星际旅行：路漫漫其修远兮？

伤我心太深

【导读】近日丽贝卡-波意尔（Rebecca Boyle）撰文指出，根据最新预测在2211年前人类将没有足够能力进行星际旅行。当然，这个预测比其他预测还是早了许多的。人类真的将无法离开地球的摇篮吗？

      

       格里泽581恒星系统（Gliese 581 System）艺术家构想的581系统中的行星。近景的行星G，被认为适宜人类居住。不幸的是，最新研究表明，在很长很长的一段时间内我们没有能力到达其他恒星系统。图片来自NASA，丽娜特-库克（Lynette Cook)。

       一位NASA的前驱动力学科学家声称经过他最近的计算，至少200年内星际航行是不可能的。届时，我们现在为航行设计的太空飞船都将被废弃。

      忽略经费、政治和所有其他因素，只考虑能量因素就至少得到2196年才能具备足够动力，马克-米尔斯（Marc Millis)如是说。他是NASA推进物理研究室的前负责人和终极宇航基地（为星际航行做理论支持）创始人。

       米尔斯通过大量的推理得到了这个结论，去年夏天在布拉格的一个天文会议上他就发布了这个结论并于这周投给了《物理档案 》（physics archive）。他仔细研究了过去27年的数据，能量发展趋势、星际任务的能量需求、个人能量消耗甚至社会优先使用能量都是他的考虑因素，最终他选择了两条航线：没有目的地的星际航行和长达75年的航行到达α半人马座。

      他调查了过去三十年发射航天飞机所需的能量，只是国家全部可用能量的一小部分。于是他假设能投入到星际航行中的能量仍是相同的比率。（他在“技术评论”arXiv博客上做了解释）

        米尔斯指出一艘500人的单向飞船需要至少1EJ（1EJ=1.0E+18 J）的能量，这只比全世界一年消耗的能量少那么一点点。而驶向α半人马座的无人飞船需要更多的能量，因为在到达的时候需要能量让它减速。这需要10EJ。甚至不考虑燃料因素，500人的飞船最早也要在2200年左右才能凑集那么多能量进行发射，α半人马座则至少要到2500年左右了。

       米尔斯的计算实际上已经比其他研究乐观很多了，他们认为至少需要全世界100倍的能量才能航行同样的距离。

        尽管如此，这意味着我们几代后的子孙将能亲身感受星际“大航海”时代的荣光。


We Won't Have Enough Power For Interstellar Travel Until At Least 2211
We Won't Have Enough Power For Interstellar Travel Until At Least 2211, According to New Calculations
On the bright side, that's sooner than others suggest
By Rebecca Boyle
Gliese 581 System This artist's concept shows the inner planets of the Gliese 581 system. Planet G, shown in foreground, is thought to be in its star's habitable zone. Unfortunately, we won't be able to visit other solar systems for a very, very long time, according to a new study. Lynette Cook/via NASA

Interstellar travel won’t be possible for at least 200 years, according to a former NASA propulsion scientist who has some new calculations. And by then, the spaceships we would design for the trip will be obsolete.

Forget cost, political will and all the other variables — simply obtaining enough energy will take until 2196, according to Marc Millis, former head of NASA's Breakthrough Propulsion Physics Project and founder of the Tau Zero Foundation, which supports interstellar travel research.

Millis did plenty of extrapolating to reach this conclusion, which he presented at an astronomy meeting in Prague last fall and posted to the physics archive this week. He crunched 27 years of data on energy trends, mission energy requirements, individual energy use and even societal priorities, and chose two possible trips: An aimless interstellar colony ship, and a 75-year-long mission to Alpha Centauri.

Millis examined the energy required to launch the space shuttle during the past 30 years, which is a fraction of the nation’s total available energy. He assumes the same ratio will hold for interstellar flight, as Technology Review’s arXiv blog explains.

For a 500-person ship on a one-way journey, Millis figures you would need at least an exajoule — that’s 1.0E18 joules — which is just a little bit less than all the energy consumed by the entire world in one year. For an unmanned ship destined for Alpha Centauri, you would actually need more energy, because you’d want to slow it down upon arrival at our nearest neighboring star. This would require 1.0E19 joules. Even without accounting for fuel, the 500-passenger ship wouldn’t be able to launch until around 2200 at the earliest, and the A. Centauri probe won’t be ready until around 2500.

Millis’ math is actually more optimistic than other studies, which have suggested you would need 100 times the world’s total energy output to cover that distance.

Still, it means we all have to live vicariously through Voyager for at least the next few generations.