The Invisible Killers

We have the technology to send astronauts to Mars. But can we return them safely to Earth?

Air & Space Magazine

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.

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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.

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