## 2 Existing Technology and Challenges
### Momentum Exchange Tethers  
Tether technology in space is not a very common practice, but it has been seen on a variety of missions. The first demonstration of this technology dates back to NASA's Gemini XI mission in 1967, which incorporated a 30 m tether connecting the crewed Gemini spacecraft with the Agena target vehicle. This system rotated and induced a light artificial gravitational acceleration for roughly 3 h, then disconnected and sent Gemini and Agena in separate opposing directions as part of a tether dynamics experiment [6]. Since this time, most missions incorporating tethers have been focused on tether dynamics similar to Gemini XI, electrodynamic tethers which rely on interactions with Earth's magnetic field for propulsion or power generation purposes, or \"space webs\" where payloads are connected by tethers for formation flying purposes [7; 8]. Momentum exchange tethers have seen some, but ultimately little usage in the space environment, and have also been used for individual deployments rather than multiple, as is being investigated by the research this article introduces. METs are a space propulsion method that rely on the exchange of momentum between two tether-connected objects, typically a payload and a deployer or counterweight. They are not to be confused with electrodynamic tethers or space webs, which operate in different ways. The MET concept involves the mechanical rotation and separation of a spinning system, converting angular momentum into linear momentum upon release, which can alter the objects' velocities and positions [7; 9; 10]. Rotation in MET systems could be induced using either propulsion systems like cold gas thrusters (as used on Gemini XI), or electric motorized systems [11; 12]. Operating this system is similar to a shepherd's sling spinning and releasing a stone, like in the Biblical story of [PERSON] and [PERSON]. Generalized visualizations of such systems' operations are included later in this article. A key advantage of METs is their minimal fuel consumption (or complete avoidance thereof), making them attractive for missions with fuel supply and storage constraints. Space tethers can serve various purposes beyond propulsion, including station-keeping, attitude control, power generation, and passive deorbiting, although this article focuses solely on the tether-based deployment of constellations, particularly orbit and spacecraft design. Despite their potential benefits, METs require precise spacecraft-tether coordination and can be affected by environmental factors like solar wind, atmospheric drag, thermal effects, and micrometeoroid impacts over extended periods. Their payload capacity is constrained by mass and geometry, limiting their suitability for certain high-load missions or missions requiring higher spin rates or torques than can be achieved by feasible drive systems [13]. METs have been tested in a very limited capacity in past space missions, and traditional propulsion systems have remained dominant due to their broader applicability [7; 9]. As a result, the technology readiness level (TRL) of MET technology can be estimated to be around level 6 or 7, where the technology has been demonstrated in the operational environment of space, but has not yet been applied outside of experimental research and development. METs that have flown specifically for momentum exchange purposes were designed as exceedingly long tethers, such as the SEDS-1 (1993) and SEDS-2 (1994) missions which utilized 20 km tethers [14], and the YES-2 (2007) mission which utilized a 30 km tether [15]. These missions involved the usage of tether systems to deploy only individual payloads. Many other efforts have conceptualized tether systems of similar or even larger scale for applications such as skyhooks or even space elevators, but most of these are not quite yet feasible with present resources. This article introduces one potential application that could benefit from using tethers which has not been previously researched: satellite constellations deployed by tethers.