The next task will be to look for smaller objects that might just destroy, say, a city, the experts told the U.S. Senate\'s Subcommittee on Science, Technology and Space.
In an update on the Near Earth Object Observation Program, experts told the Senate subcommittee that they are on schedule to finding everything bigger than 1 kilometer (0.62 mile) in diameter that might approach the planet.
\"The survey officially started in 1998 and to date more than 700 objects of an estimated population of about 1,100 have been discovered, so the effort is now believed to be over 70 percent complete and well on the way to meeting its objective by 2008,\" NASA\'s Lindley Johnson told the hearing.
There have been a few scares.
Last September, scientists spotted asteroid \"2003 QQ47\" and first measurements suggested it could hit the Earth on March 21, 2014, with an explosion the size of 20 million Hiroshima atomic bombs. But the forecast was revised: It won\'t hit, after all.
Objects certainly have hit the planet. An asteroid, or several asteroids, are believed to have kicked up so much dust and set off so much volcanic activity 65 million years ago that the resulting climate change wiped out the dinosaurs.
A smaller asteroid is believed to have leveled 400 square miles of Siberian forest in 1908.
Scientists believe an event of that size would only occur about every 700,000 years on average.
\"Although the probability of the Earth being hit by a large object in this century is low, the effects of an impact are so catastrophic that it is essential to prepare a defense against such an occurrence,\" Michael Griffin, head of the Space Department at the Johns Hopkins University Applied Physics Lab, told the hearing.
But when? \"At the current state of knowledge it is about as likely to happen next week as in a randomly selected week a thousand years from now,\" said Johnson.
If an asteroid was confirmed to be on a catastrophic collision course with Earth, the experts said it would take about 30 years to get ready to do anything about it.
\"The Space Shuttle\'s main engines and the fuel contained in the large external tank could successfully deflect a 1 kilometer object if it were applied about 20 years in advance,\" of a projected collision, Griffin said.
Using a nuclear bomb might make matters worse because the pieces of the blown-up asteroid would stay in the same orbit and eventually come back together again.
The next task will be to find smaller objects that may not destroy the Earth but could do considerable damage if they hit, the scientists said.
A single satellite orbiting the Sun just inside the Earth\'s orbit could find 90 percent of all near-Earth objects 100 meters (330 feet) or more in diameter within 10 years, Griffin said. It would cost about $300 million and could be ready within five years, Griffin said.