The meteorites
| From time to time the Earth, along its orbit,
meets pieces of rock or metallic fragments wandering in space. These rocks,
named meteoroids, are the product of the disruption a comet's nucleus,
or else are small asteroids "out of track", or rocks coming from the Moon
or Mars. The meteoroids move along very different orbits, at a speed of
a few tens of kilometers per second. |
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A meteorite found in Antarctica. (NASA-JPL)
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When one of them collides with the Earth,
two things can happen. If the meteoroid is very small, then the Earth atmosphere
friction heats it so much that it literally evaporates, consuming before
touching the ground. While it burns, it leaves a luminous strip in the
sky, which lasts a few seconds and is improperly called shooting star. |
If, instead, it is big enough, the meteoroid does not totally
consume, and it can reach the ground, even though slightly slowed down
by the atmosphere. In the impact, it produces a crater whose diameter is
a few kilometers, and destroys the surrounding vegetation.
In the first case one talks about a meteor, in
the second one about a meteorite. A meteorite can reach a speed
as high as 250,000 km/h when it touches the ground!
Meteoritic crater of Barringer, Arizona. (D. Roddy,
LPI) |
The biggest meteorite which up to now fell on the Earth,
is thought to be that of Barringer, Arizona. If fell 50 thousand years
ago: it weighed about 300,000 tons. |
The Earth is hit by about 100 meteorites each year, but fortunately,
not all of them are so big... A meteorite whose size were 100 m, or more,
would have disastrous consequences. Several scientists think that it was
the impact of a giant meteorite that caused the extinction of dinosaurs,
66 million years ago.
As we said, the meteoroids can be the remains of a cometary
nucleus, which disrupted. In that case, the fragments stay together in
a swarm, that rotates around the Sun along the same orbit of the parent
comet. In its trip around the Sun, the Earth periodically crosses a few
of these swarms, and "swallows" the rock fragments. They evaporate within
the high atmosphere, producing real meteoric showers, the well known "shooting
stars".
A shower of meteors. (NASA-JPL)
The largest of these meteoric swarms is that of the Perseid
and that of the Leonid. The first one is crossed by the Earth round about
August 9 to 12, and originates the meteoric shower named after St. Lawrence.
The second one is crossed round about half of November.
The meteoroids, disrupting in the atmosphere, form a dust
that slowly drops on the ground. Each day, about 3,000 tons of this "cosmic
dust" fall on the Earth.
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