The mechanical properties of several transparent ceramics were investigated to determine if their use might lighten next generation spacecraft windows. The measured fracture toughness and slow crack growth parameters were used as inputs to functions describing the required mass for a desired window life. Transparent magnesium aluminate (spinel, MgAlO 4 ) and AlON exhibit superior slow crack resistance relative to fused silica, which is the historical material of choice. For spinel, slow crack growth, strength and fracture toughness are significantly influenced by the grain size, and alumina rich phases and porosity at the grain boundaries lead to intergranular fracture in coarse grain spinel. The results imply that transparent ceramics can lighten window panes from a slow crack growth perspective.