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Largest asteroid to come near Earth in 22 years Maggie McKee The largest asteroid to come near the Earth in more than 20 years will make its closest approach on Tuesday, venturing as close as 1.4 times the distance to the Moon. Already, the first radar observations of the space rock reveal it may have formed from two separate asteroids that fell together and stuck. The asteroid, named 2007 TU24, was discovered by the Catalina Sky Survey in Arizona, US, on 11 October 2007. Based on its apparent brightness, astronomers estimated that it was between 150 and 600 metres wide, but it has been too far away for ground-based telescopes to resolve it and determine its actual size and shape. However, on Wednesday, it came close enough to the Earth – about 11 times the Moon's distance – for radar signals to be bounced off its surface. Five hours of observations using NASA's Goldstone radio telescope in California, US, reveal that the asteroid is about 250 metres wide.
The last time an object of about the same size was observed to approach Earth at about the same distance was in September 1985, says Don Yeomans, head of NASA's Near Earth Object programme at the Jet Propulsion Laboratory in Pasadena, California, US. "This will be the closest approach by a known asteroid of this size or larger until 2027," he adds. But similar close passes probably occur more often and simply go undetected. "There's a lot of objects zipping by that are not seen," Yeomans told New Scientist, adding that about 7000 asteroids of about the same size are expected to venture near Earth's orbit, and only 20% have been discovered so far. The Goldstone observations have not only helped pin down the asteroid's size, but also its orbit, says JPL's Steven Ostro, who led the radar observations. "We can guarantee absolutely that there's zero chance of any hazardous close approaches to Earth until 2170."
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