When astronaut Buzz Aldrin shut his eyes during Apollo 11's return from the moon in July 1969, he noticed something unusual: fireworks. Explosions of light, apparently on the insides of his eyelids. Back on Earth, he told flight surgeons what he had seen. And he learned that the pyrotechnics were not a cause to celebrate. Each was what scientists call a retinal flash—a physiological marker that occurs when a galactic cosmic ray slashes through a person's brain.
Galactic cosmic rays come from supernovas—exploding stars—outside our galaxy, and they often travel very fast, close to the speed of light. They can be any element (up to the atomic weight of iron), but because they have lost or gained electrons, they are said to be ionized, meaning that they carry a negative or positive charge. Elements with the greatest atomic weight are the most dangerous to astronauts.
You might expect lead—that familiar prophylactic against dental X-rays—to provide shielding. But when a heavy ion collides with an atom of lead, it dislodges charged particles that can be just as destructive as the original heavy ion.
Imagine a cosmic ray as "a bullet flying around with speed and mass," says Marcelo Vazquez, a researcher at the Brookhaven National Laboratory on Long Island in New York who studies the effect of radiation on brain cells and tissue. When the rays "go through matter—it can be a rock, a body, or your brain—they have so much energy and charge that they produce a kind of hole. But they also produce secondary particles, like a shower."
Hydrogen compounds offer the best shielding against secondary particles, because hydrogen atoms contain one electron for one proton. The ideal Mars-bound ship would have an aluminum or carbon-composite exterior, with two to four inches of polyethylene around the crew compartment. Water is another efficient shield; on the International Space Station, some astronauts sleep surrounded by their water bags. But nothing—not even an artificial electromagnetic field around the spacecraft—will block the rays entirely.
This matters because cosmic rays kill brain cells—or, in any event, brain cells in vitro. Vazquez's laboratory experiments have shown that cosmic rays induce what is called programmed cell death in neurons. "The cells have a kind of sensor that detects damage to DNA and other cellular material," he says. "And they say, 'Oh, I have so much damage that I can't live anymore.' So they kill themselves."
Moreover, as a heavy ion passes through a column of cells, it forms a "track"—a line of destruction that can easily be seen in microscopic images. Dislodged particles called delta rays career out from this track, inflicting yet more damage on adjacent tissue. (On a cellular level, the delta-ray pattern of damage is more like that of X-rays or gamma rays.)
During Apollo, astronauts had only a brief exposure to cosmic rays. Likewise, on Mir and the International Space Station, they have largely been shielded by Earth's magnetic field (except in a small, quirky patch of orbit above Brazil known as the South Atlantic Anomaly). But if Bush's plans for establishing a base station on the moon are realized, astronauts would face six months of chronic exposure. And on the two-and-a-half-year mission proposed for Mars, they would be struck both in transit and on the planet, where, Vazquez says, the problem is compounded because cosmic rays interact with the surface to produce secondary radiation. Unlike Earth, Mars does not have a magnetic field.
Just what exactly is the long-term effect of cosmic rays? "We don't know," says Cucinotta. Vazquez, however, raises a gloomy possibility. "Depending on the size of the cell, we estimate that between 13 percent and 40 percent of brain cells will be hit once by cosmic rays," Vazquez explains. "We have millions and millions of cells in the brain. But 40 percent is a lot. Some areas of the brain are very tiny," he adds. "But they play an important role in functioning. If you wipe out those cells, you don't need to worry about the hundreds of billions of cells. A few million—you're gone." (Alzheimer's patients, for example, often lose about 5 percent of their brains per year.)
Vazquez is quick to point out that the results of his in vitro research may not ultimately reflect what happens to cells in vivo, nor will the results of animal experiments provide the final word on people. "To go from petri dishes to humans is a big jump," he says. But the transition from brain-cell cultures to the brains of intact animals does not seem to bode well for astronaut resilience.
In addition to imaging heavy-ion tracks, Vazquez has studied the effect of chronic cosmic-ray exposure on the brains of rats, as measured by their ability to move around in a box. Groups of laboratory rats exposed to heavy ions—as well as groups bombarded with conventional radiation—show an impairment of motor skills. Over a period of 11 months, the rats exposed to X-rays and gamma rays regain some of their lost coordination. Rats exposed to cosmic rays, however, appear to never fully recover.
In a test on an early shuttle flight, astronauts were pricked in the arm with various antigens, to which they showed a diminished immune response. Latent viruses also express themselves in flight. NASA microbiologist Duane Pierson has published several papers documenting the presence in astronaut saliva of various viruses, including Epstein-Barr, which has been linked to human mononucleosis. Other common latent viruses could be more uncomfortable if they reactivated on a trip to Mars—chicken pox, for example, which usually returns as shingles. And no data exist on whether antiviral drugs, such as acyclovir, will work in flight.
One key question faces NASA scientists: Does the immune system remain severely depressed, or in the words of immunologist Clarence Sams, does it "adjust to a new normal"? Some researchers are optimistic that if they could collect more midflight data, instead of data during liftoff and landing, they might show less grim results. But medical evaluations of researchers working in Antarctica—which, like space, is a remote, nerve-racking outpost—show chronic depression of their immune response. "No matter what you do," Vazquez observes, "you put a guy in a can for six months and it's a big stress."
Microgravity adds a wild card to the mix. "We don't know if there will be a synergism between radiation and microgravity," Vazquez says. "The idea is, microgravity will modulate the radiation response. And the worst-case scenario is, microgravity will weaken the cells. We've done a lot of research on the ground, in 1 g—and we say, OK, we'll have the same response in space. But we don't know. So the nightmare situation will be—we go up there, and maybe we have radiation response at even lower doses."
Radiation also takes a toll on pharmaceuticals that astronauts with compromised immune systems might otherwise use to fend off ailments. The antibiotics currently carried on NASA missions—Cipro, Augmentin, Bactrim—barely retain their potency during a two-week shuttle flight and "would not last on a trip to Mars," said Lakshmi Putcha, a pharmacotherapeutics researcher at the Johnson Space Center. Moreover, out of concern for rapid drug degradation, the standard medicine kit kept on board the International Space Station is replaced every six months. If Mars-bound astronauts had infections, she continued, "we would have no way of treating them."
Putcha is optimistic that the drug problem could be solved if her group had greater resources to solve it. The degradation, she believes, involves the way antibiotics are formulated. "What we need to do is come up with instant preparations. Remember in the olden days you used to get the powder separate from the liquid; then you mixed the powder with the liquid, and you took it as an oral dose?"
The more high-tech version of this idea is microencapsulation, a feat of chemical engineering that involves coating the active ingredient of a drug with some materials or compounds that preserve its effectiveness. In the case of antibiotics and radiation, the idea would be to encapsulate the active compound in an antioxidant or other shielding material. For example, lidocaine, a local anesthetic, tends to remain stable in space—possibly, Putcha says, because it is in a water solution, and water functions as a stabilizer.
Nor is antibiotic stability NASA's greatest pharmacological challenge. Promethazine, the agency's antidote to motion sickness, is highly photosensitive on the ground and deteriorates severely on a two-week shuttle flight—so severely, Putcha says, that the dose in tablets or suppositories would not meet FDA guidelines.
Even if drugs could be made to remain stable on a trip to Mars, astronauts would still have to guess about how much of them they needed to use. The body absorbs medications differently in microgravity from the way it does on the ground. "Any drug, to have its effect, first has to get into your blood," Putcha says. "If it is an oral dose, it has to go through your stomach and intestines." Because gastrointestinal motility is dramatically decreased in space, "it takes a longer time for anything that you ingest or swallow to go through your body. Motion sickness is also known as sleeping stomach syndrome. Your stomach essentially becomes like a rock."
Gender differences also affect human metabolism in space. Motion-sickness drugs, for instance, take longer to reach a level of effectiveness in women. And women show greater orthostatic intolerance—light-headedness after getting up from bed or being in space.
These issues may not justify excluding women from a long-duration flight, but radiation could—if the sole criteria for picking a Mars crew were medical and not political or psychosocial. Women's unique body parts—breasts and ovaries—cause them to be more vulnerable to radiation-caused cancers. Apparently, the prostate is not similarly susceptible.
The word impossible is not a part of the NASA lexicon. So, not surprisingly, the can-do folks at the space agency have bandied about a variety of potential ways to deal with the risk of radiation:
Go faster to reduce exposure. Make the engine not nuclear but thermonuclear. And place the reactor far away from a well-shielded passenger compartment.
Surround the entire Mars spacecraft with an artificial electromagnetic field—an expensive solution that Cucinotta called "pie in the sky" and one that will not stop all cosmic rays.
Surround the entire crew quarters with five feet of water in a tank—daunting to engineers because of the expense of getting that weight, or mass, of water off Earth.
Ingest drugs or foods that are radioprotectants. Scientists are currently investigating the antioxidant properties of flavonoids in blueberries and strawberries. Neal Pellis, associate director of the Biological Sciences and Applications Office at the Johnson Space Center, suggests that buckyballs and other nanomolecules may have free radical–scavenging possibilities. So far, research has not shown that blueberries and strawberries diminish the effect of heavy ions on brain cells, Vazquez says. But in the next 30 years or so, there may be a breakthrough.
Learn more about genetic vulnerability. Some people are genetically radioresistant; in defiance of odds, for example, some atom bomb survivors have not developed soft-tissue cancers. Because of concerns about the medical privacy of astronauts, it is not currently legal to screen a crew based on genetics. But protocols could change for a Mars mission. "One option may be to say, well, I'm an astronaut, I will allow the flight surgeons to make an analysis of my genetic material," says Vazquez. "And they will tell me, 'OK, Marcelo, you have a bad mutation over there, so you will be at risk to have more cancer than other people'—so it's my decision to accept that level of risk." Or, alternatively, NASA might one day create radioresistant astronauts through the wonders of genetic engineering.
Send a bunch of aging space cowboys. If, after a 10-year latency period, astronauts in their seventies developed a cancer, critics could not charge that they had been struck down in the prime of life. While no one at NASA will speak on this subject officially, many confirm that it often surfaces informally. "My kids said that NASA should send me to Mars," Lucid jokes. "They said, 'If NASA would send you when you're 80, Mom, then you could live up there, do something, and they wouldn't even have to worry about bringing you back. And we wouldn't have to worry about taking care of you.'" Yet with the nearest emergency room 30 million miles away, and Earth-style surgical anesthetics unworkable in microgravity, elderly astronauts would have to be freakishly sturdy to weather the medical challenges of the trip.
Growing up in his native Argentina, Marcelo Vazquez had a dream—of becoming an astronaut, of exploring the solar system. As this was not a realistic possibility, he became a radiation oncologist. But after six years at the San Martín Hospital in La Plata, Argentina, he threw over his practice for his current research at Brookhaven. He has not put aside this dream, however; a vast image of the Spirit rover's landing site hangs over his video display terminal.
When Vazquez fires up his computer to show a PowerPoint presentation on his work, an ancient map appears on the screen. It shows the world surrounded by dragons—a metaphor, he says, for the radiation threat astronauts face in deep space. "My goal is to define what is the risk," he says. In the case of galactic cosmic rays, the dragons may be all too real: metaphorically fire breathing, ineluctable, lethal.
Even if that proves to be the case, NASA will no doubt have no shortage of volunteers willing to put their lives on the line. "Lots of internal discussion is going on now about what level of risk is acceptable for trips like the Mars flight," says John Charles of NASA's Space Life Sciences division. "And what is it going to mean in real-world manifestations, including: What is the likelihood of losing a person—having somebody die on a trip to Mars? We may have a case where they only have so much morphine and so many antibiotics. If somebody's really sick, do you just keep pumping them full of morphine—and then use up morphine that somebody else might need tomorrow? Or does something else have to happen? And what that something else is we all dance around because nobody wants to talk about it."
"We constantly remind each other that our examples would be people like those who settled the North American continent," Charles adds. "When they left Philadelphia or Boston, it was a one-way trip to the West Coast, and they weren't planning on turning around and coming home. If they got sick along the way, somebody buried them."
The power of positive thinking has taken us a long way since the days of the pioneers. But just as we are poised to begin exploring the frontiers of deep space, a sad truth is beginning to emerge. Far from being a naturally spacefaring species, we are frail creatures who may not be able to function for long periods outside the gravity, atmosphere, and magnetic field of mother earth. And even Mike Finke concedes that the spectacular views of home that he found so restorative during his stay on the International Space Station will not be there to comfort astronauts who travel to Mars: "Along the way, they're going to watch the Earth get smaller and smaller."
