Watching And Waiting

摘要

Ever since 1980, when Luis and Walter Alvarez came up with the idea that the dinosaurs had been wiped out when the Earth was hit by an asteroid, people have worried that something similar might happen again. Indeed, on a much smaller scale it did, 100 years ago. In 1908 an object about 50 metres (150 feet) across entered the atmosphere above Siberia. When it blew up it flattened some 2,000 square kilometres (800 square miles) of forest.rnExperts in the field put the chances of a 1908-style event happening some time this century at about one in ten. That would be devastating if it occurred over a densely populated area-of which there are many more than there were a century ago. This, plus the more remote chance of a big collision, has stimulated efforts to map the orbits of potential threats so that people will be prepared. They might, by attaching rocket motors to the incoming rock, or nudging its orbit with nuclear explosions, even be able to avert disaster. Until now, these efforts have been carried out with existing telescopes, and researchers think they have found about three-quarters of the 1,000 or so neighbouring asteroids that have a civilisation-wrecking diameter of 1km or more. But to locate the rest, and to look for smaller objects that could still wreak local devastation, they need more specialised tools.