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Asteroid Shaves Past
Earth's Atmosphere

By Jeff Hecht
NewScientist.com
8-23-4
 
The closest observed asteroid yet to skim past the Earth without hitting the atmosphere, was reported by astronomers on Sunday.
 
The previously unknown object, spanning five to 10 metres across, has been named 2004 FU162. It streaked across the sky just 6500 kilometres - roughly the radius of the Earth - above the ground on 31 March, although details have only now emerged.
 
The MIT Lincoln Laboratory's asteroid-hunting LINEAR telescope in Socorro, New Mexico,US, observed the new object four times over a 44-minute period, several hours before its closest approach in March.
 
Lincoln astronomers, who have discovered over 40,000 asteroids and comets since 1980, quickly recognised the object came exceptionally close, and posted their findings for confirmation on a web page run by the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics.
 
However, by the time it was posted the object had moved into the daytime sky, so follow-up observations were impossible and the listing was quickly removed. A search for prior observations yielded no results.
 
Dissipated harmlessly
 
Despite having only four positions for the object, Steven Chesley of NASA's Jet Propulsion Laboratory was able to calculate its orbit because it was moving rapidly across the sky.
 
He also calculated that the encounter with the Earth shifted the asteroid's orbit closer towards the Sun. Previously orbiting the Sun once a year in an orbit that ranged as far inside the Earth's orbit as outside, 2004 FU162 now has a nine-month orbit centred closer to Venus than the Earth. The Minor Planet Center published Chesleyís results on Sunday in its electronic circular.
 
"This was an extraordinarily close encounter and so the orbital change was quite extraordinary. 2004 FU162 was deflected by about 20 degrees because of the Earth's gravity. I've never seen anything like that before," Chesley told New Scientist.
 
The previous record for the closest asteroid approach to Earth was set on 18 March by an object called 2004 FH which missed the Earth by about 40,000 kilometres.
 
That was a much larger object, around 30 metres in diameter - big enough to produce a one-megaton explosion in the atmosphere. Although it was likely to have exploded so high that the energy would have dissipated harmlessly. The smaller 2004 FU162 would have burned up as a fireball ending with a smaller explosion, had it ventured into the Earth's atmosphere.
 
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http://www.newscientist.com/news/news.jsp?id=ns99996307




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