Figure 1: Experimental Plan of In Vivo Animal Exposure Mouse cohorts (n=5-15 / group) were exposed to a double-hit challenge of 0.25 Gy total body ionizing gamma irradiation (IR) and 100% pO2 for 8 hours followed by intermittent normoxia (21% pO2). This cycle was performed 3 times a week for 1, 2, and 4 weeks. Mice were sacrificed at weeks 1, 2, and 4 weeks (early) and 8, 12, 16, and 20 weeks (chronic). A total of 24 hours of hyperoxia / radiation exposure weekly simulates the maximum allowed weekly extravehicular activity (EVA) exposure for crewmembers on lengthy spaceflights.