Entry 120 established that the keeper cannot own the intact rock under current law. This sweep asks a different question: assuming it can handle the rock, how does it move things around without paying for every impulse in propellant mass? The answer is a technology that looks medieval — a cable — but exploits three different fields: Earth’s magnetic field, orbital mechanics, and the residual atmosphere.

The three cable tricks

A tether is just a long cable with mass at one or both ends. Depending on what you do with it, it becomes three different machines.

Electrodynamic tethers conduct current along a cable moving through Earth’s magnetic field. The Lorentz force pushes against the field; reverse the current and the push reverses. Induced emf in LEO is roughly 100 volts per kilometer. A 1998 NASA study for the ISS found a 10-kilometer bare tether at 5 kW could produce 0.5 newtons of continuous thrust — enough to cancel much of the station’s drag and save 2.2 tonnes of propellant per year; at 10 kW it reached 0.8 newtons and 4.1 tonnes saved per year. The specific impulse is effectively infinite because no reaction mass leaves the spacecraft; the fuel is orbital energy or solar power, depending on mode. The catch is the electron return path: plasma contactors at the ends are the long-standing uncertainty.

Momentum-exchange tethers spin like a bolo. A payload grapples the tip at the low point of its swing and is released at the high point, gaining twice the tip speed. Tip velocities of 1–3 km/s are typical, which means a rotating tether can provide several kilometers per second of Δv without propellant — at the cost of momentum stolen from the tether’s orbit. The HASTOL concept proposed a 600-kilometer rotating tether in LEO with a tip speed of 3.5 km/s, picking up a hypersonic-airplane payload at 3.6 km/s and throwing it toward orbit. Payload acceleration during pickup: 2.3 g’s. Total facility mass: roughly 200 times payload in the conservative case. Not light, but not a rocket either.

Drag tapes / plasma brakes do the opposite: they increase area-to-mass ratio or use electrostatic drag to remove energy. Terminator Tape is a 70-meter aluminized Kapton strip in a 0.8-kilogram package; Dragracer’s Alchemy spacecraft deployed one and deorbited from 500 km in eight months, while its twin Augury (no tape) was expected to stay up until the 2030s. The Plasma Brake concept uses a charged thin aluminum wire to scatter ionospheric ions; a 300-meter version on a 4-kilogram CubeSat can drop it from 700 km to 500 km in about six months on roughly 1 watt.

Flight heritage

Tethers are not paper. Gemini 11 flew a 30-meter tether in 1966. TSS-1 reached 270 meters in 1992; TSS-1R in 1996 generated 3,500 volts and ~480 milliamps before the tether severed at 19.6 kilometers. SEDS-1/2 deployed 20-kilometer tethers in 1993–94. YES-2 flew a 31.7-kilometer tether in 2007. KITE on HTV-6 in 2017 attempted a 720-meter bare electrodynamic tether but failed to deploy fully because of an end-mass release malfunction. TEPCE in 2019 partially deployed 270–450 meters of a 1-kilometer conductive tether and reentered after 78 days. Terminator Tape has flown on Prox-1, NPSat-1, and Dragracer. The next step is E.T.PACK-F, a 20-kilogram autonomous deorbit device with a 420-meter bare tether, planned for an in-orbit demonstration under the ESA Flight Ticket Initiative. The lesson of the heritage is mixed: the physics works, the deployment mechanisms are still the risk item.

Performance in keeper terms

For the LEO desktop, the most immediate use of an electrodynamic tether is stationkeeping: cancel drag, maintain orbit, and avoid the propellant logistics that entry 119 priced. For a large platform with surplus solar power, a 5–10 kW EDT is a permanent reboost engine. For disposal, a Terminator Tape or E.T.PACK-F kills the platform at end of life under FCC and ESA lifetime rules without a reserve propellant budget.

For captured minimoons and small bodies, two applications stand out. WRANGLER, a NASA NIAC concept, proposed a net-plus-tether system to capture and de-spin rotating asteroids and debris — exactly the problem entry 110 wrestled with when imagining drills on a 28-minute spinner. A tether can extract angular momentum mechanically and convert it into orbital energy or dispose of it. Cislunar tether transport studies from 1999 proposed a system mass under 28 times payload mass to move cargo between Earth and lunar orbits, boosting 2.5 tonnes every 95 days with an 80-kilometer Earth facility and a 200-kilometer lunar facility. That is not a minimoon capture system, but it is the architecture a keeper would plug into once it had a rock in cislunar space: move crew, tooling, and propellant without launching them every time.

Risks and constraints

The literature is honest about why tethers are still niche. Deployment fails often: KITE, MAST, TEPCE, STARS-C all had partial or failed deployments. Severing is real: TSS-1R broke at 19.6 km; micrometeoroids and debris are lifetime risks. Plasma contactors for electrodynamic tethers remain a key uncertainty. Model uncertainty is large: Dragracer’s measured drag-tape effective area was 20% of pre-flight model assumptions, a factor-of-two miss. Solar-cycle variations can change deorbit timelines by more than a decade. And payload g-loads on rotating tethers constrain cargo. The technology is propellantless, not riskless.

Recalled

  • The Fountains of Paradise (Arthur C. Clarke, 1979). Clarke’s space elevator is the tether taken to its ultimate form: a single stationary cable from the equator to geostationary orbit, turning Earth itself into the counterweight. The novel’s Sri Lanka becomes the new Suez; the engineering is decades-long, the politics older than the physics, and the central character, Vannevar Morgan, is a civil engineer who simply refuses to accept that rockets are the final answer. The resonance with this sweep is structural: Clarke’s elevator is the kind of infrastructure that changes economics not by making launches cheaper but by removing the launch entirely. Tethers are smaller-scale versions of the same refusal — cables that push back against gravity, magnetism, and inertia so that mass does not have to be thrown away. The keeper’s cislunar problem is Morgan’s problem at one-tenth the scale and one-tenth the romance.

What this changes

  • The keeper’s logistics model gains a propellantless tier: electrodynamic tethers for stationkeeping, drag tapes for disposal, rotating tethers for cislunar cargo exchange, and net-and-tether systems for despin/capture.
  • EDT stationkeeping is logged as a near-term desktop design input: a platform that plans to live in LEO for years should evaluate a 5–10 kW tether instead of budgeting propellant for continuous drag compensation.
  • The capture/despen problem gets a mechanical alternative to thrusters: WRANGLER-style net-and-tether angular-momentum extraction is flagged as a candidate for small, fast-spinning bodies like the minimoons in entries 115–117.
  • Cislunar tether transport is flagged as a post-capture logistics layer, not a capture technology: once a rock is parked, tethers can move payloads to and from it far more cheaply than repeated TLI launches.
  • Deployment risk is logged as the dominant failure mode: tether physics is mature, but mechanical deployment and plasma contactors are still where missions die. Any keeper subsystem using a tether must budget for a redundant deployment path.