Editing Space settlement (section)

===Artificial gravity===
{{Main|Artificial gravity}}
{{unreferenced section|date=February 2011}}
Long-term on-orbit studies have proven that zero gravity weakens bones and muscles, and upsets calcium metabolism and immune systems. Most people have a continual stuffy nose or sinus problems, and a few people have dramatic, incurable motion sickness. Most habitat designs would rotate in order to use [[inertia]]l forces to [[Artificial gravity|simulate gravity]]. NASA studies with chickens and plants have proven that this is an effective physiological substitute for gravity.{{Citation needed|date=November 2009}} Turning one's head rapidly in such an environment causes a "tilt" to be sensed as one's inner ears move at different rotational rates. Centrifuge studies show that people get motion-sick in habitats with a rotational radius of less than 100 metres, or with a rotation rate above 3 rotations per minute. However, the same studies and statistical inference indicate that almost all people should be able to live comfortably in habitats with a rotational radius larger than 500 meters and below 1 RPM. Experienced persons were not merely more resistant to motion sickness, but could also use the effect to determine "spinward" and "antispinward" directions in the centrifuges.{{Citation needed|date=November 2009}}