An object in space has an induced charge on its surface. Due to the nature of the space plasma and the surface charges on the object a sheath develops around it, e.g. around a spacecraft. The electrostatic sheath is a layer of net space charge that screens the distant plasma from the surface charges.
The sheath confines all of the significant electric fields, therefore its geometry relative to the spacecraft dimensions is of great importance for estimating the interaction between the spacecraft and the surrounding plasma including the collection and emission of particles. Conversely, ambient charged particle fluxes (ions and electrons), photoelectrons, backscattered and secondary electrons from the surfaces as well as actively emitted particles from, e.g. thrusters are taken into account together with spacecraft geometry and velocity, and any external magnetic fields when determining the properties of the sheath.