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Cosmic debris bombards the Earth by day and night

Posted in Astronomy, Science, Space on Monday, 29 October 2012

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This edited article about space originally appeared in Look and Learn issue number 773 published on 6th November 1975.

Meteor in 1868, picture, image, illustration

The Great Meteor of October 7, 1868

Hurtling through the heavens, a huge mass of rock weighing a thousand tons or more streaked Earthwards. From somewhere in the vastness of space it had been torn away by cosmic forces from its parent body – perhaps an asteroid that had shattered during a collision with another body – and found itself grasped by the Earth’s gravity.

Astronomers in their observations anxiously plotted its course. Its destination appeared to be the western United States where, if it landed, it could tear a huge crater in the Earth’s surface. Buildings would crumble, people would be killed. It would be a disaster.

Fortunately, the date of its appearance, 10th August, 1972, passed without the expected catastrophe. For the monster from outer space skimmed past the Earth just missing it by a mere 58 kilometres, or about 33 miles.

It was lucky for us that it did so, although the Earth is no stranger to visitors – not living ones it should be emphasised – from outer space. Scientists calculate that about twenty average-sized comets have hit the Earth since it was created. These can be imagined as large, dirty snowballs of particles of rock and dust held together by frozen gases.

We are also in the firing line for asteroids, which are made of rock and metal and are the rocky remnants of comets which broke up when the frozen gas, which was holding them together, escaped. Really big ones crash on to the Earth about once every 50,000 years and make a crater about 1 km (over half a mile) in diameter. There is one of this size in Arizona, U.S.A., estimated to be about 50,000 years old.

But these rare arrivals are no justification for the scientists’ description of the Earth as an interplanetary dustbin. This is earned by its unwilling acceptance of about 500 meteorites which hits its surface every year. The biggest of these weigh several tons and the smallest weigh about 3 kilograms (about 7 lbs.). One hits Britain every two or three years, the most recent being two which landed in Ireland in April, 1969.

Too tiny to be seen are particles called micrometeorites which are cushioned by the atmosphere and drift slowly to the surface. Although these cannot be detected by instruments on the ground, they can be spotted by the sensitive equipment on orbiting satellites.

These have measured an enormous stream of particles pounding towards the Earth. So vast is this flow that the particles add about 25 tons to the Earth’s weight every day.

Some of them are individual particles that move around in their own orbits and are called sporadics. Others are grouped in meteor showers, where meteoroids in the same shower all have similar orbits.

Three streams move in paths which intercept the Earth’s orbit. When the Earth passes through a meteor stream, showers of particles are captured by the Earth’s gravity and rush towards their new home.

Interplanetary space is full of such debris, some of which could eventually collide with the Earth. The most recent example to be discovered is an asteroid found by an American astronomer, Eleanor Helin, and unofficially given her surname.

Helin moves in an orbit which brings it between the Earth and the Sun, and it will reach its closest point to Earth in the 1990s, when it may be the target for a space probe. The likelihood of Helin colliding with the Earth is very low, although other less damaging objects are constantly streaming our way.

These are fireballs; meteors which burn up completely in the Earth’s atmosphere. Thousands of brilliant ones occur each year – about 15 by night over Britain – presenting onlookers with a free interplanetary fireworks display.

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