Meteors and meteorites
When a comet enters our solar system, it is heated up by the Sun. Then it looses matter as dust particles and small lumps of rocky material, which is left behind in the comets orbit. When this planetary debris comes within the range of the gravitational field of Earth, it enters the atmosphere with a velocity between twenty to sixty kilometers per second. The smallest particles evaporate due to friction and air resistance, larger ones sometimes smash into Earth.
Meteors.
When we see these particles from Earth, we call them 'shooting
stars', but to be more correct be should call them meteors. Sometimes we see
large quantities of 'shooting stars', this happens because Earth comes very close to
a comet orbit. When large quantities of dust particles are left behind by the comet,
it is known as a meteor shower.
Many meteor showers occur yearly at fixed intervals. Every year
between November 3 and 15 for example a meteor shower may be seen at night,
when Earth crosses the Taurids. This swarm of debris
is coming from the Encke comet, which return every 3.3 years.
Every year between June 24 and July 6 the Earth again crosses Taurids,
but during this time, there is almost nothing to see.
Meteorites.
Only
seldom are the fragments large enough to reach the surface of Earth, in that case
we call them meteorites.
Only the larger and more dense bodies reach the Earth's surface in
pieces that stay intact. On their way down, friction in the atmospere slows down the meteorites,
however the larger the size of the body the less the friction affects it. Large, massive bodies hardly decelerate and therefore smash
into the Earth with a greater velocity.
When they hit the ground, all of their kinetic (motion) energy is suddenly
transformed into heat. The meteorite explodes and evaporates. Sometimes this causes
large craters to be created.
As a rule of thumb a crater diameter is about twenty times larger than
the diameter of the meteorite that caused it.
Tunguska.
The forces of such an impact sometimes are gigantic, on June
31 of 1908 an enormous explosion occured above the Tunguska River
in Siberia. Eyewitnesses saw a huge fireball in the sky up to 500 kilometers away, that radiated light even brighter than
the Sun. Panic broke out up to fifteen hundred kilometers away due
to severe gusts of wind, shaking of the ground, and a roaring sound.
The
explosion flattened the forests over an area of about fifteen hundred
square kilometers, it also uprooted trees and peeled their branches and leaves.
The explosion was estimated to have had an impact equivalent to twenty to thirty
tons of TNT. Which is more or less about the same as about five hundred
atom bombs used in World War II.
The explosion was caused by a comet fragment or a rocky meteorite with a
mass of approximately 200,000 tons. After its entrance into the atmosphere
the object exploded at a height of about eight kilometers and evaporated,
because of the heat of friction. It was the largest major meteorite impact
ever recorded in human history.
Although the Tunguska meteorite was a big one, their may have been a
larger one from time to time over the course of history. It is assumed that the Dinosaurs
died out, because the Sun was covered for several months by
dust in the atmosphere after a meteorite impact. This meteorite most
probably had a size of several kilometers and weighed of at least3 billion
tons. The impact must have relased millions of mega tons of energy.
A bombardment from space?
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