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
At 5:54 a.m. on Tuesday, October 28, 2003, a giant flare exploded on the surface of the sun, sending a cloud of hot gas and charged particles hurtling toward Earth at nearly five million miles per hour. While managers of satellites and electric utilities braced for disruptions in power grids and satellite and radio communications, technicians at NASA’s Johnson Space Center in Houston hurriedly radioed astronaut Mike Foale and cosmonaut Alexander Kaleri, who were orbiting Earth in the International Space Station.
To protect them from the blast of charged particles, mission control had the pair climb into the back of the station’s massive Zvezda module (which contains the sleeping quarters, galley, and lavatory), where the shielding is thickest. During five 20-minute peak exposures that day, the team sought shelter.
NASA officials later calculated that if the two had not retreated to Zvezda, their exposure would have been minimal: In 20 minutes they would have been exposed to the amount of radiation they usually received in the station over a period of 24 hours. Yet NASA also acknowledges that the two onboard instruments designed to measure astronaut exposure to radiation were malfunctioning, so the agency does not know how much radiation the astronauts were exposed to.
And what would have happened if, during one of those times of peak exposure, Foale and Kaleri had been spacewalking? Or what if they had been on the moon, or on Mars?
After close to half a century of manned flight, we still know very little about the dangers astronauts face from radiation in space. Only the 27 Apollo astronauts who orbited or landed on the moon have gone beyond Earth’s magnetic field—which protects us from most space radiation—and then only for a short time. We do know that on the Apollo 14 moon mission, for example, between takeoff and landing, the three astronauts each received about 1,140 millirem of radiation—a little more than three times the amount people are exposed to on Earth during the same period.
Last January, President George W. Bush announced plans for much longer space missions, including lengthy manned missions to the moon as early as 2015, followed by flights to land a human on Mars. Bill Anders, an astronaut on Apollo 8 and a retired nuclear engineer, believes that Bush’s vision of future manned exploration “greatly underestimates or ignores the risk of high-energy radiation.” He points out that astronauts can be endangered by a number of sources of radiation: “What’s the point of building a nuclear rocket ship—the only way we’re going to get to Mars—if the astronauts get singed on the way there?”
But Robert Zubrin, independent mission planner and president of the Mars Society, scoffs at concerns over radiation risks. In the trade publication Space News, Zubrin wrote an article entitled “The Great Radiation Hoax,” in which he declared: “Mars mission cosmic radiation doses [are] well within the range of existing spaceflight experience.”
Who’s right? Scientists don’t yet know. From the World War II atomic bomb detonations in Japan and the 1986 accident at the Chernobyl nuclear reactor near Kiev, Russia, we know the effects of brief but intense pulses of radiation: nausea, immune system shutdown, central nervous system damage, and death within minutes or hours. And scientists have documented the effects of the constant, naturally occurring radiation found on Earth—the ultraviolet rays from the sun that cause melanoma, for example. But the forms of radiation found in space are different creatures entirely. While data from space probes and sophisticated computer modeling provide a good idea of how much and what kind of radiation normally exists between here and Mars, “we just don’t know how the human body will react to it,” says Frank Cucinotta of NASA’s Space Radiation Health Project at the Johnson Space Center. Walter Schimmerling, NASA program scientist for space radiation research, elaborates: “We don’t know if a three-year mission to Mars is equivalent to an astronaut sitting at home for the same period smoking cigarettes, or the equivalent of smoking for 30 years and living in a coal mine.”
Cucinotta and Schimmerling are at the forefront of a community of researchers working on what may be the most complex example of risk analysis ever undertaken. The study of space radiation is forging collaborations between researchers in widely different disciplines, from spacecraft engineering to solar physics to molecular biology. But so far, the results have not produced a detailed picture of how space radiation would affect human beings over long periods. And until that information is available, “we just can’t send [astronauts] into space and see what happens,” says Cucinotta. “Until we better understand the risks, NASA won’t send astronauts on long-duration spaceflights.”
The Alpha Beta Gammas
Solar storms, largely unpredictable, are not the only radiation danger in space. Far outside our galaxy, violent events such as the explosions of stars produce particles called cosmic rays. The atoms in cosmic rays are charged, or ionized: Because they have either lost or gained an electron, they carry a negative or positive charge. Heated to very high energies, these particles race through space at extraordinary speeds.
Cosmic rays can be made up of any element on the periodic table up to iron (the table lists elements by increasing atomic weight). Cosmic rays made up of heavy elements are particularly dangerous. A charged particle of iron, for example, slams into atoms in a cell and sends them careening like a cue ball hitting the rack. These newly energized particles hit others, setting off a cascade of destruction. Lead, for instance, while highly effective at shielding bodies against X-rays in the dentist’s chair, is such a heavy element that atoms set loose from it could prove lethal to astronauts using it for shielding.
Other forms of radiation populate deep space and may pose a danger to astronauts: X-rays, alpha-rays, beta-particles, gamma-rays, and neutrons. All contain excess energy and, in an attempt to stabilize themselves, throw off mass or energy. The high energy of these particles enables them not only to travel at or near light speed but also to penetrate shields and burrow deep into human tissue.
In the space between here and Mars, the distribution of cosmic rays is not dense enough to induce acute radiation sickness. But what if the exposure consisted of a low, steady level of ionizing radiation over a two- or three-year mission in deep space? Would that cause subtler health problems? Scientists estimate that an astronaut in a conventional spacecraft on a 900-day Mars mission might encounter as much as 130,000 millirem—a dose equivalent to what you’d be exposed to living 370 years on Earth.
To help build a database that relates levels of radiation exposure with adverse effects, NASA runs the Space Radiation Laboratory at the Department of Energy’s Brookhaven National Laboratory in New York. Adam Rusek oversees the daily operations of the new $34 million facility, which is the only one in the United States devoted exclusively to studying the effects of radiation on living creatures.
The NSRL is housed in an unimpressive low gray building in the woodlands of central Long Island. Here, Rusek and his team of physicists operate a particle accelerator that can replicate deep space’s highly charged subatomic particles, accelerate them to nearly the speed of light, and then slam them into vials of tissue and cells, laboratory animals, and various shielding materials.
Rusek also runs a “summer camp” for biologists to learn the rudiments of particle physics. Sitting in the NSRL’s cramped kitchen, which serves as an informal command center, Rusek comments with a wry grin: “You’d be surprised how many biologists don’t know what a Gaussian wave is.” (It’s a phenomenon of quantum physics.)
To simulate particles found in space, Rusek and his colleagues begin with ordinary materials, such as iron and carbon. They energize the particles by heating them until they are dangerously unstable. During experiments, Rusek mans a computer near the large steel door that marks the opening to the accelerator. From here he operates a Sony webcam that provides views of the 400-square-foot room where the speeding particles end up. Because of the danger involved in the experiments, opening the door can take up to five minutes, requiring an iris scan (to confirm the researchers’ identities), a sign-off from an operator watching on a video camera in another building, and a series of key insertions into a bank of instruments.
Once the door opens, the white-painted cinderblock hallway cuts left, then right, then left again, a precaution against errant particles escaping. The hallway ends at the chamber, which contains a 30-foot track of parallel stainless steel bars; the bars follow the path of the particles and disappear into a hatch in the wall. As the particles travel down the track toward this room, a series of powerful magnets attached to the bars accelerates them and focuses their path.
Marcelo Vasquez, an energetic Argentinian-born biologist and physician, is chief of medical research at NSRL. Presently, he is using mice to look at the effect of ionizing radiation on cognitive function. Vasquez and his colleagues built a three- by three-foot plexiglass pool with a small platform within. They trained mice to swim to the platform and climb on it. After the mice grew proficient at the task, the scientists recorded their times.
Vasquez then strapped three trained mice at a time to a small block of Lucite and irradiated them in the accelerator chamber. Next, he put them back into the water and found that it took the mice longer to find the platform than it had before. The radiation exposure, says Vasquez, caused the animals to lose brain cells quickly. He does acknowledge, however, that in his experiments he administers high doses of radiation, and one can’t necessarily extrapolate directly from the mouse results to what humans will experience.
Vasquez brings up on a computer screen a series of slides showing mouse brain cells exposed to increasing levels of ionizing radiation. The network of axons and dendrites, the structures that enable cells to communicate with one another, first appears as a field densely packed with shapes and fibers. By slide four, the picture is sparsely filled: Cell nuclei look like they have exploded, their contents spread randomly. The image is reminiscent of a block in World War II-era Dresden after a bombing run. The radiation, explains Vasquez, doesn’t kill all the cells, but it severely disrupts the flow of signals. Vasquez’s colleague, Derek Lowenstein, chairman of Brookhaven’s collider accelerator program, has given voice to deep fears among scientists by asking: “Will astronauts come back blithering idiots or not?”
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.
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.