Microgravity

  Once in orbit, the space vehicle and everything inside it experience a condition called microgravity. The vehicle and its contents fall freely, resulting in an apparently weightless floating aboard the spacecraft. For this reason, microgravity is also referred to as zero gravity. However, both terms are technically incorrect. The gravitation in orbit is only slightly less than the gravitation on Earth. The spacecraft and its contents.

  continuously fall toward Earth. But because of the vehicle's tremendous forward speed, Earth's surface curves away as the vehicle falls toward it. The continuous falling seems to eliminate the weight of everything inside the spacecraft. For this reason, the condition is sometimes referred to as weightlessness.

  Microgravity has major effects on both equipment and people. For example, fuel does not drain from tanks in microgravity, so it must be squeezed out by high-pressure gas. Hot air does not rise in microgravity, so air circulation must be driven by fans. Particles of dust and droplets of water float throughout the cabin and only settle in filters on the fans.

  The human body reacts to microgravity in a number of ways. In the first several days of a mission, about half of all space travelers suffer from persistent nausea, sometimes accompanied by vomiting. Most experts believe that this "space sickness," called space adaptation syndrome, is the body's natural reaction to microgravity. Drugs to prevent motion sickness can provide some relief for the symptoms of space adaptation syndrome, and the condition generally passes in a few days.

  Microgravity also confuses an astronaut's vestibular system -- that is, the organs of balance in the inner ear -- by preventing it from sensing differences in direction. After a few days in space, the vestibular system disregards all directional signals. Soon after an astronaut returns to Earth, the organs of balance resume normal operation.

An apparently weightless floating makes some tasks challenging inside an orbiting spacecraft. In this photograph, a shuttle astronaut struggles with a floating computer printout. Image credit: NASA

Recording medical information on a spacecraft enables physicians to identify any abnormal changes in the body that could indicate physical disorders or stress. Image credit: NASA