|Fig. 1: A depiction of the series of events that occur in antiproton-induced fission of a single nucleus. (Source: A. Altman, after von Egidy et al. )|
Proton-antiproton interactions are complicated and can result in many energy and particle configurations. A common reaction between the proton-antiproton pair at rest is that which produces charged mesons.  A meson is simply a quark-antiquark pair. The details of the mechanism by which this occurs are beyond the scope of this paper, but in general, the particles will get close enough for their components, quarks and gluons, to interact and form new particles. There are many ways for such interactions to happen, which is why there are several different outcomes. Other reactions can result in high energy pions, photons, and kaons. It is important to note that such reactions can also occur with a neutron-antiproton pair.
The rest-mass energy of a proton or antiproton (given by E = mc2) is approximately 938 MeV. When they interact at rest, conservation of energy dictates that the total energy of the products is about 1876 MeV. For example, the case where the products are a positive and negative pion, the total rest-mass energy of which is about 472 MeV, they must have 1876 - 472 = 1404 MeV of kinetic energy.  These are very high energy particles - which is important for starting the fission reaction, as will be discussed in the following section.
When an antiproton collides with the nucleus of an atom (with more than 26 protons), it reacts with either a proton or neutron on the surface of the nucleus, as described above, which results in high energy mesons being injected into the nucleus. These particles disrupt the existing, stable configuration of nucleons, triggering the expulsion of protons, neutrons, and alpha particles from the nucleus, and the eventual breakdown of the nucleus into smaller atoms. This is depicted in Fig. 1. The released neutrons carry on to kickstart a traditional fusion reaction; i.e. a chain reaction of neutron absorption, atomic decay, and neutron expulsion.
The most immediate and long-standing problem with this version of nuclear power is that antimatter is extremely difficult to make, and even harder to store. Current production capabilities average about 106 antiprotons per second.  At that rate, it would take about 30 years to produce 1011 antiprotons, the amount hypothesized to initiate such a fission reaction.  As mentioned above, antimatter particles are attracted to their matter counterparts, and must be levitated with magnetic fields in vacuum. Additionally, the production and storage of antimatter costs energy, so much so that it is unlikely for antimatter-based energy systems to be energetically favorable.
One of the main attractions of an antimatter initiated fission reactor is its size - it is orders of magnitude smaller than traditional fission reactors. Thus, a way to justify the energy expense of such a reactor is to use it on spacecraft, where it is important to be as lightweight as possible. Palke discusses this in detail.  Additionally, Lanham and Meza explain various methods of converting nuclear fission into thrust. [5,6]
© Aaron Altman. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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 A. Gsponer and J.-P. Hurni, "The Physics of Antimatter-Induced Fusion and Thermonuclear Explosions," in Emerging Nuclear Energy Systems, ed. by G. Velarde and E. Minquez (World Scientific, 1987), p. 166.
 A. Palke, "Hybrid Antimatter-Fusion/Fission Propulsion for Interstellar Exploration," Physics 241, Stanford University, Winter 2011.
 G. Gaidos et al., "Antimatter Initiated Microfusion for Pre-Cursor Interstellar Missions," Acta Astronaut. 44, 183 (1999).
 T. Lanham, "To Deep Space and Beyond: Nuclear Electric Rockets," Physics 241, Stanford University, Winter 2017.
 Z. Meza, "NASA's NTREES and Nuclear Thermal Rocketry," Physics 241, Stanford University, Winter 2016.
 T. von Egidy et al., "Antiproton Induced Fission," Nucl. Phys. A 558, 383 (1993).