Perhaps you didn't know, but NASA has been taking a census of asteroids in our solar system, using the Wide-field Infrared Survey Explorer (WISE) spacecraft that launched in 2000. On Friday, it announced the result of that tally: It's accounted for 90 percent of the "giant, potentially Earth-threatening asteroids, including ones as big as the one thought to have killed the dinosaurs eons ago," the Associated Press reported, and none of those are on a collision course with the Earth. So that's a plus. We now know of 911 of the 981 thought to exist. And the estimate of potentially city-destroying asteroids actually went down, from 35,000 to 19,500. As for the remaining 10 percent, NASA wasn't so comforting on that: "Fewer does not mean none," Amy Mainzer, NASA's Jet Propulsion Laboratory, told the AP. "There are still tens of thousands out there that are left to find." That remainder still does nag, though.