WHILE media attention has focused on the dangers of a giant asteroid hitting the Earth, debate is hotting up over the risk from smaller rocks, like the one that exploded above the Tunguska region of Siberia in 1908.
Experts are coming up with dramatically different estimates of the risk from small asteroids, those about 50 or 60 metres across. An astronomer at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, estimates only one impact every 1000 years, but a senior official at the US government’s Space Command says this figure is too low. He claims that during the past century alone, there were two further strikes besides Tunguska. He wants a military centre specifically to watch the skies and warn of impending impacts.
Small asteroids do not threaten global devastation, but can cause extensive local damage. The blast from the 60-metre Tunguska object flattened about 2000 square kilometres of forest when it exploded 8 kilometres above the ground. There is little evidence of how often such events occur, as small asteroids are hard to spot in space and once they’ve hit leave no craters. History records only one undisputed event, Tunguska.
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Extrapolations of the risk from larger objects imply infrequent strikes. Lunar craters imply only two strikes every 10,000 years, and asteroid surveys suggest one every several hundred years, says Alan Harris of JPL. He estimates one strike every 1000 years.
But Doug ReVelle of the Los Alamos National Laboratory in New Mexico says military records of smaller meteor explosions near the top of the atmosphere suggest Tunguska-like events occur about every 120 years.
Simon Worden of US Space Command gives an even more unsettling estimate– three events in 100 years. He says he has evidence that a Tunguska-sized asteroid hit the Amazon in the 1930s, while another exploded over central Asia in the 1940s. Civilian astronomers give little credence to these events, which are based on scattered human reports. But military analysts are taught to worry about poorly defined threats. At a Senate lunch last month, Worden warned of a catastrophe if such an asteroid hit a populated area, and urged the Pentagon’s surveillance centre in Colorado to take on about 10 people just to watch for asteroids.
Astronomers are happy for the Pentagon to use early-warning satellites to monitor small atmospheric blasts, and Worden promises to speed up the release of data, which security rules currently delay by weeks. But they are not happy about the idea of an impact warning centre in military hands.
“Impacts are an international problem,” says Brian Marsden, head of the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics, which coordinates asteroid observations.