The age of space science began less than thirty years ago．But already people have made GREat discoveries and have done exciting things．They have walked in space，visited the moon，and lived in space laboratories for long periods of time．Some scientists are now working on ways to make it possible for people to live on other planets．They are trying to solve the problems of living in space，even though they don't yet know what all of the problems willbe．For example，they don't yet know what problems people would have if they lived in a weightless（无重力的）environment for several years．
If you were a space traveller you might have to spend several months，or even years，in a spaceship． While you were in the ship you wouldn't live as you usually do．You might have to find new ways to spend your time．For example，you would have to take a huge supply of food with you because you can't go to the supermarket when you're halfway to another planet．You would also need fresh water and a way to get rid of waste． In fact，there would be many problems to solve in order to live in space．
