Stable Constellations of Frozen Elliptical Inclined Lunar Orbits
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Stable Constellations of Frozen Elliptical Inclined Lunar Orbits ToddA. Ely!
Abstract Higher altitude orbits (in the 500 to 20,000 km range) at the Moon are dominated by Earth perturbations and result in motions that do not ascribe to the standard notions of orbits dominated by nonspherical gravity effects (such as from oblateness). This fact complicates orbit design of lunar orbiter constellations that require specific and persistent coverage over a selected lunar region. Using a combination of analytical theory and numerical simulation, a technique is developed for designing a lunar constellation of three spacecraft where two spacecraft are always in view from the lunar surface for the polar regions.
Introduction A great deal of scientific interest exists regarding the permanently shadowed craters near the poles of the Moon where there may be frozen volatiles [1]. These regions, particularly the Moon's South Pole, have been proposed for extensive robotic and human exploration [2]. Unfortunately, they are typically not in view of Earth, and would require some form of communication relay to facilitate exploration via robotic and/or human missions. One solution for such a relay is a longlived constellation of lunar telecommunication orbiters providing focused coverage at the pole of interest. Robust support requires this coverage to be continuous and redundant [3]. Finally, in order to minimize costs, this constellation should have three satellites or fewer and possess coverage properties that persist [3].
Constellation Coverage and Orbit Considerations The combination of redundant, continuous, and focused coverage with a small number of satellites leads to the consideration of orbits with the following orbital characteristics: 1. Sufficiently large semimajor axis values to produce continuous single and double coverage with a minimal number of satellites. 'Senior Engineer, Guidance, Navigation, and Control Section, Jet Propulsion Laboratory, California Institute of Technology, MS 301-125L, 4800 Oak Grove Drive, Pasadena, CA 91109-8099. Email: [email protected].
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2. Large eccentricity values to focus coverage near apoapsis for longer contact durations. 3. Inclination values to orient the coverage swaths over the poles (rather than the equator). 4. Argument of periapsis values set to either 90° or 270° depending on whether apoapsis should be over the South Pole or North Pole, respectively. In response to this need, a new class of stable altitude orbits at the Moon has been found that enables the presence of a constellation of lunar orbiting telecommunication spacecraft. The orbits are elliptical with their line of apsides librating in the polar region (a.k.a. "frozen" orbits), and exhibit lifetimes in excess of ten years, the expected mission duration for a lunar telecommunications system. This paper describes the processes for selecting the orbital parameters for the constellation and the mechanisms behind its subsequent stable, long-term evolution. It is also shown that, with appropriate sel
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