We already know that living in space is no picnic: astronauts frequently suffer sickness and disorientation.
Now it seems that zero-gravity can also adversely affect their ability to judge size and distance. The new finding, from a study by French and US researchers, 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," Gilles Clément of France's National Centre for Scientific Research in Toulouse, the lead author on the paper, told New Scientist.
This, he suggests, 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%. 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.
That might explain some space mishaps, like an accident aboard Mir in 1997, when an unmanned supply craft called Progress punched a hole in the space station. The crash has been attributed in part to difficultly interpreting the craft's velocity from images on a monitor.
Space station tests
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.
That means, unless steps are taken to address the problem of perception without gravity, repeats of incidents such as the Mir collision could be likely.
"This issue is going to become more important as we try to land on Mars or on the Moon after very prolonged space flights," says Donald Parker of the University of Washington in Seattle, who has studied the impact of microgravity on spatial awareness and movement.
But combating distorted perspective might be tricky. Tests could be used to calibrate computer displays for each astronaut, says Clément. But adjusting vision for astronauts out on spacewalks or exploring other planets could be more difficult.
However, the brain might be able to adjust to such visual distortions with time. Clément is currently exploring that possibility aboard the International Space Station, where 10 astronauts are being tested, each over a span of six months, to look for changes in vision.
In addition to the cube test, astronauts are asked to estimate the distance to a point in a number of photographs. The first data is expected to return with NASA's next shuttle mission to the ISS in late November.