Figure 1. Hypothetical starship powered by antimatter.
Figure 2. Diagram of Penn State's proposed ICAN-II (ion compressed antimatter nuclear II) spacecraft.
Devotees of Star Trek will need no reminding that the starships Enterprise and Voyager are powered by engines that utilize antimatter. Far from being fictional, the idea of propelling spacecraft by the annihilation of matter and antimatter is being actively investigated at NASA's Marshall Space Flight Center, Pennsylvania State University, and elsewhere. The principle is simple: an equal mixture of matter and antimatter provides the highest energy density of any known propellant. Whereas the most efficient chemical reactions produce about 1 × 107 joules(J)/kg, nuclear fission 8 × 1013 J/kg, and nuclear fusion 3 × 1014 J/kg, the complete annihilation of matter and antimatter, according to Einstein's mass-energy relationship (E = mc2), yields 9 × 1016 J/kg. In other words, kilogram for kilogram, matter-antimatter annihilation releases about ten billion times more energy than the hydrogen/oxygen mixture that powers the Space Shuttle Main Engines and 300 times more than the fusion reactions at the Sun's core.
However, there are several (major!) technical hurdles to be overcome before an antimatter rocket can be built. The first is that antimatter does not exist in significant amounts in nature – at least, not anywhere near the solar system. It has to be manufactured. Currently the only way to do this is by energetic collisions in giant particle accelerators, such as those at FermiLab, near Chicago, and at CERN, in Switzerland. The process typically involves accelerating protons to almost the speed of light and then slamming them into a target made of a metal such as tungsten. The fast-moving protons are slowed or stopped by collisions with the nuclei of the target atoms, and the protons' kinetic energy converted into matter in the form of various subatomic particles, some of which are antiprotons – the simplest form of antimatter. So efficient is matter-antimatter annihilation that 71 milligrams of antimatter would produce as much energy as that stored by all the fuel in the Space Shuttle External Tank. Unfortunately, the annual amount of antimatter (in the form of antiprotons) presently produced at Fermilab and CERN is only 1–10 nanograms (a nanogram is a million times smaller than a milligram).1 On top of this production shortfall, there is the problem of storage. Antimatter cannot be kept in a normal container because it will annihilate instantly on coming into contact with the container's walls. One solution is the Penning Trap – a supercold, evacuated electromagnetic bottle in which charged particles of antimatter can be suspended. Antielectrons, or positrons, are difficult to store in this way, so antiprotons are stored instead. Penn State and NASA scientists have already built such a device capable of holding 10 million antiprotons for a week. Now they are developing a Penning Trap with a capacity 100 times greater.2 At the same time, FermiLab is installing new equipment that will boost its production of antimatter by a factor of 10–100.
A spacecraft propulsion system that works by expelling the products of direct one-to-one annihilation of protons and antiprotons – a so-called beamed core engine – would need 1–1,000 grams of antimatter for an interplanetary or interstellar journey.3 Even with the improved antiproton production and storage capacities expected soon, this amount of antimatter is beyond our reach. However, the antimatter group at Penn State has proposed a highly efficient space propulsion system that would need only a tiny fraction of the antimatter consumed by a beamed core engine. It would work by a process called antiproton-catalyzed microfission (ACMF).4 Whereas conventional nuclear fission can only transfer heat energy from a uranium core to surrounding chemical propellant, ACMF permits all energy from fission reactions to be used for propulsion. The result is a more efficient engine that could be used for interplanetary manned missions. The ICAN-II (ion compressed antimatter nuclear II) spacecraft designed at Penn State (see Figure 2) would use the ACMF engine and only 140 nanograms of antimatter for a manned 30-day crossing to Mars.
A follow-up to ACMF and ICAN is a spacecraft propelled by AIM (antiproton initiated microfission/fusion) in which a small concentration of antimatter and fissionable material would be used to spark a microfusion reaction with nearby material. Using 30-130 micrograms of antimatter, an unmanned AIM-powered probe – AIMStar – would be able travel to the Oort Cloud in 50 years, while a greater supply of antiprotons might bring Alpha Centauri within reach.
1. Schmidt, G., Gerrish, H., Martin, J. J. "Antimatter Production for
Near-term Propulsion Applications," 1999 Joint Propulsion Conference.
2. Smith, G. A., et al. "Antiproton Production and Trapping for Space Propulsion Applications," unpublished paper. Read on-line here.
3. Forward, R. L. "Antiproton Annihilation Propulsion," Journal of Propulsion, 1 (5), 370-74 (1985).
4. Smith, G. A., Gaidos, G., Lewis, R. A., Meyer, K., and Schmid, T. "Aimstar: Antimatter Initiated Microfusion for Precursor Interstellar Missions," Acta Astronautica, 44 183-86 (1999).