Largest asteroid to come near Earth in 22 years
23:37 24 January 2008
NewScientist.com news service
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."
Spinning rock
Moreover, the asteroid "has a very interesting shape – we see asymmetries and evidence for concavities", he told New Scientist. He says the preliminary images hint that 2007 TU24 may be a so-called contact binary – created when two asteroids orbiting each other lose enough energy to eventually "fall into each other". He says about 10% of known near-Earth asteroids have shapes that suggest they were formed by such mergers and another 10 to 15% have orbiting partners.
The radar observations also suggest the asteroid is rotating slowly – perhaps once every 10 to 30 hours, he estimates based on the initial observations. Studying rotation rates can provide clues about whether an asteroid is a solid rock or a loose collection of rubble, says Yeomans. That's because asteroids that rotate faster than about once every 2.2 hours would fly apart if they were rubble piles.
Since 2007 TU24's rate is slower than that, "it doesn't tell us anything about whether it's a monolithic rock or a rubble pile", says JPL's Lance Benner.
If a particular space rock appeared to be on a collision course with Earth, knowing its size, shape and rotation rate would also be crucial for planning ways to deflect it, says Ostro. "If we had to change its orbit, we'd have to figure out what kind of object we're dealing with," he says.
Early detection
Fortunately, 2007 TU24 will come no closer than 1.4 times the Moon's distance. But its discovery only a few months before its closest approach to Earth highlights the importance of finding potentially dangerous space rocks, astronomers say. Recent research suggests small asteroids, just a few tens of metres across, can cause significant destruction if they impact the Earth.
Currently, NASA is hoping to turn up 90% of potentially hazardous asteroids larger than 1 kilometre wide by the end of 2008. But it says it does not have the funding to comply with a directive by the US Congress to find 90% of all space rocks down to 140 metres across. "If we could do that, then 99% of the risk associated with all near-Earth objects of all sizes would be retired," says Yeomans, explaining that the rocks would be discovered early enough to deflect them.
Further radar observations of 2007 TU24 will be made with the giant Arecibo radio telescope in Puerto Rico when the rock is much closer to Earth. And for a brief time, amateur observers with telescopes 8 centimetres (3 inches) or larger should be able to see the asteroid under good observing conditions on the night of 29-30 January. Its closest approach will take place on the 29th at 0833 GMT.
23:37 24 January 2008
NewScientist.com news service
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."
Spinning rock
Moreover, the asteroid "has a very interesting shape – we see asymmetries and evidence for concavities", he told New Scientist. He says the preliminary images hint that 2007 TU24 may be a so-called contact binary – created when two asteroids orbiting each other lose enough energy to eventually "fall into each other". He says about 10% of known near-Earth asteroids have shapes that suggest they were formed by such mergers and another 10 to 15% have orbiting partners.
The radar observations also suggest the asteroid is rotating slowly – perhaps once every 10 to 30 hours, he estimates based on the initial observations. Studying rotation rates can provide clues about whether an asteroid is a solid rock or a loose collection of rubble, says Yeomans. That's because asteroids that rotate faster than about once every 2.2 hours would fly apart if they were rubble piles.
Since 2007 TU24's rate is slower than that, "it doesn't tell us anything about whether it's a monolithic rock or a rubble pile", says JPL's Lance Benner.
If a particular space rock appeared to be on a collision course with Earth, knowing its size, shape and rotation rate would also be crucial for planning ways to deflect it, says Ostro. "If we had to change its orbit, we'd have to figure out what kind of object we're dealing with," he says.
Early detection
Fortunately, 2007 TU24 will come no closer than 1.4 times the Moon's distance. But its discovery only a few months before its closest approach to Earth highlights the importance of finding potentially dangerous space rocks, astronomers say. Recent research suggests small asteroids, just a few tens of metres across, can cause significant destruction if they impact the Earth.
Currently, NASA is hoping to turn up 90% of potentially hazardous asteroids larger than 1 kilometre wide by the end of 2008. But it says it does not have the funding to comply with a directive by the US Congress to find 90% of all space rocks down to 140 metres across. "If we could do that, then 99% of the risk associated with all near-Earth objects of all sizes would be retired," says Yeomans, explaining that the rocks would be discovered early enough to deflect them.
Further radar observations of 2007 TU24 will be made with the giant Arecibo radio telescope in Puerto Rico when the rock is much closer to Earth. And for a brief time, amateur observers with telescopes 8 centimetres (3 inches) or larger should be able to see the asteroid under good observing conditions on the night of 29-30 January. Its closest approach will take place on the 29th at 0833 GMT.
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