Deep impact: An artist's impression of a huge meteorite striking Earth 65 million years ago, sending the dinosaurs and many other life forms into extinction. Photo: Reuters

Add this to your worry list: orbiting somewhere near Earth are an estimated 13,000 asteroids big enough to possibly level a country and NASA has no clue as to when, where or whether they might strike.

Worse, astronomers think their sky maps might still be missing an additional 50 to 100 asteroids so massive roughly a kilometre across or larger that they could end civilisation if they hit Earth.

With those cosmic threats in mind, the Obama administration on Tuesday unveiled a "grand challenge" that would redouble efforts by NASA and challenge amateur astronomers to catalogue every asteroid near Earth that's large enough to cause significant damage. NASA also is being asked to lead a new campaign to figure out how to defend Earth from doomsday rocks.

"We want to prove that we are, in fact, smarter than the dinosaurs," said NASA deputy administrator Lori Garver, referring to the massive asteroid or comet that scientists think killed the dinosaurs about 65 million years ago.

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Though her example was dramatic, Ms Garver didn't have to reach that far back to show what a space rock can do to Earth. Just this February, an asteroid 17 metres in diameter exploded over Russia, injuring more than 1000 people. And car-sized asteroids enter and burn up in the atmosphere almost weekly.

As it stands, NASA has located about 95 per cent of the asteroids big enough to annihilate everyone on Earth and none poses any immediate danger of hitting the planet. But they are a lot less certain about where to find the smaller ones.

NASA scientists estimate that about 13,000 asteroids larger than 140 metres which have the potential to level a country remain undiscovered. And there could be millions more that are close in size to the Russian asteroid.

"We have done a very good job on the big ones. It's the smaller ones that could be a potential threat and where we have a lot of work to do," said Jason Kessler, of NASA's chief technologist office.

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NASA aims to reduce asteroid threat