Asymmetrical Space Tether System

Space Tethers: The Next Giant Leap for Asymmetrical Payload Delivery?

Theo Raines in Tech & Innovation December 2025 • 4 min read.

"Discover how motorized momentum exchange space tethers can revolutionize spacecraft propulsion, even when faced with payload imbalances."

Momentum exchange tethers have captured global attention as a promising technology for eco-friendly and cost-effective propulsion of payloads from Low Earth Orbit (LEO). Imagine a space elevator, but instead of a fixed cable, it's a rotating tether system that 'throws' payloads into different orbits. This concept, known as a motorized momentum exchange tether, offers a revolutionary approach to space travel.

A typical motorized momentum exchange tether (MET) operating in a circular LEO could achieve orbit velocities of approximately 7.6 km/s. Furthermore, it can generate an increment of around 3.1 km/s. This increment has the potential to accelerate an outer payload, at its optimal release position, to about 10.7 km/s, which is sufficient for Earth escape. However, the operational design requires a counter-inertia system to satisfy Newton's third law of motion, ensuring stability and balance during rotation.

While symmetrical payload distribution is ideal for stable tether dynamics, unexpected events like payload loss or retrieval failure can lead to asymmetry. This paper explores the dynamics of asymmetrical tethers and presents conceptual designs for tethered payload release from LEO and lunar tether delivery and retrieval, offering solutions for future development.

The Challenge of Asymmetrical Payload Mass

The core issue with asymmetrical payload mass distribution is that the tether's center of mass shifts, resulting in dynamic disturbances that can de-orbit the tether. Payload mass symmetry is essential to maintain orbit and harness orbital harmonics, facilitating a smooth flow of payloads between the host planet and the destination.

However, perfectly symmetrical conditions are not always guaranteed. For instance, one payload could be lost, or perhaps neither payload can be loaded on either end of the tether. Therefore, it's essential to investigate the asymmetrical mass distribution problem and devise strategies to handle such scenarios.

Catastrophic Failure: A small mass asymmetry can lead to catastrophic failure, causing the propulsion side to collide with the outrigger counter-inertia system.
De-Stabilization: In an asymmetrical configuration, the tether will be de-stabilized and pushed out of the spin-and-orbit plane, generating moments that could reach up to 300 kNm.
Translation: There will also be the problem of whole-body translation of the tether to consider, due to subsequent re-positioning of the CoM.

Corrective measures are needed to address these issues. One solution involves using low-thrust propulsion on board the payloads, which can counteract the imbalance and help maintain the tether's stability.

Looking Ahead: The Future of Space Tethers

Despite the challenges, motorized tethers offer a promising path to sustainable and reusable space propulsion. Continued research and development in GNC, compensation propulsion, and innovative designs like lunar lunavators are essential for realizing the full potential of this technology. As we continue to push the boundaries of space exploration, space tethers could become an indispensable part of future mission architectures.

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Everything You Need To Know

What is a Motorized Momentum Exchange Tether (MET) and how does it work?

A Motorized Momentum Exchange Tether (MET) is a rotating tether system designed to 'throw' payloads into different orbits, acting like a space elevator but with a rotating cable. This technology utilizes momentum exchange to propel payloads, achieving velocities necessary for various space missions. A MET operating in Low Earth Orbit (LEO) can achieve orbit velocities of approximately 7.6 km/s, and can further increment velocity by 3.1 km/s. This enables an outer payload to reach escape velocity (about 10.7 km/s). The system requires a counter-inertia system to maintain stability and balance, crucial for its operation. It contrasts with traditional propulsion methods by offering a potentially more eco-friendly and cost-effective solution for moving payloads in space.

Why is symmetrical payload distribution crucial for MET stability, and what happens if asymmetry occurs?

Symmetrical payload distribution is critical for Motorized Momentum Exchange Tether (MET) stability because it keeps the center of mass (CoM) of the tether in the correct position. If payloads are asymmetrically distributed, the CoM shifts, causing dynamic disturbances. This can lead to destabilization, where the tether is pushed out of its spin-and-orbit plane, generating significant moments, potentially up to 300 kNm, and causing the tether to de-orbit. This asymmetry can arise from payload loss, retrieval failure, or uneven loading. Consequently, the tether will experience whole-body translation due to the CoM's repositioning, making it harder to maintain orbit and utilize orbital harmonics effectively. Corrective measures, such as low-thrust propulsion on board the payloads, are necessary to mitigate the impacts of asymmetry.

What are the potential applications of METs in space missions?

Motorized Momentum Exchange Tethers (METs) have several promising applications in space missions. They offer a potentially cost-effective and eco-friendly way to transport payloads in Low Earth Orbit (LEO). This technology is not limited to LEO; it also has applications in lunar missions. By adjusting the tether's rotation and release points, METs can deliver payloads to the Moon and retrieve them. This capability makes them a versatile tool for future space exploration and resource utilization. Furthermore, METs can facilitate the acceleration of payloads to Earth escape velocity, allowing for missions to more distant destinations within our solar system.

What are the challenges associated with asymmetrical METs?

Asymmetrical Motorized Momentum Exchange Tethers (METs) face several challenges. The primary issue stems from the shift in the tether's center of mass (CoM) due to uneven payload distribution. This can lead to catastrophic failures, where the propulsion side collides with the outrigger counter-inertia system. Asymmetry destabilizes the tether, pushing it out of the spin-and-orbit plane, generating significant moments. Whole-body translation of the tether due to the repositioning of the CoM complicates maintaining the desired orbit. Addressing these challenges requires corrective measures, such as low-thrust propulsion on board the payloads, to counteract the imbalance and maintain stability.

What future developments are needed to advance space tether technology?

To advance space tether technology, several key developments are crucial. Continued research and development in guidance, navigation, and control (GNC) systems are essential to precisely manage the tether's movement and payload delivery. Implementing compensation propulsion systems can help counteract payload imbalances and maintain the tether's stability. Innovative designs, such as lunar lunavators, are important for expanding the applications of tethers in lunar missions. Further development in these areas will help realize the full potential of Motorized Momentum Exchange Tethers (METs), making them an indispensable part of future space mission architectures and advancing sustainable and reusable space propulsion.