After getting their ship secured and ready, the crew will release the hold down clamps on the booster and begin deploying the tether: the Anti-Gravity Assist System that Tom had been testing as he orbited the moon. The tether will be reeled out fifty meters to start with. Then, the small rockets engines mounted on the sides of the Hab will be fired to get the two pieces rotating around the center of gravity, amid the tether, like a bolo.
Since the tether is attached to the top of the Hab, akin to spinning a rope attached to the handle of a filled bucket, the centrifugal force from the rotation will push the crew toward the floor, thereby simulating gravity. There are two ways to alter the amount of centrifugal force and the resultant level of gravity. One is to change the length of the tether, while keeping the spin rate constant. The other way is to keep the length of the tether the same and alter the rate of the spin. Mission planners believed that a spin rate over two revolutions per minute could cause severe dizziness. To keep the spin rate down around one rpm, the tether needed to be 910 meters from the Hab to the center of gravity.
If the Hab and the booster were of the same mass, the center of gravity would be halfway between the two and the tether would then be twice that length. But, if the booster were lighter, this would require the tether to be much longer, as the center point of gravity would be closer to the Hab than to the booster, like the way a teeter-totter’s fulcrum can be shifted when one child is heavier than the other.
This variable tether concept will be especially useful on the return trip to Earth. As Mars’ gravity is only about 38% that of Earth’s, the crew will lose a lot of their physical strength during the 18-month stay on the surface. If they had to make the trip home in Zero Gee, the sudden return to Earth’s gravitational strength could very likely prove fatal. Similarly, going from Mars gravity to Earth gravity, rather than a gradual introduction, is not ideal either. But, by starting off at 38% Earth Normal Gravity (ENG) when they leave Mars, and steadily increasing the gravity effect by about 2.5% per week, the crew will be able to build up their musculature to survive the change.