Asteroid to come near Earth in 22 years On Tuesday January 29th 2008
Asteroid to come near Earth in 22 years On Tuesday January 29th 2008.
Special Report
Since it formed over 4.5 billion years ago, Earth has been hit many times by asteroids and comets whose orbits bring them into the inner solar system. These objects, collectively known as Near Earth Objects or NEOs, still pose a danger to Earth today. Depending on the size of the impacting object, such a collision can cause massive damage on local to global scales. There is no doubt that sometime in the future Earth will suffer another cosmic impact; the only question is “when?”. There is strong scientific evidence that cosmic collisions have played a major role in the mass extinctions documented in Earth’s fossil record. That such cosmic collisions can still occur today was demonstrated graphically in 1994 when Comet Shoemaker-Levy 9 broke apart and 21 fragments, some as large as 2 km in diameter, crashed into the atmosphere of Jupiter. If these fragments had hit Earth instead, we would have suffered global catastrophes of the kind that inspire science fiction movies. The dangers posed by these intruders in the inner solar system are now the subject of serious scientific investigation. Observations by astronomers tracking near-Earth asteroids have raised a new object to the top of the Earth-threat list. 
The asteroid could strike the Earth in 2102. However, Don Yeomans, manager of NASA’s Near Earth Object Program at the Jet Propulsion Laboratory in Pasadena, California, US, told New Scientist: “The most likely situation, by far, is that additional observations will bring it back down to a zero.”
He adds: “We’re more likely to be hit between now and then by an object that we don’t know about.”
On 23 February, new observations allowed researchers to more accurately calculate the orbit of the asteroid, named 2004 VD17, which was originally detected by the Massachusetts Institute of Technology’s LINEAR project. Since the improvement did not rule out a potential collision with the Earth on 4 May 2102, they increased the asteroid’s rating to level 2 on the Torino Scale, a relatively rare event. Since 1998, NASA has had a US Congressional mandate to locate 90% of all NEOs of 1 kilometer or larger by 2008. Yeomans says that 830 out of a predicted 1100 have been found so far, along with thousands of smaller objects. 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 meters 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 meters 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 gyan.com, 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 gyan.com 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 meters 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 kilometer 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 meters 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 centimeters (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.
Gyan.com
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