Satellites spy on meteorite explosionsDefence data lower forecast for asteroids exploding in Earth's atmosphere.
21 November 2002PHILIP BALL
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| An impact crater in Arizona. |
| © alamy |
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Every ten years an explosion equivalent to three Hiroshima atom bombs rips through the Earth's upper atmosphere, scientists in Canada and the United States report. These explosions are not clandestine nuclear tests. They are natural events, caused by the collision of asteroids with the Earth. Peter Brown of the University of Western Ontario in Canada and co-workers have used military satellite data to figure out how heavily our planet is being bombarded by cosmic missiles1. Their findings make sobering reading, but things aren't as bad as scientists had feared. They had thought that one ten-megaton explosion, equivalent to the biggest hydrogen-bomb detonations at the height of the Cold War nuclear tests in the 1950s, happened every two or three centuries. Brown and colleagues say that these occur only once a millennium. If that's so, perhaps we can relax a little. The last 10-megaton explosion took place only about a century ago. In 1908, a meteorite exploded about 6 km up in the atmosphere above the uninhabited region of Tunguska in Siberia. It levelled forests over an area of hundreds of square kilometres. People 60 kilometres away were thrown to the ground; reindeer herders 30 km away were blown into the air - one was allegedly killed when he hit a tree. Statistically speaking, it should be a very long time before we see the like of the Tunguska explosion again. Impact factor Impacts of large asteroids can be catastrophic, and leave obvious footprints such as the awesome Meteor Crater in Arizona. But they are very rare, and invariably happened long ago. Colliding bodies called bolides, less than 100 metres or so across, tend to break up in the atmosphere and so often leave no traces.
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| Tunguska 1908: a meteorite explosion flattened forests. |
| © Smithsonian Institution |
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Brown and colleagues have gained access to a unique window on this rain of space rock falling onto the Earth. They have scanned observations made by classified US satellites, which the Departments of Defense and Energy use to look for explosions that signal nuclear-weapons tests. Between the start of 1994 and September 2002, these satellites spotted 300 events that Brown's team attribute to high-altitude bolide impacts. The satellites see a flash lasting just a few seconds. From these flashes, the researchers estimate the amount of energy released by the explosion. In six years they have seen events ranging from equivalent to a tenth of a kiloton of TNT to a few tens of kilotons. These correspond to bolides between about 1 and 10 metres across. A few ground-based telescopes are now dedicated to looking for bodies at least several metres across on trajectories that will cross Earth's path. The Lincoln Near Earth Asteroid Research (LINEAR) program in Socorro, New Mexico, and the Spacewatch telescope in Arizona are on this watch. LINEAR and Spacewatch have seen far fewer bolides because big ones are rarer. But Brown's satellite data for small objects matches up well with the telescope data for bigger objects. This, the first direct measure of the smaller impact events, indicates that there is, on average, a 5-kiloton explosion every year - that's one-third the size of the Hiroshima bomb. |
