15-11-2009 Unwelcome Visitors from Space (Article 163)

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Simon Kenny
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15-11-2009 Unwelcome Visitors from Space (Article 163)

Post by Simon Kenny » Tue Dec 30, 2008 8:26 pm

Unwelcome Visitors from Space (Article 163)
The kind of unwelcome visitors described in this article aren’t found in ‘Space Invaders’ or ‘Mars Attacks’, but they still can cause widespread damage to our planet and have done so periodically in the distant past. The dangers referred to are large chunks of material that come hurtling in from space, causing widespread, sometimes global destruction. We are familiar with their tiny relatives and call them ‘shooting stars’ or meteorites. Normally, these can be seen on any clear night and far from posing any danger, they are fascinating to watch, especially in times of the year when they are plentiful. The best times to watch for them are when the Earth crosses the dust trails of various comets and asteroids on its yearly journey around the Sun. Examples of these are the Lyrid Meteor shower in late April, the Perseid shower in mid August, arguably the best of them all, and the Leonids in mid November. These meteorites are merely sand and dust particles burning up in the atmosphere long before they reach the ground.

Such visitors from space are benign, even welcome visitors to our night skies. However, when they come in the size of boulders as large as a house, or up to several kilometres across, the fireworks get seriously out of hand. The havoc created by a relatively small asteroid is illustrated by the Tunguska incident in Siberia in 1908. A massive explosion in this unpopulated region flattened an estimated 2 million square kilometres of forest. An estimated 80 million felled trees pointed away from the centre of the blast. The explosion likely came from the breakup of a loosely compacted stony asteroid about 60 metres in diameter, several kilometres above the ground, causing an explosion 1000 times as great as the Hiroshima bomb. Catastrophic consequences would have followed had the asteroid exploded over a large metropolitan area.

Much larger missiles from space have struck the Earth during its four billion year history. It is thought that the infant Earth was so heavily bombarded by the meteoritic remnants of the solar system’s creation, that a solid crust could not form for hundreds of millions of years. It is now thought that the young Earth was struck by an object the size of Mars about 4.6 billion years ago. The collision was so great it sent a great spiral of debris into orbit around the new planet. Eventually, this debris coalesced through gravity to form our Moon. Luckily, collisions on this scale are not likely to be repeated. However, global destruction and mass extinctions can occur when an object only a few kilometres across hits the Earth. This was demonstrated 65 million years ago when a meteorite ten kilometres across struck the Northern Yucatan peninsula, creating devastation worldwide and leading to the extinction of the dinosaurs. So, we can see why scientists are busily looking for Earth-crossing asteroids and studying ways to divert any potentially dangerous ones away from Earth.

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