单选题 The future of space exploration depends on many things. It depends on how technology evolves, how political forces shape competition and partnerships between nations, and how important the public feels space exploration is. The near future will see the continuation of human space fright in Earth"s orbit and unpiloted space flight within the solar system. Piloted space flight to other planets, or even back to the moon, still seems far away. Any flight to other solar systems is even more distant, but a huge advance in space technology could drive space exploration into realms currently explored only by science fiction.
The 1968 film 2001: A Space Odyssey depicted commercial shuttles flying to and from a giant wheel-shaped space station in orbit around Earth, bases on the moon, and a piloted mission to Jupiter. The real space activities of 2001 will not match this cinematic vision, but the 21st century will see a continuation of efforts to transform humanity into a spacefaring species.
Perhaps the most difficult problem space planners face is how to finance a vigorous program of piloted space exploration, in Earth"s orbit and beyond. In 1998 no single government or international enterprise had plans to send people back to the moon, much less to Mars. Such missions are unlikely to happen until the perceived value exceeds their cost.
One belief shared by a number of space exploration experts is that future lunar and Martian expeditions should be aimed at creating permanent settlements. The residents of such outposts would have to "live off the land," obtaining such necessities as oxygen and water from the harsh environment. On the moon, pioneers could obtain oxygen by heating lunar soil. In 1998 the Lunar Prospector discovered evidence of significant deposits of ice, a valuable resource for settlers, mixed with soil at the lunar poles. On Mars, oxygen could be extracted from the atmosphere and water could come from buried deposits of ice.
The future of piloted lunar and planetary exploration remains largely unknown. Most space exploration scientists believe that people will be on the moon and Mars by the middle of the 21st century, but how they get there, and the nature of their visits, is a subject of continuing debate. Clearly, key advances will need to be made in lowering the cost of getting people off Earth, the first step in any human voyage to other worlds.

单选题 A flight to other solar systems will be made more possible by

单选题 It can be inferred from Paragraph 2 that human society will become increasingly

单选题 According to this text, piloted space missions will need to be

单选题 It is predicted that people who will live on other planets would have to

单选题 It is difficult to send people to other planets because of