通常这称作“终极界限”， 不仅仅是步柯克船长（Captain Kirk）和企业号船员（USS Enterprise）等探险家后尘的人会这样说；对于那些未来的空间探索
“你会发现从不缺少志愿者，” NASA 探索系统集成主管 John Olson 说，“这实际上与历史上的探索先锋没什么不同，就像那些穿越大洋或者是那些向西穿
1961 年 5 月，肯尼迪总统让美国接受一项挑战：最多用 10 年要把人安全送上月球并让其安全返回。为此估计花费了 1 万 4 千亿美元（按照 2009 年美元
价格计算） ；阿波罗 11 号宇航员尼尔·阿姆斯特朗和奥尔德林在 40 年前的今天成为第一批踏上月球的人。现在 NASA 希望重燃公众对载人航天飞行的
如果就像 Olson 预言的那样，人类将在本世纪中叶踏上火星，那么工程师和宇航员就可能把目光投向那些更远的冰冻行星、炙热的卫星以及诸多恒星
“我们重回月球不是为了留下旗帜和脚印，而是为了能够持久呆在那儿，” Olson 说，“我们要把月球所为登录火星的跳板，我们要看看别的有意思的地
从 肯尼迪演讲以来，美国已经失去了 17 名宇航员。3 名死于阿波罗计划早期实验的火灾中，14 名在挑战者号和哥伦比亚号航天飞机失事中牺牲。1970
年，NASA 的工程师拯救了阿波罗 13 号上的 3 名宇航员，当时它在离地 200,000 英里远的地方发生故障。但至今没有任何美国宇航员体验过缓慢的缺氧
Olson 说 NASA 目前还是遵从肯尼迪的指示要把宇航员带回来的。但是其他国家迅速 发展的空间计划可能已突破了该限制，那些可能取代国家角色的商
业公司也许亦然。“空间不再只属于权力和威望；它事关真正的经济利益，”阿波罗 11 号飞行主 管 Eugene Kranz 说，“从高风险、高级别、极端困难任务
使用目前可预见的科技，从月球前哨站出发的火星往返之旅要花上 2 到 3 年时间——来回各需要 6-9 个月，在火星上执行任务需要 1 年。
离地球所在的太阳系最近的星球是半人马座阿尔法星（Alpha Centauri），在 4.37 光年外，或者说是 2.5 万亿英里外，要往返飞行的话，飞船要携带足够
Robert Park 是一名著名的载人航空批评家和物理学家，他表示即便是一次单程的半人马座阿尔法星飞行都超出了物理学法则。
为推动该飞船完成 50 年抵达的任务所需速度所耗能量之大，人们几乎无法想象。他说那样的数字相当于全世界人类总耗能的好多好多倍。
“我们可没有曲速引擎（warp drive），” 他提到了科幻电影《星际迷航》中星际旅行时使用的引擎，“这就像是历经几代人的空间诺亚方舟，里面的孩子
Space exploration volunteers wanted (The catch? It's a one-way ticket)
It is often described as "the final frontier", and not just by those who follow the adventures of Captain Kirk and the crew of
the USS Enterprise. The phrase, though, may take an even more literal meaning for those exploring space in the future.
The next generation of astronauts may hurtle through the cosmos for years or decades on a mission to explore distant
planets and stars – and never return.
A senior Nasa official has told the Guardian that the world's space agencies, or the commercial firms that may eventually
succeed them, could issue one-way tickets to space, with the travellers accepting that they would not come back.
Dr John Olson talks about Nasa's plans for the moon, Mars and one-way tickets into space Link to this audio
The prospect of spending years cooped in a spacecraft would not deter people from applying, he said.
"You would find no shortage of volunteers," said John Olson, Nasa's director of exploration systems integration. "It's really
no different than the pioneering spirit of many in past history, who took the one-way trip across the ocean, or the trip out
west across the United States with no intention of ever returning."
In May 1961, President John F Kennedy challenged the US to put a man on the moon by the end of the decade and return
him safely to Earth. In an effort costing an estimated $1.4tn in 2009 dollars, Apollo 11 astronauts Neil Armstrong and Buzz
Aldrin became the first humans to set foot on the moon 40 years ago today. Now, Nasa hopes to reignite the public's interest
in manned space flight and win support for a massive investment in new trips to space.
If, as Olson predicts, humans reach Mars by the middle of this century, engineers and astronauts may then set their sights on
the frozen planets, fiery moons and stars beyond.
"We're going back to the moon, not for flags and footsteps but for a sustained presence," Olson said. "We're going to use the
moon as a stepping stone to Mars and we're going to look at other interesting spots, like asteroids and near-Earth objects,
and we're going to look at all the other exciting places to go in this solar system."
Since Kennedy's speech, the US has lost 17 astronauts. Three perished in a fire during early testing for the Apollo
programme and 14 died in the wreckage of the space shuttles Challenger and Columbia. In 1970, Nasa engineers saved
three astronauts when Apollo 13 malfunctioned 200,000 miles from Earth. But no US astronaut has ever suffered the slow
oxygen starvation and freeze that would doom a spacecraft lost beyond the Earth's orbit.
Nasa is currently bound by Kennedy's directive to bring its astronauts home, Olson said. But the other nations rapidly
developing space programmes may shed the constraint, as could the commercial companies that may supplant national
efforts. "Space is no longer for power and prestige; it's truly for economic benefit," the Apollo 11 flight director Eugene
Kranz said. "The technology that emerges from high-risk, high-profile, extremely difficult missions is the technology that
will keep the economic engine of our nation continuing to go through the years."
With currently foreseeable technology, a round trip to Mars launched from a lunar outpost would take two to three years – a
journey of six to nine months each way and a year-long mission on the surface.
The star nearest Earth's solar system, Alpha Centauri, is 4.37 light years away, or more than 2.5tn miles, and a round-trip
spacecraft would have to carry enough fuel to brake and propel itself back to Earth.
Robert Park, a physicist and prominent critic of manned space flight, said that even a one-way trip to Alpha Centauri was
beyond the laws of physics.
The energy required to push a spacecraft up to the speed needed to get to the star within 50 years was so great as to be
barely conceivable. He described the measurement as a fantastic multiple of the energy consumed by the entire world in a
"We don't have a warp drive," he said, referring to the interstellar engines of Star Trek fantasy. "A multigenerational space
ark would doom the children raised to continue the mission never to see Earth and would decide their destiny before their
birth, raising profound ethical questions."
Rather than devote immeasurable resources to sending humans into space, Park said science should instead build stronger
telescopes to better study distant stars and planets.