The small near-Earth asteroid 2012 DA14 passed very close to Earth on Feb. 15, 2013, as NASA's Near-Earth Object Program Office, Jet Propulsion Laboratory explains

NASA chief Charles Bolden has advice on how to handle a large asteroid headed toward New York City: Pray.

That's about all the United States - or anyone for that matter - could do at this point about unknown asteroids and meteors that may be on a collision course with Earth, Bolden told lawmakers at a U.S. House of Representatives Science Committee hearing.

An asteroid estimated to be have been about 15m in diameter exploded on February 15 over Chelyabinsk, Russia, generating shock waves that shattered windows and damaged buildings. More than 1500 people were injured.

Later that day, a larger, unrelated asteroid discovered last year passed about 27,000km from Earth, closer than the network of television and weather satellites that ring the planet.

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NASA has found 95 percent of the asteroids nearest Earth that are big enough to wipe out civilization - and none poses any immediate threat to humanity.

But before you breathe a sigh of relief, there's also this: NASA admitted that the agency has little idea about the location of so-called "`city killers,'' asteroids smaller than the half-mile-wide "world enders'' but still big enough to crater New York City if it were hit exactly right.

Worse, there's considerable question what NASA and the military could do if scientists did spot a killer rock hurtling toward Earth, officials said at the hearing.

"A big segment of the population thinks it's just a matter of calling Bruce Willis in,'' said U.S. Rep. Bill Posey, R-Fla., a reference to the popular movie Armageddon.

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"What would we do if you detected even a small one ... headed toward New York City in three weeks? What would we do? Bend over and what?'' said Posey to laughter.

Replied NASA Administrator Charlie Bolden: "If it's coming in three weeks, pray.''

Right now, he said, building a spacecraft to deflect the asteroid would take years, which is why Bolden and John Holdren, the top science adviser to president Barack Obama, both used the hearing as a springboard to champion the White House plan to send NASA astronauts on a mission to an asteroid by 2025.

"That mission will benefit from current efforts to detect track and characterize (near-Earth objects) by speeding the identification of potential targets for exploration,'' said Holdren.

"And in return, such a mission will generate invaluable information for use in future detection and mitigation efforts.''

Prompting the hearing - and another scheduled Wednesday in the Senate - were two reminders last month about the dangers of rocks from outer space.

Earth pelted daily by basketballs

Though encounters with asteroids of any threatening size are rare, Earth is pelted every day with the gravel of the universe.

"Objects the size of a basketball arrive about once per day and objects as large as a car arrive about once per week,'' said Bolden.

These objects burn up in the atmosphere and are often seen as "shooting stars.''peBut bigger strikes can - and do - happen.

In 1908, an asteroid about 50m in diameter knocked down millions of tree in Siberia, and scientists think a massive asteroid about 10km in diameter wiped out the dinosaurs 65 million years ago when it hit near what is now the Yucatan Peninsula.

Holdren said that strikes like the one that hit Siberia are expected to happen only once every 1000 years. But, he added, "the potential consequences are so large that it makes us take the threat seriously.''

Though NASA has found most of the biggest nearby asteroids, officials estimate they've identified 10 per cent or fewer of those about 150m in diameter - big enough to"`devastate the better part of a continent,'' Holdren said.

Bolden said that, with NASA's current budget, it would take until 2030 or later to catalogue space rocks in this range. (NASA spent $US7.8 million on near-Earth-object observations in 2011 and $US20.4 million in 2012, according to agency documents).

"The U.S. has come a long way in its ability to track and characterize asteroids, meteors, comets and meteorites,'' said U.S. Rep. Lamar Smith, R-Texas, chair of the House science committee.

"But we still have a long way to go.''