Phillip Cunio, ExoAnalytic Solutions; Marcus Bever, ExoAnalytic Solutions; Brien Flewelling, ExoAnalytic Solutions
Keywords: Cislunar, spaceborne SSA, optical payloads
Abstract:
As the predicted and realized volume of governmental and commercial activity in orbit continues to grow, the number of regimes in which activity can be found may similarly be expected to increase. The Low Earth Orbit (LEO) regime will soon be densely populated by megaconstellations, providing an extensive infrastructure for communications that may link any part of the Earth’s surface to any system above the Earth, and plans for activity in the cislunar regime are being developed as well.
The cislunar regime may be thought of as the region extending from above the Geosynchronous Earth Orbit (GEO) neighborhood – about the altitude of a stationary orbit – to the Lagrange point on the far side of Luna, Earth’s moon, but a few key loci in this regime will be more populated than others. Near-rectilinear halo orbits (NRHOs) provide relative stability near Luna itself, as do two of the aligned Lagrange points (L2 and L3), and the L4 and L5 points provide long-term orbital stability for emplaced hardware, as well as comparatively simple access to solar power. These loci, all of which are possible locations for a lunar gateway station or long-term scientific emplacements, and all transit routes and communications relay sites linking these loci, may be of critical interest for a successful expansion of human economic activity into the trillion-dollar opportunity that the near-term future in space represents.
As such, it will be very important to maintain space situational awareness (SSA) overwatch on these loci and the routes and links connecting them, to preserve the investment made and protect the activity these infrastructure elements support. A key part of this overwatch will be maintaining custody of objects in this regime, including active objects and any debris, to support appropriate traffic management. This overwatch should be maintained as constantly and consistently as possible, to avoid situations where collisions are not detected until after the become unavoidable.
Because the cislunar regime includes regions which are extremely distant from the surface of the Earth, it is challenging to use radar systems to track smaller objects there. Optical and infrared systems face the challenge of solar and lunar exclusions, when bright emissions or reflections from astronomical bodies can obscure the space objects in this regime. Given these physical limitations, the most viable solution is to operate a ground network supplemented by spaceborne sensors. As commercial SSA networks continue to proliferate and supplement government sensors, the physical limitations will be minimized but not fully defeated. Flying sensors in space can completely eliminate astronomical body exclusions (including lunar, solar, and terrestrial exclusions) through clever constellation design, which may include the use of sensors at Lagrange points, in GEO, or in highly elliptical orbits with apogees at multiple of the GEO radius (called high orbits with profound eccentricity), and can also address range limitations by approaching Luna more closely than the Earth’s surface ever does.
This paper will focus on the initial selection and design of a sensor payload which can operate from orbit to support a cislunar SSA network, flying in a constellation uniquely designed to deliver constant coverage while requiring the smallest amount of cost outlay. Such a payload, if matched to the appropriate orbit design, could offer protective overwatch to the extensive orbital infrastructure that the cislunar regime may soon contain.
Major design trades, such as precise sensor phenomenology and desirable detection limits, will first be addressed, using a combination of heuristic design and best practices for historically analogous sensor networks.
Secondly, a notional payload design (with initial budgets for mass, power, pointing, and cost) will be generated, and this payload will be matched with a novel orbit constellation design. Finally, this payload and constellation will be evaluated for performance against coverage metrics and cost metrics, and the performance of this novel constellation and a few in-family variants will be considered against similar constellations comprised only of systems in more traditional orbits (such as circular LEO only, e.g.).
A summary of this performance, the payload design used to achieve it, and a potential timeline for actual development and deployment of such a system will close the paper. Such factors as radiation tolerance, launch capacity, and orbital debris mitigation will also be considered, leading to a broad discussion of the relative utility of and need for additional improvements in a sensor architecture calculated for maximum efficacy at the cislunar regime.
Date of Conference: September 15-18, 2020
Track: Cislunar SSA