Standing on Nothing Much

On Earth the horizon is about 4.8 kilometres away, give or take, depending on how tall you are. You can’t really feel that. It’s just where the land meets the sky, somewhere in the middle distance, and you don’t think about it much.

On Ceres, the horizon is just over a kilometre away. And because Ceres is only 945 kilometres across, the curve of it is actually visible. The ground falls away from you in every direction. You’d be standing on something you could clearly see was a ball, and you’d feel enormous on it, the way you feel enormous standing on top of a large boulder, except the boulder is 945 kilometres across and you somehow still managed to feel too big for it.

I find that image hard to shake. It’d feel less like a planet and more like a very large piece of furniture.

And of course you wouldn’t be briskly walking anywhere.

This is the bit that nobody really talks about when we discuss low gravity. We get the headline, the Moon is one sixth of Earth’s gravity, you’d weigh less, you could jump higher, but the practical reality is weirder than that, and starts falling apart much faster than you’d expect.

Walking works because gravity is impatient. It pulls you back to the ground quickly enough that your feet stay roughly in charge of events. Loosen gravity’s grip and that stops being true. On the Moon, astronauts had to develop a sort of loping bounce, a shuffling canter, because anything resembling a normal stride left them airborne for too long. The footage looks funny. For them it was just frustrating.

On Ceres, even that wouldn’t get you far. Gravity there is 2.8% of what you feel here. A walking stride that lifts you a centimetre off the ground, barely anything, something you do a thousand times a day without noticing, would leave you floating for several seconds, just waiting for the ground to come back and collect you. And that’s just walking. A skip, which on Earth sends you maybe twenty or thirty centimetres into the air, would launch you seven or eight metres up on Ceres, and you’d spend something like fifteen to twenty seconds getting back down, tumbling with no way to steer or correct yourself, hoping you land feet-first. Running would turn into a series of uncontrolled long jumps, each stride launching you into a long shallow arc, gliding several metres between footfalls with no traction in mid-air and no good answer to the question of how you stop.

What actually works, apparently, is a flat-footed shuffle with as little upward push as possible. A penguin waddle. Or you stop pretending to walk and just grab things, which is how astronauts get around outside the ISS. On Phobos, the small lumpy moon of Mars that we may one day visit, the working plan involves tethers and anchor points, because tripping, or sneezing inside a pressurised suit, could send you slowly tumbling away from the surface with nothing to push against on the way back.

That visible curve would be right there the whole time. A constant reminder that the thing you’re carefully clinging to is not, in any meaningful sense, trying to keep you.