A secondary objective was to protect the astronauts and ground crews against sources of man made radiation, such as the radioisotope thermal generator used to conduct the lunar surface experiments, and some radioluminescent sources on the spacecraft.
The Apollo radiation protection program focused on avoiding the harmful radiation effects encountered in space by limiting the radiation dose to the lowest level judged consistent with achieving beneficial goals.
Solar Particle Radiation
It is difficult to forecast individual eruptions on the solar surface, which made planning for the Apollo missions difficult. In the event of a solar surface eruption during the Apollo missions, NASA planned to estimate the particle dose, given either visual or radiofrequency confirmation of an eruption from the Solar Particle Alert Network (SPAN), a NASA-sponsored network which provided continuous data on solar flare activity. In the heavy, well-shielded Command Module, even one of the largest solar particle events would not have caused any impairment to the astronauts. However, radiation doses to astronauts in the thinly shielded Lunar Module during an event like this would have been extremely serious. To monitor particle activity, the Apollo missions were equipped with a nuclear particle detection system, designed to have a relatively narrow acceptance angle. This device measured the isotropic proton and alpha particles derived from solar particle events.
Cosmic ray fluxes consist of completely ionized atomic nuclei originating outside the solar system. They are accelerated to very high energies and provide an average dose rate of 1.0 millirad per hour in cislunar space, and 0.6 millirads per hour on the lunar surface. During Apollo, the effect of high-energy cosmic rays on humans was not a concern unless exposures were of duration of a few years.
Neutrons created by cosmic ray collisions with lunar materials were a potential hazard to Apollo astronauts. Furthermore, the astronauts might have encountered neutron exposure from the radioisotope thermal generator used to power the Apollo lunar surface experiments. Two methods for neutron dose assessment, initiated on the Apollo 11 mission, were used: whole body counting and neutron resonant foil.
Other Radiation Measurements
To allow accurate determination of overall radiation exposure of the astronauts, each carried a personal radiation dosimeter (PRD) and three passive dosimeters. As the mission progressed, the PRD provided a visual readout of the accumulated dose to each astronaut. Each of the three passive dosimeters was placed in the astronaut's garment at various locations (ankle, chest, and thigh) to determine radiation doses for various body portions. A radiation survey meter allowed astronauts to determine radiation levels in any desired location in their compartment. This allowed the astronauts to find a habitable low dose region of the spacecraft in the event of a radiation emergency.
Radiation was not an operational problem during the Apollo missions. Doses received by the astronauts were significantly lower than the yearly average of 5 rem set by the US Atomic Energy Commission for workers who used radioactive materials in factories across the United States.
Average radiation doses for all astronauts on a particular mission were computed for each of the Apollo missions, and ranged from skin doses of 0.16 to 1.14 rads. The maximum dose limit was 400 rads to the skin for each of the Apollo missions, so actual radiation levels were well below the limit. Individual readings varied 20 percent because the astronauts were in different parts of the spacecraft, which had different shielding effectiveness, and because of their different duties and movements. Doses to blood forming organs were 40 percent lower than doses measured at the body surface.
The main reason the doses were low is that no major solar particle events occurred during the Apollo missions. One small event was detected by a radiation detector outside the Apollo 12 spacecraft, but no increase in radiation dose to the astronauts inside the spacecraft was detected. One possible effect of cosmic ray radiation was the light flash phenomenon observed during Apollo 11 and subsequent flights; although ionizing radiation can produce visual phosphenes, a correlation was not established between cosmic rays and the observation of the light flashes. (Further information on the light flashes can be found in this archive.)
In terms of sources of man-made radiation aboard the Apollo spacecraft, there were two key problems that were resolved. First, the leakage of radioactive materials from radioluminescent sources was eliminated by a change in encapsulating material. Emission of soft x-ray radiation from radioluminescent sources was resolved by applying a layer of plastic to the panels.
Although the Apollo missions did not encounter any major space radiation event, procedures for handling any contingency were in place so that space radiation events would be recognized and dealt with immediately for the safety of the crew and mission objectives. The possible deterent to manned spaceflight by large radiation doses was avoided during the Apollo program.