BETs

We name Space debris to all those objects orbiting the Earth that were created by humans, but no longer have any useful purpose. Inside this category of objects we include used upper stages from rockets, non-functional satellites and fragments coming from different spacecraft manoeuvres, explosions, collisions, etc.

Currently, the US Space Surveillance Network is using ground-based radar, optical and infrared sensors to track more than 12000 objects. The minimum size of a traceable object is about 5 to 10 cm for a low Earth orbit, and about 30 cm to 1 m for the geostationary orbit. Only about 6% of these catalogued objects are active satellites, whereas the 40% are fragments of disintegrated satellites and upper stages of rockets, and the remaining 54% corresponds to more than 200 in-orbit fragmentations that have been recorded since 1961. Except for a few collisions, most of such events were explosions related to spacecraft and upper rocket stages.

The vast majority of man-made debris comprises objects smaller than 10 cm, which are not tracked during routine operations. The number of objects larger than 1cm is estimated around 600.000 (ESA, 2009).

The basic hazard in Space caused by debris is damage or even Spacecraft destruction by collision. In fact, the first accidental collision between satellites occurred on February 2009, when an Iridium telecommunication satellite collided with a Russian military satellite Kosmos-2251. Nevertheless, current risk levels are small, although steadily increasing. For some Space missions, Space debris has already become a safety issue and active protection through shielding will be needed; the International Space Station is an example.

Present debris mitigation measures aim at reducing the growth rate, but the total amount and mass of debris is still rising, since rate of removal of debris by natural mechanisms, such as air-drag and orbit perturbations, is exceeded by Space utilization forecasts. In an extreme, this scenario could lead to a “Kessler Syndrome”, characterizing a situation in which the population of debris fragments in LEO is so high that collisions among them produce new fragments, increasing its density in a sort of “Avalanche Effect” and making the use of satellites unfeasible for many years.

Nowadays, the notion that more efficient mitigation measures must be taken is widely accepted. Those will likely include the removal of rocket bodies and spacecraft from Space upon completion of their missions.

A Space tether consists on a long cable that is used to couple two spacecraft together, which enables the transfer of energy and momentum between them, thus producing a form of Space propulsion.

There are two categories for Space tethers: Passive tethers, which allow just mechanical connection and are intended mainly for momentum transfer applications, and Electrodynamic tethers, which are made of conductive wires, allowing for flow of electric current between the ends.

The simplest application involves the use of the electrodynamic tether as a de-orbit system, using a dissipative mechanism arising from its motion relative to the co-rotating plasma and Earth’s magnetic field, which induces tape current and exerts drag on it producing orbital decay. This system is passive like air drag and propulsive like rockets and electrical thrusters. That concept is the cornerstone of the BETs project.

Moreover, it is possible to invert the electrodynamic tether operation injecting electric current from the Spacecraft power plant in the reverse direction, producing the opposite effect: transforming electrical energy into orbital one. Therefore, an electrodynamic tether interacting with the surrounding plasma and the Earth’s magnetic field can operate as an orbital control system.

Electrodynamic tethers have been inspiring aerospace engineers for many years, as they could be used for a broad range of applications like upper atmosphere probes, spacecraft orbit control, planetary exploration, auxiliary power generation, radiation mitigation, etc.

Although the complexity of their operation precluded rapid use in the past, aside of some test missions, current State-Of-the-Art in materials and components technology are opening numerous exciting opportunities for their application in current Space engineering.

The BETs Project consists in Research and Technology development of an efficient deorbit system based in Electrodynamic Tethers, to be carried in the future by every launched spacecraft.

A dedicated system is needed because LEO satellites naturally orbit at ionospheric altitudes where air drag is negligible. The system considered involves magnetic drag on a current-carrying conductive tether, uses no propellant and no power supply, while generating power for on-board use. It beats alternative systems (enhanced air drag, and rocket and electrical thrust) in simplicity and the combined basic metrics: Frontal Area x Deorbit Time and System-to-Spacecraft mass ratio. Like air drag, magnetic drag is a dissipative mechanism arising from the orbital tether motion relative to the corotating magnetized plasma, which induces the current in the tether.

The work programme of the project includes studies of plasma-tether interaction under ambient-plasma variations along orbit, performance dependence on orbital altitude/inclination, and trade-off against alternative systems; numerical simulations of current to a bare tether; and studies of orbit/tether dynamics, and of both tether survival and the tether itself as debris. Deorbiting a satellite representative in both orbit and mass in Low Earth Orbit is considered. Project tasks also include:

1.- Design and manufacturing the tether as a tape with possible materials-structure both lengthwise and in its cross section, and a study of materials involved.

2.- Deployment strategy, and design / manufacturing of subsystems: tether-deployment mechanism, end mass, electric control and driving modules, electron- ejecting plasma contactor, and interface elements.

3.- Microgravity, and hypervelocity-impact and tether-current laboratory tests.

The picture above represents the two operation modes of an orbit control system based in electrodynamic tether.

In the figure on the right, electric energy is transformed into orbital energy applying electric potential enough to generate a current from the Spacecraft to the anode. The tether interacts with the Earth's magnetic field generating a Lorentz Force that applies thrust to the spacecraft.

On the left, the opposite effect is produced as orbital energy is transformed into electric energy, thus dragging (deorbiting) the spacecraft.