We are now very familiar with the fact that living in less than the 1-gee gravity humans were born in, has proven to be hazardous to our long-term health: Muscles weaken; bones thin out; stress is placed on blood vessels; and it can take serious effort to recover from the stays in orbit or on the Moon that put us in a low- or no-gravity situation.
And yet, we are actively talking about returning to the Moon for long-term settlement, and doing the same on Mars… knowing that prolonged exposure on either low-gravity world could make it tough-to-impossible to return to the full gravity of Earth. Are we doomed to extended rehabilitation after every such stay, a “recompression” regimen that could take days, weeks… months?
Maybe not. There is another way to simulate a full Earth gravity on a low-gravity world; a real solution that doesn’t require the invention of gravity-generators or any such hand-wavium. It’s based on two very familiar concepts, one of which has been applied in science fiction regularly, while the other is applied on a regular basis… in amusement parks and on aircraft.
We’ve seen spacecraft in 2001, 2010, Babylon 5, in uncounted novels (including some of mine), etc, utilizing the principle of centripetal force to create an artificial “gravity” aboard a craft: Spin a habitation space at a rate calculated to match one Earth gravity, 9.8 m/s2, and your occupants can move about as if they are on Earth. This method works great in a zero-gee environment like space. But a planet’s surface already has its own gravity, and your spinning habitation ring cannot negate that force as it spins; much like people experience on a roller coaster’s loop, you would be heavier as your ring approaches and rises from the ground, and lighter as you reached the top and proceeded back downward.
But there is a solution: Turn your spinning ring to be parallel to the ground and perpendicular to the planet’s existing direction of gravity. Create a carousel.
A carousel revolves on its axis, creating its own centripetal force directed outward. Some amusement park rides take advantage of this, spinning fast enough to cause the ride’s seats or cars to tilt outward with the force caused by rotation. What the seats are, in fact, doing is seeking a point at which the downward gravity and the outward centripetal force combine to create a complementary vector of force. The resultant “down” force is at an angle between the outward centripetal force and the downward force of gravity, but to the human body, it feels just like regular gravity directed outward instead of straight down.
Aircraft passengers are also familiar with this effect: As a plane banks into a turn, the combination of downward gravity and outward centripetal force create a complementary force that becomes “down” to the passengers, allowing them to continue to stand and move about the cabin, even when the plane is tilted at extreme angles.
A habitation carousel could accomplish the same effect by anchoring a base to the planet’s surface, then placing the carousel atop it and spinning it up to the appropriate speed to find the complement between the planet’s lower gravity, and the centripetal force of the carousel, to match 9.8 m/s2 and provide a full gravity to its residents. It would look much like a jumbo version of the Gravitron rides at many amusement parks, maintaining a constant spin to simulate gravity on the inside of the structure.
To the outside, the carousel would look like it had a permanent angle to its body—to insiders, the floor would curve upward (much like the curving floor seen in 2001‘s space station and the spaceship Discovery), but it would curve at an oblique angle. Still, as long as the carousel was large enough to minimize the deleterious effects of rotation on the inner ear (and as long as there were no windows to give away the actual rotational motion), the human body would interpret it as being under one gravity, and one could walk around inside its curved “floor” normally. The hub of a carousel would be a location where inhabitants could go to connect to corridors and move under local standard gravity from one carousel to another, either stacked one atop the other or next to each other as needed or convenient.
This relatively simple method of providing a full Earth gee to human settlers would only require familiar mechanical systems to provide rotation. When the first humans arrived on a planet, or perhaps when their robots arrived before them, it would be their job to set up the carousel, get it rotating, and make it ready for permanent habitation.
A future settlement on a low-gee planet might look like this: Imagine that the outer ring of this settlement was moving at just the right rate of rotation to add centripetal force to the existing gravity for a sum of 1 gee for its inhabitants. The hub would not rotate, providing an easy transition from the rotating ring to the non-rotating center and transiting to the rest of the planet. Moving to the central hub (by ladder, or a car that would adjust its tilt to match the apparent gravity at different distances from the center) would enable visitors to come and go, boarding spacecraft or perhaps walking or riding a car from one such settlement to another.
A system like this would thereby allow settlers to avoid the detrimental effects of low-gravity habitation, and make it easier for them to return to Earth when they were ready, fit and healthy as when they left.