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  3. (1)Three decades after the first Apollo landing on the moon, the debate betw

(1)Three decades after the first Apollo landing on the moon, the debate betw

游客2024-11-21  0
问题     (1)Three decades after the first Apollo landing on the moon, the debate between proponents of manned and unmanned space missions has not changed a great deal. But many space scientists, who work with robotic satellites including me, have gradually moved from opposing human spaceflight to a more moderate position. In special situations, we now realized, sending people into space is not just an expensive stunt but can be more cost-effective than sending robots. Mars exploration is one of those cases.
    (2)The basic advantage of astronauts is that they can explore Mars in real time, free of communications delays and capable of following up interesting results with new experiments. But the question arises: Where should the astronauts be? The obvious answer—on the surface of Mars—is not necessarily the most efficient At the first "Case for Mars" conference in 1981, one of the more provocative conclusions was that the Martian moons, Phobos and Deimos, could serve as comparatively inexpensive beachheads.
    (3)Most current mission scenarios involve a pair of spacecraft. The first positions propellants and other heavy components, such as spare modules and re-entry vehicle, on or near Mars. Because the journey time is not crucial, it can use electric propulsion and gravity-assist procedures to reduce the cost The story is rather different for the second spacecraft, which transports the astronauts. It must traverse Earth’s radiation belts rapidly, and to save on supplies, the transit time to Mars should be as short as possible.
    (4)The carious mission plans part ways when it comes to deciding what should happen once the crew ship and the freight ship link up at the Red Planet. In order of increasing difficulty and expense, six possible scenarios are: a Mars flyby analogous to the early Apollo missions, with immediate return to Earth; a Mars orbiter, permitting a longer stay near the planet; a Phobos-Deimos(Ph-D)mission, involving a transfer to a circular, equatorial orbit, with a landing and base on a Martian moon, preferably Deimos; a hybrid mission(Ph-D-plus)that adds a brief sortie to the Martian surface; a full-scale Martian landing, with a longer stay on the surface and a complete program of research; and finally, an extended stay on Mars, during which astronauts erect permanent structures and commence continuous habitations of the planet.
    (5)The trick will be to make sure the first manned mission is ambitious—the adventure is, after all, part of the attraction—but not too ambitious, lest it not win funding. The Ph-D and Ph-D-plus missions offer a compelling balance of cost and benefit and would provide the greatest return for science.
    (6)Deimos would offer an excellent base for the study of Mars. From there the astronauts could deploy and control atmospheric probes, subsurface penetrators and rover vehicles all over the Martian surface. The moon’s near-synchronous orbit permits direct contact with a rover for about 40 hours at a time. Phobos, being closer to the planet, orbits faster and therefore lacks this particular advantage. But astronauts on either moon could analyze returned samples without fear of containing Earth with any Martian life-forms.
    (7)The ready availability of a vacuum would make it easier to operate laboratory instruments such as mass spectrometers and electron microscopes. By relocating the spacecraft to different locations on Deimos—an easy task in the minuscule gravity—astronauts could protect themselves from solar storms and meteor streams. Besides, the moons are fascinating bodies in their own right; direct sampling would investigate their mysterious origins.
    (8)In comparison, an operating base on the surface of Mars would suffer many handicaps. Rovers deployed elsewhere on the planet would still have to be operated by remote control, which would require a satellite communications system to relay the commands. Returning samples from distant locations to the base would be more difficult. Heavy backup batteries or nuclear generators would be needed to power the base at night or during dust storms.
    (9)In the more distant future, the moons could serve as way stations for descent to or ascent from the surface via tethers. Scientists on Deimos could safely direct large-scale climatological experiments, such as altering weather patterns or melting the polar caps—thereby testing techniques for terraforming Mars or mitigating climate change on Earth.
    (10)Although the costs and benefits of various mission scenarios are difficult to analyze at this early stage, I conducted a poll of Mars mission experts during a conference several years ago. It offers the full spectrum of science more cheaply and quickly, and it would set the stage for an eventual base and colony on the surface. [br] What allows a longer stay near the planet?
选项
答案 A Mars orbiter．
解析 根据第4段中的a Mars orbiter，permitting a longer stay near the planet可知，Mars orbiter可长时间停留在行星附近，所以答案为A Mars orbiter。
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