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DTUsat-1: Tether
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Tether


The primary payload is an electrodynamic tether. The tether is an unisolated copper wire of about 450 m length, picking up free electrons in the plasma trapped in the earth’s magnetic field. By emitting electrons from the satellite body, the tether (and satellite) will be positively charged. It will attract electrons, and a current will flow in the string. As the satellite, and thus the tether, moves in the magnetic field of the earth, this gives a resulting force on the satellite, as the return current will flow in the plasma. The mechanical system consisting of tether and satellite will therefore see a force in only one direction, making it possible to change the orbit without using fuel.

We will attempt to dump the satellite. This is simpler than lifting it, as the tether does not need to be isolated, and as the entire length of the wire acts as a plasma contactor. This eliminates arcing through the isolation as a failure mode. The tether itself is a 0.2 mm copper wire. It is deployed from a yo-yo-like spool, which is ejected by a spring, receiving both linear and angular velocity balanced to keep the satellite attitude stable and the tether tension low. The mechanical subsystem is very simple, and the risk of entanglement of the tether in the vicinity of the satellite is greatly reduced. The spool is braked at the end of travel by using a sticky glue to fasten the inner layers, again minimizing tension in the wire without generating space debris. The tether is electrically connected to the satellite structure. The electron emitter was projected as a chip scale device using field effects on a micrometer scale to emit electrons at low driver voltage and high efficiency. The maximum tether current using this emitter was 29 mA, which would lower the satellite orbit by 5 km/hour! However, the final metallization failed, so the tether will operate in passive mode and at considerably lower currents. Also, the tether can be shown to be dynamically unstable at this current, a problem which we have now avoided.

The mean life, the expected time before the tether is cut due to collision with space debris, for a 0.2 mm wire, is only about a week. By rolling the tether to a band of 1 mm width, the mean life could be extended to approximately 6 years, but a shipment from a Japanese manufacturer of aluminium band failed, leaving us with cupper wire as the only viable choice given our deadline.

A possible application of this device is removal of dead satellites, booster rockets etc. from orbit. If a cheap device is sold to be fitted before launch, these objects could be removed from orbit in a couple of months instead of leaving them in orbit for thousands of years.