But the US space agency said the chances of a new "Near-Earth Object" (NEO), like the one that is believed to have wiped out the dinosaurs 65 million years ago, were too remote to divert scarce resources.

Scott Pace, head of program analysis and evaluation at NASA, said the agency could not do more to detect NEOs "given the constrained resources and the strategic objectives NASA already has been tasked with."

Pace and other NASA officials were grilled at a House of Representatives hearing on the NEO program, which seized the public imagination in the 1990s through movies like "Armageddon" and "Deep Impact."

The hearing of the House of Representatives space and aeronautics subcommittee highlighted one small asteroid named Apophis, which some scientists say could come perilously close to Earth in 2029.

NASA now only tracks NEOs larger than one kilometer (.62 mile) in diameter, which come near Earth only once every few hundred thousand years.

Objects of that size can cause global disaster through their immediate surface impact and by triggering rapid climate change.

"Extinction-class" objects measuring at least 10 kilometers, such as the object that crashed into Mexico's Yucatan peninsula about 65 million years ago, would be rarer still.

Lawmakers complained that NASA had failed to come up with a budget in line with a 2005 act of Congress that mandated an expanded search for NEOs that are at least 140 meters (153 yards) in diameter.

Objects of this size are relatively common and "could still inflict large regional impacts if they struck the Earth," Republican Representative Tom Feeney said.

The subcommittee's Democratic chairman, Mark Udall, said he was "disappointed and concerned" that NASA had neglected to abide by the act's recommendations.

Apophis is about 250 meters (273 yards) in diameter and is on track to approach Earth on Friday the 13th, April 2029. NASA says there is a one in 45,000 chance that it could pass through a "gravitational keyhole" and hit the planet in 2036.

"It's a very unlikely situation and one we can drive to zero, probably," said Donald Yeomans, who manages the NEO program at NASA's Jet Propulsion Laboratory.

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