The amount of antiprotons has always been the most controversial topic for antimatter propulsion. If one uses the "beamed core engine" approach, that is, direct one-to-one annihilation and expulsion of antiproton and proton atoms, 1-1000 grams of antimatter would be required. Given that currently 1-10 nanograms of antiprotons are produced a year at Fermilab (Chicago) and CERN (Switzerland), a beamed core engine is not feasible in the near future. Other concepts must be developed first.

One of the discoveries made by the Antimatter group at Penn State was that of antiproton induced fission. Whereas current nuclear fission can only transfer heat energy from a uranium core to surrounding chemical propellant, antiproton catalyzed microfission (ACMF) permits all energy from fission reactions to be used for propulsive purposes. The result is a more efficient engine (Isp = 13,500 sec) that can be used for interplanetary manned missions. The ICAN-II spacecraft, developed at Penn State, utilizes the ACMF engine for a manned mission to Mars, which only requires 140 ng of antimatter for a 30-day transit time. Although nuclear fuel is present, the spacecraft can be assembled in Earth's orbit (for example, the ISP) to offset any environmental impacts.

A follow-up to the ACMF and ICAN is Antiproton Initiated Microfission/fusion (AIM) and AIMStar. Here, a small concentration of antimatter and fissionable material is used to spark a microfusion reaction with nearby material. The amount of antimatter ranges between 30 - 130 micrograms, which is larger than that for ICAN. However, less fissionable material is required, and the higher specific impulse (61,000 sec) makes AIMStar, an unmanned spacecraft, more attractive to reach the Oort Cloud within 50 years. An extension to Alpha Centauri may be achieved as antiproton production increases.

The AIM engine requires just 5x10⁸ antiprotons per reaction; this amount can be readily obtained from Fermilab and CERN. Experimentation with such an engine can take place after methods of storing and transporting antimatter have been realized. One of the Antimatter group's chief projects in the past decade has been the design, fabrication, and testing of a portable antiproton trap (Penning trap) named "Mark I", which can store 10¹⁰ antiprotons for one week. The experimental results from Mark I are currently being used in the development of a NASA Penning trap that can store 10¹² antiprotons, large enough to support hundreds of reactions over a 2 minute timeframe.

Storage of antimatter is a challenging task, but reaps several benefits. One of which is the generation of O¹⁵, a radioisotope used for Positron Emission Tomagraphy (PET) of the human brain. Currently, only certain research hospitals across the world have the ability to create Oxygen-15. Due to its portability, a "radioisotope generator" antimatter trap may be transported to more remote areas for patients who cannot reach these hospitals. A second medical application concerns antiproton radiotherapy of tumors. The NASA Penning trap is being designed with these medical applications in mind.

The Fermi National Laboratory is currently constructing the Main Injector ring, which can produce 14 ng of antiprotons in one year's time. A recycling ring can boost production by a factor of 10. This is adequate for ICAN, but under the levels for AIMStar. It is hoped that successful experimentation of the NASA AIM engine will ignite public interest of antimatter as a viable propulsion source. Increased funding and/or construction of an antimatter laboratory will bring projects such as AIMStar and the beamed core engine, and several antimatter applications, to fruition.

¹Sutton, G.P., Rocket Propulsion Elements, 6th Ed., John Wiley & Sons, Inc., New York, 1992.

²Mallove, E.F., et al., The Starflight Handbook, John Wiley & Sons, Inc., New York, 1989.

Written by Kirby J. Meyer. Portions excerpted from the introduction of: Werthman, W.L., "Antiproton-Catalyzed Microfission/Fusion Space Propulsion," M.S. thesis, Pennsylvania State University, August 1995. Photographs courtesy of the NASA photo gallery, with the exception of the CERN photograph, which is courtesy of the CERN homepage.