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The end of life on Earth?
It weighted about 10,000 tons, entered the
atmosphere at a speed of 64,000 km/h and exploded
over a city with a blast of 500 kilotons. But on 15
February 2013, we were lucky. The metereorite that
[05] 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, when a relatively small
[10] 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
[15] 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
[20] 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
[25] years ago. An enormous object, 10 to 16 km in diameter,
struck the Yucatan region in 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
[30] 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.
[35] 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
[40] 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.
[45] 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
[50] diameter. Unlike comets, asteroids have no tail. Most
are to small to cause any damage and burn up in the
atmosphere.
• Meteoroid – part of an asteroid or comet.
• Meteorite – what a meteoroid is called when it
[55] hits Earth.
Taken from: http://learningenglishteens.britishcouncil.org - Access on 29/06/2020
In the sentence “the dangerous comets and asteroids which might cross Earth’s orbit” (lines 39 and 40), the underlined word is similar to
