Can asteroids have moons?
Yes. Many asteroids have moons. The first was Dactyl, found orbiting Ida in 1993, and DART's target Dimorphos is the moon of Didymos.
Asteroids can have their own small moons, and astronomers have now found hundreds of such systems.
First discovery. In 1993, NASA's Galileo spacecraft, on its way to Jupiter, flew past the asteroid 243 Ida and discovered a tiny moon, about 1.4 kilometres across, later named Dactyl. It was the first confirmed asteroid moon.
How they are found. Many asteroid moons are detected from changes in brightness as one body passes in front of the other, using telescopes. Others are seen in radar images or with large telescopes using adaptive optics. Spacecraft flybys sometimes reveal unexpected moons. In 2023, NASA's Lucy mission found that the small asteroid Dinkinesh has a moon, itself a contact binary of two lobes.
How common? About one in six near-Earth asteroids larger than a few hundred metres is thought to be binary. Some asteroids have more than one moon: 87 Sylvia has two, and 130 Elektra has three.
How they form. Some moons form when a rubble-pile asteroid spins faster, gradually sped up by sunlight through the YORP effect, until loose material is thrown off from its equator and gathers into a moon. Others form from debris after collisions.
Why they matter. A moon's orbit reveals the mass of the system, allowing scientists to calculate density. Low densities show many asteroids are rubble piles.
DART. NASA's DART mission crashed into Dimorphos, the moon of Didymos, in 2022, changing its orbit around Didymos. Studying moons like this helps planetary defence.