STATION-KEEPING STRATEGIES FOR TRANSLUNAR LIBRATION POINT ORBITS

G. G\'omez,   Departament de Matem\`{a}tica Aplicada i An\`{a}lisi, 
              Universitat de Barcelona, Gran Via 585, 08007 Barcelona, Spain, 
K. Howell,    School of Aeronautics and Astronautics, 1282 Grissom, Purdue
              University, West Lafayette, Indiana, USA 47907,
J. Masdemont, Departament de Matem\`{a}tica Aplicada I, Universitat 
              Polit\`{e}cnica de Catalunya, E.T.S.E.I.B., Diagonal 647, 08028 
              Barcelona, Spain, 
C. Sim\'o,    Departament de Matem\`{a}tica Aplicada i An\`{a}lisi,
              Universitat de Barcelona, Gran Via 585, 08007 Barcelona, Spain.

Spacecraft in orbits near the collinear libration points are proving to be 
excellent platforms for scientific investigations of various phenomena.  
Since such libration point trajectories are generally unstable, spacecraft 
moving on such paths must use some form of trajectory control to remain 
close to their nominal orbit.  The primary goal of this effort is the study 
of two particular station--keeping approaches applied to the maintenance of 
a spacecraft in an quasiperiodic Earth--Moon $L_2$ halo orbit. The Target 
Point strategy and the Floquet Mode approach both use maneuvers executed 
(impulsively) at discrete time intervas. The analysis includes some 
investigations of a number of the problem parameters that affect the overall 
cost.  Preliminary results are summarized.