How should we draw the line to distinguish an acceptable risk from an unacceptable one? For cancer, the number is currently based on the 1989 “NCRP Report Number 98,” which recommends that cancer mortality for the population of workers in question should be no more than three percent above the average cancer mortality in the United States. The “three percent above” guideline is based on the additional mortality facing Americans in the most physically hazardous occupations, such as mining. Because a 40-year-old American man has a 20 percent chance of developing a fatal cancer in his lifetime, the NCRP added 20 percent and three percent to determine that 23 percent is the acceptable level of cancer risk that an astronaut can assume. In 2000, the NCRP revisited its recommendations and reaffirmed this basic risk calculation. But, based on follow-up studies of the survivors of the two atomic bombs dropped in Japan in 1945, the NCRP cut the maximum acceptable radiation doses significantly, by nearly half or more.
And that’s just for cancer risks. The challenge facing researchers, says Brookhaven’s Vasquez, “is integrating all the various risk factors for radiation into a model.” For example, says Cucinotta, astronauts develop cataracts much more frequently than average.
Based on a 2001 study of cancer patients undergoing radiation therapy and epidemiological studies of the atomic bomb survivors, Cucinotta has calculated that the added cancer risk of a 1,000-day Mars mission in an aluminum spacecraft, which would shield half the cosmic rays encountered, falls between one and 19 percent. A one percent increase is a risk most people would find acceptable. But taking the highest risk number and adding that to an astronaut’s normal incidence of getting cancer (20 percent) results in a whopping cancer risk of 39 percent.
Cucinotta’s best guess estimate is that without extra hydrogen shielding, Mars missions of 660 and 1,000 days would push 40-year-old astronauts over the NCRP risk thresholds.
The Blueberry Fix
As they build up their database, scientists may determine that astronauts’ radiation exposure should be reduced significantly. The low-tech fix would be to simply limit each astronaut to fewer trips. But then more astronauts will need to be trained, and space agencies will need bigger budgets.
Another approach is to pull astronauts from flight duty when they show signs of an imminent health problem. Late last year, NASA awarded $9.7 million to Colorado State University in Fort Collins to study how acute myelogenous leukemia develops. AML, a cancer of the bone marrow, is commonly associated with exposure to radiation. The CSU scientists will look for clues that cells are going to turn malignant. NASA hopes that the research will help physicians analyze tissue samples to determine when an astronaut is in danger of developing cancer.
Astronauts will also carry agents that will help their radiation-damaged cells repair themselves. “Over time, the DNA repair process doesn’t catch everything and mistakes can begin to add up,” says James Joseph, a biologist at the Human Nutritional Research Center on Aging at Tufts University in Massachusetts. He and his colleagues have discovered that the antioxidants in certain foods, particularly blueberries and strawberries, can help aid damaged cells repair themselves correctly. And Ann Kennedy of the University of Pennsylvania School of Medicine and her colleagues have discovered that selenomethionine, a compound of the element selenium and an amino acid, enhances the ability of DNA in irradiated mouse cells to repair itself.
And, while the scenario remains science fiction for now, future astronauts could one day travel into space with stem cells—undifferentiated cells ready to change into any kind of specialized cell—and use them to repair damage to their bodies.
So many of these proposed solutions are speculative, or unrealistic, at least with today’s technologies. Could radiation ultimately prove to be a showstopper? No matter what data the scientists come up with, not everyone involved in spaceflight will interpret the risks the same way. Says astronaut Tom Jones, a veteran of four shuttle missions, “Telling me that I may get cancer 30 years from now if I go to Mars doesn’t seem like a big deal, because sitting atop a rocket and going there is itself so risky.” But even the astronauts most gung-ho to push on to Mars acknowledge that the potentially severe effects of radiation are something to be worried about.