Shuttle, Mir and ISS crewmembers have also sometimes experienced spatial disorientation episodes while performing spacewalks ("Extra Vehicular Activity", EVA). EVA astronauts typically move about using handrails, trailing a backup safety tether. They stabilize their body with one hand while working with the other, or install foot restraints and use both hands. Since the body tends to drift while working, crews must remain conscious of their orientation, and be careful not to inadvertently bump antennae, optics, or other sensitive equipment. They must avoid thruster keep-away-zones. Working upright within the Shuttle payload bay is disorienting, since the area can be illuminated with flood lights, and the floor and side walls define a convenient visual reference frame. However when crews work on the rounded exteriors surfaces of the Mir or ISS modules, fewer global visual landmarks are available, particularly during the dark portion of each orbit, when the only lighting comes from helmet mounted lamps. Crews prepare for EVA by memorizing landmarks and routes during their preflight underwater neutral buoyancy training. NASA EVA teams also train using an interactive immersive virtual reality display system. Once in orbit they use a laptop computer graphics program to review anticipated translation paths (Homan, 2001; Walz, 2002). Nonetheless most EVA crews admit they occasionally become disoriented and sometimes must even radio for advice, or await daylight.
Some EVA astronauts have described a 0-G form of height vertigo, apparently triggered by a VRI. Early reports came from Shuttle astronauts working in the open payload bay while it faced earthward. If the astronaut happened to float into an inverted orientation, looked toward their feet and saw the Earth moving by rapidly by several hundred kilometers away, their mental allocentric reference frame apparently jumped from the payload bay to the surface of the Earth below. Perceived orientation suddenly changed from floating inverted in the payload bay with the globe of the Earth "above" to hanging from a handrail with the surface of the Earth far "below" (Fig. 13-3).
Figure 13-3. Floating inverted in Shuttle payload bay can cause EVA Height Vertigo (NASA photo)
Height vertigo reports have also come from astronauts egressing from an ISS airlock through an Earthward facing hatch, or while standing in foot restraints on the end of the Shuttle remote manipulator arm, or while hanging on the end of a crane used on the Mir station to transfer crew from one module to another (e.g. Linenger, 2000).
Some of the afflicted have extensive parachuting or rock climbing background, so it is hard to think that acrophobia is a contributing factor. In many respects the phenomenon resembles physiological height vertigo (Brandt et al., 1980) that people describe on Earth when standing at the edge of a cliff or the roof of a tall building. However, some astronauts say they also experience enhanced awareness of the spacecraft's orbital motion, and the sensation that both they and the entire vehicle are falling toward Earth. In some cases, the compulsion to "hang on for dear life" for fear they will fall to Earth is disabling. The most common etiologic factor is that the Earth's surface is perceived as beneath the body, rather than as a blue planet floating above. Veterans say the best defense against EVA height vertigo is to look at their hands, and concentrate on the vehicle as the frame of reference. Changing relative body position so the Earth is "above" should also be effective (Oman, 2002).
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