Libration Point Missions, 1978 – 2002 David W. Dunham and Robert W. Farquhar Johns Hopkins University – Applied Physics Laboratory Abstract This paper summarizes the six missions to the vicinity of libration points that have been flown up to the time of this conference in June 2002. The first libration-point mission, the third International Sun-Earth Explorer (ISEE-3), is emphasized because it laid the groundwork for so many later missions, most of which are covered more thoroughly in other papers given at this conference. First, the authors present some basic properties of libration-point orbits, and some history of their development for early missions. Only brief information is given here; details can be found in the references. Introduction In 1772, the French mathematician, J. L. Lagrange, showed that there are five positions of equilibrium in a rotating two-body gravity field. Three of these “libration points”, or Lagrangian points, are situated on a line joining the two attracting bodies, and the other two form equilateral triangles with these bodies. All five libration points lie in the orbit plane of the two primary bodies. Fig. 1 shows a sketch of the libration points near the Earth. Their proximity makes them most attractive for possible space mission applications. Figure 1. Libration Points near the Earth. Fig. 2 shows the basic linearized equations of motion near the Sun-Earth L1 libration point. The out-of-plane frequency is slightly different from the in-plane frequency, resulting in motion that describes a Lissajous figure as viewed from the Earth. It is well- known that the collinear libration points are unstable and that the equilateral points L4 and L5 are only quasi-stable in the Earth-Moon system. Therefore, some form of station keeping control is needed to maintain a spacecraft at or near a libration point, especially the collinear ones. G. Columbo first showed that the ∆V cost for maintaining a satellite