Vasquez is also concerned about other factors that may exacerbate radiation damage. “We’re testing mice here on Earth in a comfortable 1 G environment,” he says. Put people in space, and “their physiology will be stressed and that can’t help their response to radiation damage.”
Another NSRL researcher, Betsy Sutherland, is studying the cellular destruction wrought by ionizing radiation. If an ionizing particle hits DNA in a cell’s nucleus, it can cut one or both strands of the double helix like a chainsaw ripping through a tree branch. Evolution has ensured that organisms have mechanisms to repair insults to genes, which occur regularly from such sources as the sun’s ultraviolet rays and natural toxins contained in food. Proteins move quickly to reattach broken strands and splice in new sections of DNA if necessary. Cells too badly damaged to be fixed get tagged by the p53 gene, which orders the cell’s death. From the organism’s perspective, it’s better that a cell die than become fixed incorrectly: Cells with mutations could lead to cancer or defects that can be passed on to the next generation. Sutherland says that ionizing radiation appears to impede the p53 gene from doing its job.
Sutherland and other biologists have noted other disturbing effects of radiation on cells, such as “the bystander effect,” in which damage to one part of DNA causes damage to other DNA segments far away.
The researchers at the NSRL are well aware of the limitations of the work here. For example, they can shoot only one type of heavy-ion radiation at a time; in space, astronauts will be exposed to a barrage of many kinds. It is also difficult for researchers to design a lab simulation that shows how space radiation is distributed among various parts of the human body. The European Space Agency built a simulated human torso, Phantom, which was attached to the outside of the International Space Station in 2001. The dummy contained actual human bone, plastic material simulating soft tissue, lighter material representing lung tissue, and a covering of Nomex to simulate skin. Phantom was also enclosed in material simulating a spacesuit. About 350 radiation meters were placed throughout the torso, including the sites of critical and susceptible organs, such as the brain, heart, thyroid glands, and kidneys. The results from Phantom’s exposure turned out to be similar to those predicted by NASA models. The experiment also showed that more than 80 percent of the radiation that hit the dummy came from cosmic rays; protons, on the other hand, were weakened by passing through the spacecraft and Phantom’s skin.
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.