Disposal of spacecraft at end of life in geosynchronous orbit

Abstract

A number of national and international organizations have adopted a policy of removing spacecraft at the end of life from the geosynchronous ring. However, a resolution of several technical issues still remains before an international policy can be developed and implemented by all users of geosynchronous orbits. The present study reviews the cataloged population of objects in the geosynchronous ring. The probability of collision is computed as a function of range and orbital inclination of an object. Long-term stability of supersynchron ous disposal orbits and of the geopotential stable points is examined. Several disposal options are considered. The results of the study show that the use of the supersynchronous orbits for disposal purposes is economical and effective in significantly reducing the collision hazard. The use of the geopotential stable points for disposal of spacecraft at the end of life, on the other hand, was found to be impractical. I. Introduction B ECAUSE of the uniqueness and usefulness of the geosynchronous equatorial orbit (GEO), the population of objects in this orbit has increased continuously. Reference 1 shows, for example, that at the beginning of 1988, there were 286 cataloged satellites residing in this orbit, not including spent upper stages. Of these, 110 to 130 operating satellites were on station along with 150 that are nonfunctional or abandoned. ,. Reference 2, on the other hand, shows that of the approximately 453 cataloged objects traversing GEO altitudes, only about 150 are geostationary. The others are either geosynchronous or in highly elliptical (Molniya) semisynchronous orbits. There are also a number of military spacecraft, the orbjts of which are not generally availably in published form. To reduce the collision hazard within GEO, approximately 30 to 35 satellites have been removed from GEO at the end of life. Several strategies are available for disposing of GEO satellites, suqh as moving them to stable longitudes or raising them to orbits above geostationary altitude. The benefits of removing old spacecraft from GEO are a) the freeing of desirable longitudinal positions for repl^cement spacecraft and b) the reduction of collision damage potential, which can become high for collocated satellites.3 The primary issues involved are a) the protection of GEO as a unique natural resource and b) operational considerations such as cost effectiveness of the maneuver, risks of explosion, structural breakup, or other malfurigtions in carrying out the maneuver and national and international policy considerations. Other issues include the potential impact on future traffic to GEO and the perturbative effects on disppsal orbits (e.g., solar radiation and the sun/moon gravitational perturbations). In other words, abandoned rocket stages or satellites should not be a hazard to future missions, and their orbits should not intersect GEO over long periods of time.