3. Astrodynamics

To specify the functional requirements of STS, it is essential to understand the orbital motion of the injected satellites under the influence of a central gravitational force and other disturbing forces. This chapter deals with astrodynamics which explains the motion of celestial bodies as well as human-made satellites under the influence of gravitational force field of celestial bodies and other external forces. Orbital motions of Low Earth Orbit (LEO) satellites are the solutions of two-body problems i.e. the Earth and the satellite, in the specified reference frame, considering the Earth’s gravitational force as the primary central body force field. The deviation of the gravity force away from the central force field and other disturbance forces affect the orbital motion of the satellites. In addition to the central force field, gravitational forces of other planets and Moon also influence the higher altitude orbital motions. Solutions for such motions are achieved by solving restricted three body problem, considering the Earth’s gravity force as the central gravity field whereas the perturbing gravitational force is from the third body such as Moon. Depending on the type of trajectory, different reference frames are used and theses aspects are explained first. Then, this chapter discusses the orbital mechanics of satellites and various aspects of orbital motions of two-body problems. The restricted three body problem and the resulting orbital motion are also briefly explained. Even though the launch vehicles are capable of injecting the satellites in the near Earth orbits, for certain scientific applications, these satellites have to reach and orbit around Moon or distant planets. The interplanetary trajectories of such satellites from the Earth bound orbits to the target planets are also included. In an integrated mission management, optimum strategies like transferring the satellite at suitable time from the initial orbit to the required one in an optimum fashion are required. Various optimum orbital transfer maneuver strategies are explained in this chapter.