大约一年之后，如果一切顺利，一个面包般大小的小方盒将会由火箭发射到距离地球500英里的太空中。它将会在真空中展开4个闪闪发光的三角帆，除此而外就再没有什么东西了。然后，它将会在太阳光的驱动下，飞越星际。

    光帆1号（LightSail-1），像它的名字一样，将不会驶向梦幻岛。在最好的情况下，这个设备只会飞行数小时并且只升高几英里。但是这几个小时却是一个古老梦想的里程碑，这个梦想从火箭诞生的年代就开始了，而且同样充满了浪漫主义：像在地球海洋上借着风航行了几千年的水手那样在星际间借着星光之风航行。

    “依靠星风飞行的技术是有朝一日带我们航向遥远恒星的唯一方法。”路易斯-弗里德曼说，他是世界航天爱好者组织——行星协会（Planetary Society）的主席。

    尽管美国航空航天局还在这项技术的研究上犹豫不绝，弗里德曼博士在星期一已宣布，行星协会在一位匿名资助者的帮助下已经使这项未来价值巨大的科学幻想迈出了第一步。在未来的三年中，协会将会制造一系列称为“光帆”的借助太阳帆飞行的太空飞行器，首先会在环绕地球的轨道上飞行，最终将会驶向深空。

这一系列航行是协会与在纽约的宇宙工作室（Cosmos Studios）长期合作的成果。工作室由安-朱利研领导。她是一位电影制片人，同时也是已故天文学家、科普作家卡尔萨根的遗孀。

    卡尔萨根是行星协会的创始人，1980年，他与弗里德曼博士以及当时的喷气推进实验室主任布鲁斯莫里一起创办了行星学会。这一消息在华盛顿哈特参议院大楼为萨根举行的75岁生日庆祝会上宣布。萨根在1996年去世。朱利研女士，光帆计划的首席筹款人，称这一计划是萨根的“泰姬陵”，他生前非常喜欢这一概念，认为这是富有智慧的运用技术的象征。

    这是一个由来已久的幻想，可以追溯到俄罗斯火箭先驱康斯坦丁-齐奥尔科夫斯基和弗里德里希-赞德尔以及支持这一想法的科幻作家阿瑟-克拉克。“太阳帆，是宇宙航行的一种奇妙的方式，”新泽西普林斯顿高等研究院的弗里曼-戴森说，“但需要很长的时间来使想象变为现实。”

    太阳帆获得驱动力，源自一个简单的事实，那就是太阳光不仅仅携带能量，而且拥有动量——彗星被太阳风从彗核吹出的长长彗尾会告诉你这一点。在太阳帆上的力如果说不是软弱的那么至少可以说是温和的，但不同于火箭仅仅只燃烧几分钟，它是持久的。一张足够大的帆（一个侧面有几英里宽），通过经年累月的积累，可以达到数十万英里的时速，足以在5年内穿越整个太阳系。通过驾驭强大的光源，太阳帆可以在100年内抵达另一个星系，也就是说在一个人的寿命范围内。

    人类是否采取这样的旅行方式，取决于一个人看未来的眼光有多么的“不切实际”。弗里曼博士表示，将人送到像火星那么远的地方要花费极长的时间和暴露在大量的辐射下，所以星际旅行的乘客——即使通过200年的技术发展——由于需要保持飞行器很轻的质量，只有可能是机器人或者携带基因编码的芯片，就像一只巨大的太空风筝。

    理论上，太阳帆可以做到普通的帆能做到的任何事情，比如抢风行驶。不同于其它航天器，它可以作为一个反引力器，利用太阳风的压力平衡太阳的引力，从而使飞船停留在太空中的任何地方。而且，它显然无需携带成吨的火箭燃料。正如民歌手乔纳森-艾伯哈特在歌中所写的是一个“太阳能掠夺者”：

    没有冷液氧罐或罐式反应堆，只有一英里聚酯薄膜。

    没有风暴撞击樯桅, 清冽和真实的风。

    只要拖拽、转向，动作迅速，如同水手们那般。

    这些都是长期的愿景。“想想几百年或几千年后，而不是几十年，”戴森博士说，他说他认为行星协会的项目是可行的。

    “我们将要做的事情是浪漫的，”他补充说，目前还没有人知道该如何制造能进行真正旅行所用的足够大和薄的帆。“你必须有设备能展开帆并且不会打卷——这是目前还没有做到的一项大型工程。但是这项技术的有趣之处就是它是不可预知的。”

    某一段时间里，美国宇航局的许多实验室都在研究太阳帆。喷气推进实验室的科学家们甚至考虑在1986年哈雷彗星经过时发射一个太阳帆与它会合，并沿着它的轨迹飞行。

    但是这些机构为寻求资金以保证载人飞行计划开展的努力已经终止了，NASA先进概念研究所的前任主席和工程师唐娜·谢利这样说。谢利博士说，太阳帆的设想是可行的，唯一的问题是“你愿意为它花钱吗？”除非这项技术已经被证明，否则没有人会使用它。

    日本一直有这样的计划，并且测试通过卫星和火箭部署的太阳帆，但是目前还没有人试着让帆飞行到任何地方去。

    但是弗里德曼博士（他在哈雷彗星计划上受了不小挫折）一直寻求能从太空中起航。2005年6月份，由行星协会和俄罗斯科学院合作完成的，被朱利研女士称为“我们漂亮的飞行器”的宽约100英尺的宇宙一号（Cosmos-1），从俄罗斯一艘潜艇上发射，但最终落入了巴伦支海的海底。

    就在朱利研女士和弗里德曼博士为宇宙2号四处筹款时，NASA询问行星协会是否愿意接手一个小型太阳帆项目——Nanosail。这是一个只有18英尺的小型太阳帆，旨在增加大气阻力以帮助卫星脱离轨道。

    因此，光帆计划诞生了。它改进自Nanosail计划，帆面由镀铝聚酯薄膜做成，厚度大约只有普通垃圾袋的四分之一。飞行器的主体由3个微型卫星CubeSats组成，一侧有四英尺宽，它最初由斯坦福大学的学生设计，现在能在网上和其它地方买到。弗里德曼博士介绍说，其中一个小立方体中将携带电子设备，另外两个将携带折叠起来的太阳帆。

    像砖块一样组装起来的系统，整个的重量不超过5公斤（或11磅）。“硬件设备是最小的部分，”弗里德曼博士说。“你不会在一个5公斤重的系统上花费太多。”

    另一个突破来自于资金，当弗里德曼博士向一些潜在捐助者们谈论起光帆计划的时候，一名男子，用朱利研女士的话来说——“一位非常温和的人”——问起这项计划的花费，并承诺将支付其中的两部分，假如一切顺利的话，或许会还有第三部分。

    在会谈结束后，这位不愿透露身份的人士询问了协会的银行账户。几天之后，款项就汇入了协会的户头。光帆计划将会准备一年多的时间，在2010年末开始实施，具体的时间取决于火箭在何时可以发射。博士称，这个项目将会搭载在发射普通卫星的火箭上。美国或俄罗斯的火箭都有可能。

    弗里德曼博士说，光帆的首次飞行中，如果能被哪怕很小一部分的轨道器控制并且监测到任何它被太阳光加速的迹象，那么计划便取得了成功。“对第一次飞行来说，任何可测量的值都是很有用的。”他说。此外，将会有一个舷外摄像机来捕捉朱利研女士所说的“小鹰时刻”。

    下一次的飞行将会有更大的帆，并会持续好几天，产生足够大的速度将它的轨道提升几十或几百英里，博士说。

    第三次飞行，弗里德曼博士和他的同事们想要将带有科学仪器的太阳帆设置于地球轨道之外，用以监测太阳能量的输出，这将提供太阳风暴的早期预警（太阳风暴会破坏电网甚至损坏太空飞行器）。该计划会设置在地球引力和太阳引力的平衡点上——这个位置被称为L1（拉格朗日点），距离地球大约90万英里——一个传统科学卫星的好去处。他同时承认，这需要一个小火箭（比如像航天飞机上的姿态控制器）将它送到地球轨道外。

    但是，反过来说，大多数帆船都配有马达，以便在港口内行驶，弗里德曼博士这样描述在地球轨道的情况。因为太阳的位置不断变化，他说，当你停留在港口里时，所想的是就是驶向外面的海洋。

星辰大海在等待着，他这样说到。

Setting Sail Into Space, Propelled by Sunshine

About a year from now, if all goes well, a box about the size of a loaf of bread will pop out of a rocket some 500 miles above the Earth. There in the vacuum it will unfurl four triangular sails as shiny as moonlight and only barely more substantial. Then it will slowly rise on a sunbeam and move across the stars.

LightSail-1, as it is dubbed, will not make it to Neverland. At best the device will sail a few hours and gain a few miles in altitude. But those hours will mark a milestone for a dream that is almost as old as the rocket age itself, and as romantic: to navigate the cosmos on winds of starlight the way sailors for thousands of years have navigated the ocean on the winds of the Earth.

“Sailing on light is the only technology that can someday take us to the stars,” said Louis Friedman, director of the Planetary Society, the worldwide organization of space enthusiasts.

Even as the National Aeronautics and Space Administration continues to flounder in a search for its future, Dr. Friedman announced Monday that the Planetary Society, with help from an anonymous donor, would be taking baby steps toward a future worthy of science fiction. Over the next three years, the society will build and fly a series of solar-sail spacecraft dubbed LightSails, first in orbit around the Earth and eventually into deeper space.

The voyages are an outgrowth of a long collaboration between the society and Cosmos Studios of Ithaca, N.Y., headed by Ann Druyan, a film producer and widow of the late astronomer and author Carl Sagan.

Sagan was a founder of the Planetary Society, in 1980, with Dr. Friedman and Bruce Murray, then director of the Jet Propulsion Laboratory. The announcement was made at the Hart Senate Office Building in Washington at a celebration of what would have been Sagan’s 75th birthday. He died in 1996.

Ms. Druyan, who has been chief fund-raiser for the society’s sailing projects, called the space sail “a Taj Mahal” for Sagan, who loved the notion and had embraced it as a symbol for the wise use of technology.

There is a long line of visionaries, stretching back to the Russian rocket pioneers Konstantin Tsiolkovsky and Fridrich Tsander and the author Arthur C. Clarke, who have supported this idea. “Sails are just a marvelous way of getting around the universe,” said Freeman Dyson, of the Institute for Advanced Study in Princeton, N.J., and a longtime student of the future, “but it takes a long time to imagine them becoming practical.”

The solar sail receives its driving force from the simple fact that light carries not just energy but also momentum — a story told by every comet tail, which consists of dust blown by sunlight from a comet’s core. The force on a solar sail is gentle, if not feeble, but unlike a rocket, which fires for a few minutes at most, it is constant. Over days and years a big enough sail, say a mile on a side, could reach speeds of hundreds of thousands of miles an hour, fast enough to traverse the solar system in 5 years. Riding the beam from a powerful laser, a sail could even make the journey to another star system in 100 years, that is to say, a human lifespan.

Whether humans could ever take these trips depends on just how starry-eyed one’s view of the future is.

Dr. Friedman said it would take too long and involve too much exposure to radiation to sail humans to a place like Mars. He said the only passengers on an interstellar voyage — even after 200 years of additional technological development — were likely to be robots or perhaps our genomes encoded on a chip, a consequence of the need to keep the craft light, like a giant cosmic kite.

In principle, a solar sail can do anything a regular sail can do, like tacking. Unlike other spacecraft, it can act as an antigravity machine, using solar pressure to balance the Sun’s gravity and thus hover anyplace in space.

And, of course, it does not have to carry tons of rocket fuel. As the writer and folk singer Jonathan Eberhart wrote in his song “A Solar Privateer”:

No cold LOX tanks or reactor banks, just Mylar by the mile.

No stormy blast to rattle the mast, a sober wind and true.

Just haul and tack and ball the jack like the waterlubbers do.

Those are visions for the long haul. “Think centuries or millennia, not decades,” said Dr. Dyson, who also said he approved of the Planetary Society project.

“We ought to be doing things that are romantic,” he said, adding that nobody knew yet how to build sails big and thin enough for serious travel. “You have to get equipment for unrolling them and stretching them — a big piece of engineering that’s not been done. But the joy of technology is that it’s unpredictable.”

At one time or another, many of NASA’s laboratories have studied solar sails. Scientists at the Jet Propulsion Laboratory even once investigated sending a solar sail to rendezvous and ride along with Halley’s Comet during its pass in 1986.

But efforts by the agency have dried up as it searches for dollars to keep the human spaceflight program going, said Donna Shirley, a retired J.P.L. engineer and former chairwoman of the NASA Institute for Advanced Concepts. Dr. Shirley said that the solar sail was feasible and that the only question was, “Do you want to spend some money?” Until the technology had been demonstrated, she said, no one would use it.

Japan continues to have a program, and test solar sails have been deployed from satellites or rockets, but no one has ever gotten as far as trying to sail them anywhere.

Dr. Friedman, who cut his teeth on the Halley’s Comet proposal, has long sought to weigh anchor in space. An effort by the Planetary Society and the Russian Academy of Sciences to launch a sail about 100 feet on a side, known as Cosmos-1, from a Russian missile submarine in June 2005 ended with what Ms. Druyan called “our beautiful spacecraft” at the bottom of the Barents Sea.

Ms. Druyan and Dr. Friedman were beating the bushes for money for a Cosmos-2, when NASA asked if the society wanted to take over a smaller project known as the Nanosail. These are only 18 feet on a side and designed to increase atmospheric drag and thus help satellites out of orbit.

And so LightSail was born. Its sail, adapted from the Nanosail project, is made of aluminized Mylar about one-quarter the thickness of a trash bag. The body of the spacecraft will consist of three miniature satellites known as CubeSats, four inches on a side, which were first developed by students at Stanford and now can be bought on the Web, among other places. One of the cubes will hold electronics and the other two will carry folded-up sails, Dr. Friedman said.

Assembled like blocks, the whole thing weighs less than five kilograms, or about 11 pounds. “The hardware is the smallest part,” Dr. Friedman said. “You can’t spend a lot on a five-kilogram system.”

The next break came when Dr. Friedman was talking about the LightSail to a group of potential donors. A man — “a very modest dear person,” in Ms. Druyan’s words — asked about the cost of the missions and then committed to paying for two of them, and perhaps a third, if all went well.

After the talk, the man, who does not wish his identity to be known, according to the society, came up and asked for the society’s bank routing number. Within days the money was in its bank account. The LightSail missions will be spread about a year apart, starting around the end of 2010, with the exact timing depending on what rockets are available. The idea, Dr. Friedman said, is to piggyback on the launching of a regular satellite. Various American and Russian rockets are all possibilities for a ride, he said.

Dr. Friedman said the first flight, LightSail-1, would be a success if the sail could be controlled for even a small part of an orbit and it showed any sign of being accelerated by sunlight. “For the first flight, anything measurable is great,” he said. In addition there will be an outrigger camera to capture what Ms. Druyan called “the Kitty Hawk moment.”

The next flight will feature a larger sail and will last several days, building up enough velocity to raise its orbit by tens or hundreds of miles, Dr. Friedman said.

For the third flight, Dr. Friedman and his colleagues intend to set sail out of Earth orbit with a package of scientific instruments to monitor the output of the Sun and provide early warning of magnetic storms that can disrupt power grids and even damage spacecraft. The plan is to set up camp at a point where the gravity of the Earth and Sun balance each other — called L1, about 900,000 miles from the Earth — a popular place for conventional scientific satellites. That, he acknowledges, will require a small rocket, like the attitude control jets on the shuttle, to move out of Earth orbit, perhaps frustrating to a purist.

But then again, most sailboats do have a motor for tooling around in the harbor, which is how Dr. Friedman describes being in Earth orbit. Because the direction of the Sun keeps changing, he said, you keep “tacking around in the harbor when what you want to do is get out on the ocean.”

The ocean, he said, awaits.