Servicing and Deployment of National Resources in Sun-Earth Libration Point Orbits

Spacecraft travel between the Sun-Earth system, the Earth-Moon system, and beyond has received extensive attention recently. The existence of a connection between unstable regions enables mission designers to envision scenarios of multiple spacecraft traveling cheaply from system to system, rendezvousing, servicing, and refbeling along the way. This paper presents examples of transfers between the Sun-Earth and Earth-Moon systems using a true ephemeris and perturbation model. It shows the AV costs associated with these transfers, including the costs to reach the staging region from the Earth. It explores both impulsive and low thrust transfer trajectories. Additionally, analysis that looks specifically at the use of nuclear power in libration point orbits and the issues associated with them such as inadvertent Earth return is addressed. Statistical analysis of Earth returns and the design of biased orbits to prevent any possible return are discussed. Lastly, the idea of rendezvous between spacecraft in libration point orbits using impulsive maneuvers is addressed. Introduction Satellite servicing has received a great deal of study and significant execution. Several satellites were designed for servicing using the Multi-Mission Modular Spacecraft design, including Solar Max Mission, Landsat IV & V, Upper Atmosphere Research Satellite, and Extreme Ultra-Violet Explorer. Rescue missions have been performed on geostationary satellites trapped in low earth orbit, such as WESTAR-IV, PALAPA-B, Intelsat-VI, and LEASAT/SYNCOM-IV. More routine human servicing work occurs(ed) at various space stations (International Space Station, Skylab, Salyut, and Mir). The servicing of the Hubble Space Telescope (HST) has become a successful landmark, allowing HST to become one of NASA’s most productive missions. 1,2,334 Copyright @ 2002 by the American Institute of Aeronautics and Astronautics, Inc. No Copyright is asserted in the United States under Title 17, U.S. Code. The US. Government has a royalty-fiee license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner I Some obstacles to human servicing are the orbital mechanics, the cost of lifting mass, and the problems associated with travel time and thermal and instrument environmental conditions. As more ambitious missions are planned, such as the placement of the Next Generation Space Telescope (NGST) and several other missions into a Sun-Earth L2 (SEL2) or SELl libration orbit, servicing by the Shuttle and the use of low-Earth orbits (LEOS) will be limited. Development of robotic satellite servicing capabilities, such as DARPA’s Orbital Express, NASA’s Robonaut, and the University of Maryland’s Ranger, may provide for the possibility of robotic satellite servicing at various orbital locations in the nearor mid-range time frame. An enabling set of circumstances for an expansion of satellite servicing would be the placement of humans and valuable robotic assets in close proximity to one another, A space architecture that includes these conditions is a servicing facility in a lissajous or halo orbit about one of the Earth-Moon LI (EMLI), Earth-Moon L2 (EML2), or EarthMoon L3 (EMLJ co-linear libration points6. From such an orbit, spacecraft have access to a wide variety of interesting orbits at a relatively lower AV cost. Use of the EarthMoon stable L4 and L5 Lagrange regions provides additional scenarios. These servicing locations are also an excellent staging point for lunar surface and Earth-Moon orbital exploration.’ The orbits also have ready access to geostationary orbits and transfer back to LEO orbits. Table 1 provides a brief overview of Earth-Moon libration orbit staging node characteristics. Table 1. Earth/Moon Libration Orbit