Fusion Propulsion Constraints Required by Robust Exoplanet Exploration

Robust exoplanet exploration requires deceleration and orbital insertion, first around the target star, and ultimately around the exoplanet itself. Even for the nearest star Proxima Centauri, voyage durations under a century require spacecraft velocities of at least a few percent of the speed of light. There are few deceleration mechanisms feasible for such small stars. The propulsion system should optimally have a mass comparable to the scientific instrumentation. Nuclear fusion-based propulsion systems are one candidate technology. This paper reviews some of the physics constraints of such systems and reviews some promising fusion fuels such as helium-3 to assess their utility in an interstellar propulsion system.