Is Antimatter a Viable Starship Fuel?
Hope springs eternal for die-hard Star Trek fans that scientists will one day build an actual, working antimatter propulsion engine similar to the one that powers the fictional starship Enterprise.
A paper published earlier this year by a pair of enterprising (get it?) physicists should fan the flames of that fantasy even further.
Ronan Keane (Western Reserve Academy) and Wei-Ming Zhang (Kent State University) report that the latest results from their computer simulations indicate that at least one key component of realizing a working antimatter propulsion engine — highly efficient magnetic nozzles — should be far more efficient than previously thought. And such nozzles are feasible using today’s technologies.
Before everyone chimes in with a resounding “Squee!”, let’s back up a moment.
First, its true: matter/antimatter propulsion is not just the stuff of science fiction. As he did with many technical aspects of the series, for the Enterprise propulsion system, Star Trek creator Gene Roddenberry drew on science fact.
Antimatter is the mirror image of ordinary matter. So antiparticles are identical in mass to their regular counterparts, but the electrical charges of antiparticles are reversed. An anti-electron would have a positive instead of a negative charge, while an antiproton would have a negative instead of a positive charge.
When antimatter meets matter, the result is an explosion. Both particles are annihilated in the process, and their combined masses are converted into pure energy — electromagnetic radiation that spreads outward at the speed of light.
Remember in Star Trek III: The Search for Spock: when Kirk sabotages the Enterprise after surrendering his ship to the Klingons? He programs the computer to mix matter and antimatter indiscriminately. Ka-boom! The ship is destroyed.
Despite that whole annihilation thing, as recently as October 2000, NASA scientists were developing early designs for an antimatter engine for future missions to Mars.
Antimatter is an ideal rocket fuel because all of the mass in matter/antimatter collisions is converted into energy. Matter/antimatter reactions produce 10 million times the energy produced by conventional chemical reactions such as the hydrogen and oxygen combustion used to fuel the space shuttle.
We’re talking reactions that are 1,000 times more powerful than the nuclear fission produced at a nuclear power plant, or by the atomic bombs dropped on Hiroshima and Nagasaki. And they are 300 times more powerful than the energy released by nuclear fusion
Alas, the only way to produce antimatter is in large accelerators at places like CERN. Even the most powerful atom smashers only produce minute amounts of antiprotons each year — as little as a trillionth of a gram, which would barely light a 100-watt bulb for three seconds.
It would take tons of antimatter to fuel a trip to distant stars. It would take CERN roughly 1,000 years to produce one microgram of antimatter.
Should an ample supply of antimatter be found, a secure means of storage must then be devised; the antimatter must be kept separate from matter until the spacecraft needs more power. Mixing can’t occur all willy-nilly, because then the two would annihilate each other uncontrollably, with no means of harnessing the energy.
But these are trivial engineering concerns, surely. The point is, Keane and Zhang think they’ve solved one part of the conundrum. Any rocket’s maximum speed depends on the configuration of the rocket stages, how much of the total mass is devoted to fuel, and a little something called exhaust velocity that provides the all-important thrust.
Keane and Zhang focus on the latter in their paper, i.e., how fast all those particles resulting from (hypothetical) matter-antimatter annihilation are traveling as they whip out of the rocket engine. Their premise relies on charged pions resulting from proton-antiproton collisions. A nozzle that emits a strong magnetic field could channel the emitted charged particles into a focused stream of charged pions, accelerating them to produce stronger thrust.
All this is old hat. And here’s the sticking point to that plan. The exhaust velocity of those pions depends partly on how fast they’re moving as they emerge from the annihilation event, and partly on the efficiency of the magnetic nozzle design.
Past calculations have shown that while the pions’ initial speed would be over 90 percent the speed of light, the magnetic nozzle would only be 36 percent efficient, so the largest escape velocity that could be achieved would be a disappointing one-third of light speed.
There isn’t much human beings can do to jack up the pions’ initial speed, so clearly the way to tackle this problem is to focus on the design of the magnetic nozzle. That’s exactly what Keane and Zhang did, relying on CERN software designed to simulate the complex interactions between particles, matters and fields so physicists can better understand the behavior of all those particles produced in collisions at the Large Hadron Collider.
The simulations showed that prior assessments of the magnetic nozzle’s efficiency were much too low; it should be possible to build a nozzle with 85 percent efficiency using technology available to us today.
True, they also found that the initial speed of the pions was lower than previously estimated — only about 80 percent of light speed. That still averages out to a far more promising final exhaust velocity of about 70 percent light speed.
There’s still the little problem of acquiring sufficient antimatter to fuel an entire rocket, even if we could work out all the engineering kinks. Keane and Zhang hypothesize that rather than creating antimatter on board, as the Enterprise does, it might be more feasible to mine deposits of antimatter in space.
Last year the PAMELA mission found that Earth is ringed by antiprotons. Unfortunately, it only detected 28 protons over the course of its two-year mission. That’s less than CERN produces each day.
Okay, so maybe we’re not ready for antimatter powered spaceships just yet. But it’s fun to play around with these kinds of ideas. Perhaps one day, one of these crazy schemes will pay off, and a future generation of astronauts will boldly go where only the fictional Enterprise has dared to venture before.
Editor’s note: This article was originally published on May 16, 2012.
Images: (top) Proposed schematic of an antimatter propulsion engine, NASA. (bottom) The fictional U.S.S. Enterprise.