ANALYSIS OF ORBITAL MOTION AROUND 433-EROS

The Near Earth Asteroid Rendezvous (NEAR) mission spacecraft will arri ve at asteroid 433 Eros in February 1999. Following an initial period of characterization, the science phase of the mission will commence, a nd unprecedented information concerning an asteriod's shape, mass, den sity, composition and rotational dynamics will be sent to Earth for an alysis. It will also mark the start of orbital operations about the sm allest mass and the most complex and irregular gravity field ever orbi ted in the history of space exploration. The smallness of Eros' mass a nd the severity of its distortion from the usual spheroid bodies encou ntered in planetary exploration leads to fundamental differences in th e orbital dynamics about it. This paper reviews the expected orbital d ynamics of the NEAR spacecraft at Eros and discusses the problem of or bit control of the NEAR spacecraft orbit. The dynamical environment ma y be differentiated according to whether the spacecraft is close to or far from Eros. When far from Eros, the spacecraft must contend with t he solar tide and solar radiation pressure forces which are large pert urbations to the Eros attractive force. When close to Eros, the spacec raft must contend with the irregular gravity held of the body. Orbits exist which are quite unstable and may crash onto the asteroid surface in a matter of days. Obviously, it is important to understand when an d where such orbits exist, and how they may be avoided. There are a nu mber of mission specific constraints which the NEAR spacecraft must op erate under. These include pointing, operational, and safety concerns as well as science directives and goals. The application of these cons traints in the NEAR spacecraft environment at Eros is discussed, inclu ding some specific strategy implementations.