Astronauts Maintain Accurate Distance Perception in Microgravity

Even people with little coordination can, while standing on Earth, readily tell where their arms and legs are moving and whether they are in motion, falling or upright. This relies on information supplied by several useful bodily systems. Remove gravity, put on a virtual-reality headset or travel in a moving car, however, and the sensation can become rather disorientating.

Fortunately, astronauts are remarkably capable of adjusting to microgravity while living aboard spacecraft in orbit. A new study reports no obvious decline in their capacity to assess movement either in space or once they have returned to Earth.

Microgravity and astronauts’ perception of movement

The almost complete lack of gravity changes how astronauts perceive motion. As they move through their confined, weightless surroundings, they can feel as though they are travelling faster than they truly are. Nevertheless, the research showed that they can accurately estimate how far they have travelled along a corridor in visual simulations, both during spaceflight and after coming home.

"Based on our findings it seems as though humans are surprisingly able to compensate adequately for the lack of an Earth-normal environment using vision," says Laurence Harris, a sensory physiologist at York University in Toronto and the study’s senior author.

Our bodies detect changes in position partly through the movement of fluid within several curved channels deep in each ear. This is known as the vestibular system, and it gives us a sense of acceleration, tilt and rotation.

In space, microgravity interferes with the vestibular system because it removes information the system would usually receive: Earth’s continuous gravitational pull, which indicates which direction is up.

Researchers believe this effect can be reproduced on Earth by having people lie either on their backs or on their sides. In these horizontal postures, the fluids in the ear channels are not quite as precise at assessing position.

Ordinarily, the vestibular and visual systems work together to determine how far, how quickly and in what direction we have moved. In orbit, however, astronauts experience less ‘load’ on the tiny particles moving through the inner ear fluids, and may compensate by relying more heavily on visual cues.

Testing distance estimates during spaceflight

Harris and his colleagues wanted to establish how this could affect astronauts’ judgement of distance travelled in a visual simulation during and after long-duration spaceflight. They also examined whether changing body position produced a comparable effect.

The team assessed 12 astronauts - six men and six women - before, during and after their year-long International Space Station (ISS) missions. Their results were compared with those of 20 controls who remained on Earth.

Astronauts completed the tests while sitting upright and while lying on their backs before launch and after returning to Earth. During their missions, they floated freely while secured to a backrest.

The health effects of spaceflight are a major concern for space missions, especially as agencies including NASA and nations including China prepare to send astronauts to Mars.

Distance perception after returning to Earth

Soon after their flights, the astronauts marginally overestimated distance in the visual task when lying down, compared with sitting upright before their mission. Those differences subsequently faded.

Overall, though, the astronauts’ estimates of distance travelled through a virtual hallway did not alter significantly while they were in space, one week after landing or 85 days after their return. The result aligns with earlier findings by Harris and colleagues indicating that astronauts do not struggle to orientate themselves in space.

The researchers say this is encouraging for emergencies, in which astronauts may need to move swiftly through the ISS to find equipment or get to escape hatches.

Harris says it was "very reassuring" that astronauts could judge "quite precisely" how far they had travelled through virtual environments after only a few days, or even several months, in orbit.

Naturally, this was a small study involving a limited number of highly trained people. Further research is needed to clarify the vestibular system’s role in estimating forward movement and to determine whether gravity affects perception in other ways.

The study has been published in npj Microgravity.