NASA continues to get a better handle on the asteroids buzzing around in space saying today that there are roughly 4,700 potentially hazardous asteroids, or as NASA calls them PHAs. NASA says these PHAs are a subset of a larger group of near-Earth asteroids but have the closest orbits to Earth's - passing within five million miles (or about eight million kilometers) and are big enough to survive passing through Earth's atmosphere and cause damage on a regional, or greater, scale.

NASA points out too that ''potential'' to make close Earth approaches does not mean a PHA will impact the Earth. It only means there is a possibility for such a threat.

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The new numbers come from asteroid observations made by NASA's Wide-field Infrared Survey Explorer, (WISE) satellite which looked at the objects that orbit within 120 million miles of the of the sun into Earth's orbital vicinity, NASA said. WISE scanned the celestial sky twice in infrared light between January 2010 and February 2011, continuously snapping pictures of everything from distant galaxies to near-Earth asteroids and comets. It has since entered hibernation mode, NASA stated. The asteroid-hunting portion of the WISE mission called NEOWISE has seem more than 100 thousand asteroids in the main belt between Mars and Jupiter, in addition to at least 585 near Earth, NASA noted.

Specifically NASA said NEOWISE sampled 107 PHAs to make predictions about the entire population as a whole. Findings indicate there are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger than 330 feet (about 100 meters). So far, an estimated 20 to 30% of these objects have been found, NASA stated. Previous estimates of PHAs predicted similar numbers, they were rough approximations, NASA said.

"The NEOWISE analysis shows us we've made a good start at finding those objects that truly represent an impact hazard to Earth," said Lindley Johnson, program executive for the Near-Earth Object Observation Program at NASA Headquarters in Washington. "But we've many more to find, and it will take a concerted effort during the next couple of decades to find all of them that could do serious damage or be a mission destination in the future."

Asteroids have been in the news a lot lately. It has been widely reported that NASA could announce this month a manned project to land on an asteroid in the future. And in April Google executives Larry Page and Eric Schmidt and filmmaker James Cameron said they would bankroll a venture to survey and eventually extract precious metals and rare minerals from asteroids that orbit near Earth. Planetary Resources, based in Bellevue, Wash., initially will focus on developing and selling extremely low-cost robotic spacecraft for surveying missions.

And of course Doomsday 2012 scenarios have abounded in the news for a long time. NASA has spent some time shooting these theories down - including one of a world ending asteroid. "The Earth has always been subject to impacts by comets and asteroids, although big hits are very rare. The last big impact was 65 million years ago, and that led to the extinction of the dinosaurs. We have already determined that there are no threatening asteroids as large as the one that killed the dinosaurs. For any claims of disaster or dramatic changes in 2012, where is the science? Where is the evidence? There is none, and for all the fictional assertions, whether they are made in books, movies, documentaries or over the Internet, we cannot change that simple fact," NASA stated.

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