Radiation & Systems

Earth's surface is a remarkably sheltered place. The atmosphere and the planet's magnetic field together absorb or deflect the overwhelming majority of cosmic radiation, holding background dose to roughly 1–3 millisieverts per year. Low Earth orbit, where the ISS flies, still sits largely inside the magnetosphere and retains much of that protection. Beyond it — on the way to the Moon or Mars — crews lose the shield entirely and face two distinct threats.

The first is galactic cosmic rays (GCR): a constant rain of high-energy heavy ions originating outside the solar system. Because they carry so much energy and mass, GCR particles punch through spacecraft hulls and tissue, leaving dense tracks of ionization that cause complex, clustered DNA double-strand breaks the cell struggles to repair. This raises lifetime cancer risk and may injure the central nervous system, and — frustratingly — thin aluminium shielding can make it worse by shattering particles into secondary showers. The second threat, solar particle events (SPEs), are sudden storms of protons from the Sun that can deliver an acute, potentially incapacitating dose within hours.

Radiation rarely acts alone. Microgravity, psychological stress, and disrupted sleep dysregulate the immune system, shifting cytokine balance and reactivating latent viruses such as Epstein–Barr and varicella-zoster. With sixteen sunrises a day on the ISS, circadian rhythm and sleep suffer, and the isolation and confinement of a multi-year mission add a serious behavioral-health burden. Space medicine therefore treats radiation as one thread in a tangle of interacting systemic stresses — and GCR remains the single hardest unsolved problem standing between humans and Mars.