Half-year, and occasional one-year stints aboard the ISS, and especially lifetime totals nearing 2.5 years are possible ethically because these missions take place within the Earth's magnetosphere where exposure to ionizing radiation which has both steady, slowly varying, and episodic/catastrophic spikes is significantly lower than beyond it in cis-lunar space.
Qestion: What radiation data was actually available at the time of the Apollo missions to the Moon?
Where did the data available in the late 1960's come from? Are there specific satellites that can be identified which provided much of the particle rate data that were then converted to estimates of whole-body radiation dose?
Were predictions of accumulated dose possible with any reliability? When compared to dosimetry data presumably measured aboard the Apollo missions to Lunar orbit and landing, did they turn out to be close? Were the dangers of solar flares spiking the dose received appreciated back then?