Electrodynamic Tether (ET) history started in 1960, when Beard and Johnson discussed drag on a conductive spacecraft moving across a geomagnetic field B. As shown by our logo and acronym, the ET, i.e. a long conductor, is PACKaged in a reel on-board the spacecraft. Once activated, it is deployed along the local vertical where the gravity gradient keeps it taut. In the frame of the ET orbiting at velocity vrel, there is a motional electric field Em = vrel×B in the faraway plasma. As corroborated by NASA PMG mission, the field Em drives a steady current I along the ET and the Lorentz force I×B appears. In Low Earth Orbit (LEO), Lorentz force is a drag that produces the re-entry (deorbit) of the spacecraft while giving power for on-board use (generator mode). Neither propellant nor power supply are needed. If equipped with a power supply that reverses the natural direction of the current (given by Em), then the spacecraft is re-boosted (thruster mode). ETs are reversible devices that convert orbital into electrical energy and vice versa. The standard concept around 1990 was NASA tethered satellite system TSS-1, i.e. a 20-km insulated wire exchanging current at cathodic and anodic ends by using an active electron emitter (EM) and a big spherical conductor for anode (left panel in the Fig. 1). Several key developments in the following 25 years transformed old ETs into LWTs, that are more robust, simple and efficient.

First, the bare tether concept with EM (ET+EM) was introduced in 1993 [Sanmartin J., et al, J. Propulsion and Power, 1993]. The wire was left bare of insulation with no big end sphere. Electrons were captured by the ET itself and emitted back to the plasma by an active EM (middle panel in Fig. 1). A bare tether demonstration flight (ProSEDS) was prepared and qualified by NASA for flight in 2003, but it was cancelled following the Shuttle Columbia accident. A second revolution happened in 2012 when the cross-fertilization of ideas among partners in the FP7 project named BETs yielded the thermionic tether concept: if coated with a thermionic material that has a low work-function (loosely bound electrons), the tether itself emits electrons to the plasma passively and no active element would be needed (Williams J., et al IEEE Trans. Plasma Science, 2012). Few years later, it was pointed out that the photoelectric effect can be an important electron emission mechanism for coated tethers and the term Low Work-function Tether (LWT) was coined [Sanchez-Arriaga G and X. Chen, J. Propulsion and Power, 2018]. In a LWT, a steady current is reached in the tether without any active element: a tether segment captures electron passively as a giant Langmuir probe and the complementary segment emits them back to the plasma through the thermionic and photoelectric effects (right panel in Fig. 1).

Besides this trip towards simplicity and efficiency on current collection and emission, ETs have been also favoured dramatically by moving from the old (round) wires to thin tapes because the latter hold multiple advantages. Tapes are much more efficient collecting and emitting current, exhibit an excellent survivability in the orbital debris environment and, its three dimensions (length, width and thickness) allow scalability over a broad spacecraft mass ranges and orbit conditions. As a consequence, state-of-the-art ETs are much shorter (from hundreds of meters to a few kilometres), robust, and deorbit faster (typically within few months) than old round tethers.