At the beginning of the space age the dangers of hurtling into space w
ere considerable. Despite this fact, radiation risks were examined in
the U.S.S.R. and the U.S.A. and recommendations were made to limit the
exposure of the crews to radiation. To date the radiation exposures o
f crews on missions in low-Earth orbits have been low. Now that missio
ns in low-Earth orbit are becoming longer in duration and new missions
into deep space are being considered, radiation protection guidelines
become more important. Recently the estimates of the risks of radiati
on-induced cancer have been increased and new guidelines on radiation
exposure limits for crew members must be developed. For deep space mis
sions the guidelines take into account the risks posed by heavy ions.
Unfortunately, knowledge about these risks is insufficient. If the new
risk estimates are applied, current career dose limits may have to be
reduced by a factor of two.