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.
NASA is basing its data on its Wide-field Infrared Survey Explorer, which is an infrared space telescope. Though the total number is similar to past estimates, the survey found that twice as many asteroids as previously thought are in "low-inclination orbits," meaning their paths are similar to Earth's. "Our team was surprised," one NASA researcher said, but on the bright side, they "provide the best opportunities for the next generation of human and robotic exploration." (Like this, maybe?)