Hybrid Nuclear Spacecraft for the Outer Planets

Chemical rockets are never going to allow us to exploit outer solar system resources. That is going to require massive spacecraft capable of transporting heavy loads of cargo. It is also going to require reducing trip durations to economically viable times, and this must be done at a cost sufficiently low that resources from beyond the asteroid belt can compete with those obtainable within it. Chemical fuels simply cannot produce the power density and specific impulse needed to meet these requirements. Nuclear fuels can provide orders of magnitude greater power density, but conventional nuclear thermal propulsion still limits specific impulse to insufficient values. However, a hybrid nuclear reactor can employ a fusion reaction at less than 5% of breakeven in a gas dynamic mirror thruster to generate a flux of neutrons that can drive a subcritical fission reaction in a fissile fuel, or via the thorium cycle, in which fissile U233 is bred. The combined fusion and fission reactions can, in theory, generate specific impulse as high as several hundred thousand seconds, with continuous power levels in excess of 100 terawatts. If achievable in practice, this would enable a ship with economically viable cargo capacity to travel under continuous acceleration to all outer solar system bodies, including distant Eris, in economically acceptable times – days or weeks – without requiring prohibitively high amounts of propellant. But the bulk of the power would come from the fission reaction, and converting all or most of the potential fission energy to thrust presents a daunting challenge that present and foreseeable future technology may not be fully able to meet. Nevertheless a more modest capability appears possible and could still enable high-capacity cargo ships to reach Jupiter and possibly Saturn in commercially viable times. Outer solar system resources could be tapped to meet terrestrial needs, or for use by facilities or colonies in orbit or on the Moon or Mars. But this requires that they be either uniquely obtainable from locations beyond the asteroid belt or cost competitive with terrestrial and inner solar system resources. While it may very well be that the outer solar system contains needed resources unobtainable closer to home, the more conservative assumption is that they will have to compete on cost. Distance alone does not preclude that, as current global trade in resources demonstrates.