Missions to the stars carrying human crews sound much more like science fiction than science. The American Association for the Advancement of Science is the world's largest federation of scientific societies, and its business is very much science and not at all fiction. So when two colleagues and I were invited to come to the AAAS Annual Meeting in Boston last week and lead a session devoted to interstellar flight, we wondered what sort of reception we were likely to enjoy. We didn't expect to be laughed off the stage, but we certainly anticipated a good deal of skepticism.

That's why we approached the topic with a great deal of caution. First, we emphasized that the distances involved in travel to the stars are enormous, and have been known to be enormous since 1838, when Friedrich Bessel measured the parallax of, and hence the distance to, the star 61 Cygni. It takes light more than eleven years to reach us from 61 Cygni. It takes light more than four years to reach us from even the closest stellar system, Alpha Centauri. Most places where you might wish to go are considerably farther off. Sirius, the brightest star in the sky, is 8.7 light-years from us, while Betelgeuse is about 520 light-years.

Next we pointed out that you would probably not wish to set out at all unless you believed you would find a planet somewhat like Earth waiting for you. We don't yet know where any such earthlike world might be located. Perhaps it is a thousand light-years away, maybe even more.

Then there is the problem presented by the theory of relativity. As part of that theory, published in 1905, Albert Einstein proved to the satisfaction of almost everyone (there are still a few unbelievers) that no material object can be accelerated to travel as fast as light. If something is to be sent to the stars, no matter whether it is as small as a grain of sand, or as big as a spaceship containing hundreds or thousands of humans, the travel time cannot be less than light would take to complete the journey. Times will at the very least be measured in years, and more likely in centuries.

We warned the audience not to be misled by fictional devices such as hyperspace, warp drives, wormholes, ansibles, portals, stargates, or Bose networks. These are nice for story telling, but in terms of real physics they are only one step removed from sprinkling the top of your head with fairy dust and wishing yourself somewhere else.

With those preliminary warnings, we went on to discuss a couple of ways in which today's science allows us, at least in principle, to send a mission to the nearer stars getting there in a reasonable length of time - say, a few centuries.

One of these ways is to build a flying world. This would be a self-contained biosphere, able to provide its own power, grow its own food, and recycle its own wastes over a long period of time. Within would exist a human society in microcosm, people living their whole lives aboard the ship and finding nothing unnatural in this, any more than we find it unnatural to spend our whole lives moving through space within the particular biosphere of the Earth. Our flying world would necessarily be slow, because it takes enormous power to accelerate such a large object. Travel times would be correspondingly long, not perhaps centuries but millennia.

Once again, we went to some pains to point out some of the other difficulties. Experiments with self-contained biospheres show that it is difficult to keep them going for more than a few years. This would be unfortunate if you were to find yours failing when you were already outside the solar system and headed for Sirius.

Also, when we said we would need enormous power, this was perhaps an understatement. One proposal would propel a starship attached to a huge sail by using a giant laser, constructed in space and operating using solar energy. For reasonable travel times, the sail must be at least five hundred miles across. The rate of power consumption would be forty thousand times as much as the whole world produces today.

Another technique for interstellar travel would not send people who would live and breed in space for many generations, but would send people in "cold sleep," their body temperature lowered dramatically and body metabolic processes proceeding imperceptibly slowly. They would wake up only when they reached their destination. There is one rather large objection to cold sleep: we have no idea how to do it.

In fact, as we were careful to emphasize, every proposed method for sending people to the stars faces problems that are at least enormous, and at worst insoluble.

And how did the audience react to our long string of ifs and maybes and probably nots? To our amazement, they took the problems in their stride. Ignoring all cautions, they went on to talk about such things as what language should be used on the starship (English, not surprisingly, was favored), what should be the male/female mix, and what type of on-board society would be the most stable.

Afterwards, my co-presenters and I puzzled over what we had just seen. We came to a conclusion that I still favor. Much of present-day science borders on the incredible. We had heard amazing new results at the AAAS meeting, in everything from optics to microbiology to computer miniaturization. Since we did not offer actual proof that interstellar travel is impossible, our audience assumed that it is bound to happen.

One of my colleagues said he had recently attended two meetings. In the first, a scientist discussing a particular procedure in neurology said, "This is quite simple, you know. It's not rocket science." In the other meeting, a scientist analyzing a problem of planetary orbits said, "This isn't hard. It's not brain surgery."

We tend to assume that what we do is easy, while what other people do is hard. Meanwhile, they are making the same assumption about our efforts. This is exactly what our audience had done.

Interstellar travel? It may call for science that is still a little tricky, but here are three people telling us how to do it. So it must be possible. Otherwise, why would they be standing up at a science meeting and talking about it?

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