Exploring moon Triton

    The NASA program exploring habitability places in the outer Solar System searches for simple forms of life in icy moons of Giant Planets, with internal oceans off thermal equilibrium between cold, iced surface and rocky center warmed, possibly by dissipation of tides from the Planet; H₂O molecules might lose oxygen to reduced bottom minerals, and get oxygen back when moving upwards. This Ocean World concept was confirmed in Jupiter’s moon Europa, and Saturn’s moons Titan and Enceladus, the last one ejecting a local plume through ice, though not Titan, with the Europa-plume case apparently confirmed. In 2017, H₂ molecules, considered sign of life, were detected at the Enceladus plume. We here propose a mission to explore the only large Neptune moon, Triton, which is usually considered an important Kuiper Belt Object as captured dwarf planet. More relevant, here, is that Voyager 2, in 1989, found evidence of Triton ejecting a kind of plume.

      Ice Giants, however, have only received flyby missions, at difference with Gas Giants: An effective orbiter-type mission to faraway Neptune is acknowledged as extremely expensive, trip-average solar power is almost nil, chemical capture leads to high wet-mass, with scientific load and orbital maneuvers very constrained. NASA had set a nuclear reactor in space for on-board S/C power in April 1965 but afterwards, for safety considerations, it always used a Radioisotope Pu 238 power system (helpful in mission Cassini). In a sense constraints trade occurred in 2009, between proposed Triton missions a) with full modern technology but just a flyby (EPSC/Potsdam) and b) captured orbiter though with minimum technology (National Research Council / Planetary Science Decadal Survey). Aerocapture and Radioisotope, for propulsion and power, have been considered for Neptune missions, but decayed consideration.

Magnetic Solution to Power and Propulsion Issues

   Icy moon missions could use, however, thin, multi-kilometer conductive tethers, with no power or propulsion needs, available to a S/C if passing near the planet. The planetary magnetic dipole allows free –thermodynamically dissipative- S/C capture, and afterwards maneuvering: The relative S/C velocity induces a motional electric field E_m (in the tether frame) in the highly conductive plasma around, which polarizes the tether and drives a current I inside, with  I ·E_m > 0,  the field B  then exerting Lorentz drag on the current. Driving, however, was ineffective in tethers standard in the 90`s such as TSS1 and TSS1R, insulated all along, with a conductive sphere large enough at the anodic end, typically implying radius much greater than ambient Debye length and thermal electron gyroradius, strongly reducing electron collection. Bare tethers [1] eliminated sphere and insulation to allow electron collection over a resulting anodic tether segment, as a giant cylindrical Langmuir probe, in the optimal OML (orbital-motion-limited) regime. Laframboise and Parker [2] had proved that OML current to cylindrical probes of convex cross-section and equal perimeter p are equal. Thin tape cross-section will reduce tether mass when compared to an equal p tether with round cross-section. Bare tethers, which made standard tethers rapidly obsolete, will thus be characterized by all 3 dimensions, length, width and thickness, widely differing, allowing particular design easier.

    Lorentz drag decreasing as the inverse 6^th power of radial distance to dipole, capture periapsis would need be very close to the Planet. Keeping current on following capture would hardly affect it, whereas apoapsis could be lowered to finally reach 1:1 Laplace resonance of S/C and moon of interest.

Bare Tether Validation and Use

      Bare-tether analysis was validated in early 2001 in an unintended way [3]. Measurements at the International Space Station involving 3 NASA Centers, NOAA/Boulder. Univ-Houston and SAIC, showed on-board plasma-contactors ejecting substantial electron current as if ISS structure had become very negative. Authors acknowledged in the Abstract that tensioning rods of solar array masts could collect ambient electrons as bare-wire tethers, with modeling “based upon J.R. Sanmartin’s  bare-wire collection theory’, which was incorporated into the complex ISS computer code, Environment Work Bench, including models of Station, orbital motion, earth’s magnetic field, and ionosphere.

    Further, in 2007, Vlasov-code simulations in the high–bias OML regime, at an Unuversity of Michigan PhD Thesis by E. Choinière on the electron density versus radial distance [4] were shown equal to a high approximation when compared [5] with detailed results from the bare-tether theory.

     To date, the bare tether analysis has been applied in considering missions to moons Europa [6] and Enceladus [7], and to Ice Planet Neptune  [8]}

References

[1] JR Sanmartin, M Martinez-Sanchez, E Ahedo, J. Prop & Power 9, 353-360, 1993

[2] JG Laframboise, LW Parker, Phys. Fluids 16 629-636, 1973

[3] EA Bering et al, Paper 2002-0935, 40^th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, 14-17 Jan 2002

[4] E Choinière, BE Gilchrist, IEE Trans on  Plasma Science 35, 7-22, 2007

[5] JR Sanmartin, E Choinière, BE Gilchrist, J-B Ferry, M Martinez-Sanchez, IEE Trans in  Plasma Science 36, 2851-2858, 2008

[6] JR Sanmartin et al, J. Prop & Power 33, 338-342, 2017

[7] JR Sanmartin, J Pelaez,  Acta Astronautica 168, 200-203, 2020

[8] JR Sanmartin, J Pelaez,  Acta Astronautica 177, 906-911, 2020