Why astronauts fall

If you've ever watched this video, you might wonder whether an astronaut's suit is too ungainly to be graceful, or alternatively, if astronauts might just lack coordination.

A study out this week in PLoS ONE suggests otherwise. It's gravity that trips up an astronaut skipping from place to place on the moon.

Here on Earth, unless you happen to be an astronaut, you are exposed to approximately 9.8 m/s2, or 1 g of gravity from the moment of your conception until you die. Astronauts who travel to the International Space Station (ISS), which is in orbit around Earth at a distance of 205 miles (330 km) experience an effective zero-g because the station is in freefall. The pull of the Earth on the ISS and its inhabitants is about 89% of what you feel at sea level.

On the moon, astronauts are tugged on by a gravitational force of about 1.622 m/s2, which is just about the amount the researchers found necessary for a person to figure out which way is up. By spinning people around in a centrifuge at various gravity settings, they determined that our brains need a force equal to 15% of Earth's gravity to tell up from down.

Interestingly, only 20% of the perception of being upright is influenced by gravity. The rest is made possible by cues from the visual, vestibular, and proprioceptive systems. And, in low gravity environments more influence is placed on the visual inputs. This might explain why when asked about their perception of gravity, astronauts reported that they "experienced no disorientation" on the lunar surface, but still proceeded to trip and fall.

The researchers conclude: "This observation may partially explain the instability of moonwalkers but is good news for future missions to Mars." But, only because Mars has a higher gravity.

And, I leave you with a final video of an astronaut falling down.