NASA estimates 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. It would cost $300 million if a asteroid locating telescope was piggybacked on another vehicle. The report was previewed Monday at a Planetary Defense Conference in Washington.

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.

A cheaper option would be to simply piggyback on other agencies’ telescopes, a cost of about $300 million, 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 1-in-45,000 chance that it could hit in 2036.

They think it would mostly 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. “And we can’t find things that can cause massive damage.”

Lifeboat has an asteroid shield project