Many people know that rubbish is a big problem on planet Earth. What many people don't know is that junk has become a problem in outer space too.

According to BBC News, there are more than 22, 000 pieces of space junk floating around the earth. And these are just the things that we can see from the surface of the earth by telescopes (望远镜). There are also millions of smaller pieces of junk that we can't see.

Objects, like bits of old space rockets or satellites, move around the planet at very high speeds, so fast that even a very small piece can break important satellites or become dangerous to astronauts. If the tiniest piece of junk crashed into a spaceship, it could damage the vehicle (交通工具).

To make things worse, when two objects in space crash, they break into many smaller pieces. For example, when a U. S. satellite hit an old Russian rocket in 2009, it broke into more than 2,000 pieces, increasing the amount of space junk.

To reduce additional space junk, countries have agreed that all new space tools can only stay in space for 25 years at most. Each tool must be built to fall safely into the earth's atmosphere after that time. In the upper parts of the atmosphere, it will burn up.

 Many scientists are also suggesting different ways to clean up space junk. In England scientists are testing a metal net that can be fired into space junk. The net catches the junk and then pulls it into the earth's atmosphere to burn up. The Germans are building robots that can collect pieces of space junk and bring them back to Earth to be safely destroyed.

 "The problem is becoming more challenging because we're sending more objects into space to help people use their mobile phones and computers," says Marco Castronuovo, an Italian space researcher.

 "The time to act is now. The longer we leave the problem, the bigger it will become," he says.

Venus (金星) is close to the earth. It is the second planet from the sun. The earth is the third planet from the sun. Venus is similar (相似的) to the earth in size and mass (质量). It is often described as the earth’s “sister”.

Scientists have been learning about Venus for a long time. First they looked at Venus through telescopes (望远镜). But Venus is covered with thick clouds. Scientists could not see Venus well through the clouds. So they made guesses about it.

For a long time, scientists thought that Venus had water and plants. They thought Venus might have animals too. But they didn’t know for sure.

Then scientists found a way to learn more about Venus. In the 1960s, they began sending probes (航天探测器) to Venus. The probes flew through the thick clouds. They took pictures of Venus up close. They found out many things too. The probes sent the pictures and things they found out back to the scientists.

The scientists learned a lot from the probes. First they learned that most of their guesses were not right. And they learned that Venus is very hot.

After that, scientists knew they could not send people to Venus. But they still wanted to know more about it. So they sent new probes. These new probes took great pictures of the planet. The pictures showed that Venus has plains (平原) and mountains! And in some ways, Venus looks like the earth.

At the moment, it may be difficult to imagine, but many people believe that, by the year 2100, we will live on the planet Mars. Our own planet, Earth, is becoming more and more crowded and polluted. Luckily, we can start again and build a better world on Mars.

First of all, transport should be much better. At present, our spaceships are too slow to carry large numbers of people to Mars—it takes months. However, by 2100, spaceship can travel at half the speed of light. It might take us two or three days to get to Mars!

Secondly, humans need food, water and air to live. Scientists should be able to develop plants that can be grown on Mars. These plants will produce the food and air that we need. However, can these plants produce water for us? There is no answer now.

There is a problem for us to live on Mars. Mars suck us much less than the Earth does. This will be dangerous because we could easily jump too high and fly slowly away into space. To prevent this, humans on Mars have to wear special shoes to make themselves heavier.

Life on Mars will be better than that on Earth in many ways. People will have more space. Living in a large building with only 10 bedrooms is highly possible. Many people believe that robots will do most of our work, so we have more time for our hobbies.

There will probably be no school on Mars. Every student will have a computer at home which is connected to the Internet. They can study, do their homework and take exams in online schools. Each student will also have his own online teacher called “e-teacher”.

However, in some ways, life on Mars may not be better than that on the Earth today. Food will not be the same—meals will probably be in the form of pills and will not be as delicious as they are today. Also, space travel will make many people feel ill. The spaceship will travel fast, so the journey to Mars will probably be very uncomfortable.