High temperature atomic oxygen testing for ESA’s Envision mission to Venus

Abstract

EnVision will be ESA’s next Venus orbiter, providing a holistic view of the planet from its inner core to upper atmosphere to determine how and why Venus and Earth evolved so differently. The spacecraft will be equipped with a suite of instruments including a sounder to reveal underground layering, and spectrometers to study the atmosphere and surface. The thermal fluxes in orbit around Venus are high, the solar flux at Venus distance is twice its value on Earth (around 2600 W/m2), and the reflected sunlight flux has a similar order of magnitude due to the albedo. Besides, there will also be an aerobraking phase, when a third thermal flux, the aerothermal flux, needs to be considered, with a similar order of magnitude as the two others. These high total thermal fluxes, together with the cold instruments’ requirements and high-power dissipation make the thermal environment a design driver for the mission. The chemistry of the atmospheric environment during aerobraking needs also to be taken into account. Atomic oxygen exists in Venus atmosphere at the altitude range foreseen for aerobraking, with densities of up to 1017 oxygen atoms per cubic metres at aerobraking altitudes. Though the concentrations are small, their accumulation over up to 2000 atmospheric passes lead to total atomic oxygen fluences on the exposed spacecraft surfaces which are comparable to those encountered on typical Low Earth orbiting satellites (upwards of 1021 atomscm-2). This drives the selection of spacecraft materials on the most exposed surfaces. Whilst significant data exists about atomic oxygen erosion effects on materials for Low Earth Orbit, there is little knowledge about high temperature atomic oxygen effects on materials under environmental conditions representative of the Venus atmosphere. To analyse the potential risks for the Envision mission, and to assist the designers in selecting the best materials, an atomic oxygen test campaign is being performed in the ESA ESTEC atomic oxygen ground-based simulator (LEOX facility). The facility has been adapted so that material samples can be simultaneously exposed to atomic oxygen and high temperatures up to at least 200°C. For the test campaign, the Envision instrument teams and designers of the prospective satellite systems are providing candidate materials to be exposed. This includes a variety of materials to be utilised on the outside of the satellite such as thermal control coatings, optical materials, coatings for appendages and antennas and multi-layer insulation. The paper describes the modifications to the facility and the first results of the testing which has been performed.