for National Geographic News
When a massive asteroid, measuring ten kilometers (six miles) across, smashed into Earth off Mexico's Yucatán 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) acrossout there. They concluded that none are on a collision course with the Earth during the next century.
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, and to make a serious search for them would cost a fair bit of money."
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.
Debris from asteroids and comets 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 the 1990s started a program known as Spaceguard. Its goal: to identify 90 percent of the large near-Earth asteroidsthose bigger than a kilometer in diameterby 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.
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