Closest asteroid yet flies past Earth

An asteroid about the size of a small house passed just 88,000 kilometres from the Earth by on Saturday 27 September - the closest approach of a natural object ever recorded. Geostationary communication satellites circle the Earth 42,000km from the planet's centre.

The asteroid, designated 2003 SQ222, came from inside the Earth's orbit and so was only spotted after it had whizzed by. The first sighting was on Sunday 28 by the Lowell Observatory Near-Earth Object Search program in Arizona, US.

Amateur astronomer Peter Birtwhistle of Great Shefford, Berkshire, UK, then photographed it on Monday 29. This provided data that helped Brian Marsden, of the Harvard-Smithsonian Center for Astrophysics, to calculate its orbit.

The asteroid's 1.85-year orbit is quite eccentric, indicating it cannot be a man-made object, Marsden says. He estimates the asteroid measured less than 10 metres. This is too small to have posed a danger to Earth, although it would have made a spectacular fireball had it entered the atmosphere.

House fires

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The passage came at about 2300 GMT, only 10 hours after a bright fireball streaked over the Orissa region of India. Indian villagers have found pieces of the meteorite, which reportedly cause two house fires. However, this event was not connected to the fly past of 2003 SQ222, says Marsden.

The previous record for closest approach of an asteroid - 108,000km measured from the centre of the Earth - was set in 1994 by another 10m object named 1994 XM1.

But the third-closest approach - at 120,000km - was object 2002 MN, which was about 80m in diameter. If on target, that could have exploded in the Earth's lower atmosphere and devastated a couple of thousand square kilometres on the ground.

Another small asteroid, 2003 SW130, missed the Earth by 160,000km on 19 September, making it a busy month for asteroid watchers.

Jeff Hecht