[填空题]

Until the very latest moment of his existence, man has been bound to the planet on which he originated and developed. Now he had the capability to leave that planet and move out into the universe S1.（） to those worlds which he has known previously only directly. Men S2.（）have explored parts of the moon, put spaceships in orbit around another planet and possibly within the decade will land into another planet S3.（） and explore it. Can we be too bold as to suggest that we may be S4.（） able to colonize other planet within the not-too-distant future S5.（） Some have advocated such a procedure as a solution to the population problem: ship the excess people off to the moon. But we must keep in head the billions of dollars we might spend in carrying out the project. S6.（） To maintain the earth’s population at its present level, we would have to blast off into space 7,500 people every hour of every day of the year.
Why are we spending so little money on space exploration S7.