Space / Space Flight

Astronauts are renowned for their can-do attitude and cheery stoicism. Take Mike Finke, for instance. Shortly after he returned from the International Space Station in October 2004, he appeared genuinely nonplussed when asked what he considered the most stressful aspect of his 188 days in space.

"One person's stress is another person's fun," Finke said, smiling.

"But what about the confinement?"

"It's a really big space station, and there are only two people."

"You didn't feel isolated?"

"We had e-mail and the telephone," he said. "And any time we felt bad or grouchy or frustrated or anything, we'd just take a look out the window, and it was one of those magic things—all the frustration would go away."

No doubt seeing Earth from space produced a frisson. But was it enough to make up for the inconvenience of having to communicate with his wife via her cell phone while she gave birth to their second child at a Houston-area hospital?

"I didn't let that bother me, because there wasn't anything I could do about it," Finke explained, grinning yet more broadly.

Such positive thinking is pervasive among just about everyone involved in the space program. That includes President Bush, who in January 2004 unveiled an ambitious plan to return American astronauts to the moon by 2020 and to create a lunar base for future missions to Mars. "Mankind is drawn to the heavens for the same reason we were once drawn into unknown lands and across the open sea," he proclaimed. "We choose to explore space because doing so improves our lives and lifts our spirits." Never mind that Bush offered up a paltry budget increase of $1 billion over five years to help seed the effort. NASA has subsequently shifted its long-term focus away from the space shuttle and the International Space Station toward the creation of a crew-carrying vehicle to fulfill its new mandate, and engineers at the agency are supremely confident that they have the technical know-how to build such a spacecraft. There's just one catch. In recent months, biomedical researchers working in relative obscurity have begun to raise a big unanswered question that tends to be conveniently overlooked: Can the human body withstand a prolonged journey into deep space?

The positive thinkers at NASA have always assumed that a technological fix could be devised for any medical risks posed by space travel. Prolonged weightlessness, for example, is known to cause severe loss of bone and muscle mass. By one estimate, astronauts will lose bone density at a rate of roughly 1 percent per month—suffering the same debilitating decline in the two and a half years it would take to get to Mars and back as they would over an entire lifetime on Earth. One countermeasure NASA doctors came up with is to have astronauts load up on bisphosphomates—medications used to treat osteoporosis in menopausal women. Another is to provide muscle-toning exercise equipment in an artificial gravity setting created by spinning all or part of a spacecraft. Jacob Bloomberg, a neuroscientist at the Johnson Space Center in Houston, is developing an omnidirectional treadmill training system that would allow astronauts to jog or walk through a simulated environment—a favorite country lane, say, or Central Park in New York.

While logging hours on virtual-reality gym equipment may help astronauts survive the wasting effects of weightlessness, a much more serious threat looms during an extended campout on the moon or a journey to Mars: radiation. New research suggests that repeated exposure to galactic cosmic rays and other forms of radiation would be debilitating—astronauts could suffer brain damage or develop leukemia after reaching the Red Planet—and ultimately deadly.

Indeed, the dangers posed by cosmic radiation are so daunting that even some members of the normally upbeat astronaut corps are beginning to question whether a human mission to deep space will be feasible anytime in the near future. "Radiation could be a showstopper," says Shannon Lucid, the blunt-spoken 63-year-old astronaut who has spent the longest time—223 days, 2 hours, and 50 minutes—of any American woman in space. "You don't want to send dead astronauts to Mars."

The squat, beige, utilitarian buildings at the Johnson Space Center could not be more different from the ersatz rococo architecture and pulsating neon of the casinos in Las Vegas. Yet what goes on within the structures is surprisingly similar. Spaceflight—particularly long-duration spaceflight—is about gambling: computing odds and taking risks.

As the chief scientist of NASA's space radiation program, Francis Cucinotta is the agency's chief oddsmaker, an erudite, respectable Jimmy the Greek who occasionally seasons his otherwise flat, technical speech with words like roulette. The NASA formula for overall radiation exposure is 3 percent, Cucinotta says. In other words, the probability of death from cancer among astronauts should be no more than 3 percent above the average probability of death from cancer among the general population in the United States. The agency has four concerns: one, cancer, and two, at the same level, brain damage. The third is tissue degeneration—will astronauts, as a consequence of radiation, develop such linked infirmities as cataracts and heart disease? Finally, there is the problem of acute exposure—what devices will shield an astronaut during a solar flare, the blast of lethal plasma that spews out at intervals from our sun?

Astronauts are well aware of these hazards. Equal to the threat of mechanical failure, medical risk underlies the "culture of safety" mantra that vibrates through the space center. Until the Occupational Safety and Health Administration recently decided that space is not under its purview, astronauts were officially classified as radiological workers, a category that also includes employees of nuclear power plants. And lest they ever be tempted to forget their vulnerability, they wear badges during flight that act as dosimeters—measuring exposure to radiation, which NASA restricts to a point that should ensure no more than that overall 3-percent-risk threshold. Astronauts concerned about reaching the limit will sometimes refuse X-rays—anything to keep their scores down so they can continue to fly.

NASA estimates risk based on data from atomic bomb survivors and nuclear reactor workers. Such people, however, have been exposed to different types of radiation from what one encounters in space, and at best the correlation of data is imprecise. The big corrections are dose rate, type of radiation, and the differences between the populations who are exposed. These differences involve background, dietary, environmental, and genetic factors. In some ways, NASA values the imprecise correlation of data. It enables the agency to adhere to its dogged optimism about radiation.

"There's a hint that we're overestimating risk," Cucinotta says. "The way these committees work, they build in a lot of conservative factors. What NASA has been doing is coming up with its own system, where you take all the conservatism out and instead use a probabilistic approach, where you consider the range of data in a Monte Carlo sampling of how you would estimate risk."

"Monte Carlo?"

"You replace all the conservatism with a probability distribution that gives you the range of the likely solutions. You use that for deciding how long the mission should be, what kind of shielding, all these operational things."

NASA scientists are confident they have a solution for one major radiation problem: solar-particle events. These are bursts of hot plasma that episodically erupt from our sun and journey outward at high speeds through the solar system. Concern for solar flares, some say, was among the forces that grounded the Apollo program. A month before the last mission, Apollo 17 in December 1972, a severe solar storm broke out, the particles of which would have fatally seared the astronauts had they been on the moon when it occurred.

If and when astronauts return to the moon, the vehicle for touring the lunar surface could be equipped with a solar-particle shield made up of two to four inches of polyethylene in an aluminum shell. Or astronauts could carry shielding material for constructing lunar shelters into which they could bolt if a solar storm strikes. On the moon, they will have to move quickly—the radiation will hit within an hour of their learning of it, and doses could build up to an unacceptable level in three to four hours. On Mars, which is farther from the sun, astronauts will have more time—about three hours—to seek refuge.

Solar particles are just one form of radiation astronauts will have to contend with on a deep-space mission, however, including X-rays, gamma rays, and—above all—galactic cosmic rays. "Right now, the radiation has an uncertainty of a factor of six," says John Charles, an assistant director in NASA's Space Life Sciences division. "Which means that if you want to protect your biological specimen on a trip to Mars to a set conservative limit, you will probably be carrying six times too much shielding, just because the uncertainty is a factor of six—a 600 percent uncertainty in the measurement." The possibly unnecessary addition of shielding, he continues, "means a whole lot more mass, which means the rocket is heavier, which means it can't launch as easily."

Yet some scientists suggest that even bulky shielding will not protect a Mars-bound human cargo.