Title: Bare-Tether Missions Paradigm for Exploration of OceanWorlds in Plumes of Icy Moons Enceladus, Europa, and Triton
Abstract: I.-IntroductionThe case of Ice Giants, which only received flyby missions, is of particular interest as regards exploration.There are multiple issues of interest in exploring Uranus and Neptune as different from Gas Giants: Composition is definitely different; rocky mass-percent is much larger; both planetary dynamics and magnetic structures present striking differences (quite relevant for tether interaction); exoplanet statistics suggests Ice types are well more abundant than Gas types.This raises questions about exomoons and exomagnetics…As regards considering tethers for a Neptune mission, beyond just a flyby, there would seem to exist a basic problem with standard methods.The Introduction section of NASA's "Ice Giants" Pre-Decadal Mission Study Report, JPL D-100520, June 2017 (529 pp), in Sec.2.3.3, recalls the multiple studies, over the last half-century, on mission design options for exploration of Uranus and Neptune, ranging from just chemical propulsion to electric propulsion, both with and without gravity assists, and a variety of mission architectural concepts.First, such faraway missions appear quite costly.In the Report the estimated cost of a Flagship Neptune mission was $1.972B, $300M less for Uranus, whereas total budget for both planets was $2B.Secondly, available solar power is practically nil in a large part of the trip, spacecraft capture by chemical propulsion leading to high wet-mass, with scientific load limited to a small mass fraction, and orbital maneuvers quite reduced after capture.Mission concept typically involves an orbiter and an atmospheric probe.Main objectives would be setting constraints to the planet interior conditions, including layers structure, with convective versus stable regions, bulk composition, and improved knowledge of the dynamo process.Those objectives could certainly lead to fundamental advances in planetary science, and efforts to carry out such missions will surely be kept, either accepting high costs or finding new ways to reduce them.In particular, Radioisotope (plutonium-238 oxide) power systems and Aero-capture were repeatedly considered to deal with power and propulsion issues but decayed consideration. II.-A Second Exploration ParadigmPresently, however, there is an important variant of all Giants exploration, involving just Icy moons and the OceanWorld concept, which are not mentioned in the JPL D-100520 Report.In May 2015 the US Congress approved a bill to finance a new NASA program to explore habitability places in the outer Solar System, with internal oceans that might allow simple forms of life.That led to the Icy Moons concept: Life would require source of energy, water, and proper chemistry.The ocean would lie between a rocky bottom, warmed by tidal forces from their giant planets, and a top colder ice shell.In the simplest scheme of hydrothermal (redox) processes, water molecules could lose oxygen to reduced minerals at the floor and, moving upwards, get again oxidized by CO 2 molecules, generating both H 2 O and CH 4 (methanogenesis).OceanWorld missions could include Saturn's moon Enceladus foremost, a plume at its Southern Pole being explored in the Cassini mission [1]; Jovian moon Europa, which might also present a plume, as recently suggested from reviews of Galileo data, apparently overlooked [2]; and Neptune's moon Triton, where geysers, i.e. plumes, were detected at the Voyager 2 flyby.Missions minor in a sense, would involve multi-kilometers long electrodynamic bare tethers, the planetary magnetic field allowing free S/C capture and orbital maneuvers for visits to moons. III.-Bare Tethers and Icy moonsMissions to the Icy Moons may require, indeed, non-conventional concepts.Electrodynamic Tethers, being thermodynamically dissipative, do not use propellant and do not need electrical power but can generate it if convenient, when close to the planet [3].Several geometrical issues involved in bare-tether operation would make it effective for Icy Moons exploration.