moons

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Most asteroids with moonlets started off as solitary bodies that split in two while sunbathing, new computer simulations suggest.
Once thought to be rare, dozens of binary asteroid systems – kilometre-sized rocks orbited by small moonlets – have been found in the main belt between Mars and Jupiter since the first asteroid pair was spotted by the Galileo spacecraft in 1993. And 15% of all near-Earth asteroids, which cross Earth's orbit, boast satellites.
Various theories have been proposed to explain how they formed. One suggests the pairs were created by collisions between older asteroids, but because of the huge distances between objects in space, such impacts are very rare.
Collisions may account for some large pairs of asteroids in the main belt, where asteroids orbit for billions of years, but the smaller near-Earth asteroids are much more likely to crash into the Sun or one of the inner planets before colliding with each other.