The Astral Mirror Read Free

acceleration period, the crew would feel 1 g, their normal Earth weight.
    “After several months of this acceleration, the ship would be traveling at a relativistic speed—fast enough for time effects to come into play. It would then shut down its engines and coast the rest of the way to Alpha Centauri. The same procedure would be followed for the return trip: a few months of 1 g acceleration, then coasting flight back to Earth.
    “The 29 years would seem slightly shorter to the ship’s crew,” the engineer says, “because of the relativistic time-dilation effect.”
    “And that’s using power that we know we can harness,” the science fiction writer adds excitedly. “Why, maybe early next century we could reach Alpha Centauri! People alive today might make the trip!”
    “Excuse me,” says the astronomer. “Have any of you heard of the Bussard interstellar ramjet?”
    “R. W. Bussard was a physicist at the Los Alamos Scientific Laboratory when he thought of the interstellar ramjet idea,” the astronomer explains.
    “Bussard realized that one of the main drawbacks to any rocket engine is that it must carry all of its propellant with it. Spencer and Jaffe’s five-stage fusion rocket, for example, must be more than 90 percent hydrogen propellant—allowing very little payload for such a huge vehicle. The rocket must also spend a considerable amount of its energy just lifting its own propellant mass. The situation becomes a vicious circle. As long as you must carry all the rocket’s propellant along with you, any increase in speed must be paid for by more propellant mass. When you’re considering flight at close to the speed of light, this becomes a serious obstacle. It poses a fundamental limitation on the amount of energy you can get out of the fusion rocket.
    “But suppose the interstellar ship didn’t have to carry any fuel at all? It could carry much more payload. And its range would be unlimited—it could go anywhere, at close to light speed, as long as it could somehow find propellant to feed to its engines.
    “Interstellar space is filled with propellant for a hydrogen fusion rocket—hydrogen gas. There is enough hydrogen gas floating freely among the stars to build billions of new stars. This is an enormous supply of propellant.
    “However,” the astronomer admits, “when I use the word filled I’m being a little overly dramatic. The hydrogen gas is spread very thinly through most of interstellar space... no more than a few atoms per cubic centimeter. By contrast, there are more than 10 19 atoms per cubic centimeter in the air we’re breathing. That’s ten million trillion atoms in the space of a sugar cube. Out among the stars, there are fewer than ten atoms per cubic centimeter.
    “Bussard calculated that the ramjet will need a tremendously large scoop to funnel in a continuous supply of hydrogen for the fusion rocket engines. For a ship with a payload of 1,000 tons—about the size of a reasonable schooner—a funnel some 2,000 kilometers in radius would be needed.”
    The mathematician smiles. “I’m tempted to say that such a scoop would be astronomically big.”
    “Yes,” the engineer says, “but there’s plenty of open space out there.”
    “And the scoop needn’t be solid material,” the physicist adds. “If you could ionize the hydrogen with laser beams, so that the atoms are broken up into electrically charged ions, then the scoop could be nothing more than an immense magnetic field—it would funnel in the electrified ions quite nicely.”
    “Such a ship,” the astronomer goes on, “can reach the nearest stars in a few years—of ship time, that is. The center of the Milky Way would be only about 20 years away, and the great spiral galaxy in Andromeda could be reached in about 30 years. Of course, the elapsed time on Earth would be thousands, even millions of years.”
    “Even forgetting that for a moment,” the science fiction writer asks, “don’t you think the