  As part of the NASA Starlight collaboration, we look at the implications of
radiation effects from impacts with the interstellar medium (ISM) on a directed
energy driven relativistic spacecraft. The spacecraft experiences a stream of
MeV/nucleon impacts along the forward edge primarily from hydrogen and helium
nuclei. The accumulation of implanted slowly diffusing gas atoms in solids
drives damage through the meso-scale processes of bubble formation, blistering,
and exfoliation. This results in macroscopic changes to material properties
and, in the cases of blistering and exfoliation, material erosion via blister
rupture and delamination. Relativistic hydrogen and helium at constant velocity
will stop in the material at a similar depth, as predicted by Bethe-Bloch
stopping and subsequent simulations of the implantation distribution, leading
to a mixed hydrogen and helium system similar to that observed in fusion
plasma-facing components (PFC's). However, the difference in location of
near-surface gas atoms with respect to the direction of exposure means that
previously developed empirical models of blistering cannot be used to predict
bubble formation or blistering onset. In this work, we present a model of the
local gas concentration threshold for material blistering from exposure to the
ISM at relativistic speeds. Expected effects on the spacecraft and mitigation
strategies are also discussed. The same considerations apply to the
Breakthrough Starshot mission.
