ESA Science & Technology05-Jul-2005 16:48:49

Asteroid Encounters

Asteroid Moons

A few asteroids are known to have satellites which move around them in close orbits. The Galileo spacecraft was the first to discover one of these moons when it flew past the asteroid 243 Ida on 28 August 1993. By chance, Galileo's pictures revealed a tiny object, now called Dactyl, which orbits within 100 km of its much larger neighbour.

In 1999, an international team of astronomers announced the discovery of a moon orbiting the large main belt asteroid 45 Eugenia. The pictures, taken with the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, were the first of an asteroidal satellite taken from Earth.

The observations could only be accomplished because of a new technique, called adaptive optics, that reduces the blurring caused by the Earth's atmosphere. Previous attempts to photograph such satellites, using both ground-based telescopes and the Hubble Space Telescope, were unsuccessful.

A surprising result of this discovery is the very low density of Eugenia - only about 20 per cent denser than water. Researchers estimate that the diameter of the satellite is about 13 km, compared with Eugenia's diameter of 215 km. The satellite has a circular orbit about 1190 km away from Eugenia. Each orbit lasts about five days.

In October 2000, the companion was permanently designated (45) Eugenia I (Petit-Prince). Since then, many more asteroids have been found to have companions.

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