星际航行:还要多久才能实现?

今年2月23日，NASA宣布首次在距离地球约40光年的地方发现七颗类地行星，其中有三颗位于其母恒星的宜居带。这一发现为地外生命的探寻开辟了新的方向，也再一次点燃了人们对星际航行的兴趣和热情：我们将利用星际航行前往何处？怎样才能实现星际航行？具体需要突破哪些技术限制？且随本文一起了解星际航行的发展现状和前景吧！

科幻小说家和电影制作人已经为我们展现了人类足迹遍布宇宙的无数种构想，所以你如果觉得星际航行这件事已十拿九稳，那也情有可原。令人失望的是，我们还需要突破不少技术限制――比如我们目前所理解的物理学定律――才能走出我们的太阳系和银河系，开始征服新世界的旅程。

尽管如此，近年来还是出现了诸如Tau Zero基金会、伊卡洛斯计划以及突破摄星等多个由私人出资或志愿发起的项目，希望使我们距离跨越宇宙的目标更近一些。我们8月份发现了一颗与地球大小相似的行星（编注：英文原文发表于2016年10月），围绕离我们最近的恒星运行，这也为我们造访外星世界增添了新希望。

真的有可能航行去其他星系吗？如果可能的话，我们需要怎样的太空飞船才能实现这一目标呢？继续读下去就能（以超光速）了解最新进展。

我们会去哪儿？

我们不会去哪儿呢？宇宙中恒星的数量比地球上沙子的数量还多，大约有700万亿亿颗，其中数十亿恒星估计都有一到三颗行星位于所谓的“金发姑娘区”：那里既不是太热，也不是太冷。

既然我们刚刚起步，目前最有希望抵达的恒星是距离我们最近的恒星邻居――由三颗恒星组成的半人马座α星系统，位于4.37光年以外。今年，欧洲南方天文台的天文学家们发现了一颗与地球体积相当的行星，围绕半人马座α星的红矮星――比邻星运行。这颗行星名为“比邻星b”，至少是地球质量的1.3倍，但是由于其运行轨道与比邻星非常贴近，只需11个地球日就可以环绕比邻星一周。让天文学家和外行星观测者兴奋不已的是，这颗行星具有适合液态水存在的合适温度，而这是宜居性的有用表征。

它的缺点在于，我们不知道它是否有大气层，而且鉴于它和比邻星的距离如此之近――比水星环绕我们太阳的轨道还要近――它很可能暴露在危险的母恒星耀斑和辐射之下。而且它还是潮汐锁定的，这意味着这颗行星总是用同一面朝向它的母恒星。这会彻底改变我们对于黑夜和白天的认识。

我们如何到达那里？

这是个价值64万亿美元的问题。即使以我们目前科技所能实现的最快速度去比邻星b转一圈，大概也要花上18，000年才能抵达，而在这么长的时间里，我们在地球上的人类后代大有可能已经早早超越我们抵达那里，抢走所有的风头。但是许多聪明的头脑和鼓鼓的腰包都加入到了寻找更快穿越广阔空间方法的挑战中。

“突破摄星”是一项由俄罗斯亿万富翁尤里和茱莉亚・米尔纳夫妇私人投资的价值一亿美元的项目，专门研究如何驱动微型无人探测器，其驱动方法是使基于地面的强大激光束作用于探测器上重量超轻的光帆。这个项目认为，如果宇宙飞船足够小――小到不足一克重――光帆又足够轻，激光的驱动力就足以使飞船逐渐加速到接近光速的1/5，用大约20年的时间抵达半人马座α星。

米尔纳夫妇期望微型化技术能使这个微型飞船携带摄像机、推进器、电源供应装置、通讯及导航设备，以便能够在与比邻星b擦肩而过时，将所观测到的内容发送回来。希望我们能等来好消息，因为这会为星际航行的下一个难度更大的阶段――载人航行――打下基础。

曲速引擎怎么样？

虽然在《星际迷航》里看起来很容易，但是我们目前所知的一切物理定律都告诉我们，超光速飞行甚或是光速飞行，都是不可能的。这并不是说科学已经就此认输了。受科幻小说家们想象出来的另一种驱动系统的启发，NASA的演进氙离子推进器引擎项目正在研发离子引擎，希望将太空飞船的速度提升到90，000迈（145，000千米/小时），而所耗燃料比传统火箭要少得多。

但即便是以这样的速度，我们在一代太空探索者的时间里也无法走出太阳系太远。在搞懂如何实现时空翘曲之前，星际航行意味着要搭乘一艘非常缓慢的太空船去向未来。也许最好将这种太空之旅本身视为目标，而不是将其看作实现某种目标的手段。

我们在星际旅行中如何生存？

曲速引擎和离子推进都很诱人，但是如果我们的星际旅行者们甚至在远未离开自己的太阳系之前就因饥饿、脱水或难以呼吸而死去，那么这些技术终究还是没什么用。瑞秋・阿姆斯特朗研究员认为，我们需要开始思考星际旅行的人们在途中身处怎样的生态系统这一问题了。“我们正从以工业视角考虑现实转向以生态视角考虑现实。”她说。

作为英国纽卡斯尔大学试验建筑专业的教授，阿姆斯特朗谈到了“构建有机世界”这一概念，她说道：“该理念所关心的是如何在空间中居住，而不仅仅是设计一个标志性的物体。”她认为，今天的太空飞船或空间站的内部毫无生机，非常工业化。阿姆斯特朗相信，我们应该转而思考一下我们飞船的生态性――在其中栽培什么样的植被，甚至是我们携带什么样的土壤。在未来，她设想了大型的生物群落，充满有机生命体，而不像今天那样，都是些冰冷的金属盒子。

我们不能全程睡眠吗？

如何使人们在超过人类寿命长度的太空旅程中保持生存状态是个棘手的问题，针对这一问题受推崇的解决方案是低温休眠、冬眠或某种形式的静止状态。阿尔科生命延续基金项目装满了低温贮藏的身体和头部的设施证明，人们乐观地相信，我们终有一天能够搞清楚如何安全地将人们冷冻和解冻，但同样地，这样的技术目前不存在。

电影《星际穿越》和尼尔・斯蒂芬森的小说《七夏娃》等作品提出了一种建议：由于胚胎不用饮食或呼吸，可以将冷冻胚胎送上星际旅程使其（理论上讲）能够在旅程的艰难环境中存活下来。但是这又引发了“鸡生蛋还是蛋生鸡”的问题：他们抵达目的地之后，谁来养育这些初到人世的婴儿呢？

星H旅行真的会发生吗？

虽然在本文读者的有生之年可能无法实现，但是在更长的时间范围内，还是有理由持乐观态度的。“自人类存在之初，我们就抬头仰望群星，向它们寄托我们的希望和恐惧，焦虑和梦想。”阿姆斯特朗说道。随着诸如“突破摄星”这样的项目来解决工程问题，“这已经不再只是梦想，而是进入了实验阶段”。

Science fiction writers and moviemakers have shown us countless visions of humanity spread out across the Universe， so you might be forgiven for thinking that we’ve already got this in the bag1）. Unfortunately， we still have more than a few technical limitations to overcome―like the laws of physics as we understand them―before we can start colonising new worlds beyond our Solar System and galaxy.

That said2）， several privately funded or volunteer initiatives such as the Tau Zero Foundation3）， Project Icarus4） and Breakthrough Starshot5） have emerged in recent years， each hoping to bring us a little bit closer to reaching across the cosmos. The discovery in August of an Earth-sized planet orbiting our nearest star has also raised fresh hopes about visiting an alien world.

Is travelling to other galaxies possible？ And if so， what kinds of spacecraft might we need to achieve it？ Read on to get up to （warp6）） speed.

WHERE WOULD WE GO？

Where wouldn’t we go？ There are more stars in the Universe than there are grains of sand on Earth―around 70，000，000，000，000，000，000，000―and billions of these are estimated to have one to three planets in the so-called “Goldilocks zone7）”： not too hot， not too cold.

As we’re just starting out， the best contender so far is our nearest stellar neighbor―the triple star system of Alpha Centauri8）， 4.37 light-years away. This year， astronomers at the European Southern Observatory9） discovered an Earth-sized planet orbiting Alpha Centauri’s red dwarf star10） Proxima Centauri11）. The planet， named Proxima b， is at least 1.3 times the mass of the Earth but has a very tight orbit around Proxima Centauri， taking just 11 Earth days to complete the trip. What has astronomers and exoplanet12） hunters especially hot under the collar13） is that this planet is in the right temperature range for liquid water， which is a useful proxy14） for habitability.

The downside is we don’t know if it has an atmosphere， and given its closeness to Proxima Centauri―closer than the orbit of Mercury around our Sun―it would likely be exposed to dangerous solar flares15） and radiation. It is also tidally-locked16）， which means the planet always presents the same face to its star; something that would completely alter our notions of night and day.

HOW WOULD WE GET THERE？

That’s the $64 trillion question. Even at the fastest speeds of our current technology， a quick jaunt17） to check out Proxima b would see us arriving in around 18，000 years， by which time there’s every chance our Earth-bound descendants would have arrived there well ahead of us and grabbed all the glory. But many smart minds―and deep pockets18）―are being turned to the challenge of finding a faster way to cross vast distances of space.

Breakthrough Starshot―a $100 million initiative privately funded by Russian billionaires Yuri and Julia Milner―is focusing on propelling a tiny unmanned probe by hitting its extremely lightweight sail with a powerful Earth-based laser. The idea is that if the spacecraft is small enough―and we’re talking barely a gram―and the sail light enough， the impact of the laser will be enough to gradually accelerate the craft to around one-fifth of the speed of light， taking it to Alpha Centauri in around 20 years.

The Milners are counting on miniaturisation technologies to enable this tiny craft to carry a camera， thrusters19）， a power supply， communication and navigation equipment so it can report on what it sees as it flashes past Proxima b. Hopefully the news will be good， because that will lay the foundation for the next and more difficult stage of interstellar travel： people-moving.

WHAT ABOUT WARP DRIVE20）？

Star Trek made it all look so easy， but everything we currently know about the laws of physics tells us that faster-than-light travel―or even travel at the speed of light―is not possible. Not that science is throwing in the towel21）. Inspired by another propulsion system that has captured the imagination of science fiction creators， NASA’s Evolutionary Xenon Thruster22） project is developing an ion23） engine which is hoped to accelerate a spacecraft to speeds up to 90，000mph （145，000km/h） using only a fraction of the fuel of a conventional rocket.

But even at those speeds， we won’t be getting far out of the Solar System within a single generation of spacefarers. Until we work out how to warp time and space， interstellar travel is going to be a very slow boat to the future. It might even be better to think of that travel period as the end itself， rather than a means to an end.

HOW WOULD WE SURVIVE ON AN INTERSTELLAR VOYAGE？

Warp drives and ion propulsion are all very sexy， but they’re not much use if our interstellar voyagers starve， dehydrate or suffocate long before they even leave our own Solar System. Researcher Rachel Armstrong argues we need to start thinking about the ecosystem that interstellar humanity will occupy out there in between the stars. “We’re moving from an industrial view of reality to an ecological view of reality，” she says.

As professor of experimental architecture at the University of Newcastle in the UK， Armstrong talks about “worlding”： “It’s about the inhabitation of spaces， not just the design of an iconic object，” she says. The inside of a spacecraft or space-station today is sterile24）， and industrial， she argues. Armstrong believes we instead need to think ecologically about our vessels―about the vegetation that is grown， and even the kinds of soils we take with us. In the future， she envisages giant biomes25）， full of organic life， not the cold， metal boxes of today.

CAN’ T WE JUST SLEEP ALL THE WAY THERE？

Cryosleep26）， hibernation or some form of stasis27） are favoured solutions to the prickly problem of how to keep people alive on a voyage that might take longer than a human lifespan. A facility full of cryopreserved28） bodies and heads at the Alcor Life Extension Foundation29） are testament to human optimism that we will one day work out how to safely freeze and thaw humans， but again， no such technology currently exists.

One suggestion， which is explored in movies such as Interstellar and books such as Neal Stephenson30）’s Seveneves， is to send frozen embryos that could―presumably―survive those hardships by virtue of not needing to eat， drink or breathe. But this raises the very “chicken and egg” problem of who would raise these fledgling humans when they arrive at their destination.

SO， WILL IT ACTUALLY HAPPEN？

Probably not in the lifetime of anyone old enough to read this article， but in the longer term， there’s cause for optimism. “From the outset of human existence we’ve looked up at the stars and projected our hopes and fears， anxieties and dreams there，” says Armstrong. And with the launch of projects to tackle the engineering， such as Breakthrough Starshot， “this is no longer just a dream， this is an experiment now.”