There is no "biggest danger" in setting up a permanent lunar presence or sending people to Mars, says John Charles, an enthusiastic proponent of both ideas and a NASA analyst of the costs and risks of human spaceflight: "There are several."
Launch, landing and re-entry are perhaps the riskiest moments of any space venture, history shows. But on long missions, what would otherwise be minor threats could become at best serious limitations or at worst deadly disasters.
Basking in the glow of President Bush's call for sending humans back to the Moon and then eventually to the Red Planet, Charles, who works at the Johnson Space Center in Houston, offered up his danger list Monday:
"Radiation is a potential show stopper," Charles told Space.com, quickly adding that researchers are "getting on top of that" while also learning how to clear the other hurdles.
Any grand leap into the cosmos, as last week, will start with dangerous baby steps as explorers cautiously venture into the hazardous, radiation-laden space beyond Earth's protective magnetic field. Scientists are still working to characterize the dangers and develop the technologies necessary for safe and
Particle radiation in space goes right through the human body and can tear apart strands of DNA, the software of life that resides inside a cell nucleus. Damaged cells can lose the ability perform normally and to repair themselves.
There are two primary forms of hazardous space radiation particles. (These particles are different from electromagnetic radiation, such as X-rays, visible light or the ultraviolet rays that cause skin cancer.)
High-energy particles emitted by the sun during intense flares are one type. They move outward at millions of miles an hour and can strike the Earth-moon system in a day or two. Earth's magnetic field shields the planet from most of these. Some get through, though, especially in intense streams lasting several hours when a storm's magnetic field is aligned in a certain way with that of the planet.
Earth's atmosphere blocks out most of the rest of these particles.
An astronaut would not want to be caught outside during a solar storm. Even airlines reroute flights when the sun gets nasty to avoid polar regions, where more of the radiation leaks in.
Someone walking on the moon, even in a fancy spacesuit, would be as good as naked in the face of the sun's worst fury.
"If one were exposed to the full brunt of a solar event, that could cause acute effects in the very short term," Zeitlin explained in a telephone interview. "Quite severe illness" could result. NASA says the radiation sickness from a solar flare could kill an unprotected astronaut.
"These are atomic nuclei stripped of electrons," he explained. "They're able to penetrate many centimeters of solid matter."
Planets and moons offer natural protection against cosmic rays by blocking half the sky.
"When you're in free space the radiation comes at you from all directions," Zeitlin said. "When you're on a planetary surface it's only coming at you from above."
Exposure is therefore about twice as bad while travelling through space compared with being on the lunar or Martian surface.
Earth's atmosphere protects us against the cosmic particles as well as the solar. The Martian atmosphere, about 1 percent as dense as Earth's, manages to stop just about all of the solar particles, scientists figure, but it lets most of the cosmic rays through.
Only last year did scientists get the first solid measurements of radiation at Mars. Zeitlin is the principal investigator for MARIE, a radiation detection instrument aboard NASA's Mars Odyssey orbiter.
Zeitlin's team combined Odyssey data with Earth-orbiting satellite measurements of cosmic rays to