Image: Mars habitat
John Frassanito & Associates
An artist's conception shows a Mars crew erecting an inflatable habitat on the surface. Dwellings on the moon or Mars could be shielded from radiation by special coverings, or even by soil or water reservoirs.
By Senior Science Writer
updated 1/20/2004 4:02:19 PM ET 2004-01-20T21:02:19

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 as early as 2015 and then eventually to the Red Planet, Charles, who works at the Johnson Space Center in Houston, offered up his danger list Monday:

  • Lack of a medical facility could turn a mundane injury into a life-threatening situation.
  • "Psychosocial" pressure will be high in a small group isolated for months or years.
  • Zero or reduced gravity causes bone and muscle loss.
  • Dangerous radiation particles are abundant beyond Earth orbit.

"Radiation is a potential show stopper," Charles told, quickly adding that researchers are "getting on top of that" while also learning how to clear the other hurdles.

Total exposure
Any grand leap into the cosmos, as outlined by Bush 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 suits and ships.

This much they know:

Any trip beyond Earth orbit will involve radiation threats not faced by residents of the international space station, which sits inside the planet's magnetic field.

A 2½-year trip to Mars, including six months of travel time each way, would expose an astronaut to nearly the lifetime limit of radiation allowed under NASA guidelines.

The moon, with no atmosphere, is more dangerous than the surface of Mars. Lunar forays will have to be brief unless expensive shielded habitats are built.

Mission planners knew the Apollo astronauts would be at grave risk if a strong solar flare occurred during a mission. The short duration of each trip was a key to creating favorable odds.

"A big solar event during one of those missions could have been catastrophic," said Cary Zeitlin, a radiation expert at the National Space Biomedical Research Institute at Baylor College of Medicine in Houston. "The risk was known. They gambled a bit."

The White House plan calls for a permanent lunar base.

NASA already spends millions of dollars every year on research into space radiation and its biological effects, and more money goes into research on other health risks of long-term spaceflight. The new plan would refocus space station activities to support these investigations.

Double dose
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.

Cosmic ray risk
Cosmic rays, the other big space-particle worry, come from undetermined galactic sources and pose a greater long-term risk for cancer, cataracts and other illness, Zeitlin said. Cosmic ray particles are more energetic than their solar cousins.

"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.

Martian irony
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 project the radiation risk to an astronaut in free space and on the surface of Mars. The combined solar and cosmic ray particle exposure is measured in sieverts.

An astronaut in a six-month journey to Mars — the time required with conventional propulsion — would be exposed to about 0.3 sieverts, or 0.6 on a round trip. Eighteen months on the surface (if it takes so long to get there, you might as well stay awhile!) would bring another 0.4 sieverts, for a total exposure of 1 sievert.

Limits set by NASA vary with age and gender, but range from 1 to 3 sieverts.

When the Odyssey result was announced, several news reports misrepresented the risk, stating that it might prevent human missions to Mars. Zeitlin allows that it is close to the limits, but he says now, as he did then, that it is a "manageable dose." Further, the limits tend to drop as more is learned about the effects on humans. And, of course, the dose could be lowered with creative shielding technology.

(Interestingly, the best way to protect spacefarers aboard a Mars transport ship might be to surround them with the water they'd need for their journey. The hydrogen in water, scientists have learned, is one of the best absorbers of particle radiation.)

There is more to learn about the risk, however.

For one thing, Zeitlin's estimate, based on the Odyssey data, comes with 20 to 30 percent uncertainty. (In a form of cosmic irony, a solar storm last October disabled the MARIE instrument while leaving Odyssey otherwise in good order. The orbiter is currently serving as a relay for pictures and data coming from the Mars Spirit rover. Only after the rover mission is complete will engineers try to revive the radiation experiment.)

Perhaps more important, long-term space travel might make people more susceptible to radiation harm. It also appears some people are more susceptible than others. More research is needed to understand both these issues, however.

Also, the Odyssey data was collected just after the peak in a known 11-year cycle of solar activity. The levels would be greater during the peak and less at the trough. It might seem, then, that the first human trip to Mars should take place at solar minimum, a 2- to 3-year stretch every 11 years when sunspots and flares are almost nonexistent.

But there's a catch: "Galactic particle intensity picks up during solar minimum," Zeitlin said. They are higher-energy and more difficult to shield in a space habitat and "impossible to shield completely" on a spaceship.

Nuclear reactors use concrete to contain similar particles.

"You can't lift concrete blocks into space," Zeitlin points out, at least not at the going rate of about $10,000 per pound for launch costs.

Unfriendly moon
Bush's first planned stop in the cosmos is the moon. Its danger is, interestingly, not as well charted as that of Mars.

The Apollo astronauts made some measurements of radiation on the moon, but the results don't provide as complete a picture as what scientists now have of the Red Planet. But Zeitlin said the moon would be more dangerous since it has no atmosphere — probably about half as dangerous as free space (again, someone on the moon would be protected by the moon itself on one side).

"The moon would be worse than Mars and worse than the space station," he said. Short stays, perhaps one to two months, will be the norm early on. "That's a small enough dose of the galactic stuff that you're actually going to be more concerned about the solar particles, especially if you're near solar max," the intensely active part of the 11-year solar cycle.

Lunar visitors won't have the option of just sitting inside some protective shell. The president's vision makes exploration NASA's primary goal. So astronauts will presumably be called on to inspect the lunar countryside. That would present the risk of someone getting caught on a long rover excursion as a solar storm hits.

Warning times for sun storms can be as little as 18 hours. Far less time is available to make firm predictions of the expected effect of a flare-up.

Zeitlin says a more extensive warning system will need to be established. This is especially true for Mars, which when it's on the far side of the sun can be hit by solar tempests that don't even register with terrestrial scientists.

And there's more to do.

"We have to get smart about how we design the spacesuits and do everything we can to limit exposure to solar particles," Zeitlin said. "You can't stop everything with a spacesuit, but there are better and worse ways to design it."

The Apollo-era suits were not well-equipped.

"They would not have done much," Zeitlin said. "We will try to improve on that."

'Ready to go'
NASA employees are ready for the challenge.

"We're ready to go," said John Charles, the human spaceflight analyst. "I've been waiting for this for 20 years."

Charles is not among those space buffs dismayed that their ultimate desire to put humans on Mars will first involve a lengthy lunar effort. In the president's vision, the moon will serve as a testing ground for landing and surviving on Mars.

Charles said the proximity of the moon and familiarity with it offer a level of comfort.

"We're foolish not to go to the moon first," he said, "because that's a place where we can practice with some degree of safety." After all, while a Mars trip would logically last two or three years, the moon is a mere three days away. Either destination, he knows, remains dangerous.

"Anybody who gets hurt is totally dependent on the rest of the crew to take care of him or her," Charles points out. "If you're having a life-threatening trauma, three days can seem like three years."

Under the president's plan, NASA has at least 11 years to ponder the all this.

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