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Archived by Raymond J. Noonan, Ph.D., Health and Physical Education Department, Fashion Institute of Technology of the State University of New York (FIT-SUNY), and SexQuest/The Sex Institute, NYC, for the benefit of students and other researchers interested in the human aspects of the space life sciences. Return to first page for background information on these pages.


Just as aircraft and spacecraft maintain their positions based on information from radar, gyroscopes, accelerometers, and other sensors, human beings rely on several neural (nerve-brain connections) orientation sensors. These sensors add to information from the eyes, the muscles and joints, and the body's accelerometers, assisting the brain in figuring out which way is up.

The accelerometers in the neurovestibular system, which help people orient their bodies, are very sensitive to gravity. For instance, the otoliths, small vestibular organs in the inner ear, respond to the acceleration of an elevator. As a person changes positions, gravity pulls the tiny clumps of calcified crystals in the otoliths down and bends hairs in the inner ear; then the more than 20,000 nerve cells sense the crystal and hair movements in each ear and tell the brain the head's position.

Figure 1. The ear and the vestibular system. [Click for enlarged view.]

There are other sensors in the inner ear called semi-circular canals which sense rotation. When you spin around in a circle, you cause tiny amounts of fluid (called endolymph) to shift inside the semi-circular canals, pressing against a tiny flap called the cupula. The pressure of the endolymph fluid against the cupola tells your brain which way you are turning.

Figure 2. The endolymph fluid, depicted here as green, moves the cupula when you rotate your head.

Nerves also constantly perceive gravity as muscles relax and contract and use this information to sense body position. The eyes see surroundings and sense the body's position relative to other objects. All of these sensing mechanisms (along with others) together provide orientation information to the brain so that you can "feel" your body's position relative to the rest of the world. In space, gravity no longer tugs at the otolith crystals, and the muscles no longer have to support the weight of the limbs.

Theory suggests that, in microgravity, information sent to the brain from the inner ear and other sensory organs conflicts with the information that the brain has been accustomed to receiving on Earth. This conflict results in disorientation. This disorientation often results in a "space motion sickness" phenomenon which affects about one-half of all space travelers. Although the symptoms of space motion sickness are similar to Earth motion sickness, scientists are unsure if the stimulus is the same since crew members who experience Earth motion sickness may not experience space motion sickness and vice versa. At the present time, there are no tests that predict which individuals will experience discomfort or to what degree.

Space motion sickness was first experienced by Soviet Cosmonaut Titov during his one-day Vostok-2 flight on August 6, 1961, and later by the crews of other Soviet flights. American astronauts did not experience space motion sickness until the early Apollo flights. Mercury and Gemini astronauts probably didn't experience space motion sickness because they were relatively immobile during the missions and because the vehicles did not rotate with respect to the Earth's horizon. However, some people experience worse symptoms than others, and some have no symptoms at all, so the lack of space motion sickness symptoms in the early astronauts may have been just a matter of luck.

The most severe symptoms of space motion sickness occur during the first few days of space flight and disappear as the body and brain adapt to the new environment. And after landing, there is some vestibular disturbance as the astronauts return to Earth's gravity. NASA wants to improve crew efficiency and comfort by eliminating space sickness. Although astronauts have used some motion sickness drugs successfully to reduce nausea, no treatment has been found to completely eliminate the symptoms. Experiments have focused on identifying the underlying causes of this problem, on ways to treat it, and on studying how the nervous system adapts to microgravity.


Three main countermeasures have been tried to prevent space motion sickness: 1) drugs, 2) pre-flight adataptation, and 3) preventing head motion by sitting still. Drugs have unfortunate side effects, including drowsiness and nausea. Pre-flight adaptation by exposing the astronauts to unusual motions before the flight have not proven effective yet. The best countermeasure so far is preventing head motion by sitting still, but this seriously impacts crew work schedules.


  1. If you close your eyes, can you still point straight up? How do you know?
  2. Have you ever gotten motion sickness? What were you doing? Could you concentrate on work, play or conversation while you were feeling ill?
  3. Where are the otoliths? What do they do?

You can go back to where you came from, or jump back to the beginning.

The next section contains the frequently asked questions about humans in space.

Last modified: Oct 8, 1994

Author: Ken Jenks


Contact Info:
Raymond J. Noonan, Ph.D.
Health and Physical Education Department
Fashion Institute of Technology of the
State University of New York (FIT-SUNY);
SexQuest/The Sex Institute, NYC
P.O. Box 20166, New York, NY 10014
(212) 217-7460

Author of:

R. J. Noonan. (1998). A Philosophical Inquiry into the Role of Sexology in
Space Life Sciences Research and Human Factors
Considerations for Extended Spaceflight
Dr. Ray Noonan’s Dissertation Information Pages:
[Abstract] [Table of Contents] [Preface] [AsMA 2000 Presentation Abstract]


First published on the Web on June 14, 1998
This page was last changed on March 25, 2002; Ver. 3a
Copyright © 1998-2002 Raymond J. Noonan, Ph.D.

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