A MYSTERY object orbiting the Earth caused serious confusion this week. Staff at the Minor Planet Center in Massachusetts roped in amateur astronomers around the globe to help them identify it, while several national newspapers decided the object was a 鈥渟econd moon鈥�.
Now the mystery is solved. The object, code-named J002E3, is almost certainly a discarded rocket from the Apollo 12 Moon mission.
Amateur astronomer Bill Yeung first spotted what he thought was an asteroid through his 0.45-metre telescope near El Centro, California, on the night of 3 September. He alerted scientists at the Minor Planet Center, who worked out that the object was actually orbiting the Earth, not the Sun. That suggested it was space junk, since the timing would have to be exactly right for an object to be captured from interplanetary space. Any asteroid coming our way would generally either hit us, or miss us completely.
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But more detailed simulations of the object鈥檚 orbit by Paul Chodas and colleagues at the Jet Propulsion Laboratory in Pasadena, California, showed that J002E3 had indeed been been captured from a solar orbit. It had dropped into that orbit from the Earth-Moon system in late 1969, the date of Apollo 12. This makes it almost certain it was one of that mission鈥檚 discarded rockets.
But the observation is still significant, as it is the first time astronomers have seen any object captured by Earth鈥檚 gravity. It is also the first man-made object known to have orbited the Sun. This confirms the long-discussed theory that space junk from Earth doesn鈥檛 just orbit the Earth, but can also get strung out along the Earth鈥檚 orbit round the Sun, and recaptured later.
But why was this relatively bright object found by an amateur astronomer when several large optical surveys already scan the skies for asteroids that might hit Earth? In fact, says Marsden, the LINEAR programme for finding near-Earth objects may have spotted the rocket earlier. At LINEAR鈥檚 telescopes at the US Air Force White Sands Missile Range in Socorro, New Mexico, operators delete known military satellites and other fast-moving possible satellites from their images before making them public.
The confusion was compounded because LINEAR does not follow up and identify its own discoveries. To plot a reliable orbit for an asteroid or space junk, astronomers need several sightings, preferably on more than one night. To do this, Marsden and colleagues at the Minor Planet Center routinely rely on amateur astronomers to follow up their sightings.
