我们会到恒星旅行吗？，本文主要内容关键词为：恒星论文,旅行论文，此文献不代表本站观点，内容供学术参考，文章仅供参考阅读下载。
If you ask whether we will travel to the stars,the answer is probably yes,but it will take a long time.Maybe 500 years.If you ask whether any human being will travel to the stars within the 21st century,the answer is certainly no.The difference between traveling to the nearest star and traveling around our own solar system is about the same as the difference between swimming across the Atlantic and swimming across the Potomac.To get across the Atlantic you need to have a boat or an aircraft.To get to the nearest star you need to have a spacecraft that we have no hope of building within 100 years.
To scoot around the solar system and return within a few years,you need a spacecraft that will cruise at 160 km a second.At that speed you will get to Mars in 10 days,to Pluto in 16 months.We can imagine a spacecraft carrying a big area of thin film to collect solar energy,with an ion-jet engine to produce thrust powerful enough to boost a spacecraft to a speed of 160 km a second.It is also possible to build a nucle-ar-powered jet to do the same job,if the political objections to nuclear spacecraft can be overcome.The quantity of energy available from sunlight or from a nuclear reactor is large enough to take us on trips around our solar system,if we decide to spend the money to do it.We may or may not decide to build a 160-km-a-second spacecraft within 100 years,but we know that it is technically possible.The cost of developing a 160-km-a-second spacecraft would be large but not absurd.
On the other hand,the nearest star is about 10,000 times as far away as Pluto.A trip to the stars within a human lifetime requires a spacecraft that cruises at more than 16,000 km a second and accelerates to this speed within 10 years.The engine would have to deliver about a megawatt of power for every pound of weight of the ship.There is no way an engine that small and that powerful could keep itself cool.Even if the fuel is something exotic like antimatter,carrying far more en ergy than sunlight or uranium,the problem of cooling the engine remains insuperable.Travel to the stars within this century,using any kind of engine we know how to build,is not going to happen.
How about unmanned space probes going to the stars? Unmanned probes can be much smaller andlighter than manned spaceships.That means the total power required for a probe to reach the stars is much less.But the unmanned probe still needs an engine delivering one megawatt per pound.The problem of cooling the engine remains the same,whether the ship is manned or unmanned,and the conclusion is the same.Unmanned probes are not going to reach the stars within this century.
Robert Forward,an engineer who used to work for Hughes Air-craft,has designed a space probe that might reach the stars,not within this century but a little later.It avoids the problem of cooling the engine by not having an engine.It is a sailing ship,not a steamship.He calls it Starwisp.It is a fishnet made of very fine wires and weighing less than 30 g.The net acts as a sail and is driven by the pressure of radio waves generated by a huge radio transmitter.The transmitter stays put,with its radio beam pointing in the direction we wish to explore,and the sail travels along the beam,picking up momentum from the radio waves.This scheme works beautifully in theory,but there axe some practical difficulties to be overcome.The transmitter has to be gigantic and must focus the energy of the beam on the fishnet as it accelerates.The fishnet must absorb only a tiny fraction of the radio waves to avoid being vaporized.The probe must carry instruments to collect information and transmit signals back to earth,and those instruments must weigh less than 30 g.There are enough problems here to keep engineers busy for several centuries,but one day a ship like this will fly.Perhaps around the middle of the third millennium.
［参考译文］如果你要问我们能否到恒星旅行，答案是很可能，但要等很长时间才能实现，也许要等500年。如果你要问是否有人能在21世纪到恒星旅行，那么答案是肯定不能。到距离我们最近的恒星旅行，与在我们的太阳系范围内旅行，两者的区别就近似游泳横渡大西洋与游泳横渡波托马克河的区别。要横渡大西洋，你需要一艘船或一架飞机；要登上距离我们最近的恒星，你需要的是一艘我们在100年内也无望建成的航天器。
漫游太阳系，并在数年内返航，你需要一艘巡航速度为每秒160公里的航天器。以这样的速度，10天可到达火星，16个月可到达冥王星。我们可以设想一艘航天器，它装载着收集太阳能的巨大薄膜，并载有一个能产生强大推力足以将航天器加速到每秒160公里的离子喷气发动机。如果能克服政治上反对建造核航天器的意见，也可以为航天器建造一个核动力喷气发动机。如果我们决定耗资建造这样的航天器，由阳光或核反应堆提供的能量，就足以使我们在太阳系范围内旅行。不管我们是否会决定在100年内建造一艘航速每秒160公里的航天器，我们知道这在技术上是可以做到的。研制这样的一艘航天器的费用将是巨大的，但还不是高得荒诞无稽。
另一方面，距离地球最近的恒星，大约是冥王星到地球的距离的1万倍。在一个人的有生之年到恒星作一次旅行，需要一艘巡航速度超过每秒1.6万公里的航天器，而且要在10年内加速到这个速度。发动机要为航天器的每磅重量提供大约1兆瓦的能量。一个体积如此之小、功率如此之大的发动机，要保持不升温是不可能的。即使使用诸如反物质之类的某种非常燃料，它所产生的能量远比阳光或铀大得多，但是发动机的冷却问题仍然无法解决。要想使用任何一种我们能够建造的发动机，在本世纪内遨游太空，都是不可能的。
那么，能派无人驾驶的太空探测器前往恒星吗?无人驾驶探测器可以比载人航天器小得多，轻得多。这意味着一艘探测器到达恒星所需的总能量要少得多。但是无人驾驶探测器仍需要一个能为每磅重量提供1兆瓦能量的发动机。不论是载人还是无人驾驶的航天器，同样存在发动机冷却的问题，因而结论也是一样的。本世纪内，无人驾驶探测器不可能到达恒星。
曾经在休斯飞机公司工作的工程师罗伯特·福沃德，设计出一种虽不能在本世纪但有可能在稍晚一些的时候到达恒星的太空探测器。这个探测器没有发动机，从而避开了发动机冷却的问题。它是一艘帆船，而非汽船。他称其为“星丝”。它是由纤细的金属丝制成的，形似鱼网，重量不足30克。这个网在一个巨大的无线电发射器产生的无线电波压力的驱动下，像船帆一样运作。发射器固定不动，它的无线电波指向我们希望探测的方向；探测器沿着电波的方向运动，从无线电波获取动力。这个设计在理论上很完美，但是有些实际难题有待解决。无线电发射器必须非常大，而且在探测器加速时必须将能量来聚集其上。为了防止被蒸发，探测器只能吸收微量的无线电波。探测器必须装载收集信息并将信号传回地球的仪器，而且这些仪器的重量必须轻于30克。这些难题足够让工程师们忙上几百年，但可能终有一天这样的航天器将遨游太空。可能要到第三个千年的中期。