London, September 23 : A new study by French and US researchers has suggested that zero-gravity can adversely affect astronauts' ability to judge size and distance, which may make them bad drivers.
According to a report in New Scientist, the new finding may have implications for the way astronauts pilot spacecraft and perform tasks while on spacewalks.
NASA has long suspected something goes wrong with our visual perception when in space. Some of the Apollo astronauts reported difficulties judging distance while on the moon, for example: far-off rocks and features seemed closer than they really were.
It is also well known that space-shuttle pilots perform better with flight simulators and training aircraft than they do landing the shuttle after real missions.
Some researchers have suggested that these effects could be the result of confinement or the absence of easy landmarks, such as trees or buildings, but the new study pins the blame on the lack of gravity.
Humans orientate in 3D by using otoliths, small crystals of calcium carbonate and protein that shift on hairs in the inner ear. Forces acting on these grains as a person moves mean they can sense acceleration and gravitational pull.
The researchers suggested that living in zero gravity would interfere with this process.
"When you arrive in microgravity, you don't have this system any more telling you whether you're tilted," said Gilles Clement, the lead author on the paper.
This impacts an astronaut's sense of perspective, causing them to misjudge common markers that are used to perceive size and distance, like an object's vanishing point. This would render them unable to accurately assess an object's dimensions.
To test this idea, the team sent subjects aboard European Space Agency flights of a 'vomit comet' - an Airbus plane that repeatedly adopts a parabolic trajectory to create brief 20-second bouts of microgravity.
Donning virtual-reality goggles, subjects were randomly positioned in mid-air.
Using a hand-held trackball connected to a nearby computer, they were asked to adjust the line drawing of a cube that was distorted in one dimension.
In normal gravity, test subjects could adjust the depth or height cube so all sides appeared to be the same length. But without gravity, however briefly, they did not perform as well.
Instead, the adjusted cubes were still distorted - sometimes by as much as 4.5 percent. Overall, objects viewed in free fall appeared taller, thinner and shallower than they did under normal gravity.
This distortion of vision could also make it difficult to judge the velocity of objects, which humans achieve in part by tracking how fast an object changes in size as it moves.
The fact that the microgravity tests only ran for 20 seconds at a time suggests that astronauts' problem with perception is a physiological issue, and not a result of adaptation to enclosed environments.