Our planet is like a bit spaceship. Our atmosphere acts【51】a shield against harmful radiation and space debris.【52】holds the air and people on the spaceship,【53】everything doesn't float away. Each of us is an astronaut and our spaceship, the earth, provides our food and water. The sun is our energy【54】and without it we would not be able to survive. 【55】every spaceship, the earth is a closed system. The only thing we receive from the outside is energy from the sun. Everything else must be used【56】again. We astronauts don't seem to realize that this spaceship is the【57】one we've got. We【58】its resources and waste the energy the plants have stored from the sun. If we【59】our spaceship, we will destroy ourselves as well. Maybe if we look at the universe which surrounds us, we will appreciate our own spaceship more and take【60】care of it. (51)