By Adam Crowl
In the 1930s Paul Dirac discovered anti-matter and its mutually destructive interactions with regular matter represented the only known method of converting mass entirely into energy. In the years since, in science fiction, the immense energy released by matter-antimatter (M-AM) reactions has propelled starships – most famously powering the Warp-cores of Federation starships in the “Star Trek” universe and most recently propelling the ISV “Venturestar” which carried Jake Sully to Pandora in “Avatar”. In the world of rocket-science pioneering work by Robert L. Forward on just how M-AM reactions might be used to propel rockets has allowed researchers to ponder real starships. Assuming we have a supply and the means, what kind of performance can be expected? There are several ways of harnessing the energies released, the highest performing approach being the Beam-Core engine because it uses the reaction products directly.
When matter and antimatter react they produce high-energy gamma-rays, which are very hard to direct for thrust, and short-lived charged particles, which can be directed by intense magnetic fields. This means that the effective push from M-AM reactions is less than the theoretical maximum of the speed-of-light (designated by ‘c’), but it’s still very high – a staggering 0.5804 c. In every day figures that’s 174,000 kilometres per second, or 626 million kph. A Starship propelled by M-AM rockets might accelerate to 0.5 c, coast at that high-speed, then use rocket braking at its destination. A trip to the nearest star, Alpha Centauri, might then take 9 years in this scenario. The amount of matter-antimatter propellant required for such a trip would mass 5.64 times the starship’s non-propellant mass. In energy terms this is the equivalent of 500,000 trillion joules (140 billion kilowatt-hours) of energy for every kilogram of the ship’s empty mass. In power-cost terms alone that’s $7 billion per kilogram of ship at $0.05 kW-hr. Matter-antimatter starships very clearly need an energy supply revolution before they become practical.
Thus there are two main problems, directing the reaction energies and manufacturing antimatter, which need to be solved before real-life M-AM starships can be launched. Presently tiny amounts of antimatter are made in large particle accelerators, which are designed for other purposes. This method isn’t very useful because the antimatter is captured very inefficiently – a tiny percentage of the energy put into the accelerated particles emerges as antimatter. Dedicated antimatter factories, using current technology, might raise the efficiency a thousand-fold, but even then the efficiency is still 0.01%. A kilogram of matter-antimatter would then cost $12.5 trillion dollars to make from energy supplied at $0.05/kW-hr. Once made antimatter could, in theory, be stored as very cold snow-flakes of anti-hydrogen ice kept levitated in magnetic fields, but this technique has yet to be demonstrated with enough anti-hydrogen to make a snow-flake. In the decades ahead the production, storage and energy problems may well be solved. The Sun, every second, produces the equivalent of 4.33 million tonnes of energy, but the challenge is tapping that immense stream to make starship fuel.
Matter-antimatter rockets thus present a very desirable technology for interstellar travel, but require much more research before they can be said to be practical.
Adam Crowl is a member of the Project Icarus Team designing an interstellar starprobe.