The Invisible Killers
We have the technology to send astronauts to Mars. But can we return them safely to Earth?
- By John F. Ross
- Air & Space magazine, January 2006
(Page 4 of 5)
NASA is also preparing to make measurements directly in space. The agency is now accepting proposals for instruments to go aboard the Lunar Reconnaissance Orbiter, an unmanned probe scheduled to launch in the fall of 2008. The first objective of the mission is the “characterization of the lunar radiation environment, biological impacts, and potential mitigation by determining the global radiation environment, investigating shielding capabilities, and validating other deep space radiation prototype hardware and software.”
The best solution to the problem of space radiation would be to prevent exposure in the first place. Ideally, during a solar flare, astronauts could protect themselves by positioning their spacecraft so that a nearby planet, moon, or other celestial object serves as a shield, but that option is not available for a trip to Earth’s next-door neighbors, the moon and Mars. Even in future explorations of the outer solar system, the unpredictability of solar weather may make that option unrealistic. While the 11-year solar cycle is well documented, the occurrence of solar flares and the related coronal mass ejections have so far defied prediction. It’s especially difficult to monitor the weather on the side of the sun not facing Earth.
Another solution would be to equip spacecraft with enough radiation-proof shielding. But while increasing the thickness of shielding material would block more radiation, the added thickness would also provide more atoms for an incoming particle to hit, and those impacts could set off others, resulting in a domino effect that ultimately damages human tissue. The net effect of increasing the thickness of conventional shielding is negative until you scale the material up to the equivalent of a substantial concrete bunker, which, of course, is too heavy to send into space.
Engineers are evaluating non-conventional forms of shielding and construction materials. The best shield, says Brookhaven’s Lowenstein, is liquid hydrogen, but its volatility makes it dangerous. Although less effective, water would also serve as a good shield. Other promising materials include hydrogen-rich plastics, such as polyethylene, the material used to make garbage bags. Engineers at NASA’s Marshall Space Flight Center in Alabama have developed a reinforced polyethylene that is 10 times stronger than a comparably thick piece of aluminum, although price may prove a problem in its deployment. Creating an electromagnetic field around a spacecraft or the development of other kinds of “active” shielding is expensive and brings with it concerns about the technology affecting the health of the crew members. But Larry Young, a space medicine expert at the Massachusetts Institute of Technology in Cambridge, says that future shielding strategies may include the use of superconducting magnetic technology.
Risky, Riskier, Riskiest
The U.S. Occupational Safety and Health Administration treats astronauts as radiation workers. Therefore, the level of radiation that an astronaut can be exposed to over his or her career falls under the guidance of the National Council on Radiation Protection and Measurements, a not-for-profit corporation created by Congress in 1964 to collect information and develop guidelines about radiation exposure for workers of all kinds. Today, the law limits the amount of radiation that nuclear workers, including astronauts, receive to 5,000 millirem over the course of their careers.
The limits have already had effects on astronauts, who are required to wear radiation-monitoring badges on missions—silicon dosimeters on aluminum. In 2002, astronaut Don Thomas, who had flown on four prior missions, for a total of 1,040 hours, was pulled off the ISS Expedition Six crew because NASA decided that the long-duration mission would put him over the lifetime radiation exposure limit. NASA’s Frank Cucinotta monitors astronauts and their badges, and often has to compare the badges of all the astronauts on a shuttle mission to see if anyone’s badge is registering particularly low levels. “They sometimes hide their badges” in a shielded area of the shuttle, he says, “because they don’t want to go over their limit.”
Even if every astronaut wore his or her badge at all times, the risk/benefit calculation is complicated by the fact that not all astronauts are created equal. Early evidence suggests that the presence of a certain gene indicates an increased susceptibility to the negative effects of radiation. In addition, radiation exposure affects older people faster and more severely than it does the young. And, because of their susceptibility to breast, uterine, ovarian, and cervical cancers, women are prone to a greater variety of cancers than men.
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.