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Japan Aerospace Exploration Agency

Research Introduction

Cleaning Up the Space Junk

– Toward removal of space debris by means of an electrodynamic
tether system –

Advanced Space Technology Research Group

(front : from left)
Satomi Kawamoto, Atsushi Nakajima
(rear :from left)
Shin-ichiro Nishida, Shoichi Yoshimura, Yasushi Okawa,
Yosuke Nagao, Takeshi Hoshino

The term "space debris" refers to any artificial object that is no longer working and left in the mission orbit, traveling at high orbital velocity, 7 km/s or more in a low altitude orbit. The debris ranges widely from a few grams of fragment to several tons of satellite. The number of the space debris has been increasing ever since the first launch of an artificial satellite in 1957. Since the debris collision risk is growing, the effective and reasonable mitigation measures are required to ensure safe space development activities in the future.

Generation of debris

How has the debris been generated? Not only the remains of a launching rocket, but also the life-ended satellite itself will become debris.
The number of debris is increasing swiftly in such useful regions as low altitude orbits below 2,000 km, and geostationary orbit around 36,000 km altitude. Thus concerns are rising that some debris might collide mutually, generating huge number of new debris, and also collide with a working satellite, damaging its functions. Some studies reported that the density of debris in a particularly useful orbit has already reached such level that a cascade reaction of the collisions could occur even without additional launches. Kawamoto believes that "a removal system will be the only way to solve the problem if this happens."

Debris studies promoted by ISTA

ISTA tackles the space debris problem from the standpoints of observation and modeling, protection, and prevention of generation.
The space debris of 10 cm or larger in low altitude orbits and 50 cm or larger in geostationary orbit are cataloged by the North American Aerospace Defense Command (NORAD). As part of its observation and modeling studies, ISTA has developed its own software for debris detection by processing the stored image data, and is carrying on the monitoring of 20-cm class debris in geostationary orbit.
As part of its study of protection, assuming the debris collision, ISTA is conducting the ground tests of the hypervelocity impact of aluminum projectile up to the speed of 5 km/sec to the carbon fiber-reinforced plastic (CFRP) which is used for structural materials of satellite. In the series of these tests, the damages by projectile impact to CFRP specimens are examined, and the results and relation between projectile energy and damaged area are contributing to the development of a protection system.
For the prevention of debris generation, ISTA is studying a debris removal system and one of the most important subsystems is high efficient propulsion system. Electrodynamic tether (EDT) system, is promising since it requires almost no fuel.

Electrodynamic tether(EDT) system and on-orbit demonstration test

EDT is a conductive thin string. The principle of EDT system for removing debris is illustrated in Figure 1.

Fig. 1 Principle of EDT<br/>(Demonstration test reviewing chart)

Fig. 1 Principle of EDT
(Demonstration test reviewing chart)

EDT system planned by ISTA consists mainly of a conductive bare tether and an end-mass with an electron emitter(FEAC). An electromotive force (EMF) is set up with in the conductive tether deployed from a rocket upper stage. Near-Earth space is filled with very thin plasma consisting of cations (electrically positive) and electrons (negative), and the electrons are collected through the surface of the bare tether (without insulation). On the other hand, the electrons are emitted to space through FEAC, and thus electric current flows through the tether. This current generates a Lorentz force in the opposite direction to the orbital motion. The force causes the orbital energy to decrease, resulting in lowering orbit.

Nakajima explains, "What we are studying is a debris removal vehicle with multiple EDT systems. The vehicle approaches a debris and installs one EDT system on it. The EDT system activates itself in a prescribed sequence and deploys the tether. The orbit of the debris can be lowered by the EDT thrust, until it reenters atmosphere." At present, the trial fabrications are promoted of prototype field-emission type electron sources (FEAC) using carbon nanotubes based on nanotechnology, and a conductive bare(electrically no-insulated) tethers, toward an on-orbit demonstration of EDT system aboard an rocket upper stage (Figs. 2 and 3).

Fig. 2 Samples of bare tethers for electric discharge tests<br/>
        Left: Braided tether with aluminum and Kevlar©.<br/>Right: Braided tether with aluminum and carbon fiber

Fig. 2 Samples of bare tethers for electric discharge tests
Left: Braided tether with aluminum and Kevlar©.
Right: Braided tether with aluminum and carbon fiber

Fig. 3 Field-emission type electron source (FEAC)<br/>
A luminous part at the bottom shows the FEAC emitting electrons.

Fig. 3 Field-emission type electron source (FEAC)
A luminous part at the bottom shows the FEAC emitting electrons.

(Public Relations)