line as tension within the tether is minimal. The line tension under steady state conditions should be less than 60 lb for a 5 km length. The allowable voltage level for a power conducting cable in the ionosphere is not presently known. Estimates from NASA personnel range from 200 volts to 4800 volts. It is known that higher voltages will suffer significant power losses due to electron discharge into the surrounding plasma unless adequate safeguards are taken. Potential solutions include laminated layers of insulation such as kapton or kevlar and encasing the insulation in a conducting shield. There is also the problem of the electromagnetic interactions of the current flowing through the cable with Earth’s magnetic field. There may be a need for an electron (or ion) collector or generator at one end of the cable in order to provide two-way current flow. A number of tether experiments are being planned using the shuttle which should provide information on the problems and possible solutions. Typical conducting cable characteristics for the two voltage extremes (200 volts and 4800 volts) are shown in Table 1. It is assumed that the 5 km three phase AC system has the same transfer efficiency (52.8%) as that of the microwave system. It can be seen that the feasibility of a power conducting cable is determined in part by the allowable voltage. If the cable is limited to 200 volts from plasma considerations, each conducting line must assume a quite substantial diameter (1.79 cm) and mass (3395 kg), even when allowing 47% of the power to be lost as heat. Insulating layers of kapton/teflon or kevlar and additional tether structural components would further increase the cable mass. Thus for low voltages a microwave power transmission system with a low mass structural tether would be preferable to a conducting tether, especially for distances greater than 5 km. CONCLUSIONS Power transfer from a remote nuclear satellite to a space station can be accomplished with either a conducting cable or microwave power transmission. The obstacles facing high voltage conducting tethers due to ionospheric discharge effects have not yet been fully identified. The feasibility of complete tether electrical insulation and the identification of other tether instabilities including current interaction with the terrestrial magnetic field will probably remain in doubt until actual space shuttle electrodynamic tether experiments are completed. (The initial shuttle tethered satellite system [TSS] experiment is now scheduled to fly in December 1987.)
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