NASA officials say the space agency is capable of finding nearly all the asteroids that might pose a devastating threat to Earth, but there isn't enough money to pay for the task so it won't get done.

'We know what to do. We just don't have the money.'—Simon (Pete) Worden, NASA

The cost to find at least 90 per cent of the 20,000 potentially hazardous asteroids and comets by 2020 would be about $1 billion US, according to a report NASA will release later this week. The report was previewed Monday at a Planetary Defence Conference in Washington.

Congress in 2005 asked NASA to come up with a plan to track most killer asteroids and propose how to deflect the potentially catastrophic ones.

"We know what to do. We just don't have the money," said Simon (Pete) Worden, director of NASA's Ames Research Center.

Explosions also a threat

These are asteroids that are bigger than 140 metres in diameter. They are a threat even if they don't hit Earth because if they explode while close enough — an event caused by heating in both the rock and the atmosphere — the devastation from the shockwaves is still immense. The explosion alone could have the power of 90 million tonnes of dynamite, enough to devastate an entire state, such as Maryland, he said.

The agency is already tracking bigger objects, at least a kilometre in diameter that could wipe out most life on Earth, much like is theorized to have happened to the dinosaurs 65 million years ago. But even that search, which has spotted 769 asteroids and comets — none of which is on course to hit Earth — is behind schedule. It's supposed to be complete by the end of next year.

NASA needs to do more to locate other smaller — but still potentially dangerous — space bodies. While an Italian observatory is doing some work, the United States is the only government with an asteroid-tracking program, NASA said.

All options rejected

One solution would be to build a new ground telescope solely for the asteroid hunt, and piggyback that use with other agencies' telescopes for a total cost of $800 million. Another would be to launch a space infrared telescope that could do the job faster for $1.1 billion. But NASA program scientist Lindley Johnson said NASA and the White House called both those choices too costly.

A cheaper option to simply piggyback on other agencies' telescopes at a cost of about $300 million was also rejected, Johnson said.

"The decision of the agency is we just can't do anything about it right now," he added.

Earth got a scare in 2004, when initial readings suggested an 885-foot asteroid called 99942 Apophis seemed to have a chance of hitting Earth in 2029. But more observations showed that wouldn't happen. Scientists say there is a one-in-45,000 chance that it could hit in 2036.

They think it would most likely strike the Pacific Ocean, which would cause a tsunami on the U.S. West Coast the size of the devastating 2004 Indian Ocean wave.

John Logsdon, space policy director at George Washington University, said a stepped-up search for such asteroids is needed.

"You can't deflect them if you can't find them," Logsdon said.