Writing a science column makes you fair game for questions of all kinds. One that I am asked often is, "When do you think we will be able to travel to the stars?"

On the face of it, that's a reasonable question. Every science fiction movie, from "Forbidden Planet" to "Dune" to "Galaxy Quest," assumes that interstellar travel can happen, and happen to people very much like us.

My answer, which is to hem and haw and be as vague as possible, does not satisfy. I can read the look on people's faces, a look that says, "The first artificial satellite went up less than half a century ago. Since then humans have walked on the moon, and we have sent unmanned spacecraft to take a close look at all the planets. What makes star travel such a big deal that we can't expect it in the next half century?"

The answer, in a word, is distance; but let's work our way into that.

For travel on Earth, most of us are (or should be) willing to walk anything up to two miles. For two to twenty miles, a bicycle is convenient and reasonable. A car is fine from twenty to two hundred, while for two hundred to two thousand miles most of us would rather fly or take a train. Each factor of ten seems to lead to a qualitative change in the way we prefer to travel. Away from Earth, that factor of ten is no longer convenient. Our closest neighbor in space, the moon, is about 240,000 miles away. Another factor of ten in distance does not take us anywhere interesting. Nor does a factor of a hundred. Even with a factor of a thousand, we have not reached Jupiter.

Ten thousand times the distance to the moon finally takes us to the outermost planets of the Solar System. We can claim to have been there and done that, at least with unmanned spacecraft. Unfortunately, we are still a long way from the stars. Another factor of ten thousand is needed to take us to the nearest star system, of Alpha Centauri. The nearest star is about a hundred million times as far away as the moon. Getting to the moon, you may recall, was considered a big deal.

Let's put this in another, and maybe more meaningful, way, as travel times. Suppose that we have a super-transportation system, one that can carry a spacecraft and its crew to the moon in one minute. NASA, or anyone interested in solar system development, will drool at the very thought of such a device. But we will still need 190 years to reach Alpha Centauri, while most of the "famous" stars are much farther away: 1,300 years trip time to Vega, over 20,000 years to Betelgeuse. As for the galactic center, visitors to that had better be ready to spend a couple of million years in transit.

Even if we could improve our transportation system so that we could go places at the speed of light (according to current theories, that's as fast as anything can possibly go) we are still looking at multi-year trips even to the closest stars

Given all this, you might think that I believe interstellar travel to be impossible. But if you change the question from "When will we go?" to "How might we go?" my answer is not vague or discouraging at all. I am going to describe, very briefly, a couple of ways in which we (or rather, our descendants) might go to the stars. The methods are consistent with today's physics. Long time scales are involved, but who is to say that, a thousand years from now, a century will be a long human lifetime? We have doubled life expectancy in just the past two hundred years.

The first method is the interstellar ramjet, first proposed forty years ago by Robert Bussard. An electromagnetic "scoop" in front of a spacecraft funnels interstellar matter into a long, hollow cylinder. Within that cylinder, the material collected by the scoop undergoes nuclear fusion. The fusion products are emitted at high temperature and velocity from the other end of the cylinder and propel the spacecraft. The higher the ship's speed, the greater the rate of supply of fuel, and thus the greater the ship's acceleration. There is interstellar matter everywhere, even in "empty" space. Also, because a spacecraft of this type can accelerate continuously, travel times are drastically reduced. If the journey to the nearest star takes 25 years, which calls for only a very modest acceleration by the spacecraft, then every bright star in the sky can be reached in a couple of centuries.

The practical problems in building an interstellar ramjet are predictably huge; but so, in their own day, were the problems faced in building the pyramids, or the Brooklyn Bridge. Suppose, however, that the interstellar ramjet simply will not work, for whatever reason. No matter. There are other techniques that can, in principle, take us to the stars.

One of the best-studied of these, by Robert Forward, is the laser-driven light-sail. The sail is a large, delicate film of material, and the "wind" that pushes it is the light provided by a massive solar-powered laser in orbit close to the sun. In Forward's design, the light-sail can be accelerated to half the speed of light in a couple of years. Travel time to the nearest star, including slowing down and stopping when we arrive, is less than ten years.

Once again, the practical engineering demands are monstrous. The power requirement of the laser is thousands of times greater than the total electrical production of all the nations on Earth. The space construction capability is generations ahead of what we can foresee in the next century.

Do I really believe that humans will go to the stars using an interstellar ramjet or a huge light-sail? No.

Do I believe that humans will one day go to the stars? Yes.

How? I don't know. But it will surely be done using science and technology far beyond what we have available today; just as our computers and nuclear reactors were far beyond the comprehension of our distant ancestors, huddled around the pile of burning sticks in the middle of the cave and congratulating themselves on having mastered the "new technology" of fire.

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