Microgravity, or weightlessness, in orbit has been an issue not for the design of manned spacecraft but rather for the effect it has on the physiology of the human occupants. Trying to determine and understand the effects of long-term weightlessness on people has been a major preoccupation of manned space programs since the first orbital flight of Yuri Gagarin in 1961. Considerable medical data have been gathered over the years, with astronauts aboard space stations such as Skylab, Salyut, Mir, and now the ISS staying in orbit for hundreds of days and acting as willing guinea pigs. This means that the effects of microgravity over a period of time typical of, say, a manned flight to Mars can be studied and evaluated. The main physiological effects of weightlessness on people can be summarized as follows:

• Motion sickness: The balance sensors we have in the inner ear rely on the movement of fluid in a normal 1g environment to give us information about how we are oriented (lying down or upside-down, for example) and how we are moving around. With the fluid in a weightless condition, the brain has difficulty interpreting what the balance sensors are saying, and there is also a conflict between this information and what the eyes see, which can result in nausea and illness, with some astronauts being affected more than others. The brain usually takes about 2 or 3 days to sort out the new sensory inputs and to adapt to the new environment.

• Redistribution of bodily fluids: On Earth the blood pressure of a standing person decreases with the height above the feet; the pressure in the brain is about one third of that in the feet. Exposure to weightlessness causes a major redistribution of blood, resulting in a bloated face and thin legs—the classic so-called "puffy face and chicken legs'' syndrome! The astronaut soon adapts to this situation in orbit, and the body appears to recover its normal function after a short period of time once back in a 1g environment on the ground.

• Muscle atrophy: Long periods of weightlessness usually mean physical inactivity, which causes muscles to waste away. This includes the heart muscle, which generally loses mass, with an accompanying reduction in heart rate. To combat this worrying trend, astronauts must spend a significant amount of time in necessary physical exercise, using complex exercise equipment designed for the microgravity environment.

• Bone decalcification: Another significant effect of weightlessness is the cumulative loss of calcium from bones, resulting in bone fragility in the long-term. This trend appears to be reversible once the astronaut is back in a 1 g environment.

Although people seem to be able to recover from most of the effects of microgravity with time, the issues of loss of bone and muscle mass are a concern in future manned exploration of the solar system. Long-duration spaceflight in weightless conditions means the astronauts are clearly not in the best of physical condition when they arrive at their destination. Intensive physical exercise for the astronauts, and possibly the use of artificial gravity (see below) during the flight may be partial solutions to this problem.

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