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.

Notable Replies

  1. I've always heard people say on the internet things like "you owe me a new monitor" and such, but this video finally brought it home un-metaphorically. The last astronaut caused my wife spit out a prodigious amount of coffee, completely soaking my lappy, both keyboard and screen.

    In other news, Macbook Pros can apparently get pretty wet and still work fine. And it turns out coffee is a pretty good screen cleaner.

  2. As someone who went to Space Camp and got to walk in a moonwalk simulator, I can say that walking in 1/6 G is a LOT harder than you'd imagine. You have hardly any traction and the instinct to push against the ground to gain more just sends you flying upwards at a precipitous pace.

  3. I think it's pretty clear you'd end up scooting forward on your toes in a skipping motion. Legs kind of apart to the front and behind, paranoid as hell that you'd fall and break something in the massive backpack that keeps you alive. Just like the astronauts did!

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