Video: Low gravity makes astronauts prone to falling over

Moonwalking astronauts often lost their balance, but they weren’t clumsy – it was gravity’s fault. It turns out that the moon’s gravity is not enough to help them distinguish up from down.

Laurence Harris of York University in Toronto, Canada, and his colleagues spun volunteers on a giant rotating arm to simulate different strength gravitational fields. As the volunteers spun, they saw images of a landscape or the letter “p”, which they read as a “p” or a “d”, depending on which way they felt was up.


The study shows that humans need to feel at least 15 per cent of the gravitational force on Earth to figure out which way is up. The moon’s gravity is 17 per cent of Earth’s, but it is also a strange environment; low gravity and unusual scenery explain why astronauts fell down even though they didn’t report feeling out of sorts.

It’s good news for future missions to Mars, where astronauts will experience gravity about 38 per cent of that on Earth. Knowing which way is up will help Mars visitors stay safe, correctly estimate the angle of hills or turn the correct way towards an escape hatch. It could also be of use for more prosaic purposes.

“Suppose you have a North American-style light switch, where pressing the ‘top’ portion of the switch indicates ‘on’. If we don’t know which way ‘up’ is off Earth, how do we know if the switch is ‘on’ or ‘off’?” says Harris’s colleague Michael Jenkin. “Of all the challenges associated with manned missions to Mars, the gravity field should provide a sufficiently strong cue to allow for a good sense of the direction of up.”

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0106207