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Meteors and meteorites
All of us are familiar with the sight of a sudden flash of light passing across part of the night sky, possibly followed by a lasting streak of light. Although there are many aeroplanes and artificial satellites which can be seen none of these look the same as a 'shooting star'.
Very occasionally the 'shooting-star' is very bright, brighter than the stars, and sometimes appears to emit sparks or even break up into pieces. On rare occasions its passage can be heard as a roar or a series of remote explosions. These very bright objects are often called fire-balls.
The trail left by a bright `shooting-star' may last for less than a second or for a fire-ball may last for minutes.
What we are witnessing when we see a shooting-star is a small piece of interplanetary matter, called a meteor, entering the Earth's atmosphere and `burning up' at a height of about 100 km.
These small particles are moving very fast relative to the Earth and when they enter the Earth's atmosphere they are rapidly slowed down. This means that they lose a lot of energy which appears as heat. Both the particle and the air that is is forcing its way past are made very hot. The particle, unless it is large, is completely evaporated and the air in the path of the meteor is ionized. We see light from the emission of radiation from the ionized gas and from the white hot evaporating particle. The trail is the hot gas gradually cooling down.
The astronauts when they reenter the Earth's atmosphere have to take severe
precautions to orientate their spacecraft correctly so that the shielding
which is designed to absorb and dissipate the heat caused by the impact with
the atmosphere can do its work. If, for some reason, the shields did not
work the astronauts would suffer the same fate as a meteor!
When large chunks of the interplanetary matter enter the atmosphere it is unlikely that all of the chunk will be evaporated. The outer layers will disappear but the centre is likely to survive and will hit the ground. The object which hits the ground is called a meteorite. The speed with which small meteorites hit the ground can be around 500 km/h.
More than 2000 meteorites have been recovered. They are of different types, Stony meteorites, iron meteorites and the rare carbonaceous chondrites. The largest meteorite that has been found is the 60 tonne Hoba iron meteorite; the largest stony meteorite weighs about a tonne and the Allende carbonaceous chondrite was a series of chunks that totalled about 5 tonnes.
Impact craters are known on the Earth that correspond to bodies far larger
than these. One of the best known is the Arizona crater in the USA which
is 1280 metres across and 180 metres deep. It was formed several thousand
years ago by a 250,000 tonne meteorite with a diameter of 70 metres hitting
the Earth at a speed of nearly 60,000 km/h.
Dates given above may vary slightly from year to year. Some showers give much better shows every few years. The presence of a bright moon will severely reduce the chances of seeing meteors.
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