There is a remarkable clustering of surface gravity levels in our solar system

Friday, December 17th, 2021

There is a remarkable clustering of surface gravity levels in our solar system:

All bodies with 9% to 250% of Earth gravity cluster near Earth, Mars, or Moon gravity. Those 3 gravity levels seem like the only levels available for us to live in this solar system. I stumbled onto this only after 34 years in aerospace.

Surface gravity clustering in our solar system

Four other planets have within 12% of Earth gravity. But all four have extreme temperatures and atmospheric pressures. And all but Venus have two to three times Earth’s escape velocity. Returning to Earth would be hard.

The eight smaller bodies near Moon or Mars gravity seem more practical. They also have much lower two-way delta-Vs. So, a key question for living beyond Earth is whether lunar or at least Martin gravity will let us avoid health problems like those we have seen in sustained free fall. But we have no health data between 0g and a full 1g!


Early in the space age, most planners assumed rotating crewed facilities to provide Earth-level artificial gravity. Apollo flights were planned to last only 6 to 12 days, so interest in artificial gravity faded after the Gemini 7 crew spent 14 days in free fall. Even 4-, 8-, and 12-week crew stays on Skylab caused few health issues. But crews who spent 6 to 12 months on Salyut, Mir, or ISS have had significant degradation of their bones, muscles, fluids, eyes, brain, and immune response. Exercise, diet, and drug “microgravity countermeasures” have slowed these trends but have not stopped them, despite decades of countermeasure refinements.


If we find that lunar gravity is enough for long-term health, humanity may expand to the six largest moons plus Mars and Mercury, and not just our Moon. If we need Mars gravity, we might settle Mars and Mercury, but not any moons. But even in 1g, exercise is critical. Any reduction in gravity is likely to require more exercise.

If we do need sustained gravity at levels higher than that of Mars, it seems easier to develop sustainable rotating settlements than to terraform any near-1g planet. And rotating settlements offer lower gravity inboard. A key attraction of such settlements may be the easy access to a wide range of gravity levels.

We’ve been talking about rotating settlements for a long, long time.


  1. Bruce Purcell says:

    Has anyone tried having a group of people live at 1.1 g for a while?

  2. Bert says:

    An idea I’ve been thinking about (which might not work out; what do I know?) is that astronauts in a space station spend their sleeping time in a small centrifuge. That should reduce the problem by one third. The centrifuge could also be set to deliver a bit more than 1 g to make a bigger difference.

  3. Sam J. says:

    Bert says, “…astronauts in a space station spend their sleeping time in a small centrifuge…”

    That’s a good idea. We know that centrifuges cause disorientation but there’s no studies I know of where this is done long term. I think it likely that you could slowly up the speed of the centrifuge ad get used to it. NASA really needs a space station that rotates and has a super long tether or beam with two habitats that can vary the speed and distance between centers to see what happens.

    I think if we are going to live in space habitats are the way to go. You can build them a little at a time and using non-terrestrial sources of materials you could build big and robust.”Have you heard of this amazing thing?

    Earth to Jupiter Via Magsail in 21 Days and Neptune in 18 Weeks

    Does this really work? The big problem is it can only go out from the center. What we need is something just like this that can tack like sailboats. THAT would be magic. We could expand into the solar system easily.

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