The end of life on Earth?
It weighed about 10,000 tons, entered the atmosphere at a speed of
64,000km/h and exploded over a city with a blast of 500 kilotons. But on 15
February 2013, we were lucky. The meteorite that showered pieces of rock over
Chelyabinsk, Russia, was relatively small, at only about 17 metres wide.
Although many people were injured by falling glass, the damage was nothing
compared to what had happened in Siberia nearly one hundred years ago. Another
relatively small object (approximately 50 metres in diameter) exploded in
mid-air over a forest region, flattening about 80 million trees. If it had
exploded over a city such as Moscow or London, millions of people would have
been killed.
By a strange coincidence, the same
day that the meteorite terrified the people of Chelyabinsk, another 50m-wide
asteroid passed relatively close to Earth. Scientists were expecting that visit
and know that the asteroid will return to fly close by us in 2046, but the
Russian meteorite earlier in the day had been too small for anyone to spot.
Most scientists agree that comets
and asteroids pose the biggest natural threat to human existence. It was
probably a large asteroid or comet colliding with Earth which wiped out the
dinosaurs about 65 million years ago. An enormous object, 10 to 16km in diameter,
struck the Yucatan region of Mexico with the force of 100 megatons. That is the
equivalent of one Hiroshima bomb for every person alive on Earth today.
Many scientists, including the late
Stephen Hawking, say that any comet or asteroid greater than 20km in diameter
that hits Earth will result in the complete destruction of complex life,
including all animals and most plants. As we have seen, even a much smaller
asteroid can cause great damage.
The Earth has been kept fairly safe
for the last 65 million years by good fortune and the massive gravitational
field of the planet Jupiter. Our cosmic guardian, with its stable circular
orbit far from the sun, sweeps up and scatters away most of the dangerous
comets and asteroids which might cross Earth’s orbit. After the Chelyabinsk
meteorite, scientists are now monitoring potential hazards even more carefully
but, as far as they know, there is no danger in the foreseeable future.
Types
of space rocks
·
Comet – a ball of rock and ice that
sends out a tail of gas and dust behind it. Bright comets only appear in our
visible night sky about once every ten years.
·
Asteroid – a rock a few feet to
several kms in diameter. Unlike comets, asteroids have no tail. Most are too
small to cause any damage and burn up in the atmosphere. They appear to us as ‘shooting stars’.
·
Meteoroid – part of an asteroid or
comet.
·
Meteorite – what a meteoroid is
called when it hits Earth.
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