Cislunar Moving Objects

Author: Tianjiang Says

Website: https://cislunarspace.cn

Definition

Cislunar moving objects are objects within the cislunar space region that exhibit apparent motion relative to background stars. The term "cislunar space" refers to the region influenced by Earth and/or the Moon. These objects fall into three main categories:

  • Spacecraft: various artificial satellites, deep space probes, lunar orbiters, etc.
  • Space debris: spent rocket upper stages, collision fragments, retired satellite remnants, etc.
  • Natural objects: near-Earth asteroids (NEAs), temporarily captured objects, etc.

Key Characteristics

Compared to objects in low Earth orbit (LEO), cislunar moving objects exhibit several distinct characteristics:

Different Motion Laws

The motion of cislunar objects "usually follows the law of three bodies instead of the usual two-body one." This means their orbital evolution is governed by the combined gravitational pull of both Earth and the Moon, resulting in far more complex motion patterns that cannot be described by simple Keplerian orbital elements.

Lower Apparent Brightness

Due to the "significantly long distance" from Earth-based observers, cislunar objects are "much fainter for the ground-based observers." This places higher demands on telescope aperture, exposure time, and image processing algorithms.

Higher Apparent Angular Velocity

Compared to near-Earth asteroids, cislunar objects "exhibit a greater apparent angular movement velocity." This means that during a fixed exposure time, the target displaces more across the image, making it more likely to be missed by traditional stationary-target survey methods.

Observation Challenges

Detecting cislunar moving objects faces multiple challenges:

  1. Faint signals: long distances result in low brightness, requiring large-aperture telescopes and long exposure times
  2. Fast motion: traditional image stacking methods may produce trailing due to target motion, reducing detection sensitivity
  3. Background interference: dense stellar backgrounds, cosmic rays, and hot pixels all increase the false alarm rate
  4. Orbital prediction difficulty: three-body dynamics reduce the accuracy of long-term orbital predictions

Applications in Cislunar Observation

Sun et al. (2026) proposed a systematic observation approach based on image stacking for optical surveys of cislunar moving objects. The approach achieves efficient detection of faint moving objects through the following steps:

  1. Image registration: aligning consecutive frames to eliminate tracking errors
  2. Background star elimination: subtracting stellar backgrounds from registered images
  3. Stacking search algorithm (SAA): stacking images under different assumed velocities to enhance moving object signals
  4. Hot pixel removal: eliminating sensor defects through median filtering

This method maintains high detection sensitivity while effectively handling the fast apparent motion characteristics of cislunar objects.

References

  • Sun, R., Zhang, Q., Yu, S., et al. Optical Survey for Cislunar Moving Objects Using Image Stacking. AJ, 2026.