I'm working on a story that involves a lot of interdimensional travel, but I know little to nothing about building the physics of entire planets. All I know about this one particular planet is that its gravity is slightly lower than Earth's- just enough to allow the characters to do more intense stunts than would be possible on Earth.

One character has a sort of mini-jet that's designed similarly to a delta wing- small, triangular, and designed to go fast more than anything. At some point, he needs to take this jet through a rift that would land him on a much smaller planet, likely with much lower gravity and atmosphere density. How would this immediate change affect the plane? My first thought is that it would be unable to stay stable in midair and just plummet, but maybe its speed might counteract that.

I have tried looking this up through google, but all the websites I find include a lot of mathematical equations and specific physics and airplane jargon that I can't understand. My brain just refuses to comprehend algebra. Pretty much just explain this to me like I'm ten.

  • $\begingroup$ When talking about lower gravity and atmosphere density, on which scale are you putting yourself? 0.5x the old one? 0.1x? I ask because gravity loss and lower atm' density gets opposite results in the way of lifting your plane up, I believe. $\endgroup$ Commented Sep 1, 2022 at 8:54
  • $\begingroup$ More density = more lift per m2 of wing, also more air resistance and more lift for balloons. Less gravity = less lift for balloons but less weight for heavier-than-air aircraft to counter with wings. But some combinations are impossible for planetological reasons, say you can't lower gravity to 0.1g with room temperature for a planet and expect it to retain atmosphere. $\endgroup$
    – Vesper
    Commented Sep 1, 2022 at 10:51
  • $\begingroup$ Yes, flying a jet into a different atmosphere poses serious risks, up to finding out that there is no atmosphere beyond the portal = SPLAT. A smaller planet will either be colder (namely down to liquid oxygen or below) or has less dense atmosphere because of solar ionization and related processes, down to retaining zero pressure at ground level. Look at the Moon for a rather close condition to "much smaller planet", merely a 1/81th mass of Earth and no atmosphere. $\endgroup$
    – Vesper
    Commented Sep 1, 2022 at 10:55
  • 3
    $\begingroup$ what-if.xkcd.com/30 xkcd did this one. $\endgroup$
    – user86462
    Commented Sep 1, 2022 at 11:12

1 Answer 1


Lower atmospheric density

Lower atmosphere density will result in less lift, however you can counteract this with air speed or velocity. As a general rule the lower the air density the faster you will have to go to produce lift and stay airborne.

Lower gravitational strength

A lower gravitational strength will result in your aircraft having a lower weight and it will therefore require less lift to stay airborne.

In summary as long as the planet has atmosphere sufficient for producing lift at realistic speeds and a strong enough gravitational force to keep that atmosphere then heavier than air flight should be possible.

  • 1
    $\begingroup$ This is a good answer, but it's worth pointing out that there is a minimum lift required to keep an airplane in the air in any trajectory other than a ballistic trajectory. Thus, if there isn't enough surface area on the wings, the jet will simply fall to the ground in a ballistic trajectory (assuming the nose is kept level, might do that, might have some rudimentary control due to thrust, like a rocket) with a burst of speed due to the lower air density. If we handwave those issues, what Stargazer explained is dead on. +1 $\endgroup$
    – JBH
    Commented Sep 2, 2022 at 15:18

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