It's no surprise that NASA is keeping track of all potentially hazardous objects, or PHOs, that surround our planet. If it's closer than 4.6 million miles away and larger than about 350 feet in diameter, NASA's watching it. And if a comet or asteroid's orbit comes close enough to ours that there's some potential for it to collide with our planet, NASA classifies it as a PHO. If something that size smacked Earth, it'd cause a major tsunami (if it hit water) or major regional destruction (if it hit land).
There are 1,397 known potentially hazardous asteroids (PHAs) at the moment, which you can see in this list.