Eventual Inability.

That's a relief - there are many fewer 'mid-sized' Asteroids near Earth than was previously thought. There are roughly 19,500 -- not 35,000. But while the risk of impact is smaller, according to NASA, the majority of them remain to be found. Mid-sized, by the way, isn't small - it refers to asteroids in a size range between 330 and 3,300 feet wide, which could destroy a city-sized area were they to hit Earth. NASA's latest scan used the Wide-field Infrared Survey Explorer, or WISE and took two infrared scans of the entire celestial sky, scanning the entire sky twice in a series of infrared photos between January 2010 and February 2011. The scan aimed to find asteroids 'near Earth' - ie within 120 million miles.
NASA's scientists were cautious about the result, saying only that the risk to Earth could be 'somewhat less' now than they previously thought. Scientists also admit there could be 'up to a million' smaller asteroids that have not yet been detected - which could 'cause damage' if they hit Earth. What is definitely good news is that the likelihood of a 'planet-killer' - the mountain-sized asteroids in the 'large-sized' range, above 3,300ft - appears to have fallen more significantly. There are only 981 of these objects near Earth according to the latest estimates, and NASA has found 911 of them. The scenario, beloved of film directors, where a huge asteroid threatens all life on Earth now looks distinctly unlikely. All near-Earth asteroids that are as big as the one that wiped out the dinosaurs (around six miles across) are thought to have been found by NASA's scans, which use infrared to detect both 'light' and 'dark' asteroids. 'NEOWISE allowed us to take a look at a more representative slice of the near-Earth asteroid numbers and make better estimates about the whole population,' said Amy Mainzer, lead author of the new study, published in the Astrophysical Journal, and principal investigator for the NEOWISE project at NASA's Jet Propulsion Laboratory in Pasadena, Calif. WISE captured a more accurate sample of the asteroid population than previous visible-light surveys because it is difficult for visible-light telescopes to see the dim amounts of visible-light reflected by dark asteroids. 'The risk of a really large asteroid impacting the Earth before we could find and warn of it has been substantially reduced,' said Tim Spahr, the director of the Minor Planet Center at the Harvard Smithsonian Center for Astrophysics in Cambridge, Mass.