pale blue dot -carl sagan-第41节
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
hazard）。 A space station is also unnecessary for human exploration of the Moon。 Apollo got there very well with no space station at all。 With Saturn V or Energiya class launchers； it also may be possible to get to near…earth asteroids or even Mars without having to assemble the interplanetary vehicle on an orbiting space station。
A space station could serve inspirational and educational purposes； and it certainly can help to solidify relations among the spacefaring nations—particularly the United States and Russia。 But the only substantive function of a space station； as far as I can see； is for long…duration spaceflight。 How do humans behave in micro gravity？ How can we counter progressive changes in blood chemistry and an estimated 6 percent bone loss per year in zero gravity？ （For a three… or four…year mission to Mars this adds up； if the travelers have to go at zero g。）
These are hardly questions in fundamental biology such as DNA or the evolutionary process； instead they address issues of applied human biology。 It's important to know the answers； but only if we intend to go somewhere in space that's far away and takes a long time to get there。 The only tangible and coherent goal of a space station is eventual human missions to near…Earth asteroids； Mars； and beyond。 Historically NASA has been cautious about stating this fact clearly； probably for fear that members of Congress will throw up their hands in disgust； denounce the space station as the thin edge of an extremely expensive wedge； and declare the country unready to mit to launching people to Mars。 In effect； then； NASA has kept quiet about what the space station is really for。 And yet if we had such a space station； nothing would require us to go straight to Mars。 We could use a space station to accumulate and refine the relevant knowledge； and take as long as we like to do so—so that when the time does e； when we are ready to go to the planets； we will have the background and experience to do so safely。
The Mars Observer failure； and the catastrophic loss of the space shuttle Challenger in 1986； remind us that there will be a certain irreducible chance of disaster in future human flights to Mars and elsewhere。 The Apollo 13 mission； which was unable to land on the Moon and barely returned safely to Earth； underscores how lucky we've been。 We cannot make perfectly safe autos or trains even though we've been at it for more than a century。 Hundreds of thousands of years after we first domesticated fire； every city in the world has a service of firefighters biding their time until there's a blaze that needs putting out。 In Columbus' four voyages to the New World； he lost ships left and right； including one third of the little fleet that set out in 1492。
If we are to send people； it must be for a very good reason—and with a realistic understanding that almost certainly we will lose lives。 Astronauts and cosmonauts have always understood this。 Nevertheless； there has been and will be no shortage of volunteers。
But why Mars？ Why not return to the Moon？ It's nearby； and we've proved we know how to send people there。 I'm concerned that the Moon； close as it is； is a long detour； if not a dead end。 We've been there。 We've even brought some of it back。 People have seen the Moon rocks； and； for reasons that I believe are fundamentally sound； they are bored by the Moon。 It's a static； airless； waterless； black…sky； dead world。 Its most interesting aspect perhaps is its cratered surface； a record of ancient ；catastrophic impacts； on the Earth as well as on the Moon。
Mars； by contrast； has weather； dust storms； its own moons； volcanos； polar ice caps； peculiar landforms； ancient river valleys； and evidence of massive climatic change on a once…Earthlike world。 It holds some prospect of past or maybe even present life； and is the most congenial planet for future life—humans transplanted from Earth； living off the land。 None of this is true for the Moon。 Mars also has its own legible cratering history。 It Mars； rather than the Moon； had been within easy reach； we would not have backed off from manned space flight。
Nor is the Moon an especially desirable test bed or way station for Mars。 The Martian and lunar environments are very different； and the Moon is as distant from Mars as is the Earth。 The machinery for Martian exploration can at least equally well be tested in Earth orbit； or on near…Earth asteroids； or on the Earth itself—in Antarctica； for instance。
Japan has tended to be skeptical of the mitment of the United States and other nations to plan and execute major cooperative projects in space。 This is at least one reason that Japan； more than any other spacefaring nation； has tended to go it alone。 The Lunar and Planetary Society of Japan is an organization representing space enthusiasts in the government； universities； and major industries。 As I write； the Society is proposing to construct and stock a lunar base entirely with robot labor。 It is said to take about 30 years and to cost about a billion U。S。 dollars a year （which would represent 7 percent of the present U。S。 civilian space budget）。 Humans would arrive only when the base is fully ready。 The use of robot construction crews under radio mand from Earth is said to reduce the cost tenfold。 The only trouble with the scheme； according to reports； is that other scientists in Japan keep asking； 〃What's it for？〃 That's a good question in every nation。
The first human mission to Mars is now probably too expensive for any one nation to pull off by itself。 Nor is it fitting that such a historic step be taken by representatives of only a small fraction of the human species。 But a cooperative venture among the United States； Russia； Japan； the European Space Agency—and perhaps other nations； such as China—might be feasible in the not too distant future。 The international space station will have tested our ability to work together on great engineering projects in space。
The cost of sending a kilogram of something no farther away than low Earth orbit is today about the same as the cost of kilogram of gold。 This is surely a major reason we have yet to stride the ancient shorelines of Mars。 Multistage chemical rockets are the means that first took us into space； and that's what we've been using ever since。 We've tried to refine them； to make there safer； more reliable； simpler； cheaper。 But that hasn't happened； or at least not nearly as quickly as many had hoped。
So maybe there's a better way： maybe single…stage rockets that can launch their payloads directly to orbit； maybe many small payloads shot from guns or rocket…launched from airplanes； maybe supersonic ramjets。 Maybe there's something much better that we haven't thought of yet。 If we can manufacture propellants for the return trip from the air and soil of our destination world； the difficulty of the voyage would be greatly eased。
Once we're up there in space； venturing to the planets； rocketry is not necessarily the best means to move large payloads around； even with gravity assists。 Today； we make a few early rocket burns and later midcourse corrections； and coast the rest of the way。 But there are promising ion and nuclear/ electric propulsion systems by which a small and steady acceleration is exerted。 Or； as the Russian space pioneer Konstantin Tsiolkovsky first envisioned； we could employ solar sails—vast but very thin films that catch sunlight and the solar wind； a caravel kilometers wide plying the void between the worlds。 Especially for trips to Mars and beyond； such methods are far better than rockets。
As with most technologies； when something barely works； when it's the first of its kind； there's a natural tendency to improve it； develop it； exploit it。 Soon there's such an institutional investment in the original technology； no matter how flawed； that it's very hard to move on to something better。 NASA has almost no resources to pursue alternative propulsion technologies。 That money would have to e out of near…term missions； missions which could provide concrete results and improve NASA's success record。 Spending money on alternative technologies pays off a decade or two in the future。 We tend to be very little interested in a decade or two in the future。 This is one of the ways by which initial success can sow the seeds of ultimate failure； and is very similar to what sometimes happens in biological evolution。 But sooner or later some nation—perhaps one without a huge investment in marginally effective technology—will develop effective alternatives。
Even before then； if we take a cooperative path； there will e a time—perhaps in the first decades of the new century and the new millennium—when an interplanetary spacecraft is assembled in Earth orbit； the progress in full view on the evening news。 Astronauts and cosmonauts； hovering like gnats； guide and mate the prefabricated parts。 Eventually the ship； tested and ready； is boarded by its international crew； and boosted to escape velocity。 For the whole of the voyage to Mars and back； the lives of the crew members depend on one another； a microcosm of our actual circumstances down here on Earth。 Perhaps the first joint interplanetary mission with human crews will be o
