It is the second-brightest object in our night sky after the moon. It is one of man's greatest scientific achievements. It's the International Space Station（ISS）that orbits（绕……运行）Earth.

Nov. 20 marks the station's 20th birthday. In 1998, the first piece of the ISS was sent into space. Since then, 230 people from 18 countries have visited it.

Five space agencies（空间站）—those of the United States, Russia, Canada, Europe and Japan—worked together to build it. Weighing about 450 tons, it contains many different parts, including labs and solar panels（太阳能板）

Generally, the ISS holds three to six people at one time. Astronauts living there work on a lot of different science projects. The ISS has hosted over 1,500 such projects, according to NASA.

But the station is becoming older. In 2011, Russia announced that the ISS will not be used in 2020 in order to prevent it from becoming dangerous "space junk". But the US later said it would use repairs to make its service life longer（to 2025）, the International Business Times reported.

The ISS costs $3-4 billion（21-28 billion yuan）each year. Many have argued that this money could be better spent improving conditions on Earth.

In February, US President Donald Trump announced plans to end support for the ISS by 2025.The US will use its money for other projects, like next-generation rockets and deep-space missions. The Washington Post reported.

What will happen to the ISS after 2025 is unknown. "There are lots of uses," former（前）astronaut Jeffrey Hoffman told NBC News. "Other ISS partners could continue scientific experiments on it. Private companies could turn it into a space hotel."

We live on the Earth. It’s our home. But how much do you know about it? Let me tell you something about our Earth.

The Earth is like a huge ball. Like the other seven planets, the Earth is running around the Sun. It’s the third nearest planet to the Sun. It takes the Earth about 365 days to run around the Sun. At the same time, the Earth is going around itself. If you are in space, you can see lots of white clouds over the surface of the Earth. Through the clouds, you can see the blue color of the oceans and the brown color of the land. About 70% of the Earth is covered with water.

Why do we have day and night? When the half of the Earth is facing the Sun, it’s daytime. As the Earth turns and this half is away from the Sun, night is coming. And it’s daytime for the other half.

We often think that the sun rules the day and the moon rules the night. That’s kind of right, but not totally.

The sun shines all the time. It shines during the day when we see it, and even at night when we don’t.

As the earth turns, sometimes we face the sun. That’s when we get day. When the earth’s spin (自转) turns us away from the sun and we are in the earth’s shadow. we get night. When people on the opposite side of the earth face the sun, those people have day while we have night.

Unlike the sun, the moon doesn’t make its own light. Moonlight is actually the sun’s light reflecting the moon back to us.

The sun lights up only the side of the moon that faces the sun. At the full moon, the moon is on the opposite side of the earth. Then the side of the moon facing the earth is all lit up. During the full moon, the moon rises around sunset, shines all night, and sets around sunrise. Since the moonlight is so bright, you can sometimes see it during the day, too. A few days before the full moon, if you look east in the afternoon, you may see the moon in the sky. After the full moon, you may find it if you look west in the morning.

It is difficult to see the moon at the new moon, when its orbit (轨道) puts it between the earth and the sun. Then the side of the moon facing the earth is dark and we can’t see the moon at all.