Astronomers rue the one that got away.
On 3 July, an asteroid zipped past Earth at a distance of some 400,000 kilometres — slightly farther away than the Moon. In theory, something that close ought to be easy to study. But astronomers have struggled to map the size and shape of the space rock — and now say they know why they found it so difficult.
The rock, dubbed 2004 XP14, is one of more than 800 'near-Earth asteroids' that have been identified in orbits that come perilously close to our planet. This particular rock is unlikely to hit us, but astronomers hoped their observations would help establish how diverse such asteroids are and so better quantify the threat they pose.
But the data obtained by the team proved surprisingly hard to analyse. “The asteroid rotates slowly, so its appearance in the images, due to rotation, hardly changed at all,” says Lance Benner, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, California, who led the effort to image the asteroid.
That's unusual. “Most near-Earth asteroids are very fast rotators,” says Vishnu Reddy, a graduate student at the University of North Dakota who also observed the object.
Benner and his colleagues imaged 2004 XP14 using a 70-metre radio antenna at the Goldstone Complex in California. At 260 metres across, the asteroid was a lot smaller than earlier predictions of up to 880 metres. This, together with its rotation rate of roughly one turn every 500 hours, meant that the images the team received barely changed during recording sessions of up to 2 hours. As a result, the researchers could not build the detailed picture of the rock that they wanted.
Earth may be safe from 2004 XP14, but there are plenty more asteroids out there that might collide with us. Having identified three-quarters of the candidates 1 kilometre or more in diameter, NASA plans to widen its search to include objects as small as 150 metres across. And in July, the International Astronomical Union formed a committee to keep it up to date about asteroids that may pose a serious threat to our planet.
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Ledford, H. Asteroid fly-by eludes study. Nature 442, 855 (2006). https://doi.org/10.1038/442855a
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