By Matt Smith, CNN

updated 8:29 PM EDT, Wed May 16, 2012

This image, taken by NASA's Near Earth Asteroid Rendezvous mission in 2000, shows a close-up view of the asteroid Eros.

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(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.

Read more space and science news at CNN.com's Light Years blog

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.

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NASA estimates 4,700 'potentially hazardous' asteroids