Nuclear pulse propulsion has the downside that the minimum size of the engine is defined by the minimum size of the nuclear bombs used to create thrust. With conventional technologies these bombs can scale down to about the kilotonne range, but making them smaller seems difficult. These large bombs require a heavy structure for the spacecraft, and a very large (and heavy) pusher-plate assembly.
A new technique is changing the nature of this equation considerably. By injecting a small amount of antimatter into a subcritical mass of fuel (typically plutonium or uranium) an interesting reaction takes place that leads to the fission of the fuel.
An anti-proton has a charge of negative 1 just like an electron, and can be captured by an atom into the electron orbitals. However this is not a stable configuration, and the anti-proton will start to radiate away energy as gamma rays. As it does so the orbital falls closer and closer to the nucleus of the atom.
Eventually the anti-proton will decay to the point where it lies inside the nucleus, where it will annihilate with a proton. This reaction releases a tremendous amount of energy, enough that the thermal effects will cause the nucleus to explode. This releases a shower of neutrons, causing the surrounding fuel to undergo rapid fission. The reaction is fast enough to also serve as the trigger for a nuclear fusion reaction if desired.
In theory, there is no lower limit to the size of this sort of device, one anti-proton is enough to start the chain reaction. There are real-world issues relating to the lifetime of the anti-protons and their chance of reacting with the fuel that impose a lower limit on the amount of anitmatter needed per reaction, and the geometry of the fission reaction imposes a lower limit on the size of the fuel as well. However these real-world numbers turn out to be entirely feasible with today's technology and infrastructure, unlike either the Orion-type system which requires huge numbers of large bombs, or the various anti-matter drives which require impossibly huge amounts of antimatter.
Several groups are actively studying such antimatter catalyzed micro fission/fusion engines in the lab (sometimes antiproton as opposed to antimatter), after its invention at Pennsylvania State University in 1992.
Tuning of the performance to the mission is also possible. Rocket efficiency is strongly related to the mass of the working mass used, which in this case is the nuclear fuel. Although the reaction energy of a fusion reaction is about 1/10th that of a fission reaction, the LiD fuel used in these reactions is much lighter. For missions requiring shorter periods of high thrust, manned interplanetary missions for instance, pure microfission is used because it reduces the number of fuel elements needed. For missions with longer periods of lower thrust, outer-planet probes for instance, microfission/fusion is used because it reduces the total fuel mass.
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