An amateur astronomer may have found another moon of the Earth. Experts say it may have only just arrived.

Much uncertainty surrounds the mysterious object, designated J002E2. 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 and reported as a passing Near-Earth Object. It was soon realised however that far from passing us it was in a 50-day orbit around the Earth.

Paul Chodas of Nasa’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.

When he detected the object Bill Yeung contacted the Minor Planet Centre in Massachusetts, the clearing house for such discoveries, which gave it the designation J002E2 and posted it on their Near-Earth Object Confirmation webpage.

Soon however, its 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.

Nasa’s 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 J002E2 is natural it will become Earth’s third natural satellite.

Earth’s second one is called Cruithne. It was discovered in 1986, and then found in 1997 to have a highly eccentric orbit, cannot be seen by the naked eye, but scientists working at Queen Mary and Westfield College in London were intrigued enough with its peregrinations to come up with mathematical models to describe its path.

The 3-mile-wide (5-km) satellite, which takes 770 years to complete a horseshoe-shaped orbit around Earth and will remain in a suspended state around Earth for at least 5,000 years. Every 385 years, it comes to its closest point to Earth, some 15 million kilometers away. Its next close approach to Earth comes in 2285.

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