Lost in Space

Shuttle mission specialist John-David Bartoe remembers his first days in orbit: "I followed the advice from my commander, Gordon Fullerton. He recommended that for the first few days we always keep ourselves oriented up with respect to the writing on the walls and with respect to the other crew members. This worked fine for me.

"After day two I was more adventurous and would turn upside down for fun - I had no problem!"

The vestibular system isn't the only one affected by the absence of weight. The proprioceptive system - that is, nerves in the body's joints and muscles that tell us where our arms and legs are without having to look - can also be fooled. Without the stresses in the joints usually caused by the pull of gravity, this sense is sometimes dampened.

more Open wide! Skylab 2 Commander Charles Conrad submits to a dental examination by Medical Officer Joseph Kerwin in the Skylab Medical Facility.

"The first night in space when I was drifting off to sleep," recalled one Apollo astronaut, "I suddenly realized that I had lost track of ... my arms and legs. For all my mind could tell, my limbs were not there. However, with a conscious command for an arm or leg to move, it instantly reappeared - only to disappear again when I relaxed."

Another astronaut from the Gemini program reported waking in the dark during a mission and seeing a disembodied glow-in-the-dark watch floating in front of him. Where had it come from? He realized moments later that the watch was around his own wrist.

These sorts of mismatches between what the eyes see and what the body feels can trigger a malady called "space sickness." Scientists think it's much like "car sickness," which you can get right here on Earth by trying to read in a moving car. The inner ear detects the motion of the car but the eyes - staring at a page filled with unmoving words - do not.