Abstract
The Dragonfly mission to Titan is proposed to use an MMRTG (Multi Mission Radioisotope Thermoelectric Generator) to supply electrical and thermal power to the spacecraft. As no radioisotope powered spacecraft has ever been flown to Titan before, the effects of the generator's radiation field on the surrounding environment are unknown. MCNP neutronic simulations were run on a model of the RTG in a coarse-grained Titan environment and the results analysed. Repurposing the model with the addition of electron detectors allowed for an attempt at determination of the degree to which the radiation fields from the generator may ionize the surrounding atmosphere and generate interference with the lander's atmospheric conductivity measurements. Following the simulations, an experiment was conducted, the apparatus primarily comprising a pressure vessel which was evacuated, filled with an analogue Titan atmosphere, and in which a neutron source was suspended. Readings were taken from a neutron detector in a moderator block below, with the source at various height within the chamber. Neutron attenuation by the atmosphere was not found to be statistically significant, and absorption by surface material was found to be most likely minimal, refuting and tentatively reinforcing the respective modelling conclusions. Ultimately it was found that neutrons from the MMRTG are unlikely to be a problem for Dragonfly's measurements on the surface of Titan, and neutronic modelling should have fidelity sufficient to determine the magnitude of minor effects. The chamber was then filled with various blends of ultra high purity nitrogen and a mixture of 95\% nitrogen / 5\% methane, simulating Titan's atmosphere with up to 50\% relative methane humidity, as well as an Earth atmosphere for comparison, with a source of gamma rays suspended therein. Readings were taken from a plate immersed in the atmosphere at the top of the chamber connected to a conductivity probe, with the source again at various distances from the plate. Gamma-induced conductivity was not found to be significant at a distance from the source equivalent to that on the surface of the hull segment on Dragonfly containing the radioisotope generator when gamma ray energies and field geometry were corrected for, however ion buildup in the experiment chamber indicated the possibility of similar buildup within the hull segment itself. The effect of methane in the atmosphere was statistically indeterminate, reflecting earlier simulation results, however molecular ions were shown more likely to be the predominant carriers of charge than the free electrons measured in the simulation.