Phillip M. Cunio, ExoAnalytic Solutions, Inc.; Marcus Bever, ExoAnalytic Solutions, Inc.; Christopher Ingram, ExoAnalytic Solutions, Inc.; Brien R. Flewelling, ExoAnalytic Solutions, Inc.; Greg McNeill, ExoAnalytic Solutions, Inc.
Keywords: Cislunar SSA, highly elliptical orbits, novel orbits, SSA constellation, stable orbits
Abstract:
As human activity and infrastructure steadily and more thoroughly invest the spherical domain stretching from Geosynchronous Earth Orbit (GEO) to the neighborhood of lunar orbit and the associated libration points (a volume collectively known as cislunar space), understanding the dynamics and mechanics of orbits in this volume becomes concomitantly more important.
At present, most experts have developed their intuition about orbital mechanics, and thus their understanding of the most effective ways to operate from space, based on knowledge of either the GEO or Low Earth Orbit (LEO) regimes. Therefore, most intuition regarding orbits is driven by a few shared presumptions: orbits are fixed rings around the Earth, constellations are strings of satellites on the same orbit, and satellite coverage is made of sliding conic sections projected onto the Earths surface.
Although these intuitions are valuable, they do not necessarily apply in the larger cislunar domain, where the presence of a second body with an additional gravitational influence (Luna itself) and the vastly larger amount of empty volume (10X the range and 1000X the volume) greatly affect all considerations. For example, the conception of orbits as fixed rings near the Earth, while still technically accurate, to an extent clashes with the physics that allow libration points and associated halo configurations to maintain a perceived comparatively stable relationship to the Earth and the Moon. Additionally, even the simple fact of some cislunar regimes having orbital periods longer than a day is at odds with the simpler intuition of orbits that pass overhead frequently or that hover in a fixed region of the sky a highly-inclined circular orbit at some multiple of the GEO belt altitude would, e.g., trace a complex sinusoid in the sky from a ground observers point of view.
The process of becoming familiar with the features of cislunar orbits can begin with the classification and description of said orbits. Much as traditional orbits are classified into a small number of regimes (LEO, GEO, MEO, and HEO are widely understood as these regimes) and some particular orbits are understood as special in these regimes (e.g., sun-synchronous, GEO walker, GPS, and Molniya), it is feasible to begin describing regimes in the much more expansive cislunar domain, and to start identifying orbits in these regimes which may be of especial interest.
This paper will begin to develop a more systematic understanding of orbits, some relatively unique, that may be of interest in the cislunar domain. The paper will clarify definitions of the cislunar domain, discuss reasonable delineations of regimes therein, describe a few possible unique orbits in some regimes, and use these as exemplars of orbit families. (Because the smallest change in any of six orbital parameters technically results in the occupation of an entirely new orbit, this paper will use the term orbit family to describe a cluster of similar orbits, rather than attempt to discuss precise orbits.) Not all unique nor even all interesting orbit families in cislunar space may be described; knowledge of the entire cislunar domain and the uses to which satellites there may be put is still maturing, and there will undoubtedly be distinctions made in the future that cannot now be expected. For described orbits, orbital parameters will be given, and other features such as orbital stability will be presented.
There is typically some match between interesting orbit families and associated missions. For well-utilized domains, such orbits as the Clarke orbit, Molniya orbit, and the Cobra Teardrop orbit have specific applications in communications, and sun-synchronous orbits (or their subfamily of terminator orbits) have particular applications in ground imaging. This paper will describe some orbit families in the cislunar regime which have similarly-precise applications (such as the HOPE orbit and vertical triad) and detail the possible applications.
These applications will stretch across the traditional spaceborne missions of communications, observation, weather, and navigation, and include also the supporting missions of space situational awareness/space traffic management (SSA/STM) and scientific observation and exploration.
Once a few regimes have been suggested, and interesting orbits and missions described, a composited picture of the cislunar domain will be presented, giving all proposed domains and orbit families in context. Any apparent heuristics for orbit family/mission match will be described as well, and ways in which this overarching picture may be utilized as an early mission-planning tool will be discussed as well.
Date of Conference: September 14-17, 2021
Track: Cislunar SSA