E.T.PACK

A H2020 FET OPEN project aming to develop a deorbit kit and related software based on low work-function tether technology. †

Current in-space propulsion technologies, such as chemical and electrical thrusters, work under the law of action-reaction. Consequently, the payload mass and mission lifetime are penalized by the need of propellant. E.T.PACK envisages a new era enabled by a free-of-propellant device named Low Work function Tether (LWT) that operates under different principles. Instead of carrying propellant, LWTs produce a propulsive force by taking advantage of the natural space environment, which includes the geomagnetic field, the ambient plasma and the solar radiation.

A LWT is a long tape of a conductor, for instance aluminium, coated with a thin layer of a special material that emits electrons when illuminated by the Sun. The typical length, width and thickness of the tether are few kilometres, a couple of centimetres and tens of microns. The LWT is packaged in a reel onboard the spacecraft and, once activated, it is deployed along the local vertical where the gravity gradient keeps it taut. Thanks to an electrodynamic effect, a tether segment captures electrons from the ambient plasma and the complementary segment emit them back through the thermionic and photoelectric effects, thus yielding a steady electric current and, consequently, a Lorentz force. In Low Earth Orbit (LEO), the Lorentz force is a drag that produces the re-entry (deorbit) of the spacecraft while giving power for on-board use. This mode of operation is called generator mode. Neither propellant nor power supply are needed, and the operation of the tether is fully passive (the reader can observe a similar drag by letting a magnet fall inside a pipe of cooper).

LWT can also be used in thruster mode to increase orbital height. This is possible using a power supply that reverses the natural direction of the current, then the spacecraft is re-boosted.

The potential impact for society is supported by a fundamental characteristic of LWTs: they are reversible devices that convert orbital into electrical energy and vice versa without using any consumable. Such a property is key for several space applications. For instance, a LWT in generator mode can be used for deorbiting spacecraft at the end of life, thus contributing to solve the space debris problem that is one of the most important space challenges of the next decades. LWTs in thruster mode can provide indefinite station keeping, which is of particular importance for satellites orbiting at very low orbit and the International Space Station that requires about 10 tons of propellant per year (see for instance the work by L. Johnson and M. Herrmann, International Space Station Electrodynamic Tether Reboost Study, 1998). The opening of new horizons for science and technology is also envisaged because LWTs can be used as scientific instruments and in missions to planet with magnetosphere such as Jupiter and Saturn.