"Tunguska" Impacts Rarer Than Thought

November 21, 2002 | Over the past 8½ years, satellite sentinels operated by the U.S. Department of Defense have detected 300 powerful explosions in our atmosphere caused by asteroidal fragments from 1 to 10 meters across. That alarming statistic is relatively good news, says Peter Brown (University of Western Ontario), because the occurrence rate of these small, harmless bodies hints that their larger and more worrisome siblings strike Earth less often than thought. In today's issue of Nature, Brown and four colleagues conclude that an asteroid strikes Earth with the kinetic-energy equivalent of 10 million tons (10 megatons) of TNT — comparable to the Tunguska blast in 1908 — about once every 1,000 years. Their finding agrees closely with an estimate announced two months ago by dynamicist Alan W. Harris. Previous estimates had suggested that Tunguska-class events occurred more frequently, every 200 to 300 years.

Nature's online summary of the paper can be found at http://www.nature.com, and a press release from the University of Western Ontario appears at http://comms.uwo.ca/media/releases/index.htm.

U.S. Expands Meteorite Hunt in Antarctica

November 21, 2002 | As winter tightens its grip on North America, a dozen researchers are heading for the 'round-the-clock summer sunshine of Antarctica, where they will search for meteorites lying atop on the continent's vast ice fields. This year's effort has been fortified with a three-year, $1.6-million grant from NASA, which will allow the main team to conduct detailed hunts for meteorites in one locale and reconnoiter promising but poorly studied or hard-to-reach areas elsewhere. This year, for example, the reconnaissance team plans to explore ice fields around the Pecora Escarpment, located some 200 kilometers from Amundsen-Scott Station at the South Pole. Funding from the National Science Foundation has permitted American scientists associated with Case Western Reserve University's Antarctic Search for Meteorites (ANSMET) program to visit the south pole's windswept ice fields for 25 years. Of the nearly 12,000 meteorites collected by U.S. teams, roughly 5 percent have unusual, scientifically valuable properties, and roughly one in 1,000 is from the Moon or Mars.

A full description of the ANSMET program appears at http://www.cwru.edu/affil/ansmet/.


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