Space radiation is considered the biggest threat to astronauts’ health in long-duration space missions, where the main concern is the potential carcinogenic effects from the continuous exposure to Galactic Cosmic Rays (GCR). The quality factor ( \(Q\) ) of GCR presents the greatest uncertainty, complicating accurate risk assessment and the establishment of safe radiation exposure standards for astronauts in missions beyond low Earth orbit. This study uses a recently developed analytic microdosimetric model to calculate \(Q\) values for space radiation. The influence of different target sphere diameters is examined, and optimum values are suggested. The predictions of the new model are benchmarked against experimental animal and in vitro data for \({RBE}_{max}\) and \({RBE}_{\gamma -acute}\) as well as against different versions of NASA’s model, and the International Commission on Radiological Protection (ICRP) Publication 60 recommendations. The comparisons involve quality factor values for individual ions ( \({Q}_{ion}\) ) over the 1 MeV/u to 1 GeV/u energy range, as well as dose-averaged quality factor values for GCR ( \(\overline{{Q }_{GCR}}\) ) for three space radiation environments (free space, Moon and Martian surface) and different shielding conditions (aluminium, polyethylene, and regolith of 0–20 g/cm2 \()\) . The results of the present model reveal strong variations of \({Q}_{ion}\) (up to a factor of ~ 6) and \(\overline{{Q }_{GCR}}\) (up to a factor of ~ 4) depending on the sphere diameter, with the values of 100 and 1000 nm yielding better agreement with experimental data. Interestingly, the \(\overline{{Q }_{GCR}}\) values calculated for a 1,000 nm sphere diameter align with NASA’s 2012 model and ICRP Publication 60 recommendations, whereas the \(\overline{{Q }_{GCR}}\) values for the 100 nm sphere diameter agree better with NASA’s most recent 2022 model.
