Poul Anderson - New America

At the “equalizing” v of 0.7 c, these figures become 10.7 years for the crew and 14.4 years for the stay-at-homes. This illustrates the dramatic gains that the former, if not the latter, can make by pushing c quite closely. But let’s stay with that value of 0.7 c for the time being, since it happens to be the one chosen by Bernard Oliver for his argument against the feasibility of star travel.

Now, Dr. Oliver, vice president for research and ‘ development at Hewlett-Packard, is definitely not unimaginative, nor hostile to the idea as such. Rather, he is intensely interested in contacting extraterrestrial intelligence, and was the guiding genius of Project Cyclops, which explored the means of doing so by radio. The design which his group came up with could, if built, detect anybody who’s using radio energy like us today within 100 light-years: Or it could receive beacon signals of reasonable strength within 1000 light-years: a sphere which encloses a million suns akin to Sol and half a billion which are different.

Still, he does not fudge the facts. Making the most favorable assumption, a matter-antimatter annihilation system which expels radiation itself, he has calculated the minimum requirement for a round trip with a stopover at the destination star, at a peak speed of 0.7c. Assuming 1000 tons of ship plus payload, which is certainly modest, he found that it must convert some 33,000 tons of fuel into energy—sufficient to supply the United States, at present levels of use, for half a million years. On first starting off from orbit, the ship would spend 10 times the power that the Sun gives to our entire Earth. Shielding requirements alone, against stray gamma rays, make this an absurdity, not to speak of a thousand square miles of radiating surface to cool the vessel if as little as one one-millionth of the energy reaches it in the form of waste heat.

Though we can reduce these figures a good deal if we assume it can refuel at the other end for its return home, the scheme looks impractical regardless. Moreover, Dr. Oliver, no doubt deliberately, has not mentioned that space is not empty. Between local stars, it contains about one hydro; gen atom per cubic centimeter, plus smalls-amounts of other materials. This is a harder vacuum than any we can achieve artificially. But a vessel ramming through it at 0.7c would release X-radiation at the rate of some 50 million roentgen units per hour. It takes less than 1000 to kill a human being. No material shielding could protect the crew for long, if at all.

Not every scientist is this pessimistic about the rocket to the stars, that is, a craft which carries its own energy source and reaction mass. Some hope for smaller, unmanned probes, perhaps moving at considerably lower speeds. But given the mass required for their life support and equipment, men who went by such a vehicle would have tp reckon on voyages lasting generations or centuries.

This is not impossible, of course. Maybe they could pass the time in suspended animation. Naturally radioactive atoms in the body set an upper limit to that, since they destroy tissue which would then not be replaced. But Carl Sagan, astronomer and exobiologist at Cornell University, estimates that a spore can survive up to a million years. This suggests to me that humans should be good for anyway several thousand.

Or maybe, in a huge ship with a complete ecology, an expedition could beget and raise children to carry their mission on. Calculations by Gerard K. O’Neill, professor of physics at Harvard, strongly indicate that this is quite feasible. His work has actually dealt with the possibility of establishing permanent, self-sustaining colonies in orbit, pleasanter to live in than most of Earth and capable of producing more worldlets like themselves from extraterrestrial resources. He concludes that we can start on it now , with existing technology and at startlingly low cost, and have the first operational by the late 1980’s. Not long afterward, somebody could put a motor on one of these.

The hardened science fiction reader may think such ideas are old hat. And so they are, in fiction. But to me the fact is infinitely more exciting than any story—that the accomplishment can actually be made, that sober studies by reputable professionals are confirming the dream.

True, I’d prefer to believe that men and women can get out there faster, more easily, so that the people who sent them off will still be alive when word arrives of what they have discovered. Is this wishful thinking? We’ve written off the rocket as a means of ultra-fast travel, but may there be other ways?

Yes, probably there are. Even within the framework of conventional physics, where you can never surpass c, we already have more than one well-reasoned proposal. If not yet as detailed and mathematical as Oberth’s keystone work on interplanetary travel of 1929, the best of them seem equivalent to Tsiolkovsky’s cornerstone work of 1911. If the time scale is the same for future as for past developments, then the first manned Alpha Centauri expedition should leave about the year 2010____

That’s counting from R. W. Bussard’s original paper on the interstellar ramjet, which appeared in 1960. Chances are that a flat historical parallel is silly. But the engineering ideas positively are not. They make a great deal of sense.

Since the ramjet has been in a fair number of stories already, I’ll describe the principle rather briefly. We’ve seen that at high speeds, a vessel must somehow protect its crew from the atoms and ions in space. Lead or other material shielding is out of the question. Hopelessly too much would be required, it would give off secondary radiation of its own, and ablation would wear it down, incidentally producing a lot of heat, less readily dissipated in space than in an atmosphere. Since the gas must be controlled anyway, why not put it to work?

Once the ship has reached a speed which turns out to be reasonable for a thermonuclear rocket— and we’re on the verge of that technology today—a scoop can collect the interstellar gas and funnel it into a reaction chamber. There, chosen parts can be fusion-burned for energy to throw the rest out backward, thus propelling the vessel forward. Ramjet aircraft use the same principle, except that they must supply fuel to combine with the oxygen they collect. The ramjet starcraft takes everything it needs from its surroundings. Living off the country, it faces none of the mass-ratio problems of a rocket, and might be able to crowd c very closely.

Needless to say, even at the present stage of pure theory, things aren’t that simple. For openers, how large an apparatus do we need? For a ship-plus-pay load mass of 1000 tons, accelerating at one gravity and using proton-proton fusion for power, Bussard and Sagan have both calculated a scoop radius of 2000 kilometers. Now we have no idea as yet how to make that particular reaction go. We are near the point of fusing deuterons, or deuterons and tritons (hydrogen nuclei with one and two neutrons respectively), to get a net energy release. But these isotopes are far less common than ordinary hydrgen, and thus would require correspondingly larger intakes. Obviously, we can’t use collectors made of metal.

But then, we need nonmaterial shielding anyway. Electromagnetic fields exert force on charged particles. A steady laser barrage emitted by the ship can ionize all neutral atoms within a safety zone, and so make them controllable, as well as vaporizing rare bits of dust and gravel which would otherwise be a hazard. (I suspect, myself, that this won’t be necessary. Neutral atoms have electrical asymmetries which offer a possible grip to the forcefields of a more advanced technolgoy than ours. I also feel sure we will master the proton-proton reaction, and eventually matter-antimatter annihilation. But for now, let’s play close to our vests). A force-field scoop, which being massless can be of enormous size, will catch these ions, funnel them down paths which are well clear of the crew section and into a fusion chamber, cause the chosen nuclei to burn, and expel everything aft to drive the vessel forward, faster and faster.