Science Science Fiction Space

The only good gravity is one gravity

Partial gravity — anything that’s not the same as Earth — used to be a special effects expense that no-one wanted. It’s 2024, we have cheap SFX, so why does everywhere in space still look like it’s either zero-g or on Earth?

After all, there’s just one place in the universe — our home — that has gravity of 9.8m/s2. Of our Earth-like neighbours, Venus has a comfy 0.82g and Mars has a puny 0.38g.

I was all set for a rant about the lack of partial gravity in TV and film science fiction, when I found the fabulously-titled paper Walking on Exoplanets: Is Star Wars Right?. It turns out that Earth-like gravity is incredibly common in the planets we’ve discovered out there in the Milky Way.

Time for a cuppa and a rethink.

SF comfort food 2: One gravity to rule them all

Ballesteros and Luque’s short paper from 2016 had only 2,000 confirmed exoplanets to look at. According to Wikipedia, there are currently (June 2024) 6,140 confirmed exoplanets in 4,527 planetary systems. However, the methods used to find them still favour larger planets over small ones like the Earth.

It’s a shame that no-one smarter than me has automated Ballesteros and Luque’s maths. I’d love to know how things changes as more exoplanet data arrives.

Earth-like surface gravities cluster together in a graph of gravity vs mass for exoplanets and local solar system objects.
Planets with Earth-like gravity cluster together when you compare exoplanets, even when they get pretty big. Image: Ballesteros & Luque.

The cluster of Earth-like surface gravities they identify still covers a broad range, from about 0.3g to 3g. At the top end, that’s the maximum most rollercoasters put you through, and nothing you’d want to live in. The lower limit’s roughly Mars gravity, which is believed to be the lower limit of long-term human comfort. 1 Surprisingly, no-one has done much research in this field since the end of the Apollo programme.

What looks like a get-out for TV and film producers doesn’t really imply that every habitable exoplanet will have gravity in which you could stroll along with the same gait you’d have in Hollywood. You might get away with it on Venus but you’d definitely be bouncing along on Mars.

The problem is that if their extrapolation works for Earth-like exoplanets, presumably it will also work for exo-moons and asteroids. They’ll need to follow a similar distribution of size and gravity, when we’re able to find them.

For example, Earth’s Moon, Jupiter’s moon Ganymede and Saturn’s moon Titan are all about one-sixth Earth gravity (0.14g). Ceres, the largest asteroid, has one-thirtieth of Earth gravity. You’ll feel 0.01g — a hundredth of your Earth weight — on Saturn’s moon Enceladus.

Walking on the Moon

How will you move in these places? You won’t fly off into space unless you’re on a very small asteroid. Even so, NASA’s lunar explorers found themselves hopping and skipping along the surface, and falling a fair bit, too.

Looks like a lot of fun, but a lot of it was down to the cumbersome space suits. Future generations of travellers will have a lot more freedom to move, but it won’t be like walking on Earth.

Gene Cernan knew how to moonwalk.

Gravity and cultural inertia

Now, Star Wars is as much swords-and-sorcery in space as it is science fiction, so I have low expectations. Star Trek, though, is overdue an escape from the shackles of 1960s TV production.

In film, The Martian made a lot of its well-researched science background. All the same, Matt Damon’s Mark Watney never looked like he was light on his feet.

Hard SF fans have flocked to The Expanse and For All Mankind because they pay some attention to real physics. Partial gravity is still a frontier too far, even for these shows. The only thing that’s reliably shown to move strangely is liquids, and that’s because they’re easier to add in post-production.

Few of For All Mankind’s lunar or Martian outdoor scenes show more than an awkward lope. The Expanse made a fuss about the effects of coming to Earth for a Martian, but it’s inconsistent about off-Earth gravity. No-one on the Moon, Mars or Ganymede walks as if they’re anywhere but on Earth. I vaguely recall that 1970s beige-punk series Space: 1999 had a few scenes of astronauts hopping along the surface, but Moonbase Alpha was one-gee all the time.

Literary SF, at least, doesn’t have to worry about special effects budgets or actors trying to learn silly walks instead of acting. SF authors have been having fun with gravity since Jules Verne went to the Moon. Only 12 people have ever walked on another world, so it’s one aspect of SF that remains firmly in the realm of the imagination.

Elves are from Mars, dwarves are from Jupiter

We know very little about the effect of zero-gravity on humans in space for long periods, but it’s not great. We know nothing about living in partial gravity, although writers have tried to imagine how it might affect people after several generations.

It’s generally assumed that weaker gravity leads to weaker bones and cardiovascular systems. Evidence from long-duration stays in Earth orbit bears this out, but it’s zero-g, not half a gravity. SF also makes low-g humans taller too, and vice versa for high gravity. As the old saying goes, space elves are from Mars, space dwarves are from Jupiter.

But genetics and epigenetics are far more complex than this. Epigenetics, in particular, can produce physical and behavioural changes that are passed on to the next generation without being inherited by subsequent generations. 2Yes, I’m vastly simplifying here.

There might be more subtle effects on our sense of balance, our vision, even how food slops around in our stomachs. That’s not accounting for the relativistic effects of living in low gravity (about which I understand nothing except that they might exist).

Toxic planets

The planets themselves will be a toxic challenge. Lunar regolith is known to be hyper-abrasive while Martian soil contains highly-reactive perchlorates. That’s something that conveniently didn’t feature in The Martian. All those reactive chemicals and increased radiation would increase cancer rates and the overall rate of genetic mutation.

I’ve opened up from gravity into wider environmental factors because they will all play out at the same time. First generation colonists will face illness. Birth defects and epigenetics will affect the second and third generations, then we’re on long road of evolutionary selection. For those who survive, ultra-harsh environments often prove to be genetic bottlenecks by restricting the population size. They can embed both adaptations like blue eyes in Nordic people and purely visual differences like the epicanthic fold in Asian populations.

What’s more, colonists to such harsh environments are likely to take advantage of any medical assistance that’s available, even if that means untested drugs and hacking their own genes. Depending on how they get there, they might not even have a choice.

Finding your space legs

I haven’t dipped into magnetic boots and spin gravity because they deserve their own rants, other than to say that I think that spin-gravity space habitats will happen. I’m not so sure about spaceships with spin gravity.

While colonists might at least get used to living in low gravity, space travellers would have a nightmare. Imagine taking a trip from a one-gee spinning space station to the Moon, and having to compensate for not just a different gravity but the change in Coriolis effect. Or moon-hopping around Saturn, from 0.01g on Enceladus to hefty Titan with its 0.14g, then back to its tiny neighbour Hyperion with one-thousandth of an Earth gravity. Astronauts making multiple stops would be constantly, comically misjudging their steps and how to put things down.

At least we don’t have to worry about the hardship of living in high gravity, because everywhere we know that has a higher gravity than Earth is also fabulously lethal. Get that close to Jupiter or Saturn, and the radiation will kill you well before the gravity. Unless you’re a rad-hardened space dwarf.

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