  The space radiation environment is a complex combination of fast-moving ions
derived from all atomic species found in the periodic table. The energy
spectrum of each ion species varies widely but is prominently in the range of
400 - 600 MeV/n. The large dynamic range in ion energy is difficult to simulate
in ground-based radiobiology experiments. Most ground-based irradiations with
mono-energetic beams of a single one ion species are delivered at comparatively
high dose rates. In some cases, sequences of such beams are delivered with
various ion species and energies to crudely approximate the complex space
radiation environment. This approximation may cause profound experimental bias
in processes such as biologic repair of radiation damage, which are known to
have strong temporal dependancies. It is possible that this experimental bias
leads to an overprediction of risks of radiation effects that have not been
observed in the astronaut cohort. None of the primary health risks presumely
attributed to space radiation exposure, such as radiation carciogenesis,
cardiovascular disease, cognitive deficits, etc., have been observed in
astronaut or cosmonaut crews. This fundamentally and profoundly limits our
understanding of the effects of GCR on humans and limits the development of
effective radiation countermeasures.
