First, however, it is worth noting that not ail disposal options are clearly distinguishable from salvage. For example, it may be desirable to collect SPS satellites that have reached the end of their useful life at a repository location in geosynchronous orbit. This location might be co-located with a manufacturing base that, over an extended period of time, would make use of the SPS material. Disposal of SPS satellites to a common geosynchronous location is a trivial matter requiring only a few m/s of velocity increment and which could be accomplished over a period of one to a few months at very little cost. This disposal option has been discussed in part in Section 4.6 and is not considered further here. The major disposal options considered here include those shown in Table 1.3; L4 or L5 supra-GEO, moon, heliocentric orbit and earth reentry. The L4 or L5 disposal option is illustrated in Figure 6.1. There exists in a two body gravitational system five points at which gravitational forces and accelerations cancel each other such that an object placed at these positions remains stationary with respect to both of the major bodies. The five points are referred to as libration or Lagrangian points. Points Lp L2 and are unstable in the sense that if the body placed there is subjected to a small perturbation from the precise position of the Lagrangian point, it will drift away or assume an orbit which diverges from the Lagrangian point. Points L^ and L^, sometimes referred to as the equilatoral Lagrangian points, however, are stable. That is to say if an object is placed near these points, it will tend to orbit stably around the Lagrangian point, at least for extremely long periods of time. Thus if SPS satellites were disposed of in these locations, one could expect that they would remain there unattended, essentially forever. The only qualification to this mode of disposal would be that it might become desirable to lash together all of the satellites located at each of these points in order to keep them from bumping violently into each other. Since
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