Why didn't the Apollo astronauts die from radiation exposure?
If space is full of deadly radiation, how could astronauts have survived a trip to the Moon? Shouldn't the radiation have killed them?
Short answer: No, the radiation exposure isn't enough to kill a person.
Long answer: There are numerous types of radiation in space and many of them present a real challenge to space travelers. In low Earth orbit, astronauts are well protected by the planet's magnetic field. The problems begin at around 600 km (400 mi) from Earth when they encounter the famous Van Allen radiation belts.
The radiation in this region is enough to threaten satellites, and would be dangerous for any human who chose to spend too long there. The solution is simple: Don't spend too long there. Apollo astronauts passed through the Van Allen belts in a few hours and received less exposure than a hospital CT scan.1
Farther out, past the Van Allen belts, astronauts must deal with radiation from the Sun as well as cosmic radiation from distant sources in all directions. That sounds bad, and it is.
In the case of the Moon landings, the solution was to monitor space weather and hope for the best. The spacecraft's aluminium skin gave some protection but would not shield against all of it. They really were just lucky that there were no unexpected radiation bursts.
Looking into the future, astronauts spending more time in deep space will not be able to rely on luck. One of the big challenges of long-term missions to the Moon and beyond is how to deal with radiation.
Now, here's an important point: The Apollo astronauts apparently did not get off scott-free from the radiation. At least one report2 concluded that exposure to space radiation caused these astronauts to have a higher than normal mortality rate due to heart disease.
As you can see, while the conspiratorial claim that space radiation would be quickly lethal is false, there is an element of reality there. Space radiation is a threat that must be taken seriously.
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1. English, R. A. et al. “Apollo experience report: Protection against radiation.” (1973).
2. Delp, M., Charvat, J., Limoli, C. et al. Apollo Lunar Astronauts Show Higher Cardiovascular Disease Mortality: Possible Deep Space Radiation Effects on the Vascular Endothelium. Sci Rep 6, 29901 (2016). https://doi.org/10.1038/srep29901