One of the beauties of antimatter is its efficiency. A fission reaction uses up about 1 percent of the available energy inside matter, whereas the annihilation of antimatter and matter converts 100 percent of the mass into energy. No wonder tiny amounts of antimatter can have such powerful effects. Put a gram of matter together with a gram of antimatter and you release the equivalent of a 20 kiloton bomb, about the size of the one that destroyed Hiroshima.
And if you really want to see antimatter’s potential, consider what it does to mass ratios, which compare the weight of a fully fueled spacecraft with that of an empty one. In his book Mirror Matter: Pioneering Antimatter Physics (New York: John Wiley & Sons, 1988), Robert Forward spoke of antimatter-driven spacecraft with mass ratios of 5 to 1 (by contrast, the Apollo missions operated with a ratio of 600 to 1). Indeed, Forward believed that a 1-ton probe to Alpha Centauri would require roughly four tons of liquid hydrogen and forty pounds of antimatter.
All of which takes us back to Gerald Smith’s intriguing work with positrons. I first ran into Smith’s ideas when he collaborated with Steven Howe, Raymond Lewis, and Kirby Meyer on a project called AIMStar (Antimatter Initiated Microfusion Starship). Here’s a link, but beware: it’s a PDF file. With a mission goal of reaching 10,000 AU — the domain of the Oort Cloud’s comets — in fifty years, AIMStar would use antimatter to ignite a fusion reaction. Building on an earlier design that used antiprotons to trigger fission, AIMStar would tap tens of micrograms of antimatter to create the reaction, injecting deuterium and helium-3 into a cloud of antiprotons.
Note: I made an error in Centauri Dreams (the book) when describing AIMStar — I mentioned it using 30 to 130 milligrams of antimatter, rather than the correct 30 to 130 micrograms. This whopper has now been added to the Errata page.
Another design to look at as you consider Smith’s ‘positron rocket’ idea is also from Penn State, where AIMStar originated. The Ion Compressed Antimatter Nuclear Rocket (ICAN-II) would use pellets of uranium and liquid hydrogen, with antiprotons to trigger the resulting nuclear reaction. In both cases, a critical factor was working within the limits of available antimatter. Note that Smith’s new design calls for antimatter in the milligram range (Smith thinks he’ll need tens of milligrams, so it’s quite a step up from the AIMStar design).
The key to all our antimatter hopes is increased antimatter production. The last time I looked, the cost of producing antimatter was about $62.5 trillion per gram, or $1.75 quadrillion per ounce. Get this: making antiprotons in particle beam collisions takes ten billion times more energy than is stored in their mass, and a few years ago, CERN pointed out that the amount of antimatter an accelerator laboratory can produce in a year is enough to make a 100-watt lightbulb shine for fifteen minutes.
So there’s the crux: we have to find ways to ramp up production and lower cost until we’re in the range Gerald Smith talks about. How to proceed? Robert Forward recommended the development of antimatter factories, dedicated facilities that wouldn’t have to rely on particle accelerator labs with larger agendas for their product. Here’s what Forward says about this in his Indistinguishable from Magic (New York: Baen Books, 1995), 25–26:
In a study I carried out for the Air Force Rocket Propulsion Laboratory, I showed that if an antiproton factory were designed properly by engineers, instead of by scientists with limited budgets and in a hurry to win a Nobel prize, the present energy efficiency (electrical energy in compared to antimatter annihilation energy out) could be raised from a part in sixty million to a part in ten thousand, or 0.01%, while at the same time, the cost of building the factory could be substantially lowered compared to the cost of the high precision scientific machines. From these studies, I estimated the cost of the antimatter at ten million dollars per milligram.
Now we’re getting somewhere — $10 million per milligram is more cost effective than chemical propulsion given the huge energy efficiency of antimatter. We won’t have Forward-style factories to rely on for the conceivable future, but designs that maximize the antimatter we can produce — and Steve Howe’s antimatter sail is certainly high on that list — will help us get antimatter into the rocket business. Both Smith’s Positronics Research and Howe’s Hbar Technologies are companies to watch as antimatter research continues.