  Achieving relativistic flight to enable extrasolar exploration is one of the
dreams of humanity and the long term goal of our NASA Starlight program. We
derive a fully relativistic solution for the motion of a spacecraft propelled
by radiation pressure from a directed energy system. Depending on the system
parameters, low mass spacecraft can achieve relativistic speeds; thereby
enabling interstellar exploration. The diffraction of the directed energy
system plays an important role and limits the maximum speed of the spacecraft.
We consider 'photon recycling' as a possible method to achieving higher speeds.
We also discuss recent claims that our previous work on this topic is incorrect
and show that these claims arise from an improper treatment of causality.
