Computerworld - By the end of this year, NASA hopes to find about 90% of the largest asteroids that could potentially strike Earth, a blast that could throw dust into the atmosphere and cause firestorms and acid rain. These asteroids can be as large as mountains but are at least 1 kilometer (3,280.8 feet) in diameter. NASA estimates that 900 of these objects are in potentially hazardous range of Earth.
But the more immediate threat is from much smaller asteroids, such as the asteroid that has a 1-in-25 chance of hitting Mars on Jan. 30. The asteroid, which has the unglamorous name of 2007 WD5, is only 50 meters (164 feet) and is barely a chip off the massive, 10-kilometer-wide (6.2 miles) asteroid that wiped out the dinosaurs some 65 million years ago. Small, yes, but such an asteroid has the explosive force of a 10-megaton nuclear weapon.
"There are thought to be about 75,000 potentially hazardous asteroids larger than 50 meters, and the vast majority remains undiscovered," Donald Yeomans, manager of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif., said in an e-mail response to questions. "Hence, at the moment, we would not have much warning time prior to a collision. That's the bad news."
But the "good news is that an object of this size would only cause local damage if it hit, or exploded above, a populated area, which seems unlikely," Yeomans said. Such a strike is unlikely given that two-thirds of the Earth is covered by ocean, he said.
A 50-meter asteroid, similar to the one inbound to Mars, will hit Earth once every 500 to 1,000 years, according to Yeomans. This is the same size of the object that struck Tunguska, Siberia, 100 years ago. The Tunguska object disintegrated in the earth's atmosphere, but its blast flattened and scorched trees over an area of some 800 square miles. The Mars asteroid is traveling at 30,000 miles per hour, and a strike could create a crater more than a half-mile wide.
But the U.S. isn't searching for the smaller, potentially hazardous asteroids, even though in 2005 Congress directed NASA to find by 2020 potentially hazardous objects of 140 meters or larger.
A midsize object of 140 meters (459.3 feet) or larger, with an impact energy of 100 megatons or more, can be expected to hit Earth once every 5,000 years -- a 1% probability of impact every 50 years. In contrast, 1-kilometer or larger asteroids have a mean impact frequency of about once every 500,000 years, according to testimony by Yeomans in November on near-Earth objects before the U.S. House Committee on Science and Technology.
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