Space motion sickness is caused by changes in g-forces, which affect spatial orientation in humans. According to Science Daily, "Gravity plays a major role in our spatial orientation. Changes in gravitational forces, such as the transition to weightlessness during a space voyage, influence our spatial orientation and require adaptation by many of the physiological processes in which our balance system plays a part. As long as this adaptation is incomplete, this can be coupled to motion sickness (nausea), visual illusions and disorientation." One understanding of motion sickness is that nausea is a pro-survival evolutionary adaptation, because the sensory stimulation of a maladapted high acceleration environment that the body is not accustomed to is recognized by the brain as being similar to the sensory conflict from eating a poisonous plant, in which case vomiting is a helpful reaction.
Modern motion-sickness medications can counter space sickness but are rarely used because it is considered better to allow space travelers to adapt naturally over the first day or two than to suffer the drowsiness and other side effects of medication. However, transdermaldimenhydrinate anti-nausea patches are typically used whenever space suits are worn because vomiting into a space suit could be fatal. Space suits are generally worn during launch and landing by NASA crew members and always for extra-vehicular activities (EVAs). EVAs are consequently not usually scheduled for the first days of a mission to allow the crew to adapt, and transdermal dimenhydrinate patches are typically used as an additional backup measure.
Space motion sickness was effectively unknown during the earliest spaceflights as these were undertaken in very cramped conditions; it seems to be aggravated by being able to freely move around and so is more common in larger spacecraft. After the Apollo 8 and Apollo 9 flights, where astronauts reported space motion sickness to Mission Control and then were subsequently removed from the flight list, astronauts (e.g. the Skylab 4 crew) attempted to prevent Mission Control from learning about their own SAS experience, apparently out of concern for their future flight assignment potential.
As with sea sickness and car sickness, space motion sickness symptoms can vary from mild nausea and disorientation, to vomiting and intense discomfort; headaches and nausea are often reported in varying degrees. About half of sufferers experience mild symptoms; only around 10% suffer severely. The most extreme reaction yet recorded was that felt by Senator Jake Garn in 1985. After his flight NASA jokingly began using the informal "Garn scale" to measure reactions to space sickness. In most cases, symptoms last from 2–4 days. In an interview with Carol Butler, when asked about the origins of "Garn" Robert E. Stevenson was quoted as saying:
Jake Garn was sick, was pretty sick. I don't know whether we should tell stories like that. But anyway, Jake Garn, he has made a mark in the Astronaut Corps because he represents the maximum level of space sickness that anyone can ever attain, and so the mark of being totally sick and totally incompetent is one Garn. Most guys will get maybe to a tenth Garn, if that high. And within the Astronaut Corps, he forever will be remembered by that.