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.

Here are billions of stars and planets in space. What’s the difference ______ stars _______ ( 71 ) planets ? Stars can give out heat and light, but planets cannot.

The sun is one of the stars . Some planets are balls of rock and some are balls of gas. We live on the planet Earth. The earth spins(自转）around once every day to give us day and light.( 72 ) The earth goes around once every year to give us different seasons. There are eight planets ________ (move)（73) around the sun. We call the family of the sun and its planets the solar system. Mercury( 水星 ) is the nearest to the sun, so it is very hot. It is the smallest planet. Venus(金星)is very bright and hot. It is the nearest to the earth. Mars is covered with orange-red dust and rocks, so it looks like a red star in the sky. It has seasons like the earth. Jupiter(木星) has more than 16 moons( 卫星 ) and it is the biggest and heaviest planet. Saturn(土星)has more than(74) 22 moons. The other planets such as Uranus(天王星) and Neptune( 海王星), are all very far from the sun and very cold. We need a telescope(   望远镜 ) to see them.

Nowadays, a growing number of countries realize the importance of space and pay more attention to their space programs. Space exploration（探索） around the world has entered a new stage of rapid development that will have great influence on human society. Here is how China plans to take actions on space programs.