A small problem of propulsion - for interstellar spacecraft

FOR THE BETTER PART OF A DECADE, GERALD SMITH HAS BEEN CHASing particles of antimatter and collecting them in magnetic bottles, where they whiz around like subatomic fireflies. Now the Penn State physicist thinks he is on the verge of making antihydrogen, the first antimatter atom. When he tells other physicists about his progress, or when he justifies his work to the people who provide the funding, he emphasizes how it will enable him to test one of the most fundamental tenets of particle physics--the idea that antimatter is a perfect mirror image of matter. Once he's got antihydrogen atoms in hand, he explains, he will use a laser beam to stimulate them to emit light. If the theory is correct, antihydrogen should emit the same color light as ordinary hydrogen. If not, so much the better: Smith's experimental data would be even more important then.

Yet despite the value of Smith's work to basic physics, his real motivation for studying antimatter is far more practical--in a manner of speaking. He wants to fashion antimatter into rocket fuel to propel a spaceship to near-light speeds. "My father wanted me to be an engineer," says Smith. "I guess I'm a strange mixture of engineer and physicist. I have in my bones a sheer enjoyment of imagining applications of this stuff down the road." Smith has done more than merely daydream. He's precisely worked out how to build an antimatter rocket, down to the amount of fuel it would take and the size of the crew's quarters. "Ten years ago people thought it was impossible to trap an antimatter particle," he says. "Now we're about to make atomic antihydrogen. Eventually we might prove that antimatter propulsion is credible."

Smith is not the only scientist who's being lured to the stars. He is one of a small, somewhat eccentric, but devoted group of scientists who assert passionately that recent advances in technology have brought interstellar travel into the realm of the remotely possible. To support this claim, they keep up a steady barrage of proposals that range from manned rockets powered by nuclear and antimatter reactors to tiny robotic probes pushed to near-light speeds by laser or particle beams. Many of their ideas, such as beam propulsion, are inspired by still-classified military work under the Star Wars missile defense program. The hope is always that one of these proposals will attract a following in the community of space enthusiasts and--who knows?--perhaps even spark a groundswell of enthusiasm among the taxpaying public.

Until that day arrives, these modern-day quixotes labor on shoestring budgets, often in their spare time, and under the constant threat of being snickered at. In self-defense, they are quick to argue the merits of deep-space flight. A trip to Alpha Centauri, the nearest star, would give astronomers reams of data about the age of the universe and other cosmic mysteries. By going a mere 50 billion miles into the interstellar void, around 14 times farther than Pluto, researchers could use the sun's gravitational field as a giant magnifying lens to peer into the heart of the galaxy. Even parking a second Hubble telescope as near as Pluto would give astronomers a stereoscopic view that would help in measuring cosmic distances.

What keeps star-flight enthusiasts going, though, is not so much curiosity about what they would find as the fabulous engineering challenge of getting there. Alpha Centauri is 4.3 light-years, or 25 trillion miles, away. The space shuttle's three chemical rockets, which provide an acceleration of 1.7 g's at liftoff--1.7 times the gravitational acceleration of an object falling to Earth--would have to maintain that acceleration for more than two months to get up enough speed to make it to Alpha Centauri in a decade. But they couldn't do it: the fuel needed for such a burn would weigh so much that the spacecraft would hardly budge.

And that's not all the physics you'd have working against you. To reach Alpha Centauri in a decade, you'd have to average nearly half the speed of light. When you start talking about such speeds, however, you have to reckon with Einstein, the cosmic traffic cop. His theory of special relativity not only makes light the fastest thing in the universe but saddles any object that approaches light speed with extra mass. With each increment of acceleration your spaceship becomes heavier, which means that for each succeeding increment you must pump even more energy into your rockets. By the time you reach about three-quarters the speed of light, your mass has ballooned to one and a half times what it was when you started. Increasing the thrust yields virtually no acceleration at all.

The limitations of special relativity make it all the more essential to keep the weight of any deep-space ship to the barest minimum. The energy requirements of even a small probe are gargantuan by today's standards. Any proposal to accelerate an astronaut-bearing payload to one-third light speed is even less practical--it calls for a power output roughly equivalent to all of Earth's power plants operating together for several years on end. Any serious plan to send a ship to deep space, whether manned or unmanned, runs up against the enormous cost of the required "space infrastructure"--the space-based power plants, factories for building equipment, mines on asteroids, space stations for housing workers, and so on. This harsh reality does not depress the true interstellar ranger. "We could do it now if it were urgent enough," says Bob Forward, a retired Hughes Aircraft physicist who now works as a parttime consultant for NASA. "It would be a monstrous undertaking, but it's not impossible."