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

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

These are asteroids that are bigger than 460 feet in diameter - slightly smaller than the Superdome in New Orleans. 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 with the power of 100 million tons of dynamite, enough to devastate an entire state, such as Maryland, they said.

The agency is already tracking bigger objects, at least 3,300 feet in diameter, that could wipe out most life on Earth, much like what is theorized to have happened to 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.

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 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.