Jeffrey Sutton, director of the National Space Biomedical Research Institute and Nitza Cintr¿n, chief of NASA's Space Medicine and Health Care Systems Office, explain.

Space is a harsh environment that affects the body in many ways. In microgravity, bone loss occurs at a rate of 1 to 1.5 percent a month, leading to an acceleration of age-related changes similar to osteoporosis. Decreases in bone density and strength are more pronounced in some skeletal regions, such as the pelvis, although much of the loss is reversible upon return to Earth. Prolonged exposure to weightlessness also increases the risks of kidney stones and bone fractures, which are both associated with bone demineralization. In addition, studies suggest that microgravity alters the ability of bones to heal after fractures.

Long stays in space also impact muscles. There is loss of muscle mass, strength and endurance, especially in the lower extremities. Changes in muscle performance, coupled with the effects of microgravity on connective tissues and the demands of activities of varying intensities, place astronauts at risk of fatigue and injury.

The heart is a unique muscle, and diminished cardiac function and the possible occurrence of heart rhythm disturbances are concerns faced during space flight. The details of these cardiovascular changes and risks are not yet completely known, however. In microgravity, body fluids are redistributed away from the extremities, which results in puffiness in the face during flight as well as changes in cardiovascular physiology. Upon return to Earth, some astronauts experience impaired orthostatic response, which means that their blood pressure drops abnormally low when they move from lying down to a sitting or standing position.

Similarly, microgravity also impacts the neurovestibular system--an integrated set of neural sensory, motor and brain circuits that allows humans to maintain balance, stabilize vision and understand body orientation in terms of location and direction. Exposure to microgravity often leads to disorientation and decreased neuromuscular coordination upon return from prolonged missions. Immediately after landing, astronauts may have problems standing up, stabilizing their gaze, walking and turning.

In space, circadian rhythms are disrupted because the 24-hour day/night cycle is absent. Sleep loss, stress related to workload, high performance expectations, and psychosocial factors all affect the body on long-duration missions. The body also suffers loss of blood volume, immunodeficiency, and transient post-flight anemia (low red blood cell levels), despite adequate nutritional intake.

Space radiation is rich in heavy ions and poses one of the greatest risks to humans on prolonged missions. Radiation can induce cataracts and cancer, as well as adversely affect many physiological processes.

As space missions grow longer, much remains to be learned about just how the space environment alters the body and how complex physiological and psychological changes vary with mission length.

The reader also inquired as to what doctors recommend astronauts do to counteract these effects.

Astronauts and crews are carefully selected, trained, and monitored to minimize risks and to promote health and safety. Improvements are constantly being made to spacecrafts, in-flight medical care capabilities and countermeasure protocols, as well as post-flight rehabilitation. During prolonged missions, exercise is effective at minimizing large muscle atrophy, and astronauts use a cycle ergometer and treadmill with downward applied pressure to maintain fitness. Certain tasks, such as extravehicular activities, are not routinely performed until bodily fluid redistribution stabilizes and astronauts have an opportunity to acclimatize to space. Prior to re-entry to gravity, increasing fluid intake is helpful in minimizing the effects of orthostatic hypotension.

Physicians and researchers for NASA and the National Space Biomedical Research Institute are pursuing ways to develop and validate medical technologies and risk management strategies for long-duration space flight. These groups want to advance in-flight diagnostic and therapeutic capabilities for crewmembers commensurate with current standards of health care delivery. They are also pursuing ways to improve the ability to remain in space for long periods by finding novel ways to decrease microgravity's ill effects.