"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 is in a 50 day orbit, and if it is natural the estimated size is a metre 50 metres diameter.

If it proves to be something not man-made, it will be our third moon - discovered in 1986 was Cruithne.

UPDATE: It is space junk after all.