Spacecraft Coating-Induced Charging: A Materials and Modeling Study of Environmental Extremes

摘要

As mankind reaches to explore extreme environments in space, the application of ceramics surface coatings isnincreasing. The 2005 mission concept for Solar Probe used a unique design to achieve the necessary thermal controlnfor a very close approach to the solar corona, including the use of a highly refractory, electrically insulating ceramicncoating over a carbon–carbon composite heat shield. The proposed trajectory takes the spacecraft froma Jovian fly-nby to within 4 solar radii of the sun, spanning 5 orders ofmagnitude in solar radiation and solar wind plasma densitynas well as spacecraft temperatures from <100 K to >2000 K. Using the NASCAP-2K charging modeling program,nthe degree of charging expected for this spacecraft design has been calculated for this range of radiationnenvironments. New measurements of the electron emission and estimates of related properties of the candidatenmaterials, Al2O3, pyrolytic born nitride, and bariumzirconiumphosphate, are presented. Absolute and differentialnsurface charging are found to depend strongly on temperature through increased conductivity at higherntemperatures and on radiation flux through enhanced charge accumulation and radiation-induced conductivity. Asnthe spacecraft approaches the sun, the competition between increased charge dissipation at higher temperatures andnincreased charge accumulation at higher fluxes leads to a maximum in differential charging between 0.2 andn2 astronomical units. Although the spacecraft charging behavior of thesematerials is found to be significant, it is notnsevere enough to endanger the mission, and a number of options exist to mitigate the degree of charging. Among thenceramics considered, the use of Al2O3 coatings is found to minimize both absolute and differential spacecraftncharging.