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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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.
“NASA uses standards to set spaceflight exposure limits to protect NASA astronauts’ health and performance, both in mission and after mission,” says Dave Francisco of NASA’s Office of the Chief Health and Medical Officer. He acknowledges that, while astronauts on Mars missions would benefit from the thin Martian atmosphere that provides some limited protection, “transit in deep space has the highest exposure levels.”
That means long-haul space trips come with the biggest risks. A stay on the lunar surface for six months or more—presuming, of course, that astronauts eventually have a presence there and do not spend most of their time in subsurface habitats—would involve nearly 200 millisieverts of exposure, a higher amount than an extended visit to the ISS. And an astronaut traveling to Mars would be exposed to even more radiation. Whether they reached the Red Planet through a lunar stopover or on a direct spaceflight, they could have experienced significant radiation exposure en route. Even before they embarked on the trip back home, they could have already exceeded the 600 millisievert limit. The entire voyage, which would likely last a couple of years, could involve well more than 1,000 millisieverts. So if astronauts—and not just robots—will be sent to Mars, NASA likely will need to request waivers for them, Hricak says, although the exact process for obtaining a waiver has not yet been laid out.
The report’s proposal for a new radiation maximum is not without its critics. “For a mission to Mars, a 35-year-old woman right at that limit could have an over 10 percent chance of dying in 15 to 20 years. To me, this is like playing Russian roulette with the crew,” says Francis Cucinotta, a physicist at the University of Nevada, Las Vegas, and former radiation health officer at NASA. Despite the supposed benefits the new limits would have for female astronauts, he is concerned that the risks are particularly pronounced for younger women in space.
On the contrary, Hricak says, in its request for new limits, NASA has sought to be conservative. The European, Canadian, and Russian space agencies all currently have a higher maximum allowed dose of 1,000 millisieverts, while Japan’s limit is age- and sex-dependent like NASA’s current one, mainly because of a shared dependence on the atomic bomb survivor data.
But unlike someone in the vicinity of a nuclear explosion, the risk to an astronaut exposed to space radiation is long-term rather than immediate. Without proper shielding (which tends to be rather heavy and thus prohibitively expensive to launch) their chances of developing cancer, as well as cardiovascular disease, cataracts and central nervous system damage, slightly increase each day they are in space. In a person’s cells, space radiation can sever both strands of a DNA molecule’s double helix. And while a few such instances might come with very limited risks, each additional severance raises the odds of developing a harmful mutation that could cause cancer.
Fortunately, however, the body has ways of repairing some kinds of DNA damage, and it is possible to study that DNA repair in space, as was demonstrated by a new study published in the journal PLOS ONE in late June.
“This experiment set up a bunch of techniques that have never been implemented before in the very complex environment of the International Space Station,” says Sebastian Kraves, a co-founder of the Genes in Space student competition, which produced the investigation, and a co-author of the study. Using yeast cells onboard the ISS, Koch herself performed the experiment, which could become a precursor to future attempts to carefully monitor DNA damage and cellular repair in astronauts.
In addition to medical technologies, propulsion systems and shielding to protect against space radiation will likely advance as well. Particles expelled from the sun, for example, could be blocked with a few centimeters of aluminum or other materials, though astronauts outside their spacecraft or outside future lunar or Martian structures would be vulnerable. And they cannot be as easily shielded from more energetic cosmic radiation sources, such as heavy ions originating from distant exploding stars.
In any case, considering how little is known about various health risks from different kinds of space radiation, compared with radiation we are familiar with on Earth, researchers will surely continue with more studies like these to protect astronauts as much as possible. “I can tell you exactly how much exposure you’re going to get from a CT scan,” Hricak says, “but there are many uncertainties with space radiation.”