Will humans colonize other worlds? This recurring sci-fi theme won't go away. It's part of our collective psyches. And along those lines, NASA now stands at a pivotal crossroad: The blue-ribbon panel appointed by the president has concluded its work, and Mr. Obama has promised a decision any time now.

Continue testing the new Ares rocket, and stick to the 2020 Bush timetable for returning to the Moon? Continue testing, but at a money-saving slower pace? Scrap the whole thing? Aim instead at an eventual manned trip to Mars?

As this historic decision nears, we've discussed the issue twice this year. But one factor rarely gets official mention. It's the simplest: risk.

Space-flight misfortunes are always the most dramatic. And humans already fear accidents. This is a bit illogical, since only one person in 25 dies in some sort of accident, while one in 88 dies in a car crash. By contrast, 23 percent of us die of cancer. And nearly half succumb to heart or arterial perils like stroke.

Nonetheless, disasters make headlines, which is why it's a bit strange that no one seems to be discussing the risks of future manned missions. You don't hear about it on TV, even though many insiders regard manned travel as a dangerous bummer.

"I found it extremely boring," assessed Wally Schirra after 11 days of commanding the first Apollo spacecraft. Okay, he was widely regarded as irritable; but what about the Russians who spent collective years aboard Mir and Soyuz? They often got depressed. True, they're Russians. Nonetheless, they'd grow plants and then check them a half-dozen times a day, obsessively fondling the only natural object in their environment.

"I suffered sadness and mental stress," explained Anatoly Grigoriev after 438 days in space. Vadim Gushin, another long-timer, adds, "The plants helped us establish ties with Earth, with Nature. This is the essence with which the human mind is based."

Duh! Of course! Humans evolved in a living environment, to interface with wind and pollen. Isolate a person with bottled air, fluorescents and plastics, and who knows what psychological damage must follow? Long-period missions might drive astronauts loony.

And it's not just the mind. As results trickle in from the ISS and elsewhere, astrobiologists grow more concerned, not less, about the long-term consequences of space travel.

In orbit, bone and muscle vanish at one percent per month, and some of the loss is irreversible. Hearts shrink in size; cardiac capacity goes steadily downhill. Astronauts might look like they're happily at the gym up there, on the treadmill in their underwear, but it's hopeless: Space is just not good for you.

Things really get bad when you leave Earth orbit and pass outside our magnetosphere, which guards us from solar and cosmic radiation. Apollo astronauts all saw flashes of light resembling shooting stars cross their visual fields about once a minute, as heavy ions ripped through their brains. Their radiation exposures were not trivial and, years later, Alan Shepard didn't deny that going to the Moon may have given him the leukemia that ultimately killed him.

Things could have been worse. An intense solar mass ejection occurred just one month before the final Apollo 17 mission. Had the astronauts been on the Moon's surface - or in their capsule going or coming - they would have been killed.

High cancer risk aside, brain neurons will be destroyed continuously by the nonstop radiation. One biologist estimates that during a two-year Mars mission, an astronaut will lose between 13 and 40 percent of his brain! Ouch! Even us smart people can't afford that. It greatly exceeds the five percent annual neuron necrosis suffered by some Alzheimer's patients.

Once there, astronauts will have to fend for themselves: There'll be no ride home for at least 18 months, when Earth and Mars again align. You're on a place without breathable air. It's always freezing. Radiation is giving you a splitting headache. The pizza is terrible.

Still want to go? Maybe, except for the astronauts, neither will anyone else.