amy mainzer is a nasa scientist who focuses on detecting asteroids. >> mainzer: so we got to move the we're going to start to follow the asteroid as it tracks across the sky. >> cooper: this telescope at the table mountain observatory in california is one of dozens all over the world that are used to track and study near earth objects. mainzer told us they're often very hard to find. >> mainzer: some of these asteroids are really, really dark, darker even than coal in some cases. kind of like the soot at the bottom of a barbecue grill. >> cooper: so you're looking for something that's darker than coal against a black sky. >> mainzer: exactly. and now you see the problem. >> cooper: another problem is that ground-based telescopes can't see objects coming from the direction of the sun because they're in the daytime sky, like the asteroid that hit russia. astronomers find asteroids by taking repeated pictures of the night sky and looking for things that change position. professionals and amateurs all over the world work together, sharing information. once paul chodas and his nasa colleag