Cosmic Cancer

In Brookhaven’s particle accelerator, scientist can speed heavy particles like those found in space. Radiation biologists expose samples, usually cells in a Petri dish, to the beam of heavy energized atoms to look for carcinogenic effects.

In one study led by Ann R. Kennedy, a radiation biologist at the University of Pennsylvania, heavy particles quadrupled the risk of lymphoma, a cancer of immune cells, in mice.

Relating the findings to space travelers, however, requires comparing animal studies to reports of humans exposed to radiation. With so little information about astronauts, scientists use the numerous data from people exposed to lower-energy gamma rays and X-rays — survivors of the World War II bombings in Japan, as well as people who work in nuclear reactors or hospitals. As Cucinotta, the chief NASA scientist, explains, “I know what happened in humans with X-rays, I know what happened in my model system with X-rays, I know what happened in my model system with [heavy atoms]. And that gives you an approach to try to relate it back to humans.”

What scientists have started to learn from their models suggests that they need to reduce the risk of radiation. The six-month return to the moon, known as the “lunar outpost mission,” bumps up against the ceiling of acceptable risk. Astronauts with any compounding risk factors, such as previous radiation exposure from a stint on the International Space Station, would surpass the limit, disqualifying them from the lunar liftoff planned for about 2024.

No matter how clear their background checks, no astronauts today would qualify for the Mars missions that NASA is tentatively planning. By the lowest estimate, a 1000-day exploration of Mars would put an astronaut at just above a four percent increased risk of dying from cancer.

If estimates, as they become more certain, continue to indicate that cancer risks are too high, scientists are trying other approaches to make long-term space missions safer. Shielding is an obvious safeguard. But the aluminum shielding on spacecrafts only does so much to protect astronauts from the heaviest particles in cosmic radiation, blocking about 25 percent of the most penetrating.

While thick lead would be a more formidable defensive line, it would also weigh down the spacecraft like the proverbial lead balloon. Burma, the Dallas researcher, is focusing on the ability of lightweight hydrogen-based shields like polyethylene to protect cultured cells from doses of heavy radiation. Polyethylene reduces the dose of space radiation by about 35 percent, depending on the shield thickness.

Compared to impenetrable lead shielding, strawberries and blueberries may seem like rather paltry countermeasures but some scientists are also looking to dietary supplements with antioxidant benefits, like berries have, as means to prevent DNA damage from radiation that does make it through the shields. The results in atomic bomb survivors foretell some promise: Those who have diets rich in fruits had about a 12 percent lower incidence of cancer, according to a report in the journal Mutation Research.

But just like the limitations of heavy lead shielding, antioxidants are not without drawbacks. As Guida, the Brookhaven scientist, points out, they might keep cells alive that have incurred more DNA damage than they can repair. In this case, antioxidants could have the paradoxical effect of increasing the risk of passing on genetic mishaps to new generations of cells.

For now, NASA is trying to gain better estimates of the cancer risks that surround long-term space missions. In the meantime, scientists will continue to chip away at the risk of space radiation through countermeasure approaches. No method will remove the risk, but Guida suggests that a combination of approaches may be the answer. “Maybe three things put together will bring the doses [of radiation] down to something that’s acceptable,” he says.

NASA Chief Scientist Cucinotta is optimistic that the agency will be able to get a good grasp of the threat of radiation in the final frontier. “Fortunately the Mars mission is quite far out,” he notes, “So we think we have time to bring down the uncertainty.”

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