Most people have a place to call home on Earth. What about in outer space?     21     It welcomed its first crew that year, paving (铺平) the way for 22 years of future research.

“You just have this impression that we’re all citizens of not a country, but of the planet,” former NASA astronaut Scott Kelly told National Geographic about working in the ISS.     22    

Over the past 22 years, the ISS has mainly been used as a science lab, hosting 241 people from 19 countries, according to NASA. They went on spacewalks, did research on how space affects the human body, and even grew plants in space.

    23     For example, the station’s water purification (净化) technology has been used in poor areas to provide clean drinking water. The station also helps to warn us of natural disasters, such as earthquakes.

However, after being our home in space for over 7,000 days, the station is starting to get old. It has leaked (漏出) air a few times.     24     After that, scientists will bring it down to Earth. It is believed that Earth’s atmosphere (大气) will burn up most of the station. Other debris (残骸) will fall into the Pacific Ocean.

In the future, space will be home to more stations like the ISS. NASA is building a space station that is smaller than the ISS near the moon.     25    

From a plane you can see the fields,     26    , mountains or rivers below. If we go into space, we see more and more of the earth. People and man-made satellites are     27     out into space to look at the earth, the stars and other     28     and learn more about them.

The moon is our     29     neighbour in space. It is our satellite and men have visited it already. It is smaller than the earth.     30     can’t grow on the moon because there’s no air or water on it.

The moon hasn’t its own light. It     31     back the light of the sun. When the sun is shining on the side of the moon that faces us, it     32     like a great round ball. We can’t see the moon when the sun is shining on the side of the moon that faces away from     33    .

In fact the moon isn’t as near as it looks. It is three     34     and eighty thousand kilometres away from the earth. Of course we can fly to the moon in a spaceship. A spaceship     35     very, very fast, and it flies at about eleven kilometres a second. So it only takes more than three days to get there by spaceship.