Extended-body effects and rocket-free orbital maneuvering

Abstract

Celestial mechanics is most often concerned with the motion of pointlike, or at least sphericallysymmetric, masses. However, the orbits of more-complicated extended bodies can differ considerably from their point-particle counterparts. This is especially so if such a body-say a spacecraft-is able to actively modulate its shape. Except where forbidden by symmetries and their associated conservation laws, shape-changing spacecraft can in fact adjust their orbits arbitrarily (though slowly) without the use of reaction mass. We show that such maneuvers can be understood using elementary methods and without constructing any detailed model of a spacecraft's interior. In many cases, all of a spacecraft's design can be abstracted to the specification of a single time-dependent eigenvalue of its quadrupole moment. Appropriately varying this eigenvalue allows the energy and eccentricity of an orbit to be increased or decreased and its apses to be rotated arbitrarily. In other contexts, we show that extended-body effects can be used instead to stabilize unstable orbits.