Only the second known satellite of an asteroid has been discovered by astronomers.

A close up view of the asteroid Gaspra

The asteroids are the tens of thousands of chunks of rock that circle the Sun mainly between the orbits of Mars and Jupiter.

They could be the wreckage of numerous worlds that broke up due to collisions during the Solar System's early years.

The largest asteroid is Ceres, with a diameter of over a thousand kilometres. In total, the mass of all the asteroids would be less than that of our moon.

Some asteroids have been seen from close-up by passing probes such as the Galileo and NEAR spacecraft. One of them, asteroid Ida, was seen to have a tiny moon called Dactyl.

Large asteroid

Dactyl in close-up

Now scientists at the Southwest Research Institute in Colorado have reported the discovery of another moon orbiting an asteroid.

What is remarkable is that it was not discovered by a spacecraft flying past the asteroid, but by a ground-based telescope.

With a diameter of 210 km, Eugenia is one of the largest asteroids. It was first seen in 1857. Detailed observations of this rock suggest that it rotates every 5.7 hours.

Using a large telescope on Mauna Kea in Hawaii, the astronomers noticed a small spec of light close to Eugenia. It was about a hundred times fainter than the asteroid.

Observations indicate that the tiny companion is in a near-circular orbit around the asteroid with a period of about 4.7 days. Its orbit is inclined to the line-of-sight by about 45 degrees.