Questão 2497720

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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

5 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

The passage “the damage was nothing compared to what had happened in Siberia nearly one hundred years ago” (lines 7 to 9) states that the incident occurred _______ a century ago.