Reaching for the Stars, Across 4.37 Light-Years
By DENNIS OVERBYE

Can you fly an iPhone to the stars?

In an attempt to leapfrog the planets and vault into the interstellar age, a bevy of scientists and other luminaries from Silicon Valley and beyond, led by Yuri Milner, a Russian philanthropist and Internet entrepreneur, announced a plan on Tuesday to send a fleet of robot spacecraft no bigger than iPhones to Alpha Centauri, the nearest star system, 4.37 light-years away.

If it all worked out — a cosmically big “if” that would occur decades and perhaps $10 billion from now — a rocket would deliver a “mother ship” carrying a thousand or so small probes to space. Once in orbit, the probes would unfold thin sails and then, propelled by powerful laser beams from Earth, set off one by one like a flock of migrating butterflies across the universe.

Within two minutes, the probes would be more than 600,000 miles from home — as far as the lasers could maintain a tight beam — and moving at a fifth of the speed of light. But it would still take 20 years for them to get to Alpha Centauri. Those that survived would zip past the star system, making measurements and beaming pictures back to Earth.

Much of this plan is probably half a lifetime away. Mr. Milner and his colleagues estimate that it could take 20 years to get the mission off the ground and into the heavens, 20 years to get to Alpha Centauri and another four years for the word from outer space to come home. And there is still the matter of attracting billions of dollars to pay for it.

“I think you and I will be happy to see the launch,” Mr. Milner, 54, said in an interview, adding that progress in medicine and longevity would determine whether he would live to see the results.

“We came to the conclusion it can be done: interstellar travel,” Mr. Milner said. He announced the project, called Breakthrough Starshot, in a news conference in New York on Tuesday, 55 years after Yuri Gagarin — for whom Mr. Milner is named — became the first human in space.

The English cosmologist and author Stephen Hawking is one of three members of the board of directors for the mission, along with Mr. Milner and Mark Zuckerberg, the Facebook founder.

“What makes human beings unique?” Dr. Hawking asked. He went on to say, “I believe that what makes us unique is transcending our limits.”

Dr. Hawking added, “Today we commit to the next great leap in the cosmos, because we are human and our nature is to fly.”

The project will be directed by Pete Worden, a former director of NASA’s Ames Research Center. He has a prominent cast of advisers, including the Harvard astronomer Avi Loeb as chairman; the British astronomer royal Martin Rees; the Nobel Prize-winning astronomer Saul Perlmutter, of the University of California, Berkeley; Ann Druyan, an executive producer of the television mini-series “Cosmos: A Spacetime Odyssey” and the widow of Carl Sagan; and the mathematician and author Freeman Dyson, of the Institute for Advanced Study in Princeton, N.J.

“There are about 20 key challenges we are asking the world’s scientific experts to help us with — and we are willing to financially support their work,” Dr. Worden said in an email.

A detailed technical description of the project appears on the project’s website.

Estimating that the project could cost $5 billion to $10 billion, Mr. Milner is initially investing $100 million for research and development. He said he was hoping to lure other investors, especially from international sources. Both NASA and the European Space Agency have been briefed on the project, Dr. Worden said.

Most of that money would go toward a giant laser array, which could be used to repeatedly send probes toward any star (as long as the senders were not looking for return mail anytime soon) or around the solar system, perhaps to fly through the ice plumes of Saturn’s moon Enceladus, which might contain microbes — tiny forms of life.

In a sense, the start of this space project reflects the make-it-or-break-it mode of Silicon Valley. Rather than send one big, expensive spacecraft on a journey of years, send thousands of cheap ones. If some break or collide with space junk, others can take their place.

Interstellar travel is a daunting and humbling notion, but Alpha Centauri is an alluring target for such a trip: It is the closest star system to our own, and there might be planets in the system. The system, which looks to the naked eye like one star, consists of three: Alpha Centauri A and Alpha Centauri B, which circle each other, and Proxima Centauri, which may be circling the other two. In recent years, astronomers have amassed data suggesting the possibility of an Earth-size planet orbiting Alpha Centauri B.

It would take Voyager 1, humanity’s most distant space probe, more than 70,000 years to reach Alpha Centauri if it were headed in that direction, which it is not.

Over the years, a variety of propulsion plans have been hatched to cross the void more quickly. In 1962, shortly after lasers were invented, Robert Forward, a physicist and science fiction author, suggested they could be used to push sails in space.

In 2011, Darpa, the Defense Advanced Research Projects Agency, got into the act with 100 Year Starship, a contest to develop a business plan for interstellar travel.

By all accounts, Mr. Milner was initially skeptical of an interstellar probe. But three trends seemingly unrelated to space travel — advances in nanotechnology and lasers and the relentless march of Moore’s Law, making circuits ever smaller and more powerful — have converged in what he called “a surprising way.”

It is now possible to fit the entire probe with computers, cameras and electrical power, a package with a mass of only one gram, a thirtieth of an ounce.

That, Dr. Loeb said, is about what the guts of an iPhone, stripped of its packaging and displays, amount to.

Power would come from a tiny radioactive source like americium, the element in smoke detectors. Propulsion would come from foil sails that would unfold to catch laser light.

The laser is the most intimidating and expensive of the challenges. It would have to generate 100 gigawatts of power for the two minutes needed to accelerate the butterfly probes to a fifth of the speed of light (subjecting its tiny innards to 60,000 times the force of normal gravity, by the way). That is about as much energy as it takes for a space shuttle to lift off, Dr. Loeb said, and about 100 times the output of a typical nuclear power plant.

To achieve that energy would require an array about a mile across combining thousands of lasers firing in perfect unison.

Moreover, to keep the beam tightly focused on one probe at a time would require an adaptive optics system that compensated for atmospheric turbulence — something astronomers know how to do over a span of 10 meters, the size of a big telescope mirror now, but not over a mile.

Posing another challenge is the design of the sails, which would have to be very thin and able to reflect the laser light without absorbing any of its energy. Absorbing as little as one part in 100,000 of the laser energy would vaporize the sail.

Another challenge might simply be to the imagination. Nobody knows what the Starshot fleet might find out.

“Looking is very different from going and visiting,” Dr. Loeb said.

As he noted, referring to recent physics experiments, “Nature teaches us that its imagination is better than ours.”

霍金打造奈米太空船 尋找星際新生命

科學家霍金12日與俄國富豪米爾納共同宣布，未來數年斥資一億美元（台幣32.4億元）打造一個巨型雷射陣列，將大批郵票大小的奈米太空船送上20年的星際旅程，抵達最近的星系南門二（Alpha Centauri，半人馬座阿爾發），傳回資料。

奈米太空船暫稱「星片」（StarChip），重一盎司（28公克），只比稍大的郵票厚一點，帶著超輕的帆。數百或上千星片裝入一隻貨櫃，由一具傳統火箭送上赤道上空3萬5680公里，大約每天釋出一批，等待來自地面的助航。

星片將由地面一個一公里寬的強大雷射陣列推進，數分鐘後達到光速的20%，巡航速度約為每秒5萬9520公里。

從這個階段開始，迷你太空船自己航行，航過4.3光年，亦即40兆公里的浩渺空間，在啟程20年後穿過半人馬座阿爾發。那是迄今太空船所航最遠距離的二千倍。那一帶說不定有行星，甚至生命。

屆時尚存的「晶片太空船」將對由三顆或兩顆星球組成的半人馬座阿爾拍照，以其內建的極微雷射將資料傳回地球。資料回傳抵達地球需時四年。

人駕太空船 需飛11.4萬年

阿波羅型太空船，史上最快的有人駕駛太空船，抵達半人馬座阿爾發要11萬4000年。

「星片」的助航來陸基雷射。米爾納手持雷射筆說，我將這支雷射筆指向一個星片，推得動星片，但沒法超快，速度大致像螞蟻，但電子元件往超微發展，材料日並極輕極薄，光電子科技進步也大，以小雷射構成的陣列形成一個力量強大的光束來推動這些星片，半人馬座阿爾發之旅成為可以驗證的想像。

米爾納用拇指和食指挾著極迷你太空船，船內五臟俱全，包括攝影機、光子推進器、電源，以及導航與通訊儀。一個星片的量產成本與一支iPhone相當。

霍金與米爾納在新紐約世貿中心頂上的觀測台宣布這項〈突破星擊〉專案。霍金說：「我們面對的限制只有我們和眾星之間的浩瀚虛空，但我們現在有辦法超越這限制。我們將在一個世代之內，用光速、輕帆和史上最輕的太空船啟動前往半人馬座阿爾發之旅。今天，我們獻身於進入宇宙的這一大步，因為我們是人，而飛翔是人性。」

總經費達數百億美元

整個任務的最終成本將會相當於目前全球最大的幾項科學專案，例如美國太空總署88億美元的韋伯太空望遠鏡，和一百億美元的大強子對撞機。

米爾納希望20年內完成研發，但總經費需要數百億美元。54歲的米爾納說，希望在世之日來得及目睹星片升空。

原文參照：
http://www.nytimes.com/2016/04/13/science/alpha-centauri-breakthrough-starshot-yuri-milner-stephen-hawking.html

紐約時報中文版翻譯：
http://cn.nytstyle.com/science/20160413/t13starshot/zh-hant/

Video：Mission to Alpha Centauri
The cosmologist Stephen Hawking and the entrepreneur Yuri Milner released a simulation that shows how a project called Breakthrough Starshot aims to send small robots to the nearest star system, Alpha Centauri.
http://nyti.ms/25XJ8vV

Video：Stephen Hawking on ‘Starshot’ Project
The renowned physicist spoke of transcending humanity’s limitations through advanced technology, using robot spacecraft to reach the nearest star system.
http://nyti.ms/23sVsp0

2016-04-13．聯合晚報．A6．國際焦點．編譯彭淮棟