In the zero gravity of outer space, the human body slowly turns to mush. We have evolved in an environment about 4,000 miles from the center of Earth with a steady pull of 9.8 meters per second squared, a pull defined as 1 g. The cells that make up our bodies expect that pull and build up their walls with support structures, called cytoskeletons, accordingly. In space, the pull doesn't exist, and so the cells' cytoskeletons collapse. Muscles atrophy and bones decalcify. The heart shrinks.
This is a large part of why we're nowhere near deploying a manned Mars mission. After traveling in zero gravity for 18 months, the first human to step down onto the Red Planet would probably snap an anklebone and collapse into a small pile of goo. NASA has nearly completed the prototype of a spacecraft that would cut travel time to three months each way, but even in that reduced timeframe, zero gravity would have a debilitating effect on the body. NASA astronaut Jerry Linenger remembers how the absence of gravity caused extreme back pain for some passengers on the Russian space station Mir. In his book, Off the Planet, which recounts five months on Mir, Linenger says he experienced a 13 percent bone loss in his hips and lower spine. Two years later, he still hadn't recovered fully. Translation: Human physiology remains the primary stumbling block to a prolonged space journey. "On a long-duration trip to Mars, there's going to be significant bone loss," Linenger said on NPR's Fresh Air. "That could be a show-stopper."
Long-term exposure to microgravity makes humans weak, so hypergravity should have the opposite effect, right? In a 2001 study, 10 Australian fighter pilots routinely exposed to 2 to 6 gs while flying experienced an average 11 percent increase in the density of their spinal vertebrae over a 12-month period. NASA figures that astronauts could use a solar-powered, onboard centrifuge to stave off muscle and bone deterioration in space.
