"As we discover more, and hopefully conclude that each one cannot hit within the next century or so, the remaining threat will shift to the smaller ones," said Clark Chapman of the Southwest Research Institute in Boulder, Colorado.
Tracking smaller asteroids is almost impossible, mainly because there are so many of themten million in Earth's neighborhood, according to David Morrison, a NASA scientist.
The Minor Planet Center receives observations of up to 15,000 new objects every day from two telescopes in New Mexico alone. An informal network of amateur astronomers around the world does much of the follow-up observation work.
While they are not potential Earth killers, smaller asteroids can cause considerable harm. "Any asteroid larger than 50 meters [164 feet] is a threat to the place it hits," Morrison said.
In 1908 an asteroid believed to be about 60 meters (197 feet) in diameter exploded in the atmosphere over Siberia. The resulting shock wave knocked down trees for hundreds of square miles.
An asteroid made of iron, on the other hand, would crash through the atmosphere intact and plunge into Earth. If it fell in the ocean, it could create a giant tsumani that could threaten coastal cities.
According to one expert scenario, there is a 10 percent chance that a 70-meter (230-foot) asteroid will impact Earth in our lifetime, striking with an energy of 10 megatons, equivalent to 700 Hiroshima-size atomic bombs.
"There is a better than even chance that an asteroid big enough to do some damage will impact in a person's lifetime," Johnson said. "However, it will probably be relatively small and the area of damage fairly isolated, so the probability of any specific person being affected is quite small, maybe one in 300,000 in any one year period."
A potential hit by a large asteroid is likely to be discovered decades in advance, allowing scientists to find ways of deflecting the object by, say, setting off a nuclear bomb on the object to change its orbit.
A smaller asteroid, on the other hand, is likely to slip under the radar. "Most likely, we'll have no warning at all," Chapman said.
On March 18 of this year, an asteroid measuring perhaps 50 meters across passed Earth at a distance of 30,000 miles (48,000 kilometers). It was announced with 23 hours notice.
"Generally, we get a couple of scares each year," Marsden said. "Most of them are pretty silly, because someone says something unwise."
Chances of getting killed by any asteroid are slim.
"If you are a smoker, or drive without seat belts, forget it," Chapman said. "If you are worried about shark attacks, or terrorist attacks, or the chances of another Three Mile Island, then pay attention. The impact hazard is more likely to kill you than any of those."
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