Evolution of orbits about comets with arbitrary comae

Abstract

AbstractSpacecraft and natural objects orbiting an active comet are perturbed by gas drag from the coma. These gases expand radially at about 0.5 km/s, much faster than orbital velocities that are on the order of meters per second. The coma has complex gas distributions and is difficult to model. Accelerations from gas drag can be on the same order of gravity and are currently poorly understood. Semi-analytical solutions for the evolution of the Keplerian orbital elements of a spacecraft orbiting a comet using simplified drag and coma models are derived using a Fourier series expansion in the argument of latitude. It is found that the mean element evolution is only dependent on the zeroth- and first-order terms of the Fourier series expansion. For an arbitrary, inverse-square, radial, perturbing force, there are no frozen orbits; however, the argument of pericenter has a stable equilibrium and an unstable equilibrium and the angular momentum vector of the orbit is constant. Furthermore, the radius of the orbit at two specific angles relative to the ascending node is preserved. The evolution of the orbit is governed by the argument of pericenter, resulting in orientations that raise and lower the radius of pericenter and implying safe and unsafe orbit orientations for spacecraft operations.