As it turns out, there are a number of options. But none of these is especially easy. This section describes some candidate antimatter storage systems.
One possibility is magnetic storage rings. Using combinations of electric and magnetic fields, antiprotons would be spun continuously around one ring at constant velocity, positrons (if necessary) around another. When reaction with normal matter in the starship’s combustion chamber is required, an appropriate mass of antiparticles could be magnetically diverted towards the target without touching chamber walls. Antiparticles have been stored in such a manner after deceleration in existing antimatter factories. But we wonder what the limits are on antiparticle density in the ring. And is it possible to reliably alter field strength in parts of the storage ring as the ship changes its acceleration rate?
Many of the potential solutions to antimatter storage have been reviewed in a paper by the American physicists Steven Howe and Gerald Smith. They describe a version of the Penning trap they constructed at Pennsylvania State University. This device might be able to store one hundred billion antiprotons per cubic centimeter. That sounds like a lot of antiprotons, but a Penning trap at least a kilometer across would be required to store a kilogram of antiprotons!
Forward, in his Air Force report, expresses the opinion that antimatter engineers will store frozen anti-hydrogen rather than antiprotons or an antiproton-positron plasma. A ball of anti-hydrogen with an electric charge could be levitated using electric fields. Care must be taken, though, to adjust the field to compensate for the starship’s acceleration. And some mechanism must be developed to cleanly remove anti-hydrogen atoms from the ice ball and transfer them to the reaction chamber without prematurely and disastrously annihilating them.
The levitated ice ball concept might be workable in the frigid wastes of interstellar space. But frozen anti-hydrogen might be very hard to store in the much hotter environment of a near-Sun antimatter factory.
We are a long way away from being able to produce and store the amounts of antimatter needed for an interstellar voyage.
Antimatter technology is in its infancy. But as it matures, its application to space flight is a natural outcome. Figure 1 presents major features of an antimatter rocket. The payload rides ahead of the fuel tanks. The fuel consists of normal matter (probably hydrogen) and antimatter. Antimatter is fed into an “annihilation chamber” where it reacts with normal matter. An electromagnetic nozzle is used to expel the charged particles as exhaust.
Figure 1. Artist concept of an antimatter rocket. (Image courtesy of NASA.)
Let’s say we desire an interstellar cruise velocity of 0.09c after all the fuel is expelled, which allows a ship to reach Alpha Centauri in about fifty years (not counting the time required for acceleration and deceleration).
If our starship has a mass of about one million kilograms, then it would require twelve thousand eight hundred kilograms of antimatter. The hypothetical Mercury-based antimatter factory discussed in a previous section could produce this mass of antiprotons in about twenty-five years.
Instead of a crewed starship, let’s say we wish to launch a robotic probe with an unfueled mass of one thousand kilograms. In this case, only 12.8 kilograms of antimatter will be required! And if further miniaturization is possible, the antimatter mass required for an interstellar probe can be reduced still further.
We next consider the acceleration process. If the ship requires about 10 years to accelerate an average of about 107 kilograms of matter will be converted into energy each second. The probe generates matter/antimatter annihilation energy at an approximate average rate of 1010 watts, roughly equivalent to that of a large city. The ship’s generated power level will be about one thousand times greater, approximating that of our entire global civilization! Antimatter propulsion is clearly not for the faint hearted!
Early antimatter history has been discussed in many archival sources. One such is H. A. Boorse and L. Motz, ed., The World of the Atom, Basic Books, NY (1966).
The story of the antiproton is eloquently told by L. Yarris in “The Golden Anniversary of the Antiproton,” Science @ Berkeley Lab (Oct. 27, 2005), http://newscenter.lbl.gov/feature-stories/2005/10/27/ the-golden-anniversary-of-the-antiproton/
For further information regarding possible biomedical antiproton applications, check out L. Gray and T. E. Kalogeropoulos, “Possible Biomedical Applications of Antiproton Beams: Focused Radiation Transfer,” Radiation Research, 97, 246-252 (1984).
Many sources have speculated on possible military applications of antiprotons. Two web references on this topic, both by Andre Gsponer and John-Pierre Hurni, “Antimatter Underestimated,” arXiv:physics/0507139v1 [physics.soc-ph] 19 Jul 2005 and “Antimatter Weapons,” http://cul.unige.ch.isi/sscr/phys/antim-BPP.html
Many astronomy texts consider the early moments of the universe when matter (and antimatter) formed. One readable text, authored by Eric Chaisson and Steve McMillan, is Astronomy Today, 3rd ed., Prentice-Hall, Upper Saddle River, NJ (1999).
Sanger’s photon rocket is described by Eugene Mallove and Gregory Matloff in The Starflight Handbook, Wiley, NY (1989). This book also discusses the decay scheme for the proton-antiproton annihilation reaction.
Robert Forward’s work is reviewed in The Starflight Handbook and other interstellar monographs. His final report to the US Air Force Rocket Propulsion Laboratory is entitled AFRPL TR-83-067, “Alternate Propulsion Energy Sources.” Many of Bob Forward’s ideas regarding antimatter (and a host of other subjects) are also published in a more accessible form: R. Forward, Indistinguishable from Magic, Baen, Riverdale, NY (1995).
Antimatter production by black holes is described by C. Bambi, A. D. Dogov and A. A. Petrov in “Black Holes as Antimatter Factories,” which was published in Sept. 2009 in the Journal of Cosmology and Astroparticle Physics, which is an on-line journal. This paper is also available from a physics archive as arXiv.org/astro-ph>arXiv:086.3440v2.
A NASA web publication, titled “Antimatter Factory on Sun Yields Clues to Solar Explosions,” describes the discovery of gamma rays in solar flares. http://www.nasa.gov/vision/universe/solarsystem/rhessi _antimatter.html.
To learn more about the surprising discovery of positrons associated with terrestrial lightning discharges, consult R. Cowen, “Signature of Antimatter Detected in Lightning,” www.wired.com/wiredscience/2009//11/antimatter-lightning/.
Information regarding the current capabilities of the Tevatron was obtained from Wikipedia and the Fermilab website. Operational details regarding the Large Hadron Collider are available on the CERN website.
Many books on SETI (the Search for Extraterrestrial Intelligence) deal with the Kardashev scheme for categorizing the capabilities of advanced technological civilizations. A very readable and authoritative one is W. Sullivan’s We Are Not Alone, revised edition, Dutton, NY (1993).
A number of researchers have considered the application of solar-sail technology to the construction of huge planetary sunshades or solar collectors. Analysis by Robert Kennedy, Ken Roy and David Fields is discussed and reviewed by L. Johnson, G. L. Matloff and C Bangs in Paradise Regained: The Regreening of Earth, Springer-Copernicus, NY (2009).
The cited antimatter-storage paper by S. D. Howe and G. A. Smith is entitled “Development of High-Capacity Antimatter Storage.” It was delivered at the Space-Technology and Applications International Forum-2000, University of New Mexico, Albuquerque, NM, July 30-February 3, 2000 and is available on line.