In 1997, Lottie Williams was walking through a park in Oklahoma, USA, with her friends. She picked the object up. It was about as heavy as a tennis ball. Later, scientists told her it was a piece of a DeltaⅡrocket that had been used to launch a satellite (发射卫星).
We use satellites to send and receive TV and radio signals (信号). To report the weather and for many other things. But when they become old or break down, we just leave them there. They become space junk.
Scientists think there are about 16, 000 objects larger than 10 centimeters wide flying around the Earth, and tens of millions of smaller objects too. That's fast! And it's their speed that makes these pieces of space junk really dangerous. If one small piece of space junk hits something at high speed, it can do a lot of harm. But in 2009, an old Russian satellite hit an American satellite and created 2,000 pieces of space junk!
Pieces of space junk enter the Earth's atmosphere (大气层). Again. Lottie Williams is perhaps the only person who has been hit. Most space junk burns up and doesn't reach the Earth's surface. It made a hole in the ground 30 centimeters deep and over 3. 3 meters wide. Lottie was lucky she wasn't hit by it!
A. Luckily, that hardly ever happens.
B. They travel at around 7 kilometers a second(秒).
C. Many countries around the world have sent satellites into space.
D. Suddenly something fell on her shoulder and then to the ground.
E. But in 2011, a 35-centimeter-wide metal ball fell to the Earth in Namibia, Africa.
