In 1993, the Galileo spacecraft discovered a tiny moon (Dactyl) orbiting the asteroid Ida. Astronomers have found many asteroids with a satellite or moon orbiting it. Moons orbit near-Earth asteroids, asteroid belt or main-belt asteroids, or Kuiper Belt objects.

Franck Marchis of the University of California at Berkeley used the European Southern Observatory’s Very Large Telescope and found a large moon orbiting the asteroid 87 Sylvia. Sylvia is a main-belt asteroid about 175 miles wide. Three colleagues found a smaller moon orbiting much closer to Sylvia. It’s the first asteroid discovered to have two moons.

Sylvia was named for Rhea Sylvia (in Mythology, mother of the founders of Rome), so the moons were named after the two founders of Rome. Romulus, the larger moon, is about 11 miles wide. It orbits Sylvia every 87.6 hours at a distance of 843 miles. Remus, the smaller moon, is just over 4 miles wide and orbits Sylvia in 33 hours at a distance of 44 miles.

The discovery team believes the moons were formed after a collision that broke the asteroid into many large fragments. Most of the chunks gravitated together and reformed the asteroid. The moons were formed from fragments too far away to rejoin the asteroid. The more distant fragments joined or accreted, forming the two moons.