through LEO are (after their functional lifetime) placed into orbits that will decay in a reasonable amount of time appears to be the most costeffective reorbiting measure. Determining exactly how long this time should be will be as much a political and economic decision as a technical one, due to the relatively large costs that such a maneuver may impose on some missions. Because of the long lead time required to develop and qualify new space hardware, however, it is necessary to begin setting standards now. For this reason, the committee recommends the following:

Recommendation 13: Spacecraft and rocket bodies in LEO and in highly elliptical orbits passing through LEO should be reorbited after their functional lifetime. This reorbiting maneuver should either remove them from LEO or reduce their orbital lifetime. Effort should be made to achieve an international consensus on the magnitude of such reorbiting maneuvers. A draft NASA guideline suggested that spacecraft in orbits that pass through LEO should be limited to orbital lifetimes in LEO of no longer than 25 years after mission completion; this standard does not seem unreasonable. However, any orbital lifetime limitation guideline that is adopted should be based on thorough scientific analysis.

Although the geosynchronous region may not be subject to collisional cascading and current GEO hazard levels from orbital debris appear to be very low, the hazard from debris left in GEO can persist for millennia. Currently, the long-term evolution of the debris environment is not well enough understood to determine the best long-term strategy for managing the debris hazard in GEO. Experts have not yet reached a consensus on the best locations for disposal orbits, or even on whether the use of disposal orbits is the optimal strategy for containing the GEO debris hazard. However, it may not be wise to let the GEO debris population grow until a permanent solution is divined. For these reasons, the committee recommends the following:

Recommendation 14: The use of GEO disposal orbits should be further studied. Until such studies produce a verifiably superior long-term strategy for dealing with the GEO hazard, operators of GEO spacecraft and rocket bodies should be encouraged to reorbit their spacecraft at EOL if they are capable of safely performing a reorbiting maneuver to a disposal orbit at least 300 km from GEO. Studies on the use of GEO disposal orbits should be focused on the development of a long-term strategy for maintaining a low debris hazard in GEO. Such studies should include the development of accurate models capable of predicting the effects of various debris reduction measures on the future hazard in GEO.



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