A Small Problem of Propulsion

It's a long way to alpha centauri, but some think antimatter could send us there in record time.

By Fred Guterl|Sunday, October 01, 1995

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 part- time consultant for NASA. It would be a monstrous undertaking, but it’s not impossible.

Gerald Smith is acutely aware of these limitations, which is why he and every other space scientist have ruled out using chemical rockets for deep-space flight. Nuclear electric propulsion would provide 10 million times more thrust per pound of fuel, but conventional nuclear technologies are problematic for spaceflight. Fission needs to be contained by an elaborate reactor, which would melt under the high temperatures required for propulsion. And besides, fission produces heavy, slow-moving ions that don’t lend themselves to fast acceleration. In theory, fusion is better suited. A pellet of fuel bombarded by laser beams could be made to produce a fusion explosion in a combustion chamber, releasing enough energy to kick a rocket to high speeds. A reliable fusion reactor, however, is probably many decades away. Some engineers are skeptical of ever getting the process to work.

Smith believes that antimatter may be the answer. Although it has the ring of science fiction, antimatter is pretty ordinary stuff to high- energy physicists. Antimatter particles such as antiprotons and antielectrons, which are also called positrons, are, theoretically, almost identical to their ordinary, matter counterparts, except they have an opposite electric charge. They also disappear in a burst of energy when they come into contact with their matter counterparts, an event physicists call annihilation. Annihilation events release tremendous energy in the form of gamma rays and pi-mesons, or pions, which is what makes them so interesting to rocket designers. In theory, a pound of antimatter fuel would yield a hundred times more energy than a pound of fission or fusion fuel. That means, according to one proposal, an antimatter rocket should be able to accelerate a one-ton payload to one-tenth the speed of light with a mere nine kilograms of antimatter fuel.

The first stumbling block to such a voyage is scraping up enough antimatter. This isn’t easy. For one thing, antimatter particles are hard to catch. At the CERN particle physics laboratory in Geneva, where Smith does a lot of his work, a billion antiprotons come whizzing off a ring- shaped accelerator every ten minutes at one-tenth the speed of light--so fast and energetic that they pass through just about anything you put in front of them. Smith manages to slow them down by throwing layers of metal foil and gas in their path. The antiprotons collide with electrons in the foil, losing energy along the way. Then he has to trap them in his magnetic bottle before they run into ordinary protons, their matter mirror images, and cease to exist. If all goes well, about a million of the antiprotons have just enough kinetic energy left to enter the magnetic bottle but too little to shoot out the other side. This way Smith reduces his rampaging herd of antiprotons into a meek flock that huddles in a dime-size space in the middle of his trap.

A million antiprotons is a promising start, but rocket fuel it isn’t. Smith still needs to capture a lot more than that--almost a billion billion times more--to get even a gram of the stuff. Since the particles can live indefinitely in their magnetic bottle, called a Penning trap, zigging and zagging and spiraling around the magnetic field, in principle Smith should be able to repeat the procedure and keep amassing more and more of them. But he still has two big problems to overcome. For one, his Penning trap would begin to burst its seams with more than about a hundred billion antiprotons. Since the particles carry an electric charge that causes them to repel one another, the more densely they crowd together in the middle of the trap, the stronger the magnetic field needs to be to contain them. Making a bigger trap than Smith’s, already the world’s largest, would require extremely powerful, and expensive, superconducting magnets.

A better way, Smith thinks, is to combine the antiprotons with positrons to make antihydrogen atoms. Just as conventional hydrogen atoms, made up of a single electron orbiting a proton, have no net charge, neither would antihydrogen--the positron’s charge cancels the antiproton’s. What would keep the atoms from escaping the magnetic trap is the tiny magnetic field created by each spinning positron and antiproton. This so-called magnetic moment, if it is oriented opposite to the field of the bottle, generates a force that is just strong enough to push the atoms toward the center of the trap without bursting it altogether.

Smith plans to try his hand at synthesizing antihydrogen atoms later this year. First he’ll put a piece of silver foil in the trap. Then he’ll inject positrons, which will collide with the foil and knock loose some electrons. The electrons and the positrons will pair off to form atomlike structures called positroniums, which can exist for a brief time before the particles annihilate each other. That brief interlude will serve to slow down the positrons, so that when a positronium collides with an antiproton, the positron will have a low enough energy to orbit the antiproton and form an atom of antihydrogen. The leftover electron will go flying out of the trap, taking leftover energy with it.

It’ll work, says Smith. All the physics in this process is understood. We’re not having to rely on any theory. The only thing that could go wrong is if we don’t get enough positroniums, or if our magnetic field doesn’t hold them. But that’s just technology. Smith thinks the procedure will serve as a precursor to an industrial-strength process to make antihydrogen in bulk quantities. Eventually he hopes to condense the antihydrogen into liquid droplets, or even tiny icelike crystals, and store them at extremely low temperatures. That would allow him to use storage chambers that are more compact and efficient than Penning traps.

But even if antihydrogen solves the storage problem, it still takes too long to produce enough particles to make fuel. Amassing even a gram of antimatter would take, for all practical purposes, forever--even with the improvements Smith envisions to make his process more efficient, and even with an investment of hundreds of millions of dollars in what would essentially be antimatter factories. Smith believes that by the end of the decade it would be reasonable to shoot for synthesizing antiprotons at the rate of one microgram--a millionth of a gram--per year. At that rate, nine kilograms of rocket fuel is 9 billion years away.

So how will antimatter take us to the stars before the stars themselves die, let alone the human race? The only way, Smith thinks, is to forget about pure antimatter propulsion for the time being. Instead he proposes using antimatter as a catalyst for a conventional fission-fusion reaction--the kind used in hydrogen bombs. These start with a piece of uranium. Bombarding it with neutrons starts a fission reaction, which in turn heats a capsule of deuterium and tritium--heavy forms of hydrogen-- thus triggering a fusion reaction.

The drawback, from the standpoint of space travel, is that these reactions produce huge explosions, equivalent to millions of tons of TNT, that are difficult to contain in a combustion chamber. Smith proposes cutting them down to size by truncating the initial fission reaction. He would inject antiprotons into a capsule of uranium containing a smidgen of deuterium and tritium. When an antiproton hits a uranium atom, it annihilates itself along with one of the protons in the nucleus. A few of the resulting pions rip through the remainder of the nucleus and blast it apart, releasing copious neutrons--more than six times the number of neutrons produced in a conventional fission reaction. The resulting fission chain reaction proceeds enormously fast, generating enough heat and pressure to trigger a fusion reaction in the deuterium-tritium core. Using antiprotons to jump-start the fission reaction in this way would allow Smith to trigger the fusion reaction with only a tiny pellet of uranium.

The result, according to Smith’s calculations, is a microexplosion equivalent to roughly 15 tons of TNT. By setting off one of these every second for a few days, a manned ship could get up enough steam to make it to Pluto in only three years, Smith reckons. Smith is aware that the idea of powering a spacecraft with hydrogen bombs sounds alarming. We would take what is obviously a very nasty thing, which we all hope will never, ever be used on Earth, and try to reduce it to an object 1,000 times smaller so we can take advantage of the physics that goes on, says Smith. Other people have looked at this, and I don’t think anybody thinks it’s crazy. It makes sense. What’s needed is a test.

But some of Smith’s peers in the interstellar community are intensely skeptical. The basic problem with any fuel-burning interstellar rocket, says Bob Forward, who has studied the feasibility of antimatter rockets for NASA, is that the rocket has to push a reaction mass out its tail in order to push itself forward. The reaction mass is dead weight that has to be carried along to the stars, and it is also extremely hot when it shoots out the tail. It’s hotter than flame, says Forward. Nobody has been able to develop an engine that doesn’t melt.

Smith has countered such criticism by designing his rocket to sort of melt as it goes along. The antimatter-induced fusion reaction releases energy chiefly as photons, in the form of gamma rays. To keep these highly penetrating rays from escaping the combustion chamber in all directions, Smith passes some of them through a lead filter, which converts them to X-rays. The X-rays then strike a titanium pusher plate, vaporizing a thin layer of the metal, which in turn is forced out the back of the ship. The titanium acts as the reaction mass, and throughout the journey it is gradually consumed. Still, Smith hasn’t addressed the problem of designing a nozzle that won’t melt. It’s messy and complicated, he admits. If it were anything else but antimatter, you wouldn’t fool with it. But the specific energy density of antimatter is so large, it seems to me it’s worth making the effort.

I’m a practical experimentalist, he adds. I’m not interested in fantasy. I don’t think I’m on the lunatic fringe. If I had to give odds, I would give antimatter propulsion a less than 1 percent chance of succeeding. But if it works, it’ll be big stuff.

Forward’s approach to interstellar travel is fundamentally different from Smith’s. Since retiring from Hughes in 1987 to devote himself to researching advanced propulsion and writing science fiction novels, Forward has become something of a head cheerleader for a group of space enthusiasts whose opposition to nuclear propulsion is almost philosophical. They argue that the limits of special relativity preclude carrying anything as cumbersome as a rocket engine and fuel to deep space. Instead, they have embraced beam propulsion as a more elegant alternative.

Back in the early 1960s, Forward was toying with an idea for a solar sail--a big swatch of aluminum foil that would catch the solar wind, the charged particles that stream constantly outward from the sun, and ride it out of the solar system. The free-ride aspect was attractive, but Forward quickly realized it wouldn’t work for interstellar space travel, because the spacecraft would hit the doldrums outside the solar system, where the wind peters out. He then read a magazine article that described the then-new ruby laser’s light as brighter than the sun’s. That gave him the idea of pushing his sail with a laser beam. As Forward worked it out, photons from the laser would strike the sail and impart some of their energy in the form of momentum, pushing the sail faster and faster.

The interesting quality of laser light, from the standpoint of propulsion, is that the beam hardly diverges, which means it will propagate immense distances before it begins to widen and its power diffuses. And since the power source would be left behind in the solar system, it could be serviced and maintained or even replaced if the need arose. Best of all, without the need for engines or fuel, the spaceship could be made much lighter, which means less power would be needed to push it to near-light speeds.

To get a spacecraft to Alpha Centauri, the laser beam would push the sail for about a year, accelerating it to one-third the speed of light. At this point the beam would be turned off, and the ship would coast. One of the niftier aspects of Forward’s idea is what would happen next. As the ship neared Alpha Centauri, the crew would detach the outer ring of the sail--the sail would be constructed in three concentric circles--and push it in front of the ship. Back in the solar system, the big sun-powered laser would be fired up again, sending a giant slug of light toward the ship. The light would bounce off the detached loop and fall onto the center part of the sail from the front, thus putting the brakes on the spacecraft.

Of course, the laser would still be pushing on the central sail in a direction away from the solar system, but since the outer ring covers an area nine times bigger than the inner two, its decelerating force would win out. When the crew was ready to head home, they would detach the second ring, and then the reflected beam would be strong enough to accelerate the ship back toward the solar system--or so Forward claims. I worked this out when I was writing my science fiction novel Rocheworld, he says. It was only later, when I plugged the numbers in, that I found it would really work.

Unfortunately, the power saved by leaving the ship’s engines behind would be offset by another inefficiency. Namely, laser light does not give much of a push to even the best sail. The push it gives comes from its magnetic field, which exerts a forward force on charged particles oscillating inside the sail, but the force is very small. As a result, Forward’s scheme requires a hugely powerful laser beam to drive the spaceship. He got a slight plausibility boost in the 1980s, when Star Wars researchers found a way of collimating, or making parallel, many laser beams, creating one giant beam. Rather than one giant laser, Forward says, you could actually use a thousand or so small solar-powered lasers. Placed in orbit around Mercury, they would convert the intense sunlight there into laser beams and then feed them into a giant collimator that would collect them into a superpowerful beam. Each of the mini-lasers, however, would still need to be about a hundred billion times more powerful than any solar-powered laser yet developed.

The lasers and the collimator are actually the more modest aspects of Forward’s scheme. The spacecraft’s sail, built from wire and aluminum foil, would need to be large enough to catch momentum from the laser beam--around 600 miles in diameter, says Forward, which is a little bigger than the state of Texas. If it’s any smaller it won’t work, he says. You would need robotic spiders to put together the sail. They would probably actually look something like spiders.

The most daunting engineering challenge would be building a laser lens. Because even a laser beam tends to diverge over long distances, Forward envisions placing a lens somewhere between Saturn and Neptune to refocus the beam and keep it powerful. The lens would be kept in position by a balance between the sun’s gravitational pull and the outward push of the laser. It would consist of rings of plastic alternating with empty space on a steel framework, and it would have to be just as big as the sail. It would weigh 50,000 tons or so. We need a space infrastructure to mine asteroids before it becomes cost-effective to build the lens, Forward says. You get one asteroid a couple hundred feet across and send a factory up there, which weighs maybe 10,000 tons. You have to think big to get it working at all. If the lens isn’t as big as Texas, the beam will spread out before it gets to Alpha Centauri, the light pressure on the sail will fall off, and you won’t get up to speed.

Not surprisingly, for a construction project whose fundamental unit of scale is Texas, Forward’s scheme has been criticized, even by fellow beam-propulsion advocates, as impractical. Even if you forgo the idea of sending a crew to Alpha Centauri and dispatch a one-ton robot instead, the sail and the lens would have to be 60 miles in diameter-- bigger than Delaware. Bob Forward’s ideas are totally outlandish, says Ed Belbruno, a mathematician at the University of Minnesota at Minneapolis.

The problem, Belbruno says, lies in the whole idea of using lasers as propulsion. Lasers have a wimpy momentum transfer, he scoffs. He believes that particle beams offer the best chance of reaching the stars. Particle beams are beams of heavier particles, such as protons, which travel slightly slower than light but which, because they have mass, are more efficient than massless photons at imparting momentum.

One particle-beam scheme is the brainchild of two down-to-earth aerospace engineers: Bob Zubrin of Lockheed Martin and Dana Andrews, chief engineer of Boeing’s X-33 project--the ship that Boeing hopes will replace the space shuttle. I spend most of my time thinking about getting things to low Earth orbit, says Andrews. Nobody’s funded research into advanced propulsion in this country for 15 years. But I’ve been interested in this stuff since I was in graduate school, and the interests you have at the beginning are the interests you keep.

In Andrews and Zubrin’s scheme, the spacecraft’s sail is just a giant loop of superconducting wire, which generates a doughnut-shaped magnetic field. When charged particles from the beam strike the field, they are deflected, just as the solar wind is deflected by Earth’s magnetic field. But in the process they transfer momentum to the sail. The particle beam itself would be powered by a fusion reactor, probably located on an asteroid, which would heat a gas to extremely high temperatures. This hot gas, or plasma, would then be funneled into a tube about half a mile long. As the particles moved down the tube, they would be deflected off the sides so that by the time they reached the end, they would all be traveling in more or less the same direction.

The drawback to a particle beam is that it tends to diverge quickly--the charged particles jostle each other as they travel, eventually deviating from their original direction, causing the beam to widen. For this reason, a particle beam would be effective for only a relatively short distance. Fortunately, since a particle beam is a more powerful accelerator than a laser, it would not need to be trained on the spaceship for as long. According to Andrews, a particle beam could accelerate a manned ship to one-third light speed using only about a sixth the energy required by Forward’s laser scheme. One problem, though, is that the crew would be exposed to around 1,000 g’s.

Could anyone survive such a crushing acceleration? Salamanders can, Andrews points out: he cites experiments by the Shimizu Institute, a research firm in Minnesota, in which several generations of salamanders were bred at extremely high accelerations with no ill effects. To keep the human skeleton from collapsing under its own weight, astronauts would have to immerse themselves, salamander-like, in some kind of liquid. As it happens, says Andrews, researchers have shown that divers can breathe highly oxygenated liquids, such as water or fluorocarbon, without too much difficulty; the liquid helps them avoid the bends.

From a human standpoint, another drawback to particle beams is that they cannot project power across stellar distances. In other words, once the crew had traveled to Alpha Centauri in its fluorocarbon baths, it would have no means of returning to the solar system. Andrews is unfazed by this difficulty. We’d have no problem finding volunteers for a suicide mission, he says. Just think about the things you’d be able to see and name. Being prone to claustrophobia, however, Andrews disqualifies himself from such a mission.

The drawbacks bother Belbruno, who argues that particle-beam- propelled robotic probes offer the only practical avenue to the stars in the next half century. In fact, Belbruno was so excited about the plausibility of such a mission that he organized a conference in New York in August 1994 to address the issues. It was supposed to be a serious look at interstellar flight, which previously hadn’t been done, he says. Until then, the conferences had been sort of way out. Bob Forward’s ideas were totally way out. I’m not putting him down, but you just can’t do them. So the point was, let’s have a serious conference and see what you can do with current technology.

After meeting for several days with experts from all walks of space travel, Belbruno concluded that there is a practical way of overcoming Einstein’s special relativity theory and, with a reasonable amount of energy, getting a spaceship to the stars. What would solve the problem, says Belbruno, would be to make a spacecraft about the size of the head of a pin. A nanotechnology spacecraft.

Weighing about a gram, a pin-size probe could be accelerated to three-quarters the speed of light without having its mass swell to overwhelming proportions. Even if researchers master the art of building a probe that small, which they have not done, other obstacles present themselves. How would we track a pinhead at Alpha Centauri? One scientist at Belbruno’s conference suggested shining a laser beam on it and then looking for the reflection with the Keck telescope. But how would the spacecraft report back to us, given that there’s no way of building a radio dish as small as a pinhead?

Anybody with his feet on the ground would throw up his hands at this point, but this is where the true space enthusiast gets really stubborn. The eternal hope is that some new technology or new physics comes along to make the problems go away. Wormholes, for instance: These tunnels in space-time, postulated by physicist Kip Thorne of Caltech, might theoretically provide cosmic shortcuts past the annoying limitations of special relativity. If a wormhole could be made big enough for a spaceship to pass through, astronauts could go anywhere in the universe in a single step. Similarly, physicist Miguel Alcubierre of the University of Wales has shown that in theory a spaceship could travel faster than light speed by warping space-time with some kind of antigravity.

But these theories prove only that such travel is not intrinsically impossible, which is still a long way from giving a hint of how to do it. It does seem that within the laws of physics as we know them today, there are ways to do faster-than-light travel, says Belbruno. However, you have to understand what gravity is, and we don’t understand it. Also, we don’t even know what 99 percent of the mass of the universe is. So we don’t know anything right now. Probably we’re going to be able to fly all over the place eventually, but right now we just don’t know how to do it.

On that assessment, at least, he and Forward agree. The best idea hasn’t been thought of yet, says Forward. That’s why everyone is concentrating on coming up with new ones.

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