The Galileo spacecraft captured this false-colour image of the asteroid 951 Gaspra. (NASA)

NASA is charged with seeking out nearly all the asteroids that threaten Earth but does not have the money to do the job, a U.S. government report says.

That is because even though the U.S. Congress assigned the space agency this mission four years ago, it never gave NASA money to build the necessary telescopes, the new National Academy of Sciences report says. Specifically, NASA has been ordered to spot 90 per cent of the potentially deadly rocks hurtling through space by 2020.

Even so, NASA says it has completed about one-third of its assignment with its current telescope system.

NASA estimates there are about 20,000 asteroids and comets in our solar system that are potential threats to Earth. They are larger than 140 metres in diameter — slightly smaller than a sports stadium. So far, scientists know where about 6,000 of these objects are.

Rocks between 140 and 1,000 metres in diameter can devastate an entire region but not the entire globe, said Lindley Johnson, NASA's manager of the near-Earth objects program. Objects bigger than that are even more threatening.

Just last month astronomers were surprised when an object of unknown size and origin bashed into Jupiter and created an Earth-sized bruise that is still spreading. Jupiter gets slammed more often than Earth because of its immense gravity, enormous size and location.

Program a 'lame duck', space observer says

Disaster movies like Armageddon and near misses in previous years might have scared people and alerted them to a serious issue. But when it comes to doing something about monitoring the threat, the National Academy of Sciences concludes "there has been relatively little effort by the U.S. government."

And the U.S. government is practically the only government doing anything at all, the report finds.

"It shows we have a problem we're not addressing," said Louis Friedman, executive director of the Planetary Society, an advocacy group.

NASA calculated that to spot the asteroids as required by law would cost about $800 million US between now and 2020, either with a new ground-based telescope or a space observation system, Johnson said. If NASA got only $300 million US it could find most asteroids bigger than 300 metres across, he said.

But so far NASA has gotten neither sum.

It may never get the money, said John Logsdon, a space policy professor at George Washington University in Washington, D.C.

"The program is a little bit of a lame duck," Logsdon said. There is not a big enough group pushing for the money, he said.

Canadian satellite to join search

At the moment, NASA has identified about five near-Earth objects that look big enough to cause serious damage and that pose a one-in-a-million risk of hitting our planet, Johnson said. That number changes from time to time, usually with new asteroids added and old ones removed as more information is gathered on their orbits.

The space rocks astronomers are keeping the closest eye on are a 130-metre diameter rock that has a 1-in-3,000 chance of hitting Earth in 2048 and a much-talked-about asteroid, Apophis, which is twice that size and has a one-in-43,000 chance of hitting in 2036, 2037 or 2069.

Last month, NASA started a new website for the public to learn about threatening near-Earth objects.

While NASA's near-Earth object program tracks the paths of near-Earth asteroids and comets from the ground, a Canadian satellite expected to launch in 2010 will be the first asteroid-tracking telescope to operate from space

The Near Earth Object Surveillance Satellite, or NEOSSat, is a suitcase-sized telescope capable of spotting asteroids and tracking high-altitude satellites and space debris. From orbit it will allow scientists to track a class of asteroids that ground-based observatories normally can't see and will help supplement data from NASA's near-Earth program.