SAN FRANCISCO - About twice a year, an asteroid smashes into Earth's atmosphere with the force of a Hiroshima-size atomic blast. And those are small ones, scientists say; the space rocks vaporize before they can do any harm.

When the big one hits, we won't be as fortunate.

Researchers at the recent American Association for the Advancement of Science meeting warned that it is inevitable that an asteroid large enough to crack the atmosphere will hit the planet.

When it does, it has the potential to be just as awful as the 6-mile-diameter rock that wiped out most life on the planet 65 million years ago.

"There is a danger of an asteroid killing the Earth," said David Morrison, an astrobiologist at NASA's Ames Research Center in California.

As for warning, we might have a few weeks. Or none at all. A rogue asteroid could easily blindside us by coming around the sun and approaching Earth with the sun behind it, obscuring views.

"You wouldn't know it until the sky lit up and the impact shook the Earth," Morrison said.

Although the chances of any single asteroid striking the globe are fairly remote, there are thousands of potential planet-killers lurking in the asteroid belt between Mars and Jupiter.

Erosion and geologic forces have softened or eliminated most signs of impacts on Earth over the past 4.6 billion years, but you need only look to the moon to see how violent space can be.

The pockmarked lunar surface provides a clear picture of how asteroids have rearranged terrain since the solar system formed. And so does the Barringer Meteor Crater, an impact site that spans nearly a mile in northern Arizona, and a crater site in Ohio's Adams County that is 5 miles in diameter.

Asteroid tracking was largely the role of amateur astronomers until a few years ago, when governments started to get involved.

NASA, for example, is about 70 percent of the way through an effort to identify all near-Earth asteroids larger than about one-third of a mile in diameter.

So far, 840 potentially dangerous asteroids have been named and charted. Comets also are a concern, but there are so many more asteroids that they get the most attention.

The closeness of a "close encounter" is relative. There were close encounters on Feb. 1 and Feb. 7, although both asteroids passed about 1 million miles from Earth, according to spaceweather-.com.

Other months are busier. In April 2004, five asteroids, each larger than 328 feet in diameter, passed relatively close to Earth.

So we can track them. What if one is coming right at us?

Scientists now say we have the technology to slightly alter the path of a planet-killer.

The 2004 discovery of Apophis, a 1,200-foot-diameter asteroid that appeared headed for a collision with Earth in 2036, moved Morrison and other scientists to call for tracking smaller objects and initiating a plan to nudge stray asteroids into safe orbits.

"This is not about science. This is about public safety," said former astronaut Russell Schweickart, who wants the United Nations to take the asteroid threat seriously.

Three years ago, scientists said there was a 2.7 percent chance that Apophis will hit Earth. Now, astronomers say the asteroid probably will clear Earth with 20,000 miles to spare.

Right now, they estimate the risk of collision is about 1 in 45,000. In 2029, Apophis will pass through an astronomical keyhole, a precise spot in space where gravitational forces could put it on a collision course.

"What Apophis would do is destroy (an area the size of) England or northern California," Morrison said.

Jay Melosh, a geophysicist at the University of Arizona's Lunar and Planetary Laboratory, said that if Apophis struck Earth, it would produce a 40-megaton blast, almost eight times larger than the most powerful nuclear bomb ever detonated.

The explosion would create a crater more than 2 miles wide and obliterate buildings and bridges in a 4-mile radius.

Melosh said everything around it would be buried beneath 20 inches of debris.

In 1908, an asteroid exploded in the air over remote Siberia. The 10-megaton explosion obliterated herds of reindeer, leveled trees over a 400-square-mile area and, reportedly, knocked a man down 60 miles away.

No matter where it strikes, if it does, Apophis is large enough to toss enough dust into the air to cool the planet, causing climate changes severe enough to disrupt worldwide agriculture and threaten civilization, Melosh said.

And if it hit the ocean, it would set off a tsunami and send enough chlorine and bromine from vaporized seawater to destroy the ozone layer, he said.

That is why NASA researchers and others are working to identify potential killer asteroids early, said Edward Lu, of the Johnson Space Center.

"It is possible to save the Earth from something like Apophis," he said.

Blowing the asteroid up, like in the movie Armageddon, wouldn't work. That would produce even more out-ofcontrol rocks, Lu said.

Far better, he thinks, is to send a large probe to rendezvous with the asteroid. By parking near it, the spacecraft could exert enough gravity that its engines could be used to "tow" the asteroid.

"You can pull an aircraft carrier with a tiny force if you pull long enough," Lu said.

For Apophis, about 2,200 pounds of force constantly applied could shift the asteroid's course half the diameter of the Earth in about 12 days.

Altering an asteroid's course, however, means that some nations will become more vulnerable until the tow is complete.

"When you start to deflect an asteroid, some nations are going to have to accept some increased risk in order to drop the probability to everyone to zero," Schweickart said.

There also is a limit to space nudging, Melosh said.

"If the asteroid was 10 times bigger (than Apophis), the space tug wouldn't cut any ice," he said. "The best thing we can do is watch and estimate where it might hit.

"If it were much bigger, we would be talking about evacuation."

Potential targets will be mapped along a series of approach lines. Early on, as many as 100 might be drawn across the globe.

"Every nation will have a line going across it," Schweickart said.

As the asteroid closes in, more precise calculations can be made until there's just one line of approach and one target.

Schweickart said politicians must think hard about Apophis and other asteroids.

"This is extinction," he said. "We can't prevent a hurricane. We can't prevent a tornado.

"But we can prevent an asteroid impact, and we can do it by reshaping the solar system. And if we do not do it, we really are not that far past the dinosaurs."