By Dr David Whitehouse BBC News Online science editor

Much uncertainty surrounds the mysterious object, designated J002E3. It could be a passing chunk of rock captured by the Earth's gravity, or it could be a discarded rocket casing coming back to our region of space.

It was discovered by Bill Yeung, from his observatory in Arizona, US, and reported as a passing Near-Earth Object.

It was soon realised, however, that far from passing us, it was in fact in a 50-day orbit around the Earth.

Paul Chodas, of the American space agency's Jet Propulsion Laboratory in California, says it must have just arrived or it would have been easily detected long ago.

Calculations suggest it may have been captured earlier this year.

Moon or junk?

When he detected the object, Bill Yeung contacted the Minor Planet Center in Massachusetts, the clearing house for such discoveries, which gave it the designation J002E3 and posted it on their Near-Earth Object Confirmation webpage.

Soon, however, the object's motion suggested it was in an orbit around the Earth. Its movements had all the hallmarks of being a spent rocket casing or other piece of space junk.

But experts are not completely sure what exactly the object is.

Observations made by Tony Beresford in Australia indicate that the object's position does not match any known piece of space junk.

Observations made in Europe have failed to see any variations in brightness that might be expected from a slowly spinning metallic object.

Paul Chodas says the object must have arrived quite recently or else it would have been easily detected by any of several automated sky surveys that astronomers are conducting.

Its trajectory suggests that it may have been captured in April or May of this year, but there is still some uncertainty about this.

If it is determined that J002E3 is natural it will become Earth's third natural satellite.

Earth's second one is called Cruithne. It was discovered in 1986 and it takes a convoluted horseshoe path around our planet as it is tossed about by the Earth's and the Moon's gravity.