In our eternal strive for progress the most important technologies are those of paradigm shift that suddenly emerges as a wholly new reap from a field of technology in saturation. Such was the emergence of the jet engine technology that superseded the reciprocal engine technology at its almost saturated stage. The tether technology is one technology of a similar, important kind. It will support our space activity in conjunction with conventional rocket technology much more efficiently, from a variety of fields and aspects. The specific features tether technology embodies as its most important aspects are 1) simplicity and low mass, 2) strength in tension 3) compact in (wound) volume, 4) autonomous construction, since deployment is possible without relying much on the limited human resource in orbit, and 5) a broad range of potential applications, particularly for the electrodynamic tether. This special issue is to summarize present status and expectations of the tether technology, namely to provide inherently sustainable space technology without a need for either propellant or carbon dioxide gas discharge.

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Tethers have been studied and validated in space since the 1960’s, and many studies show their applicability across a broad range of applications, including space propulsion, power generation, formation flying, multi-point ionospheric science, and active space debris removal. Important milestones include retrieval of a tether in space (Tethered Satellite System 1992), successful deployment of a >30-km tether in space (Young Engineer’s Satellite 2, 2007), operation of an electrodynamic tether with tether current driven in both directions-power and thrust modes (Plasma Motor Generator 1993), high tether current capability (Tethered Satellite System Reflight 1996), demonstration of a long-term (∼10 year) tether operation on-orbit (Tether Physics and Survivability Experiment 1996-2006), and the successful deployment of a tape tether (Japan Tether Experiment 2010). Various types of tethers and systems could be used for space applications. Electrodynamic tethers can use solar power to ‘push’ against a planetary magnetic field to achieve propulsion without the expenditure of propellant. Utilizing completely different physical principles, long non-conducting tethers can exchange momentum between two masses in orbit to place one body into a higher orbit or a transfer orbit for lunar and planetary missions. Tethers can also be used to support space science by providing a mechanism for precision formation flying, fixe baseline multi-point science observations, and for reaching regions of the upper atmosphere that were previously inaccessible.

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The project to verify the performance of space tether technology was successfully demonstrated by the launch of the sounding rocket S520 the 25th. The project is the space demonstration of science and engineering technologies of a bare tape electrodynamic tether (EDT) in the international campaign between Japan, USA, Europe and Australia. Method of “Inverse ORIGAMI (Tape tether folding)” was employed in order to deploy the bare tape EDT in a short period time of the suborbital flight. The deployment of tape tether was tested in a various experimental schemes on ground to show high reliability of tape tether deployment. The rocket was launched on the summer of 2010 and deployed a bare electro-dynamic tape tether with length 132.6 m, which is the world record of the length deployment of tape tether. The verification of tether technology has found a variety kind of science and technology results as the first in the humankind and will lead a large number of applications of space tether technologies.

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A 3-year Project financed by the European Commission is aimed at developing a universal system to de-orbit satellites at their end of life, as a fundamental contribution to limit the increase of debris in the Space environment. The operational system involves a conductive tapetether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. The Project will size the three disparate dimensions of a tape for a selected de-orbit mission and determine scaling laws to allow system design for a general mission. Starting at the second year, mission selection is carried out while developing numerical codes to implement control laws on tether dynamics in/off the orbital plane; performing numerical simulations and plasma chamber measurements on tether-plasma interaction; and completing design of subsystems: electronejecting plasma contactor, power module, interface elements, deployment mechanism, and tether-tape/end-mass. This will be followed by subsystems manufacturing and by currentcollection, free-fall, and hypervelocity impact tests.

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The increase in the orbital debris population is becoming a serious problem for human space activities. One of the most effective solutions to remediate the on-orbit environment is the active removal of existing large orbital debris. In order to realize low-cost active debris removal systems, simple and efficient deorbit propulsion is needed. Electrodynamic tether is a promising candidate for such a propulsion system because of its propellant-less mechanism, high-efficiency in weight and electrical power, and ease of attachment to debris. The first half of this paper describes the fundamentals of the electrodynamic tether including its advantages and disadvantages, and the strategic roadmap of the development of the active debris removal systems using the electrodynamic tether. The second half describes the current research and development status of the system and key technologies for a demonstration flight of the electrodynamic tether as the first step of the strategic roadmap.

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This paper describes the two kinds of space verification experiments for a tethered space robot. A tethered space robot is a new type of space robot system proposed in the previous work. The major advantage is that its attitude can be controlled under tether tension by its own link motion. One space experiment was done by a pico-satellite named “KUKAI” on the orbit (Altitude: 666 km, Inclination: 98 deg). Basic function of the tether tension control system was verified, though tether could not be extended enough. Also, the mother (4.2 kg) and the daughter satellites (3.8 kg) were separated away for several centimeters, and then, basic robot motion and sensor based feedback control were verified. The other space experiment was done by the sounding rocket S-520-25. It has a capability for launching far above 300 km altitude and provides more than 5 minutes for space flight environments. The tether was extended (more than 2 m) and kept its tension, and the attitude control of the robot (2.2 kg) was performed. The disturbed vibration of attitude was suppressed, and also it was converged on the desired attitude by arm link motion control of the robot.

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In recent decades much research has focused on the use of space tether systems for payload orbital transfers. Orbit raising through the exchange of momentum using hanging tether systems in addition to enhanced momentum exchange resulting from motorised torque-driven systems have both been considered. In this paper the focus is on the performance of symmetrically laden motorised momentum exchange tethers with the aim of conducting orbital transfers. These consist of two propulsion tethers symmetrically attached to a motor shaft with payloads, to be later transferred, attached to their free ends. Rotation is achieved through reaction against two further tethers and masses attached to the motor stator with an estimated mass of this system, minus the propulsion tethers and payloads, of 1.5 tonnes. By deriving the maximum tension acting on a system orbiting an oblate Earth expressions are obtained for the maximum rotational velocity that the tether systems are capable of withstanding, based upon the material properties of the tethers themselves. Having obtained these expressions the performance of both hanging and motorised tether systems for conducting orbital transfers is analysed in terms of the gain in energy when the orbital geometry and tether length are varied. From this analysis the material-specific tether length which gives the greatest gain in rotational kinetic energy to the payloads is established for a motorised system. Finally, by utilising this material-specific length, the minimum semi-major axis which is capable of performing lunar transfers and Earth escape trajectories is obtained.

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