STRATEGIES FOR MAXIMIZING A SATELLITE LIFETIME BY TETHER-MEDIATED ORBITAL INJECTION

The tether slingshot technique, which is based on the exchange of angu lar momentum between a satellite and a platform, is analyzed here with the goal of maximizing the satellite lifetime after release. Once the tether is deployed and the system librates about the local vertical, the tether is cut twice: the first time at the platform end in order t o increase the apogee height of the satellite-tether system and later on at the satellite end in order to increase the perigee height of the satellite. The goal is to define the conditions at the end of deploym ent and the timing and parameters of the two cuts in order to maximize the satellite lifetime. Two separate strategies are followed: (a) the first tether cut occurs when the libration angle reaches a critical v alue for which the tether crosses the local vertical (with prograde li bration) at the second apogee passage, i.e., when the second cut provi des the maximum increase of perigee altitude; and (b) the first cut ta kes place when the tether crosses the local vertical and the true anom aly at the second tether cut is taken as an additional free parameter of the optimization process. In both strategies we assume, at first, t hat the maximum lifetime corresponds to the minimum semi-major axis de cay rate or, conversely, to the maximum increase of the satellite peri gee. Since increasing perigee height means sacrificing apogee height, the assumption above is tested by directly computing the satellite lif etime as a function of the free parameters in order to refine the esti mate of the optimal cut parameters. The orbital-parameters-oriented ap proaches (a) and (b), together with the direct computation of the life time, allow us to select the end-of-deployment and cut parameters whic h maximize the satellite lifetime.