We all know the sun is important. It makes life on our planet possible. But how does the sun do that? It’s the sun’s energy.

Energy from the sun is called solar energy (太阳能). It comes in the form of light or heat. People have used solar energy for thousands of years. The sun’s energy can be used to heat water and even food. If you own a magnifying glass, your parents have probably warned you not to leave it in the sun. Solar energy can actually make the magnifying glass burn objects underneath it causing a fire. This type of energy from the sun is called solar thermal (热的) energy.

We can also turn the sun’s light into electricity. This is done with solar panels (板). Solar panels are made up of a material called silicon (硅). The silicon is heated and formed into very thin wafers (晶片). When the sunlight hits the solar panel, the electrons in the silicon move and flow through wires (电线). Using this technology, we can run a calculator and even power cars in an easy way.

But what happens on cloudy days? The electrons in the silicon will stop moving, which means the electricity can’t flow. But this doesn’t mean that solar energy can only be used on sunny days. People can use the batteries (电池) which are attached to (附着于) solar panels to store the electricity. So the stored electricity can be used to power cars at night or on cloudy days.

Did you know that solar energy is not as expensive as it used to be? In fact, in the last few years.solar panels have become much more affordable for people to use. Since the sun gives off more energy in one second than people have used, imagine what we could do if we used the sun’s energy to power our planet.

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 learned about Venus for a long time. First they looked at Venus through telescopes (望远镜). But scientists could not see Venus well because of thick clouds on Venus. 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 and mountains! And in some ways, Venus looks like the earth.