  Solar hard X-ray flares can expose astronauts on lunar and deep space
extravehicular activities (EVAs) to dangerous acute biological doses. We
combine calculations of radiative transfer through shielding materials with
subsequent transfer through tissue to show that hazardous doses, taken as >=
0.1 Gy, should occur with a probability of about 10% per 100 hours of
accumulated EVA inside current spacesuits. The rapid onset and short duration
of X-ray flares and the lack of viable precursor events require strategies for
quick retreat, in contrast to solar proton events, which usually take hours to
deliver significant fluence and can often be anticipated by flares or other
light-speed precursors. Our results contrast with the view that only particle
radiation poses dangers for human space exploration. Heavy-element shields
provide the most efficient protection from X-ray flares, since X-rays produce
no significant secondary radiation. We calculate doses due to X-ray flares
behind aluminum shields and estimate the required shield masses to accompany
EVA rovers.
