Why are asteroids different colours?
Asteroids look different colours because they are made of different materials and have different histories. Carbon-rich asteroids are dark, rocky ones reflect more light with reddish tints, and space weathering and fresh surfaces alter the colours further.
Seen through an ordinary telescope, an asteroid is just a point of light. But when astronomers split that light into its colours, measuring how much is reflected at each wavelength, distinct patterns emerge. These reflectance spectra reveal minerals in the same way that a fingerprint reveals a person, and they are used to group asteroids into classes.
The most common are the C-types, rich in carbon compounds and water-bearing clay minerals. They are very dark, reflecting only a few per cent of the sunlight that hits them, about as dark as charcoal, and fairly neutral in colour. Ryugu and Bennu are examples, and their material resembles carbonaceous chondrite meteorites. S-types are stony bodies made largely of the silicate minerals olivine and pyroxene mixed with some metal. They are brighter and look slightly reddish, and many are linked to ordinary chondrites, as the Itokawa samples showed. The M-types and related X-types include bodies that may be partly metallic; the large asteroid Psyche, target of a NASA mission launched in 2023, is the best-known example. V-types such as Vesta show strong signs of the mineral pyroxene, matching the basaltic HED meteorites.
Colour also changes with age. Space weathering by the solar wind and tiny impacts darkens and reddens surfaces over time, while landslides or fresh craters expose brighter, bluer material. Particle size, viewing angle and temperature all affect the measurements, which is why researchers combine spectra with other evidence before linking an asteroid to a meteorite type.
The broad pattern reflects where asteroids formed. Stony types dominate the inner belt, closer to the Sun, while dark, carbon-rich types become more common farther out, hinting at the temperature gradient in the early disc, although later shuffling by the giant planets has mixed the belt.