II. Tether History The tether has been flown on manned missions in the past (13, 50). There were tether experiments carried out on the last two Gemini missions. Gemini XI used a 100 foot long tether between it and the Agena to conduct artificial gravity experiments at two different rotation rates. Gemini XII used a similar tether to conduct gravity gradient stabilization experiments. Both sets of experiments were extremely successful and the results warrant further study. The next planned tether experiment is a proposed Italian subsatellite designed to investigate the upper atmosphere from LEO. The concept is to deploy the satellite from the orbiter on the end of a 50 km (over 30 nautical mile) long tether and sample the upper atmosphere. Engineering objectives include the behavior of a very long tether in space, dynamics of the mass-tether combination, and the testing and operation of a winch in space. This is an important proof of concept experiment that should lead to increased emphasis on tethers in the future (56). III. Tether Physics A. Gravity - Actual and Artificial A long object will tend to stabilize with its long axis pointing at the center of the earth. The combination of forces will lead to small artificial gravitational forces on either end of a stable platform. This artificial gravity is equal to 1.5 times the change of gravity gradient in LEO at about .0004 g per km. The extra third is due to centrifugal force gradients induced by the end masses attempting to pull into different orbits away from the center of mass. The presence of these forces also causes a tension on the tether. This force doubles as the length of the tether doubles (13, 56). In addition to a stationary tether, artificial gravity can also be induced by rotating the mass-tether combination. This has been proposed in several space station applications including Dr. Gerard K. O’Neill’s original High Frontier proposal (13, 37, 56, 84, 89). This proposed using rotating hydrogen tanks in a tethered facility 200 meters in diameter at a rotation rate of 3 rpm to induce internal gravity for habitation from .7 - 1.0G.
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