NASA has barred veteran astronaut Donald Thomas from serving on the International Space Station, citing concerns over his long-term exposure to cosmic radiation. What is cosmic radiation?
Perhaps best known as the mysterious force that gave comicdom's Fantastic Four their superpowers, cosmic radiation is omnipresent in the universe. Much of it consists of low-energy subatomic particles left over from the Big Bang's aftermath—a sort of interstellar background noise. These photons and neutrinos are not considered hazardous to humans.
More worrisome is the radiationproduced by solar flares, periodic eruptions during which the sun releases energy equivalent to a billion mega-tons of TNT. The Earth is protected from these lethal streams by the atmosphere and ozone layer, two luxuries not enjoyed by astronauts. If a spacewalker were to be inadvertently exposed to solar-flare radiation, odds are he or she would come down with fatal radiation sickness. The good news is that, since solar flares can be detected before the particles arrive, there is often plenty of time for a spacewalker to find a well-shielded spot aboard a spacecraft.
But even shielding does little good against so-called galactic cosmic radiation, which originates in deep space. Consisting chiefly of high-energy protons and electrons produced by stars, black holes, and gamma-ray bursts, GCR is tough to defend against. Shields help a little, but they can't stop every subatomic bit. Astronauts who spend too much time aloft are believed to dramatically raise their risk of developing cancer, although they're not in immediate danger of suffering from radiation sickness.
How much time in space is too much?A precise correlation between GCR exposure and cancer has yet to be established, but NASA was worried enough to hold back Thomas, who has spent 43 days in space on four previous missions. The Russians, however, seem a bit more carefree with their spacemen's well-being. New space-station resident Nikolai Budarin once spent nearly 10 months aboard Mir.
Bonus Explainer: NASA's not the only organization to fret over cosmic radiation. The Association of Flight Attendants has voiced similar concerns, arguing that commercial flights are subject to similar hazard. The union cites Federal Aviation Administration figures that estimate that 27,000 hours of flight time—a reasonable amount of time in the career of a flight attendant—can lead to a 1 percent increase in the odds of dying from cancer.