And twice as many as previously believed are in orbits similar to Earth's
(NEWSER) - NASA thinks it's got a pretty solid estimate on the number of gigantic asteroids that could plausibly cause regional or global catastrophe by smashing into Earth: 4,700, give or take 1,500. Included in that number are all the rocks that exist, or which they suspect exist, that are larger than 330 feet across, and have orbits that pass within 5 million miles of Earth, Space.com reports. Only about 30% of these asteroids have actually been found. More»