Although many factors affect human health during periods in space, weightlessness is the dominant and single most important one. The direct and indirect effects of weightlessness lead to a series of related responses. Ultimately, the whole body, from bones to brain, kidneys to bowels, reacts. When space travelers grasp the wall of their spacecraft and jerk their bodies back and forth, they say it feels as though they are stationary and the spacecraft is moving. The reason is based in our reliance on gravity to perceive our surroundings. The continuous and universal nature of gravity removes it from our daily notice, but our bodies never forget. Whether we realize it or not, we have evolved a large number of silent, automatic reactions to cope with the constant stress of living in a downward-pulling world. Only when we decrease or increase the effective force of gravity on our bodies do our minds perceive it. Our senses provide accurate information about the location of our center of mass and the relative positions of our body parts. Our brains integrate signals from our eyes and ears with other information from the organs in our inner ear, from our muscles and joints, and from our senses of touch and pressure. The apparatus of the inner ear is partitioned into two distinct components: circular, fluid-filled tubes that sense the angle of the head, and two bags filled with calcium crystals embedded in a thick fluid, which respond to linear movement. The movement of the calcium crystals sends a signal to the brain to tell us the direction of gravity. This is not the only cue the brain receives. Nerves in the muscles, joints, and skin—particularly the slain on the bottom of the feet—respond to the weight of limb segments and other body parts. Removing gravity transforms these signals. The inner ear no longer perceives a downward tendency when the head moves. The limbs no longer have weight, so muscles are no longer required to contract and relax in the usual way to maintain posture and bring about movement. Nerves that respond to touch and pressure in the feet and ankles no longer signal the direction of down. These and other changes contribute to orientation illusions, such as a feeling that the body or the spacecraft spontaneously changes direction. In 1961 a Russian astronaut reported vivid sensations of being upside down one space shuttle specialist in astronomy said, 'When the main engines cut off, I immediately felt as though we had inverted 180 degrees.' Such illusions can recur even after some time in space. From the first two paragraphs we learn that weightlessness is caused by ______.