Damage to Relativistic Interstellar Spacecraft by ISM Impact Gas Accumulation Author: Alexander Cohen, BS, Jr. Specialist, University of California, Santa Barbara Abstract Background: As part of the NASA Starlight collaboration we look at the implications of directed energy driven spacecraft capable of achieving relativistic flight impacting the interstellar medium (ISM) along their journey. These same considerations apply to the Breakthrough Starshot effort. Relativistic spacecraft will experience the ISM as a wide, MeV-range beam composed primarily of hydrogen and helium at constant velocity. A straightforward calculation of the sputtering yield for impacts at these energies shows that it is negligible. However, sputtering is not the only damaging process caused by particle irradiation. Bubble formation, blistering, and exfoliation are meso-scale processes driven by the accumulation of implanted insoluble gas atoms in solids, resulting in macroscopic changes to material properties and, in the cases of blistering and exfoliation, material erosion via blister rupture and delamination. Abstract Objectives: 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. Abstract Methods: Contemporary understanding of these phenomena comes from a combination of theoretical, empirical, and computer models. Careful application of these models is necessary to understand the cumulative effects of ion bombardment. To perform this analysis, we find implantation profiles of ISM gas atoms at relativistic speeds using a BCA code, calculate critical concentrations for blistering onset for hydrogen and helium individually assuming a worst-case scenario of negligible diffusion, and show the effect of non-negligible diffusion on local gas concentrations. Abstract Results: For a 20-year journey without an adequate shielding system, hydrogen bubble formation and subsequent bursting may induce damage to the spacecrafts leading edge which, as the damage continues to make its way into the spacecraft body, will eventually damage components and electronics essential to the spacecrafts performance. Moreover, secondary particle production and heavy species impacts have the potential to induce damage at a greater rate, depth, and severity than the common proton impact. Abstract Conclusions: Various shielding schemes are proposed as mitigation strategies. Since the spacecraft is designed to fly edge-on into the ISM, a circumferential shield is proposed to protect the ISM-facing edge.