So in the story I'm trying to write, I'm having trouble getting the gravity on my fictional to seem realistic, and I would like to know if it seems technically possible?

A group of astronauts travel through a portal in spacetime and land on a small moon with earthmoon-like gravity, but an oxygen rich atmosphere. A group of technically advanced aliens, thinking that the humans are native to the planet but aren't properly evolved to suit it, gather the astronauts and genetically mutate them so that their bodies are better adapted to the atmosphere. Over thousands of years, after the adapted humans see the aliens as a threatening force and destroy them, they are left with dense bones largely constructed from a superdense metal that is rich in the soils and minerals of the moon. (as well as stronger muscles and lungs and other minor physiological changes)

My question is this, with an environment supporting moon like (0.16 g) gravity but also a breathable atmosphere, would these newly dense humans be able to walk on the surface like every-day humans do in earth, their density simulating earth gravity, or would they simply move the same way humans already do on the moon, because weight doesn't matter in low gravity, even while in an environment with oxygen? Yes or no and why or why not?

Thanks folks!

  • $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Commented May 8, 2017 at 7:11
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    $\begingroup$ The question I'm interested in is how this small, light world has actually retained a breathable atmosphere. $\endgroup$
    – Monty Wild
    Commented May 8, 2017 at 7:16
  • $\begingroup$ Agree with @MontyWild. 0.16 g is not sufficient to retain an oxygen-rich atmosphere, so you need some mechanism other than gravity to explain that. Compare Graphs of escape velocity against surface temperature of some Solar System objects showing which gases are retained linked from Wikipedia: Atmospheric escape. $\endgroup$
    – user
    Commented May 8, 2017 at 7:30
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    $\begingroup$ You should get rid of most of the middle paragraph — humans modified to be extra dense is all we need. The Title doesn’t make sense. Put the question at the top of your post. $\endgroup$
    – JDługosz
    Commented May 8, 2017 at 8:09
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    $\begingroup$ Stronger muscles makes little sense. With the reduced gravity unused muscle mass should degenerate. Research also indicates 0.16G is far to low for human pregnancy. $\endgroup$
    – Mormacil
    Commented May 8, 2017 at 8:28

3 Answers 3


Acceleration remains 0.16 g

No, your humans will not walk about as they would on Earth. Your aliens have actually made it harder for them to move about.

The reason is that you have normalized their weight, that is to say the force on the ground and thereby restored friction to what it is on Earth. But you have done so by increasing their mass, and not done a thing about the acceleration.

So the astronauts will still need to resort to that bunny-hop mode of moving that the astronauts on the Moon used used, but now they will have to put much more effort into moving their extremities and body around.



The funny thing with gravity is that big and tinny masses fall with the same acceleration. (when ignoring air resistance) The equality between gravitational and inertial mass is suprising, but true. So is your humans are twice as dense, they experience two times higher force, but since their inertia is two times bigger too, they still fall with the same acceleration as normal humans.

Oxygen atmosphere is irrelevant in this matter, except that the air resistance limits a bit their jumping height.


Humans in this environment would move like humans on the moon. Their bones are heavier, but their muscles are also stronger. Given what seems to be an equivalent strength to weight ratio, these stronger, heavier humans would still move with the characteristic bounding gait of men on the moon, though atmospheric drag would lead to their leaps being less of a perfect parabola, resulting in a steeper descent than ascent.

Gravity operates as a field of constant acceleration irrespective of mass, the effective force between two objects being proportional to the combined mass. Lower gravity means slower falls. (It also usually means less atmosphere, since fast molecules can escape more easily.)


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