Undetectable Asteroids Could
Destroy Cities - Experts

By Stefan Lovgrenfor
National Geographic News

LOS ANGELES -- When a massive asteroid, measuring ten kilometers (six miles) across, smashed into Earth off Mexico's Yucatan Peninsula some 65 million years ago, it most likely changed the shape of life on Earth. The dust from the impact, perhaps exacerbated by other asteroid blasts, blocked the sun, darkening and cooling the Earth. When the dust settled, increasing greenhouse gases sent temperatures soaring. The violent climate change, most scientists believe, is what finished the dinosaurs, along with 70 percent of all plants and animals living at the time.
So, could such an asteroid strike again?
Absolutely. But while the dinosaurs didn't know what was about to hit them, humans probably would. Scientists have already identified more than 700 of the estimated 1,100 "Earth killers" - asteroids bigger than one kilometer (about a thousand yards) across - out there. They concluded that none are approaching the Earth.
The bad news, however, is there are also about ten million "smaller" asteroids out there. These could not destroy humankind, if they were to hit Earth, but could cause widespread damage, possibly even wiping out an entire city. Because they have not been identified, the smaller asteroids could potentially strike without warning.
"Finding and cataloguing the big [asteroids] is relatively easy and inexpensive," said Brian Marsden, director of the Minor Planet Center in Cambridge, Massachusetts, a clearinghouse for asteroid observations. "But when it comes to ... the likelihood that there really would be an impact in the foreseeable future, it is the smaller objects that are of more concern."
Formed during the creation of the universe, most asteroids are made of rock, but about 3 percent are made of metals like iron. They range in size from small boulders to objects that are hundreds of miles in diameter. In our solar system most asteroids orbit the vast region of space between Mars and Jupiter.
Tiny asteroids or debris - called meteoroids - collide with Earth all the time. But these simply burn up on their descent through Earth's atmosphere, producing "shooting stars."
On the other hand, the probability of a large asteroid hitting Earth is extremely slim. However, such an impact could be devastating, which is why NASA in 1998 started a program known as Spaceguard. Its goal: to identify 90 percent of the large near-Earth asteroids - those bigger than a kilometer in diameter - by the year 2008.
"To date, more than 700 objects of an estimated population of about 1,100 have been discovered," Lindley Johnson, the manager of NASA's Near-Earth Object Observation Program in Washington, D.C., told the U.S. Senate's Subcommittee on Science, Technology, and Space last week.
The estimate of the large asteroids vary. Some speculate there may be as many as 1,500. About 100 objects have been found per year in the last four years, though experts agree it's unlikely that every single large asteroid will be found.
"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.
Shock Waves
Tracking smaller asteroids is almost impossible, mainly because there are so many of them - ten million in Earth's neighborhood, according to David Morrison, a NASA scientist.
The Minor Planet Center receives up to 15,000 new observations every day from one powerful telescope 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."
No Warning
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."
© 2004 National Geographic Society. All rights reserved.



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