Space tethers are long cables made from highly durable materials that are used to connect objects in space. These objects can include satellites, spacecraft, or probes. Space tethers function in distinctive ways by harnessing gravitational forces or interacting electrically with Earth's magnetic field. Their main advantage is their ability to perform tasks like propulsion or stabilization without the need for conventional fuel systems.

The materials used to construct space tethers must be lightweight, strong, and capable of withstanding the harsh environment of space. This includes exposure to extreme temperatures, micrometeoroid impacts, and atomic oxygen, all of which can degrade conventional materials. Advanced composites and high-strength polymers, such as Spectra or Kevlar, are often employed to meet these stringent demands. 

Space tethers utilize gravity gradient attitude control, offering a cost-effective method for orientation. When appropriate for the mission, this system can generally align the tethered setup within ±10 degrees of vertical, both inside and outside the orbital plane.

Space tethers are among the most innovative technologies in modern space exploration. By harnessing the forces of gravity and electromagnetism, they offer a range of applications, from propulsion to debris mitigation.

Types Of Space Tethers

Space tethers facilitate the exchange of energy and momentum between objects, functioning as a method of propulsion in space. These systems are typically classified into two primary types:

1. Momentum-exchange tethers
2. Electrodynamic tethers

Momentum-Exchange Tethers

Momentum-exchange tethers, which are nonconductive and utilize passive propulsion, facilitate the transfer of momentum between objects in space, such as two spacecraft. These tethers redistribute momentum within a system by transferring it from one body to another while ensuring the overall momentum remains conserved. The underlying mechanism relies on the gravity gradient force, which arises due to differences in gravitational pull across the tethered objects, creating a natural alignment between them. This innovative approach allows spacecraft to adjust their orbits without expending propellant, making it a sustainable solution for orbital maneuvering.

When two objects are tethered and separated by a distance, the gravity gradient force "pulls" them into a vertical (radial) alignment. However, variations in the central body's gravitational field cause this tethered system to oscillate like a pendulum around its center of mass. This oscillatory motion, known as libration, can be harnessed to alter the orbit of the system. By carefully managing the tether's dynamics, it is possible to raise or lower the system's orbit, enabling precise orbital adjustments without relying on traditional propulsion methods.

Different types of tether systems offer diverse functionalities. For example, releasing one end-body from a tethered system transfers momentum, elevating the released object to a higher orbit while lowering the orbit of the remaining system. The bolo tether, which rotates end-over-end in the orbital plane, can propel a payload into a new orbit or even "catch" a payload. Additionally, stationary tethers maintain a constant length between two end-bodies, making them ideal for tasks like atmospheric sampling by dragging a lower payload through the upper atmosphere while simultaneously adjusting the system's orbit. These versatile systems demonstrate the potential of momentum-exchange tethers in revolutionizing space exploration and satellite deployment.

Electrodynamic Tethers

Electrodynamic tethers, made of conductive materials, offer active propulsion and power generation capabilities in space. These systems consist of a long conducting wire extended from the primary spacecraft, often supported by a secondary end-body to aid in deployment. The gravity gradient field aligns the tether vertically relative to the spacecraft. As the tether moves through Earth's magnetosphere, its orbital velocity exceeds the geomagnetic field's rotational velocity, creating drag. This interaction induces an electromotive force (EMF), producing a voltage along the tether's length, which can reach several hundred volts per kilometer.

The operation of electrodynamic tethers is determined by the direction of current flow within the system. They can function either as propulsion or drag devices. When set up as a drag system, the tether generates electrical power by converting the spacecraft's orbital energy, creating drag that reduces its orbit. The current loop, which includes the tether, ionosphere, and external plasma, captures electrons from the plasma, resulting in a net positive charge. While this process generates power for the spacecraft, it depletes the system’s orbital energy. In contrast, when configured for thrust, power is supplied to the tether, allowing it to interact with Earth's magnetic field to produce a "push," raising the spacecraft's orbit.

Electrodynamic tethers offer versatile applications beyond propulsion. A notable feature is the emission of electromagnetic waves (ULF, ELF, VLF) as electrons are transferred back into the plasma, potentially enabling their use as antennas. The drag capability also serves practical purposes, such as deorbiting defunct satellites or malfunctioning spacecraft. Additionally, tethers can be employed for various low-cost scientific and operational purposes, including atmospheric studies, gravity experiments, communication systems, and power generation, highlighting their multifaceted utility in space exploration and research.

Selection Criteria for Space Tethers

Space tethers are a promising technology for a variety of space applications, including orbital maneuvering, payload transfer, and energy generation. When selecting a space tether, several criteria must be considered to ensure it meets the mission's technical and operational requirements. These criteria can be broadly categorized into mechanical, material, operational, and environmental factors.

Material Properties

The choice of material significantly impacts tether performance. Key material selection criteria include:

• Strength-to-Weight Ratio: High tensile strength and low density are crucial to withstand the stresses of deployment and operation in microgravity. Materials like carbon nanotubes, graphene, and advanced polymers (e.g., Zylon, Kevlar) are often considered.
• Elasticity: Tethers need to balance flexibility for deployment with stiffness for stability under tension.
• Thermal Resistance: Space tethers must endure extreme temperature fluctuations without degrading, particularly in Low Earth Orbit (LEO) where temperatures can range from -150°C to +150°C.
• Radiation Resistance: The material must withstand prolonged exposure to cosmic rays, solar radiation, and atomic oxygen in LEO.

Mechanical Design

• Length and Diameter: The tether's dimensions determine its mass, drag, and functionality. Long tethers are often needed for momentum exchange, while short ones suffice for stabilization.
• Tether Dynamics: Stability during deployment, oscillation suppression, and ability to maintain tension is critical. Dynamic analyses are required to prevent tangling or uncontrolled motion.
• Mass Efficiency: The tether must add minimal mass to the spacecraft while providing maximum functionality.

Operational Requirements

• Deployment Mechanism: The tether must deploy smoothly and reliably, whether via spools or controlled release systems.
• Application-Specific Needs: For example, momentum exchange tethers require strong coupling to spacecraft, while electrodynamic tethers depend on conductivity for power generation or propulsion.
• Longevity: The tether's lifespan must match the mission duration, requiring robust materials and coatings.
• Cost and Scalability: The solution should be cost-effective and scalable to larger projects.

Environmental Considerations

• Orbital Debris Mitigation: Tether designs should minimize the risk of generating or being damaged by orbital debris.
• Atmospheric Drag (in LEO): Thicker tethers or those with larger cross-sections experience greater drag, impacting orbital decay rates.
• Interaction with Plasma: For electrodynamic tethers, compatibility with the ionospheric plasma is critical to ensure effective current flow.

Safety and Reliability

• Redundancy: Incorporating failsafe mechanisms, such as multiple tether strands, can mitigate the risks of a single-point failure.
• Testing and Validation: Rigorous ground-based and spaceflight tests are essential to verify the tether's performance under simulated mission conditions.
• Emergency Measures: Provisions to safely retract or sever the tether in case of malfunction are necessary.

Specific Mission Objectives

• Payload Mass and Dynamics: The tether must handle the payload's mass without compromising its structural integrity.
• Orbit and Purpose: Tether specifications vary depending on whether they are intended for deorbiting, power generation, or inter-orbital transfers.

Design Criteria

Criteria for determining the strength requirements for tether missions and ensuring mission success despite potential severing from micrometeoroid and orbital debris impacts. It also covers material considerations for space tethers, including electrically conductive options and dynamic factors for tether selection. Additionally, safety, quality, and reliability factors are addressed for tether projects. 

Strength Criteria

Tethers are critical structural components in experiments but can be vulnerable to single-point failures. Their design should prioritize durability and robustness, allowing them to withstand normal mishandling and potential damage. Due to their length, tethers are prone to issues like crimps, pinches, vibrations, or contact with spacecraft elements.

To ensure durability, qualification testing should simulate off-nominal conditions (e.g., tight knots or partial damage). Flight tethers should be proof-tested end-to-end, ideally at operational temperatures, or adjusted for temperature-related strength degradation. A minimum ultimate tensile strength of 100 pounds is recommended to avoid fragility. Manufacturing defects should be inspected, and repairs tested and proofed.

Tethers are generally designed for single-mission use, with re-flight considered only after a thorough evaluation of strength degradation due to space exposure and other factors. Inspections and reproofing are essential for reused tethers.

Safety factors must be verified through testing under maximum predicted loads:

Qualification tests: FS ultimate = 5.0, FS off-nominal = 2.0.

Proof tests: Proof factor = 2.0, conducted end-to-end.

Meteoroid and Orbital Debris

Early Apollo studies focused on meteoroid impacts on spacecraft. Today, manmade orbital debris is a greater threat in low Earth orbit (LEO), though meteoroids still pose risks to components like space tethers. Meteoroids, originating from comets and asteroids, are less dense than orbital debris, but both can cause significant damage depending on particle size and impact velocity. Meteoroids hit at 19 km/sec, while orbital debris impacts at 10 km/sec.

Space tether missions are vulnerable to accidental severing, which can lead to mission failure or satellite loss. Tethers, like those in the Tethered Satellite System (TSS), are especially at risk from small, undetectable debris. If a tether is severed, it can recoil, posing safety hazards, particularly for manned missions.

The probability of a tether severance due to debris is calculated using the Poisson Distribution, considering particle flux, exposure time, and tether area. NASA’s safety standards aim for a 95% chance of no tether severance for manned missions with no immediate crew danger, and less than one in a million for those with immediate risks. Engineers are encouraged to design systems that minimize safety risks.

The equation for determining the probability of no critical failure is 

For critical Failure 

Material Selection

Tether construction materials must adhere to MSFC-STD-506 standards, with exceptions evaluated under MSFC-PROC-1301. For tethers incorporating electrical conductors, additional precautions are necessary to prevent electrical arcing. All tether materials and processes should ensure reliability and performance within prescribed diameter and weight limits. Comprehensive physical, chemical, and electrical data must be collected for both electromechanical and structural tethers. Fabrication standards require tethers to be free of defects and contaminants, with clear engineering specifications for manufacturing and storage conditions. The tether must also demonstrate resistance to space radiation environments, such as Galactic Cosmic Radiation, trapped radiation, and solar proton events, as well as degradation from atomic oxygen at orbital altitudes and velocities. Additionally, tether components should function seamlessly after six months of storage on a reel and be designed to avoid degradation due to hygroscopic effects. Compatibility with the conductive plasma in low Earth orbit (LEO) is also essential, alongside strict contamination control during manufacturing and testing to minimize particulates.

Dynamic Consideration

Tether Longitudinal Flexible Body

The choice of the tether will influence the vibration between the two end masses as well as the damping of those vibrations. Due to the tether's temperature fluctuations as it moves in and out of sunlight during each orbit, it undergoes significant expansion and contraction, causing substantial changes in length. However, by selecting an appropriate tether material, the dynamics can be controlled to a manageable level. Additionally, tethers naturally experience a twisting torque, but this can be minimized if the tether is handled correctly before flight.

The tether's cross-sectional area (A) and Young's modulus of elasticity (E) of the tether material are used to determine the strain of the tether per unit of applied load.

Here, T represents the tether tension. The spring constant (k) of the tether connecting the two bodies is given.

Here, L represents the length of the tether. The stretch (w) of the tether between the two bodies is:

Here, T denotes the average tension in the tether. If the end masses are treated as rigid bodies, the bobbing frequency (ω) of the end mass can be estimated using:

where M represents the reduced mass of the end bodies, defined as:

TSS and SEDS tether missions have functioned at tension levels well below the maximum strength capacity of their tethers. Significant variations in AE have been observed when operating at low tension fractions, with AE rising considerably as the operating tension increases. AE on the TSS and SEDS tethers also increased as temperatures decreased and decreased when temperatures rose. A longitudinal wave travels along the tether at a speed of:

Here, p represents the mass per unit length of the tether. If the tether is severed, the velocity of the tether remnants moving toward the end masses is:

Here, T is assumed to be constant along the length of the tether before the break, with no reduction in velocity due to natural forces. When the tether is severed, such as by a micrometeorite, a wave travels along the tether at speed (s), relieving the stress in the tether and imparting velocity (V).

Thermal Expansion

The TSS tether had a thermal coefficient of expansion (CTE) of -0.0000098 per degree Celsius. DuPont, the manufacturer of Kevlar, reports that its CTE is -0.000002 per degree Celsius, which is lower than the value observed for the composite material in reference 13. While most materials used in the tether have a positive CTE, the Kevlar strength member predominates, though the reason for this higher CTE magnitude is unknown.

Tether Twist

Tethers will inevitably experience some twisting torque during deployment in space, regardless of preflight handling. This torque can be managed by pre-twisting the tether on the spool, ensuring no net twist upon deployment. If no twists occur during deployment, the spool will remain twist-free. Longer tethers are more prone to twisting, and tension can exacerbate this effect. While preflight tests of the TSS tether were conducted to predict torque behavior, the results were inconclusive for flight conditions. However, the torque is expected to be minimal and should naturally unwind shortly after deployment. Solar heating and cooling may shift the tether's equilibrium position, but these effects are anticipated to be minor.

Skip Rope Motion

When a tether containing a conductor moves at velocity (V) through Earth's magnetic field (B), it generates an electromotive force (EMF) between the tether's ends, which can be calculated 

where θ is the angle between the magnetic field vector and the tether's velocity vector. If an electric current (I) flows through the tether, a force (F) is created, approximated by:

where θ is the angle between the tether's length and the magnetic field. This force generally acts in the westerly direction for orbits moving from west to east, with the magnetic field lines aligned along Earth's longitudes.

This force causes the tether to move, and oscillations in current can either amplify or dampen the motion, depending on the phase relationship between the current and the tether's movement. Initially, the motion is planar, but as the magnetic field varies, it can shift to an elliptical "skip rope" motion. If the tether needs to be retrieved, this motion must typically be managed. As angular momentum is conserved, the skip rope motion’s amplitude increases as the tether is pulled in. The centrifugal force from the tether can cause the satellite to move toward the deployer before the entire tether is retracted. However, the skip rope motion may be dampened during retrieval if a current is introduced that opposes the motion, requiring careful understanding of this dynamic behavior.

Safety and Mission Assurance

The tether will be built according to NASA-approved specifications, with a quality plan submitted for approval. NASA will oversee manufacturing, ensuring materials are traceable and a clean environment is maintained. All testing, storage, and shipping will be NASA-approved and inspected by a quality assurance team.

For crewless missions, tether safety is primarily concerned with ground processing and range safety. However, human spaceflight missions face additional risks, such as tether slack or breakage due to space debris, which could hinder reentry or cause collisions. The tether system must be designed to minimize these risks and include fault tolerance.

While achieving a "fail-safe" system is difficult, provisions for crew intervention will be made, and a tether-cutting saving system will be incorporated. A failure analysis will identify critical risks, with specific safety thresholds for missions. The Safety and Mission Assurance Office will ensure compliance with safety policies and review designs.

Applications of Space Tethers

Space tethers offer a diverse range of applications, each with significant potential to transform space operations.

1. Propellantless Propulsion: Electrodynamic tethers generate thrust by interacting with the Earth’s magnetic field, offering a fuel-free alternative for orbital adjustments. This can significantly reduce mission costs and enable long-duration space missions without the constraints of fuel resupply.
2. Deorbiting Space Debris: Tethers can be used to deorbit defunct satellites and space debris, addressing the growing problem of orbital clutter. By creating drag or generating electrical currents, tethers lower the altitude of debris, causing it to reenter the Earth's atmosphere and burn up safely.
3. Formation Flying and Stabilization: Tethers enable precise control of multiple spacecraft in formation flying missions. By adjusting tether lengths and tensions, engineers can maintain relative positions between satellites, which is critical for scientific missions and Earth observation.

Challenges in Tether Technology

Despite their potential, space tethers face significant challenges that require further research and development.

1. Deployment Mechanisms: Reliable deployment of tethers in space remains a technical hurdle. Issues such as tangling, mechanical failures, and inconsistent deployment rates can compromise mission success. Engineers are developing advanced systems to ensure smooth and controlled deployment.
2. Durability and Longevity: Tethers must withstand the harsh conditions of space, including extreme temperatures, micrometeoroid impacts, and atomic oxygen erosion. Enhancing material strength and incorporating redundant designs are essential to improving tether durability.
3. Efficiency and Control: Maintaining efficient current flow in electrodynamic tethers while minimizing energy losses is a challenge. Innovations in tether materials and power management systems are critical to optimizing performance.

Key Missions in Space Tether Technology

Space tethers represent a sustainable and innovative solution to many challenges in space exploration. Their ability to provide propulsion, stabilize satellites, and mitigate debris highlights their versatility. As research and development continue, space tethers are poised to become a cornerstone technology for future missions, offering cost-effective and environmentally friendly solutions to the complexities of space travel.