.

In the context of the new brachinite result, Hera’s role becomes even more important. If Didymos and Dimorphos turn out to be less “ordinary chondrite-like” than we assumed, or if their surfaces disguise a more complex origin, Hera is the mission that can test that assumption directly. It’s a reminder that asteroids still have the power to surprise us.

Ben Rider-Stokes does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.