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Astronaut Scott Kelly famously spent an entire year residing onboard the International Space Station (ISS), about 400 kilometers above Earth, and his NASA colleague Christina Koch spent nearly that long “on station.” Each returned to Earth with slightly atrophied muscles and other deleterious physiological effects from their extended stay in near-zero gravity. But another, more insidious danger lurks for spacefarers, especially those who venture beyond low-Earth orbit.

Space is filled with invisible yet harmful radiation, most of it sourced from energetic particles ejected by the sun or from cosmic rays created in extreme astrophysical events across the universe. Such radiation can damage an organism’s DNA and other delicate cellular machinery. And the damage increases in proportion to exposure, which is drastically higher beyond the protective cocoon of Earth’s atmosphere and magnetic field (such as on notional voyages to the moon or Mars). Over time, the accrued cellular damage significantly raises the risk of developing cancer.

To address the situation, at NASA’s request, a team of top scientists organized by the National Academies of Sciences, Engineering, and Medicine published a report in June recommending that the space agency adopt a maximum career-long limit of 600 millisieverts for the space radiation astronauts can receive. The sievert is a unit that measures the amount of radiation absorbed by a person—while accounting for the type of radiation and its impact on particular organs and tissues in the body—and is equivalent to one joule of energy per kilogram of mass. Scientists typically use the smaller (but still quite significant) quantity of the millisievert, or 0.001 sievert. Bananas, for instance, host minute quantities of naturally occurring radioactive isotopes, but to ingest a millisievert’s worth, one would have to eat 10,000 bananas within a couple of hours.

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