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What is cislunar space and what makes it special?
Cislunar space is the volume containing the Earth, the Moon, and just beyond the Moon. The region is not rigorously defined; however, it is a regime where neither the gravitational forces associated with Earth, nor the Moon can be neglected. Subsequently, the unique dynamics enable the existence of exotic orbits and trajectories that are not found in near-Earth space. These exotic orbits and trajectories enable exploration of regions that are otherwise difficult to reach.
In recent years, the cislunar space has become increasingly popular for ongoing and future space programs. From science-gathering missions in search of resources like water and rare earth metals (REMs) to long-term, human space flight programs such as Artemis and the Lunar Gateway, there are many upcoming cislunar initiatives with a wide range of goals. This large influx of planned activities in such a unique and complex environment will certainly bring discoveries and innovation but also create reasonable concerns about safety in this challenging environment.
In order to protect cislunar space and the missions planned to take place in it, there is an increasing need for space situational awareness (SSA) efforts in this domain. Cislunar SSA requires collecting observations of objects and keeping tabs on them without the need for constant, direct communication. Cislunar SSA is an important and challenging undertaking, as evidenced by its inclusion in The White House’s First National Cislunar Science & Technology Strategy. The strategy summary states that space situational awareness “enables transparency and safe operations for all entities operating in cislunar space.” Despite this statement, there is a lack of plans for infrastructure and missions with cislunar SSA as their primary objective. Given the multitude of challenges operating in the cislunar regime presents, questions regarding how cislunar SSA can best be executed remain.
Cislunar Observation Challenges
Researchers are actively working on effective strategies to overcome challenges associated with the observation of objects in the cislunar domain. The more awareness of the challenges and the need to solve them, the more research will be done to confront them, and, in turn, the closer we will get to developing an effective cislunar SSA strategy. Some observation obstacles to take note of include:
Detection
- With a volume as large as cislunar space (~10,000x volume of GEO), resident space objects (RSOs) and the technologies made to observe them can be at significant distances from one another. These distances can cause the objects to become faint, and at times undetectable, to the observer. In other words, the farther the two entities move away from each other, the less likely the RSOs are to be identified.
Tracking
- Cislunar space currently faces tracking challenges as a result of limitations concerning signal-to-noise ratio observations, lunar exclusion angles, data arcs, and nonlinear dynamics. More simply put, a single traditional sensor would have difficulty continuously observing an RSO due to the domain’s unique conditions. Subsequently, scenarios such as these present a need for a communicative network of sensors throughout cislunar space.
Lighting Conditions
- Due to phenomena such as intense, direct sunlight from the lack of an atmosphere or the “Cone of Shame,” there are at times difficulties with imaging RSOs. Too much or too little light can cause inaccuracies in data collection, so it is critical to be mindful of these conditions for proper object observation.
Sensor Choice & Placement
- There is a multitude of variables to consider for sensor placement, from sensor type, such as electro-optical (EO) or radiofrequency (RF) sensors, to sensor location and platform, such as Earth-based or on-orbit-based. It is important to remember certain sensor types are better suited to specific tasks and no sensor location has the capability to observe cislunar space in its entirety. Depending on the requirements, these decisions must be made prudently for successful object observation.
To overcome these challenges and promote safe operations in cislunar space, technologies will need to be advanced. Systems that will enable more accurate and comprehensive observations in cislunar space include, but are not limited to, orbit determination, mission autonomy, and low-thrust propulsion.
As we continue to break new ground in cislunar space, it is paramount that the awareness surrounding observational challenges is heightened. The conditions in this unique domain create new opportunities for investigation and discovery. Diving deeply into the development of proper cislunar observational practices will advance how this complex environment is studied and will vastly improve how data is collected.
To address these obstacles and expand cislunar SSA capabilities, a number of new technologies are in development or have already been created. Take FreeFlyer for example, a.i. solutions’ astrodynamics software for space mission design, analysis, and operations. This software and the technology it enables have wide-ranging capabilities, many of which can be used to work toward solving observation problems in cislunar space such as autonomous station-keeping and executing cislunar transfer orbits.
