This image, taken from NASA/JPL’s Photojournal, shows all 1,397 of those PHAs as represented by their orbits that...

This image, taken from NASA/JPL’s Photojournal, shows all 1,397 of those PHAs as represented by their orbits that would cause “major devastation" if they hit our planet. Kind of amazing that we haven’t been hit by one, isn’t it?

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).

Source: http://photojournal.jpl.nasa.gov/new