填空题

The first flight of the Space Shuttle Columbia in the spring of 1981 was a revolutionary development in space (36) . Unlike the rockets of the previous 20 years, Columbia has the (37) advantage of being specially designed to return to earth and make (38) flights. Among the large number of projects scientists are (39) for its future use, the most exciting, perhaps, is that the first steps can now be taken towards (40) human (41) in space. The present Space Shuttle can only stay in space for about a week, but it could take people out to (42) "islands" where they stay for longer periods. By the year 2050, scientists estimate that man will be able to construct permanent (43) The first "islands" would be energy stations. (44) . However, if the original could (45) , allowing the Space Shuttle to refuel and stay in orbit longer periods of time. If this initial project was successful, much larger solar energy stations would be constructed later with materials brought out by the Shuttle. (46) .