Using Space Weather Variability in Evaluation the Radiation Environment Specifications for NASA's Constellation Program

NASA's Constellation program, initiated to fulfill the Vision for Space Exploration, will create a new generation of vehicles for servicing low Earth orbit, exploration of the Moon, and beyond. The space radiation environment specifications for hardware design are necessarily conservative to assure system robustness for a wide range of space environments. Design specifications for radiation environments implemented by the program range from solar wind at the lowest energies for evaluating optical properties of material surfaces to galactic cosmic rays at the highest energies for use in evaluating crew dose during long duration missions. Spectral models of the solar particle events and trapped radiation belts are necessary for the design requirements of total ionizing dose, displacement damage, single event effects, and spacecraft charging. The space environments and effects community designs to the level of threat environment a system is required to withstand. These design environments are required for guiding design and construction of systems to withstand acceptable extremes in the space environment. Therefore, design environments are based on observations and empirical based models to assure the design is traceable to credible, measured extremes in the environment. What is considered acceptable depends on the program risk posture and the consequences of risk. For example, the consequences of lost science spacecraft or science payloads, denial of service from communications systems, prematurely aborted missions, or sickness or loss of crew will drive the risks. A program will determine the appropriate threat level they will design their systems to withstand and beyond which they are willing to accept the risk of exposure. Space weather monitoring is then required for operations in environments more extreme than the design environment. The goal of this paper is to demonstrate the conservative nature of the space radiation design environments that have been established for design of Constellation systems. We first describe the solar energetic particle event and trapped radiation belt design environments specified for use in designing the Constellation hardware. We then compare the specified environments for each spectral model to space weather variations to demonstrate the conservative nature of the design specifications and the potential vulnerabilities of Constellation systems