Worse, astronomers think their sky maps might still be missing an additional 50 to 100 asteroids so massive - roughly 0.6 mile 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.
Though her example was dramatic, Garver didn't have to reach that far back to show what a space rock can do to Earth. Just this February, an asteroid 55 feet in diameter exploded over Russia, injuring more than 1,000 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 460 feet - 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.
Still, the money for this program has been minuscule compared with NASA's proposed 2014 budget of $17.7 billion (A$18.7 billion). And even a White House decision to double the amount - from US$20 million (A$21.1 million) in 2013 to US$40 million (A$42.2 million) in its 2014 budget - isn't much by NASA standards.
That's why the administration's plan also directs NASA to partner with amateur astronomers and space activists. A NASA document released Tuesday also asks the space community to submit ideas on how to detect asteroids and deflect them.
The added budget money would be spent on wringing more observations from radar and telescopes on Earth, including the Pan-STARRS facility in Hawaii that can monitor wide swaths of the cosmos.
NASA officials also hope to use some of the money to revive an orbiting telescope known as WISE - for Wide-field Infrared Survey Explorer - that can detect as many as 100 new asteroids a year.
"Our plan is to turn it back on," said Jim Green, director of NASA's planetary-science division.
NASA put WISE into hibernation in 2011 after it had completed its primary mission of surveying the entire sky in infrared light; it found a host of new cosmic objects, including millions of black holes. And even though WISE has exhausted its supply of coolant needed to chill its sensitive telescope, Green said NASA hoped it would still be useful for asteroid detection.
"We'll start using it in two wavelength bands that don't require cooling" but still can help find nearby asteroids, he said.
Though the asteroid-detection program has no firm deadline, Garver said the aim was to find all nearby - and dangerous - space rocks by 2033, about 20 years from now.
NASA's own plan to defend the Earth from asteroids still is in its infancy. As part of its 2014 budget, the administration is proposing a mission to "lasso" a small asteroid with a space probe and drag it near the moon. Agency officials said the mission could help them practice how it might redirect a dangerous asteroid.
An open question is how Congress will react to the new initiative. Though lawmakers generally have supported asteroid-detection efforts, a US House blueprint for NASA up for debate Wednesday includes no funding for the asteroid-capture mission.
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