Earth–Moon System & Cislunar Activity

Orbital visualization and research overview

The Challenges of Cislunar Tracking

Cislunar space is defined by vast search volumes and longer timescales than the Geostationary Earth Orbit (GEO) domain. Effective tracking is hindered by several factors:

  • Chaotic Dynamics: The three-body gravitational environment makes uncertainty propagation significantly more difficult, resulting in highly non-Gaussian state distributions (Iannamorelli & LeGrand, 2025).
  • Reduced Visibility: Terrestrial and Earth-orbiting sensors face periodic occultations by the Earth or the Moon, as well as blinding by solar radiation, which leads to long gaps in data collection (ESA, 2025).
  • Degraded Signal Strength: The immense distances between Earth-bound sensors and cislunar objects cause a severe loss of signal quality, limiting the precision of traditional tracking methods (ESA, 2025).

Sensor Types for Cislunar SDA

To overcome these limitations, modern SDA architectures rely on a hybrid of active and passive sensor technologies, each offering distinct advantages depending on the mission requirements.

1. Passive Electro-Optical (EO) Sensors

Passive optical sensors, such as telescopes, remain the primary tool for SDA due to their widespread availability and flexibility (ESA, 2025).

  • Operation: These sensors detect light reflected by an object (typically sunlight) or infrared (IR) emissions from the object itself (ESA, 2025).
  • Advantages: EO systems can observe from high-vantage points, such as Lagrange points (L1 and L2), where the dark celestial background enhances the detection of faint targets near the bright Sun (MDPI, 2026).
  • Innovations: Researchers are developing "Time Delay Integration" (TDI) systems for continuous scanning of the sky, improving upon the inefficient "step-stare" movement of conventional telescopes (Kirshner et al., 2024).

2. Radio Frequency (RF) and Radar Sensors

Active radar systems transmit energy and receive the backscattered signal, while passive RF sensors listen for communications or other signals emitted by a target.

  • Active Radar: High-power ground-based radars, often operating at X-band or C-band frequencies, are essential for characterizing objects. Large-scale transmitter arrays could potentially reach deep space, meeting threshold requirements for planetary defense and cislunar SSA (NASA, 2023).
  • Synthetic Aperture Ladar (SAL): Advanced laser-based radar systems are being explored to bypass the diffraction limits of real-aperture telescopes to produce centimeter-class imagery of objects at cislunar distances (Karr, 2024).

Hybrid Architectures and Future Strategies

The consensus in current research is that no single sensor type can provide comprehensive visibility. Optimal track custody is best achieved through hybrid architectures that fuse data from multiple perspectives (Iannamorelli & LeGrand, 2025).

  • Proximity Sensing: Placing sensors in cislunar orbits or on the lunar surface mitigates signal loss associated with Earth-based tracking (ESA, 2025).
  • Sensor Fusion: Advanced filtering techniques, such as the Adaptive Gaussian Mixture Model (AGMIMM), are being deployed to fuse heterogeneous data (angles-only, range, and radar) from diverse sensor nodes (Iannamorelli & LeGrand, 2025; Koblick & Choi, 2022).
  • Autonomous Tasking: Because communication delays make it impractical to downlink all imagery, next-generation space-based sensors require sophisticated onboard processing to autonomously detect and characterize space objects in real-time (Merry et al., 2023).

References