NASA's Constellation program, initiated to fulfill the Vision for Space Exploration, will create a new generation of vehicles for servicing low Earth orbit, the Moon, and beyond. Space radiation and plasma environment specifications for hardware design are necessarily conservative to assure system robustness for a wide range of space environments. Spectral models of solar particle events and trapped radiation belts are necessary for the design requirements of total ionizing radiation dose, dose rate effects, displacement damage, single event effects, and spacecraft charging. This presentation first describes the rationale in using the spectra to establish the environment design specifications. We then compare the variability within the spectral models to evaluate the applicability and potential vulnerabilities to extreme space weather events.