Wednesday, October 6, 1999 Published at 18:00 GMT 19:00 UK
Asteroid moon secrets revealed
Faint images of the moon were captured
By BBC News Online Science Editor Dr David Whitehouse
Earlier this year BBC News Online exclusively revealed that astronomers had discovered a moon orbiting an asteroid, only the second time one had been seen.
Now astronomers have given further details about their discovery in the journal Nature.
Pictures taken by the Canada-France-Hawaii Telescope of the asteroid Eugenia show it to have a tiny companion.
The only other asteroid known to have a moon circling it is asteroid Ida. In 1993, the Galileo spacecraft, then en route to Jupiter, discovered a tiny worldlet orbiting it. Astronomers called it Dactyl.
Previous attempts to look for satellites of asteroids have proved fruitless. Ground-based telescopes and even the Hubble Space Telescope have not had keen enough vision to see any.
Astronomers have likened this discovery to taking a photo of a candle 400 km (250 miles) away and seeing a firefly circling within two metres of the flame.
The new discovery was accomplished because of the technique of "adaptive optics." This involves distorting a telescope's main mirror in just the right way to correct for distortions introduced into the image by the Earth's turbulent atmosphere.
The object that orbits Eugenia has not yet been named. The little that is known about it is tantalising. Eugenia is estimated to be about 215 km (134 miles) in size, its companion only 15 km (nine miles).
The biggest puzzle concerning the discovery is that both Eugenia and its companion seem too light.
The presence of the tiny moon enables estimates about the mass and density of the two bodies to be determined.
Eugenia has a density only about 20% greater than that of water making it far lighter than rock. Rock has a density triple that of water.
"A picture is emerging that some asteroids are real lightweights," said Dr William Merline leader of the team of scientists.
"Either these objects are highly porous piles of rock, or they are mostly water ice," said Dr Clark Chapman, another member of the team.
Astronomers believe that Eugenia's companion is a chip of the old block.
It is almost certain that the satellite was formed by a collision," said Merline. "As we know from the formation of our own Moon and the craters on planetary surfaces, collisions played a large role in the formation of the solar system."
"Satellites of asteroids give us a window into these collisions and help us understand how and why our solar system looks like it does," he added.