今年2月23日，NASA宣布首次在距離地球約40光年的地方發(fā)現(xiàn)七顆類地行星，其中有三顆位于其母恒星的宜居帶。這一發(fā)現(xiàn)為地外生命的探尋開辟了新的方向，也再一次點(diǎn)燃了人們對(duì)星際航行的興趣和熱情：我們將利用星際航行前往何處？怎樣才能實(shí)現(xiàn)星際航行？具體需要突破哪些技術(shù)限制？且隨本文一起了解星際航行的發(fā)展現(xiàn)狀和前景吧！

科幻小說家和電影制作人已經(jīng)為我們展現(xiàn)了人類足跡遍布宇宙的無數(shù)種構(gòu)想，所以你如果覺得星際航行這件事已十拿九穩(wěn)，那也情有可原。令人失望的是，我們還需要突破不少技術(shù)限制——比如我們目前所理解的物理學(xué)定律——才能走出我們的太陽系和銀河系，開始征服新世界的旅程。

盡管如此，近年來還是出現(xiàn)了諸如Tau Zero基金會(huì)、伊卡洛斯計(jì)劃以及突破攝星等多個(gè)由私人出資或志愿發(fā)起的項(xiàng)目，希望使我們距離跨越宇宙的目標(biāo)更近一些。我們8月份發(fā)現(xiàn)了一顆與地球大小相似的行星（編注：英文原文發(fā)表于2016年10月），圍繞離我們最近的恒星運(yùn)行，這也為我們?cè)煸L外星世界增添了新希望。

真的有可能航行去其他星系嗎？如果可能的話，我們需要怎樣的太空飛船才能實(shí)現(xiàn)這一目標(biāo)呢？繼續(xù)讀下去就能（以超光速）了解最新進(jìn)展。

我們會(huì)去哪兒？

我們不會(huì)去哪兒呢？宇宙中恒星的數(shù)量比地球上沙子的數(shù)量還多，大約有700萬億億顆，其中數(shù)十億恒星估計(jì)都有一到三顆行星位于所謂的“金發(fā)姑娘區(qū)”：那里既不是太熱，也不是太冷。

既然我們剛剛起步，目前最有希望抵達(dá)的恒星是距離我們最近的恒星鄰居——由三顆恒星組成的半人馬座α星系統(tǒng)，位于4.37光年以外。今年，歐洲南方天文臺(tái)的天文學(xué)家們發(fā)現(xiàn)了一顆與地球體積相當(dāng)?shù)男行?，圍繞半人馬座α星的紅矮星——比鄰星運(yùn)行。這顆行星名為“比鄰星b”，至少是地球質(zhì)量的1.3倍，但是由于其運(yùn)行軌道與比鄰星非常貼近，只需11個(gè)地球日就可以環(huán)繞比鄰星一周。讓天文學(xué)家和外行星觀測(cè)者興奮不已的是，這顆行星具有適合液態(tài)水存在的合適溫度，而這是宜居性的有用表征。

它的缺點(diǎn)在于，我們不知道它是否有大氣層，而且鑒于它和比鄰星的距離如此之近——比水星環(huán)繞我們太陽的軌道還要近——它很可能暴露在危險(xiǎn)的母恒星耀斑和輻射之下。而且它還是潮汐鎖定的，這意味著這顆行星總是用同一面朝向它的母恒星。這會(huì)徹底改變我們對(duì)于黑夜和白天的認(rèn)識(shí)。

我們?nèi)绾蔚竭_(dá)那里？

這是個(gè)價(jià)值64萬億美元的問題。即使以我們目前科技所能實(shí)現(xiàn)的最快速度去比鄰星b轉(zhuǎn)一圈，大概也要花上18，000年才能抵達(dá)，而在這么長(zhǎng)的時(shí)間里，我們?cè)诘厍蛏系娜祟惡蟠笥锌赡芤呀?jīng)早早超越我們抵達(dá)那里，搶走所有的風(fēng)頭。但是許多聰明的頭腦和鼓鼓的腰包都加入到了尋找更快穿越廣闊空間方法的挑戰(zhàn)中。

“突破攝星”是一項(xiàng)由俄羅斯億萬富翁尤里和茱莉亞·米爾納夫婦私人投資的價(jià)值一億美元的項(xiàng)目，專門研究如何驅(qū)動(dòng)微型無人探測(cè)器，其驅(qū)動(dòng)方法是使基于地面的強(qiáng)大激光束作用于探測(cè)器上重量超輕的光帆。這個(gè)項(xiàng)目認(rèn)為，如果宇宙飛船足夠小——小到不足一克重——光帆又足夠輕，激光的驅(qū)動(dòng)力就足以使飛船逐漸加速到接近光速的1/5，用大約20年的時(shí)間抵達(dá)半人馬座α星。

米爾納夫婦期望微型化技術(shù)能使這個(gè)微型飛船攜帶攝像機(jī)、推進(jìn)器、電源供應(yīng)裝置、通訊及導(dǎo)航設(shè)備，以便能夠在與比鄰星b擦肩而過時(shí)，將所觀測(cè)到的內(nèi)容發(fā)送回來。希望我們能等來好消息，因?yàn)檫@會(huì)為星際航行的下一個(gè)難度更大的階段——載人航行——打下基礎(chǔ)。

曲速引擎怎么樣？

雖然在《星際迷航》里看起來很容易，但是我們目前所知的一切物理定律都告訴我們，超光速飛行甚或是光速飛行，都是不可能的。這并不是說科學(xué)已經(jīng)就此認(rèn)輸了。受科幻小說家們想象出來的另一種驅(qū)動(dòng)系統(tǒng)的啟發(fā)，NASA的演進(jìn)氙離子推進(jìn)器引擎項(xiàng)目正在研發(fā)離子引擎，希望將太空飛船的速度提升到90，000邁（145，000千米/小時(shí)），而所耗燃料比傳統(tǒng)火箭要少得多。

但即便是以這樣的速度，我們?cè)谝淮仗剿髡叩臅r(shí)間里也無法走出太陽系太遠(yuǎn)。在搞懂如何實(shí)現(xiàn)時(shí)空翹曲之前，星際航行意味著要搭乘一艘非常緩慢的太空船去向未來。也許最好將這種太空之旅本身視為目標(biāo)，而不是將其看作實(shí)現(xiàn)某種目標(biāo)的手段。

我們?cè)谛请H旅行中如何生存？

曲速引擎和離子推進(jìn)都很誘人，但是如果我們的星際旅行者們甚至在遠(yuǎn)未離開自己的太陽系之前就因饑餓、脫水或難以呼吸而死去，那么這些技術(shù)終究還是沒什么用。瑞秋·阿姆斯特朗研究員認(rèn)為，我們需要開始思考星際旅行的人們?cè)谕局猩硖幵鯓拥纳鷳B(tài)系統(tǒng)這一問題了。“我們正從以工業(yè)視角考慮現(xiàn)實(shí)轉(zhuǎn)向以生態(tài)視角考慮現(xiàn)實(shí)?！彼f。

作為英國(guó)紐卡斯?fàn)柎髮W(xué)試驗(yàn)建筑專業(yè)的教授，阿姆斯特朗談到了“構(gòu)建有機(jī)世界”這一概念，她說道：“該理念所關(guān)心的是如何在空間中居住，而不僅僅是設(shè)計(jì)一個(gè)標(biāo)志性的物體?！彼J(rèn)為，今天的太空飛船或空間站的內(nèi)部毫無生機(jī)，非常工業(yè)化。阿姆斯特朗相信，我們應(yīng)該轉(zhuǎn)而思考一下我們飛船的生態(tài)性——在其中栽培什么樣的植被，甚至是我們攜帶什么樣的土壤。在未來，她設(shè)想了大型的生物群落，充滿有機(jī)生命體，而不像今天那樣，都是些冰冷的金屬盒子。

我們不能全程睡眠嗎？

如何使人們?cè)诔^人類壽命長(zhǎng)度的太空旅程中保持生存狀態(tài)是個(gè)棘手的問題，針對(duì)這一問題受推崇的解決方案是低溫休眠、冬眠或某種形式的靜止?fàn)顟B(tài)。阿爾科生命延續(xù)基金項(xiàng)目裝滿了低溫貯藏的身體和頭部的設(shè)施證明，人們樂觀地相信，我們終有一天能夠搞清楚如何安全地將人們冷凍和解凍，但同樣地，這樣的技術(shù)目前不存在。

電影《星際穿越》和尼爾·斯蒂芬森的小說《七夏娃》等作品提出了一種建議：由于胚胎不用飲食或呼吸，可以將冷凍胚胎送上星際旅程使其（理論上講）能夠在旅程的艱難環(huán)境中存活下來。但是這又引發(fā)了“雞生蛋還是蛋生雞”的問題：當(dāng)他們抵達(dá)目的地之后，誰來養(yǎng)育這些初到人世的嬰兒呢？

星際旅行真的會(huì)發(fā)生嗎？

雖然在本文讀者的有生之年可能無法實(shí)現(xiàn)，但是在更長(zhǎng)的時(shí)間范圍內(nèi)，還是有理由持樂觀態(tài)度的?！白匀祟惔嬖谥?，我們就抬頭仰望群星，向它們寄托我們的希望和恐懼，焦慮和夢(mèng)想?！卑⒛匪固乩收f道。隨著諸如“突破攝星”這樣的項(xiàng)目來解決工程問題，“這已經(jīng)不再只是夢(mèng)想，而是進(jìn)入了實(shí)驗(yàn)階段”。

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 weve 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 wouldnt 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 were 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 Centauris 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 dont 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？

Thats 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 theres 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 were 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， NASAs 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 wont 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 theyre 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. “Were 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”： “Its 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， theres cause for optimism. “From the outset of human existence weve 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.”