From Nature magazine

A small asteroid called 2012 KT₄₂ came within three Earth radii of striking the planet on 29 May, but slipped past. The event was the sixth-closest encounter of any recorded asteroid.

In a video posted online on 19 June by researchers using NASA's Infrared Telescope Facility (IRTF) in Hawaii, the bright asteroid appears fixed, while background stars zip past (in fact, the asteroid is speeding along at 17 kilometres per second). “You get the view of riding along with it,” says Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology in Cambridge, who led the observations. At its closest, the asteroid was around 19,000 kilometres from Earth — a distance between the orbit of the International Space Station (about 1 Earth radius) and that of a geosynchronous satellite (about 6 Earth radii).

Hours after the object was discovered by a small telescope on Mount Lemmon near Tucson, Arizona, Binzel was able to obtain a few hours of time on the IRTF. The resulting in-depth study was innovative for such a small object.

By determining 2012 KT₄₂'s composition and reflectivity, Binzel was able to use the brightness of the asteroid to estimate its size: about 7 metres across. He says several objects this size cross Earth’s path every year.

2012 KT₄₂ is now continuing on its 1.5-year elliptical orbit of the Sun. Even if it had struck Earth, Binzel says, it probably would have burned up in the atmosphere. Binzel wants to discover an object that is not big enough to present a hazard to Earth, but is large enough to be spotted in space and later found on the ground as a meteorite, as was the asteroid 2008 TC₃, which came to Earth in Sudan in October 2008 (see the 'The rock that fell to Earth'). “I want them to be just the right size to deliver some fresh samples,” he says.

This article is reproduced with permission from the magazine Nature. The article was first published on June 21, 2012.

4 Comments

1. 1. Daniel35 04:58 PM 6/22/12

   If we're going to be comparing the risk of such events,shouldn't we be talking about size up front, as well as distance?
   
   What kind of near miss would it take to make it come back for another pass later? Would it have to go through the atmosphere to lose some energy?
   
   Is anyone on record as being injured by anything falling from space, except people in what's falling?
   
   How big was 2008 TC3 when in space?
   
   "He says several objects this size cross Earth’s path every year."
   
   But maybe 180 degrees away?

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2. 2. Daniel35 07:18 PM 6/22/12

   Hey SciAM. what's the point of telling me there's a new post when it always turns out to be mine? I'd be much more interested in the one AFTER mine.

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3. 3. cubeboy 04:20 PM 6/24/12

   There's a factual error in this story: it says the asteroid passed earth "between the orbit of the International Space Station (about 1 Earth radius) and that of a geosynchronous satellite (about 6 Earth radii)".
   
   But the earth's radius is about 4,000 miles, while the International Space Station has an average distance from earth of about 250 miles or 400 kilometers -- one-sixteenth of an earth radius.

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4. 4. edprochak in reply to cubeboy 04:25 PM 6/25/12

   Do the math. Earth radius =4000 miles
   ISS at 250 miles above surface OR
   4250 miles from center
   so the ISS is 1.06 earth radii.
   
   You dinged them for 6% roundoff?
   They said "about 1 Earth radius"

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