Introduction
The space debris environment in Low Earth Orbit.
Low Earth Orbit (LEO), the region up to 2,000 km above Earth’s surface, contains three major physical elements: natural meteoroids, orbital assets, and an expansive assortment of “junk” (cite: Orbital debris environment for spacecraft in low earth orbit). This junk can be defined as man-made materials left in space due to humanity’s endeavors. It ranges from tiny particles of rocket propellant to much larger fragments from explosions and collisions between substantial objects like defunct satellites and spent rocket motors (cite: The History of Space Debris). While some debris has re-entered the atmosphere or escaped Earth’s gravity, a significant amount remains in orbit due to increasing human activity in space. It is estimated that around 3,000,000 kg of human-made objects are orbiting within approximately 2,000 km above Earth’s surface. The majority of this mass comes from about 3,000 spent rocket motor cases, inactive payloads, and a small number of active payloads (cite: Overview of the Space Debris Environment). The growing presence of objects in Earth’s orbit heightens the risk of the Kessler Syndrome, magnifying threats to orbital assets and interstellar missions and rendering scientific progress exceptionally difficult.
(cite: The European Space Agency – About space debris) – This graph provides a visualization of the extent to which the orbital object count has increased over roughly the past 60 years.
The Kessler Syndrome.
Proposed by NASA scientist Donald J. Kessler in 1978, the Kessler Syndrome describes an imminent scenario in which the density of objects in LEO exceeds a certain threshold, initiating collisions between orbital assets or various forms of space debris. This would create a chain reaction in which the fragments from these collisions would begin breaking up other intact objects at an accelerating rate. The Kessler Syndrome further explains that once triggered, billions of additional fragments would be produced, enveloping Earth in debris and effectively isolating it from space. (cite: The Kessler Syndrome – Implications to Future Space Operations AND cite: Understanding the misunderstood Kessler Syndrome).
The scientific and economic significance of a clean space environment.
Maintaining a clutter-free space environment is crucial for the safety of orbital assets essential for global communications, navigation, environmental monitoring, weather forecasting, and space research. Although a post-Kessler Syndrome LEO environment would still be navigable, it would be at the cost of significant money, propellant, and time. Despite these efforts, the risk of collision would still remain extremely high (cite: What Are Satellites Used For?). Currently, the International Space Station maneuvers frequently to avoid debris; however, small fragments still damage its protective shielding (cite: How does Space Debris Threaten the ISS?). As the number of nations engaged in space exploration increases, the deployment of rockets, satellites, and other orbital assets into LEO will grow exponentially. These assets face escalating risks due to accumulating debris from previous launches, posing potential mission failures for both manned and unmanned assets, significantly hindering scientific progress in terrestrial and extraterrestrial domains (cite: A Comprehensive Study on Space Debris, Threats posed by Space Debris, and Removal Techniques). In a post-Kessler Syndrome LEO environment, the signals sent and received by orbital assets could encounter interference from the abundance of debris, impairing critical technologies like mobile communications, GPS, and military surveillance (cite: The Growing Problem of Space Debris). Therefore, maintaining a clutter-free space environment is essential for enabling future spacefaring and furthering humanity’s knowledge in science. Furthermore, maintaining a clean space environment also holds significant economic implications. Orbital assets, critical to sectors like remote sensing technology, are key contributors to the global economy. Remote sensing involves analyzing radiation emitted or reflected from a region, typically from satellites or aircraft in LEO, to record Earth’s physical characteristics. These orbital assets are rigged with sophisticated cameras that capture distant images, providing researchers with valuable insights into Earth’s features and conditions (cite: What is remote sensing and what is it used for?). Market projections indicate rapid expansion in the remote sensing technology market, with estimates suggesting it could reach USD 61.7 billion by 2033, driven by a compound annual growth rate of 19% from 2023 to 2033 (Market Us) (See FIG 1).
However, a growing concern is the escalating volume of space debris in LEO, which threatens the very infrastructure this market relies on. Collisions between debris and operational satellites already result in annual losses estimated between $86 million and $103 million for satellite operators, and these costs are only expected to increase with each additional collision and operator (cite: Space Trash Threatens the Global Economy). This can further disrupt critical services and adversely affect respective market stability. Therefore, preserving a clutter-free space environment is also crucial for safeguarding the economic stability and growth of vital industries reliant on orbital assets and space-based technologies.
The limitations of current removal techniques and the role of artificial intelligence.
Current space debris mitigation measures primarily focus on preventing future debris creation through regulations and operational guidelines. These measures aim to minimize debris created by spacecraft launches and operations and passivate orbital assets at the end of their lifespan. (cite: A Comprehensive Study on Space Debris, Threats Posed by Space Debris, and Removal Techniques). For instance, the U.S. Government Orbital Debris Mitigation Standard Practices (November 2019 Update) established guidelines for limiting the creation of new orbital debris from spacecraft and launch vehicles. The guidelines range from controlling debris released during normal operations to post-mission disposal of space structures (cite: U.S. Government Orbital Debris Mitigation Standard Practices, November 2019 Update). However, these methods do little to address the vast amount of debris already in orbit. As a result, researchers have begun developing active debris removal (ADR) technologies, such as laser-based, ion-beam shepherd-based, tether-based, satellite-based, and other techniques, to actively remove debris from LEO (cite: ACTIVE DEBRIS REMOVAL FOR LEO MISSIONS AND Review of Active Space Debris Removal Methods). Such technologies include the Gossamer Orbital Lowering Device (GOLD), Laser Orbital Debris Removal (LODR), and more. In the process of implementing these techniques in orbital spacecraft, however, scientists have encountered significant challenges. For instance, after undocking from the International Space Station, the Kounotori-6 spacecraft intended to deploy its 700-meter electrodynamic tether to remove space debris but failed due to an issue with the mechanism to release the tether (cite: A Comprehensive Study on Space Debris, Threats Posed by Space Debris, and Removal Techniques). This is where Artificial Intelligence (AI) presents itself as a transformative solution. AI encompasses a range of machine learning algorithms and computational processes that enable systems to learn, reason, and adapt without explicit programming. Specifically, machine learning enables AI to recognize patterns and make decisions based on an expansive array of historical and real-time data without being explicitly programmed to. This makes AI especially effective in debris mitigation. (cite: What is (AI) Artificial Intelligence?). Due to this real-time processing, AI can revolutionize current mitigation measures by automating complex tasks, enhancing decision-making, and facilitating the development of more efficient and adaptable ADR strategies.
Discussion
Enhanced Trajectory Prediction with Machine Learning
Over recent decades, orbital mechanics have relied heavily on classic physics principles, such as Newtonian laws, to predict the trajectories of objects in space. These methods involved solving complex differential equations that govern the motion of orbiting bodies (cite: https://spsweb.fltops.jpl.nasa.gov/portaldataops/mpg/MPG_Docs/MPG%20Book/Release/Chapter7-OrbitalMechanics.pdf). However, due to scattered and low-precision observations, unknown geometric and physical characteristics of debris, incomplete force models, and influences from the effects of general relativity, trajectory prediction based on recent orbital mechanics experiences rapid error growth over extended periods. This limits the accuracy and validity of current orbital trajectory prediction measures (cite: https://ieeexplore.ieee.org/document/9076032). This is where machine learning (ML) has emerged as a powerful tool. ML encompasses a range of algorithms that enable systems to learn from data, identify patterns, and make predictions without explicit programming (cite: Machine learning, explained (MIT)). Based on input data designed for training, an ML algorithm will create an estimate about a pattern in the data. An error function then assesses the accuracy of the model’s predictions. By comparing known examples, the error function evaluates how well the model performs. If the model needs to better fit the data points in the training set, weights are adjusted to minimize the difference between the known examples and the model’s estimates. This iterative “evaluate and optimize” process is repeated, with the algorithm autonomously updating weights, to ultimately reach the desired accuracy threshold (cite: What is machine learning (ML)?). This capability makes ML particularly valuable for enhancing orbital mechanics calculations, specifically in predicting the trajectories of space debris and orbital assets.
Trajectory Prediction Based on Machine Learning, a study by Lumin Su and Li Li, illustrates the potential of trajectory prediction utilizing ML in real-time. Su and Li proposed a spatial information extraction method based on multistage cluster and the LSTM model and the bidirectional LSTM model in deep learning which are used to predict position of debris (cite: https://iopscience.iop.org/article/10.1088/1757-899X/790/1/012032). A Long Short-Term Memory network (LSTM) model is a type of recurrent neural network (RNN) that can learn and retain information over long periods of time. Similarly, a Bidirectional Long Short-Term Memory (BiLSTM) model is a type of RNN that processes sequential data in both forward and backward directions. They outline steps for the proposed trajectory prediction method, including trajectory spatial information extraction, trajectory time and direction information addition, feature vector extraction, and trajectory prediction (cite: https://iopscience.iop.org/article/10.1088/1757-899X/790/1/012032 ).
The trajectory prediction method Su and Li utilized considers not only spatial features, but also other features such as time and direction. Experiments conducted with the LSTM model are shown in Figure 8. As more features are considered, the prediction accuracy of space debris gradually improves. The results from Su and Li’s experiments reveal that their novel method can greatly improve the prediction performance of machine learning for space debris.
Additionally, results from A Machine Learning-Based Approach for Improved Orbit Predictions of LEO Space Debris With Sparse Tracking Data From a Single Station found that using the boosting tree algorithm, a learning technique that combines the predictions of multiple prove decision trees to improve accuracy, ML models trained to fit a historical prediction error set captured more than 80% of the historical prediction error patterns. Through the error correction with the learned error pattern, the physics-based prediction errors over the next seven days reduced from thousands of meters to hundreds or even tens of meters, achieving at least a 50% improvement in accuracy (cite: https://ieeexplore.ieee.org/document/9076032). By leveraging historical information and real-time data, ML algorithms can significantly improve trajectory prediction accuracy in numerous ADR technologies, resulting in more efficient mission planning, optimized energy usage, and effective rendezvous maneuvers. As the field of ML continues to evolve, its integration with ADR techniques holds immense potential for mitigating the Kessler Syndrome.