All four giant planets, far from the Earth and sun and having deep gravitational wells, present propulsion and power mission issues, but they also have an ambient plasma and magnetic field that allows for a common mission concept Electrodynamic tethers can provide propellantless drag for planetary capture and operation down the gravitational well, and they can generate power to use along with or be stored for inverting tether current The design for an alternative to NASA's proposed Europa mission is presented here. The operation requires the spacecraft to pass repeatedly near Jupiter, for greater plasma density and magnetic field, raising a radiation-dose issue that past analyses did take into account; tape tethers tens of kilometers long and tens of micrometers thick, for greater operation efficiency, are considered. This might result, however, in attracted electrons reaching the tape with a penetration range that exceeds tape thickness, thereby escaping collection. The mission design requires keeping the range below thickness throughout, resulting in an orbit perijove only hundreds of kilometers above Jupiter and tapes a few kilometers long. A somewhat similar mission design might apply to other giant outer planets. Copies of this paper may be made for personal and internal use, on condition that the copier pay the per-copy fee to the Copyright Clearance Center (CCC). All requests for copying and permission to reprint should be submitted to CCC at www.copyright.com; employ the ISSN 0748-4658 (print) or 1533-3876 (online) to initiate your request.*

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Abstract-Deorbit, power generation, and thrusting performances of a bare thin-tape tether and an insulated tether with a spherical electron collector are compared for typical conditions in low-Earth orbit and common values of length L = 4−20 km and cross-sectional area of the tether A = 1−5 mm 2 . The relative performance of moderately large spheres, as compared with bare tapes, improves but still lags as one moves from deorbiting to power generation and to thrusting: Maximum drag in deorbiting requires maximum current and, thus, fully reflects on anodic collection capability, whereas extracting power at a load or using a supply to push current against the motional field requires reduced currents. The relative performance also improves as one moves to smaller A, which makes the sphere approach the limiting short-circuit current, and at greater L, with the higher bias only affecting moderately the already large bare-tape current. For a 4-m-diameter sphere, relative performances range from 0.09 sphere-to-bare tether drag ratio for L = 4 km and A = 5 mm 2 to 0.82 thrust-efficiency ratio for L = 20 km and A = 1 mm 2 . Extremely large spheres collecting the short-circuit current at zero bias at daytime (diameters being about 14 m for A = 1 mm 2 and 31 m for A = 5 mm 2 ) barely outperform the bare tape for L = 4 km and are still outperformed by the bare tape for L = 20 km in both deorbiting and power generation; these large spheres perform like the bare tape in thrusting. In no case was sphere or sphere-related hardware taken into account in evaluating system mass, which would have reduced the sphere performances even further.

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