NASA estimates 4,700 'potentially hazardous' asteroids
(CNN) -- About 4,700 asteroids are close enough and big enough to pose a risk to Earth, NASA estimated Wednesday after studying data beamed back from an orbiting telescope.
The figure -- give or take 1,500 -- is how many space rocks bigger than 100 meters (330 feet) across are believed to come within 5 million miles (8 million km) of Earth, or about 20 times farther away than the moon.
"It's not something that people should panic about," said Amy Mainzer, an astronomer at NASA's Jet Propulsion Laboratory in California. "However, we are paying attention to the issue."
NASA defines a potentially hazardous asteroid as one large enough to survive the intense heat generated by entry into the atmosphere and cause damage on a regional scale or worse. The figure released Wednesday is lower than a previous rough estimate had projected, but more are now thought to be in orbits inclined like Earth's, making them more likely to cross its path.
Asteroid found that dates back to early solar system
Mainzer said asteroids in orbits pitched at a similar angle offer not only a hazard, but also an opportunity. They would be easier for spacecraft to reach.
"They're a population of interest, and we want to keep an eye on them," she said.
NASA says a 40-meter asteroid would strike the Earth with an impact comparable to a 3-megaton nuclear bomb. A 2-km asteroid striking Earth "would produce severe environmental damage on a global scale," the space agency estimates, but an impact of that magnitude isn't likely to occur more than twice per million years.
The estimate comes from infrared scans of the cosmos by the 16-inch WISE telescope, which was launched in December 2009. The instruments aboard the satellite allowed scientists to spot close-in asteroids by picking up the heat they emit, Mainzer said.
"It allows us to find the very dark asteroids, the ones that are more like a piece of coal and than shiny pavement," she said. "We can also tell the difference between an asteroid that's very large and very dark and a small one that's very shiny."
Mainzer said between 20% and 30% of the estimated 4,700 potentially hazardous objects have been discovered so far.
The asteroid, DA14, discovered by astronomers at LaSagra Observatory in Spain, is estimated to come near enough to Earth on February 15, 2013, that it could disrupt geosynchronous satellites.
While Nasa have said the chance of the asteroid hitting Earth is 0.031%, if it did it would hit with the force of a 2.4 megaton explosion, similar to the mysterious Tunguska event of 1908 which levelled hundreds of square miles of Siberian forest, the Daily Mail reported.
The asteroid's exact orbital path is being determined by Nasa and astronomers are erring on the side of caution in case it does come in contact with a satellite.
"That's very unlikely, but we can't rule it out," said Paul Chodas, a planetary astronomer at Nasa's Jet Propulsion Laboratory (JPL) in Pasadena California.
While the asteroid is currently a 'fuzzy little blob' , as seen through telescopes, 2012 DA14 may eventually come to pass 33,796km away from the Earth putting synchronous satellites in the firing line, the paper said. "The orbit for 2012 DA14 is currently very Earth-like , which means it will be very close to Earth on a regular basis," said Chodas.
In the preceding months to February , Nasa will try to form a fuller picture of where and how close the satellite will get.
"We don't know exactly where it is, and that uncertainty maps through to an uncertainty in the orbit and predictions," said Steven Chesley, who also works at JPL.
The detonation of the 140,000 tonne rock would not end civilization , but would potentially cause massive loss of life if it hit a populated centre.