The asteroid, 2003 YN17, "is probably a chunk of debris" from an impact between a larger space rock and the surface of the moon, the British weekly says.

2003 YN17's orbital plane is roughly the same as the earth's, but its unusual path, compounded by a corkscrew-like track, means that sometimes it is ahead of us and sometimes it is behind.

"Since 1996, its path has taken it round the earth, making it a quasi-satellite. This phase will last until 2006," the report says.

The finders are a team led by Paul Chodas, an asteroid specialist at NASA's famed Jet Propulsion Laboratory in California.

Two other "quasi-moons" -- temporary fellow-travellers that loop around the earth for while as they girdle the sun -- have been spotted in recent years: Cluithne and asteroid 2002 AA29.

SPACE.WIRE