For now, forget buying a pod on Newt Gingrich's moon colony—besides the technological hurdles, new research suggests that the human body might not have what it takes to cut it in space for long periods of time.

A study of 27 astronauts found that nearly all of their eyes and brains showed abnormalities after returning from space. Lead researcher Larry Kramer of the University of Texas Medical School at Houston, says the abnormalities are caused by increased pressure in the brain that's consistent with idiopathic intercranial hypertension, a disease on earth without a known cause. Astronauts who were in space for longer periods of time showed the worst symptoms, which include altered vision, displaced pituitary gland, and swelling of the optic nerve.

Kramer and NASA scientists believe the astronauts' exposure to the low-gravity environment in space is causing the condition.

"We have indications that the severity and incidence goes up with time," Kramer says.

The findings are the latest biological hurdle humans will have to clear before leaving earth behind—previous studies have found that astronauts experience muscle atrophy and loss of calcium from their bones while in space. Richard Williams, chief health and medical officer for NASA, says those two conditions can be partially mitigated with exercise aboard the space station and dietary supplements, but increased brain pressure is a tougher nut to crack.

"We've put a lot in motion in an effort to understand [increased brain pressure] and to try to mitigate it," Williams says. "It's a high priority."

The symptoms experienced by astronauts can often be treated on earth, but Americans' relatively short stints in space—the longest was 215 days—seem to be enough to prevent serious harm.

"How big a risk it poses for longer term missions of 18 months, two years, we haven't gone to at this point. It's hard to say," Williams says. NASA has set a 2025 target for a manned "exploration" mission to Mars or another solar system spot, which would presumably be some of the longest manned space flights attempted. "We don't see this as precluding those missions."

But longer or permanent missions could be out of the question, Kramer says. When it comes to longer-term space travel or planetary colonies, he says "we may be limited by our own biological makeup."

Both Kramer and Williams note that some astronauts have no ill effects from prolonged space exposure. Genetics and anatomical makeup may play a role in certain humans being better adapted for space travel, but Williams says it's impossible to tell which astronauts will be better protected. With very few humans taking trips to space, and the canceled space shuttle program, research in the area is likely to continue to be slow.