Assessment of time spent in the LEO, GEO, and semi-synchronous zones by spacecraft on long-term reentering disposal orbits

The U.S. Government Orbital Debris Mitigation Standard Practices (ODMSP) released in December 2019 include a disposal option to use orbital eccentricity growth for long-term reentry within 200 years. Long-term reentry is beneficial for orbital debris mitigation because it offers a large reduction in long-term collision risk in the regions above low Earth orbit (LEO) compared to use of storage disposal orbits. As a condition for use of this option, the ODMSP specifies a 25-year limit on cumulative time a disposed spacecraft can spend in designated altitude zones that are frequently used by operational spacecraft. An analysis was performed to determine the time spent by disposed spacecraft in the geosynchronous (GEO) zone, the LEO zone, and the semi-synchronous zone for three classes of reentering disposal orbits above LEO: (1) near-circular inclined geosynchronous orbits (IGSOs), (2) eccentric IGSOs (Tundra orbits), and (3) GPS orbits. Long-term disposal orbit propagations over 200-years were performed using the Aerospace high-precision integration code TRACE. Collision probability with operational satellites in the various zones is determined using the Aerospace Debris Environment Projection Tool suite in order to check the trend with time in zone. Results of the study showed that time in zones is less than 25 years when orbital lifetime is less than 200 years, thereby clearing the way for using the new long-term re-entry option relative to this condition. The absolute numerical value of collision probability with operational satellites was found to vary substantially with disposal orbit case and zone. For most cases, results show a clear trend between time in zone and collision probability with some spread. An exception is the GEO zone, for which results show a trend but a wide spread. This may be because most GEO satellites are confined to a ring instead of the shell represented by an altitude zone.