Charging of fast spinning spheroidal satellites in sunlight

Abstract

We present models for a fast spinning, dielectric coated, spheroidal (prolate or oblate) spacecraft charging in sunlight. This work is a generalization of previous treatments of sunlight charging of spherical satellites. The main difference is that the spacecraft geometry can be characterized by a shape parameter that sets the aspect ratio of the spheroids. The models are based on an expansion of the Laplacian potentials external to the spacecraft surface in terms of products of standard Legendre polynomials, describing the polar angle dependence, and modified Legendre functions of the second kind, representing the “radial” behavior. The potential distributions are discussed relative to the corresponding monopole-dipole and monopole-quadrupole configurations in spherical geometry. A Taylor expansion is developed for the potentials when the shape parameter is large (the spherical limit) and expressions are also given in the opposite limit, when the shape parameter goes to zero. As in the spherical case, the potentials produce photosheath barriers which act to block escaping photoelectrons and lead to current balance, allowing sunlight charging to high negative levels. The sheath barrier location and height are calculated numerically, for a wide range of shape parameters. Contour plots are given to depict the potential distributions for sunlight charging of prolate and oblate spheroids at a representative aspect ratio.