Many astronauts have some flying background and are familiar with the somatogravic illusions of aerobatic flight, including a sensation of flying upside down during an aerobatic pushover due to the associated "eyeballs up" acceleration. (Cheung, 2004) Since the US Shuttle thrusts into orbit in an inverted attitude, crewmembers experience "eyeballs-in and up" acceleration, it is not surprising that crewmembers report feeling upside down during the launch phase.
Immediately after main engine cutoff and the onset of weightlessness, almost all US and Russian crews experience momentary somersaulting sensations, and thereafter frequently feel upside down for a period of time ranging from seconds to several minutes (Gazenko, 1964; Yuganov et al., 1966; Oman et al., 1986). Cosmonaut Titov reported: "the weight vanished as quickly as Vostok separated from the booster...and I felt suddenly as though I were turning a somersault and then flying with my legs up! Fortunately the sensation lasted only seconds"(Titov and Caidin, 1962). A similar illusion has been reported at the onset of the weightless phase of parabolic flight (Lackner, 1992). Almost all blindfolded subjects making their first flight experience somersaulting, or sometimes a paradoxical sensation of inversion without pitch. If vision is available, the incidence of the illusion is lower.
For a small minority of astronauts and cosmonauts, 0-G inversion illusions are more persistent, sometimes lasting for hours, and return sporadically during the first several days in orbit. Once crewmember said: "The only way I can describe it is that though I'm floating upright in the cabin in weightlessness, both the spacecraft and I seem to somehow be flying upside down". Rolling upside down in the cabin does not eliminate the inversion sensation - only the spacecraft seems right side up. A Spacelab crewmember said: " I knew I was standing upright ... in the normal way with respect to the orbiter, and nevertheless I felt upside down ... despite the fact that everything was normally oriented around me. This gave me the intellectual interpretation that the orbiter was flying upside down I just interpreted intellectually that the orbiter has to be upside down because you feel upside down and yet you see are the right way up.." Some have reported that the inversion sensation was more noticeable in the visually symmetrical Shuttle mid-deck than when on the flight deck. Certain of the afflicted have found that inversion illusion can be momentarily eliminated by standing in bungee cords, or looking at their own face in a mirror. However such methods have little practical appeal to busy crewmembers. Since 1978, Russian crews have worn "Penguin" suits that use elastic cords to load their bodies along the head-foot axis as a countermeasure against muscle and bone deterioration. In the early 1980s they also evaluated two other disorientation countermeasures, a cap that applied a load between the top of the head and the shoulders, and also sandals with insoles that could be inflated, applying pressure to the feet. The latter became known as "Cuban Boots" after the Cuban cosmonaut who first tried them. However, the extent to which these devices can reproduce haptic gravitational cues is unclear, and users reportedly still experienced illusions and space sickness (Reschke et al., 1994b). Artificial cues that reinforce the perception that "down" is beneath the feet may render freely moving cosmonauts more susceptible to VRIs whenever they float inverted.
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