New Space Radiation Limits Needed for NASA Astronauts, Report Says, https://www.scientificamerican.com/article/new-space-radiation-limits-needed-for-nasa-astronauts-report-says/   Although meant to minimize risks to human health, the proposed new limits would still be exceeded by any conceivable near-future crewed voyage to Mars

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.

Every current member of NASA's astronaut corps has received less than 600 millisieverts during their orbital sojourns, and most, including Koch, have received much less and can thus safely return to space. But a year on the ISS still exposes them to more radiation than experienced by residents of Japan who lived near the Fukushima Daiichi nuclear accidents of 2011.

"Everybody is planning trips to the moon and Mars," and these missions could have high radiation exposures, says Hedvig Hricak, lead author of the report and a radiologist at Memorial Sloan Kettering Cancer Center in New York City. Using current spaceflight-proved technologies, long-distance voyages—especially to the Red Planet—would exceed the proposed threshold, she says.

That could be a big problem for NASA's Artemis program, which seeks to send astronauts to the moon in preparation for future trips to Mars. Another problem for the space agency is that the epidemiological data it uses mostly come from a longevity study of Japanese survivors of atomic bomb blasts, as well as from the handful of astronauts and cosmonauts who have endured many months or even years in low-Earth orbit. NASA's current space radiation limit, which was developed in 2014, involves a complicated risk assessment for cancer mortality that depends on age and sex, yet more relevant data are necessary, Hricak argues. In the atomic bomb survivor study, for instance, women were more likely to develop lung cancer than men, suggesting a greater sex-based vulnerability to harmful radiation. "But with the knowledge we presently have, we know we cannot make a comparison between high exposure versus chronic exposure," Hricak says. "The environment is different. There are so many factors that are different."

NASA wants to update its standards now because the agency is on the cusp of sending so many astronauts well beyond low-Earth orbit, where greater amounts of space radiation seem destined to exceed previously mandated exposure limits. Furthermore, Hricak says, having a single, universal radiation limit for all space travelers is operationally advantageous because of its simplicity. A universal limit could also be seen as a boon for female astronauts, who had a lower limit than men in the old system and therefore were barred from spending as many days in space as their male counterparts.

The new radiation limit proposed by Hricak and her team is linked to the risks to all organs of a 35-year-old woman—a demographic deemed most vulnerable in light of gender differences in the atomic bomb survivor data and the fact that younger people have higher radiation risks, partly because they have more time for cancers to develop. The goal of the radiation maximum is to keep an individual below a 3 percent risk of cancer mortality: in other words, with this radiation limit, at most three out of 100 astronauts would be expected to die of radiation-induced cancer in their lifetime.

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