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VOA慢速英语新闻:如何发现小行星?

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Space rocks often enter the Earth's atmosphere. But they usually explode and burn before falling to the ground. About 70 percent of our planet is covered with water. As a result, most meteorites falling from the sky disappear on the oceans' floor. We would not know about them if it was not for an international agency.

You may remember the meteorite that exploded last year 23 kilometers above Chelyabinsk, in Russia. Scientists estimate it was about 18 meters long before it fell to Earth. They also think it had a mass of about 10,000 tons.

Many people would have died if the meteorite fell in a populated area. Luckily, it did not. Instead it broke apart mid-air and most of the pieces fell into Lake Chebarkul. But that is a little too close for some people.

The Comprehensive Nuclear-Test-Ban Treaty Preparatory Commission is a group based in Vienna, Austria. It says the Earth is visited by meteorites more often than we know.

The group records explosions made by meteorites that enter Earth's atmosphere. Pierrick Mialle works for the commission, also known as the CTBTO. He is an acoustics expert, specializing in the study of sounds.

He says since meteorites are sometimes entering the atmosphere, they are creating loud air bursts.

The commission controls sensors and other equipment that watch for possible violations of the Nuclear Test ban treaty. Some of the devices are seismic, meaning they measure the movement of the Earth. Others are hydrostatic, meaning they measure the movement of the Earth's water. The Commission also uses ultrasound and radioactivity sensors.

Between 2000 and 2013, the CTBTO recorded 26 powerful explosions of large meteorites entering the atmosphere.

The B612 Foundation also studies meteorite impacts. This group estimates at least four of those 26 explosions were stronger than the nuclear bomb that destroyed Hiroshima.

In addition to loud noise, explosions create low frequency infrasound. Sound waves can travel very long distances at such low frequencies. But human beings are unable to hear them.

CTBTO stations as far away as the Antarctica recorded the explosion of the Chelyabinsk meteorite.

Mr. Mialle says listening stations operate anywhere from four to 15 high-technology sensors. He says that the sensors work like large microphones.

"They are called micro barometer because they measure micro fluctuations of the atmosphere. It's like a large microphone except there's no membrane."

He adds that some of the explosions are hard to identify immediately after they are recorded.

"For instance, the first time we had this, what was later called the Super Bolide of North Pacific, in the first few weeks all that was known was a large event in the middle of the Pacific. But we didn't know what it was."

The commission keeps contact with other agencies, such as the American space agency, that follow meteorites. Space agency officials later confirmed that the mysterious explosion was caused by a meteorite.

When complete, the CTBTO system will have 337 stations worldwide. And these stations will be listening for nuclear explosions and other loud sounds in the atmosphere.

I'm Anna Matteo.

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