This month, the University of Helsinki and the European Space Agency (ESA) will test a new space tether that has less chance of snapping under the stresses of operating in orbit. Installed aboard Estonia’s ESTCube-1 cubesat, the new tether is scheduled to be launched with ESA’s Proba-V satellite atop a Vega rocket as part of an experiment in developing an electric solar sail.
Tethers have been around since the United States’ Gemini 11 deployed one in 1966 to help control the capsule’s rotation. These tethers are routinely used to despin satellites by playing out weights from a spinning spacecraft. As the weights move further away, the satellite’s rotation slows down. When the spin rate is suitably slow or stopped, the tethers are released in an outer space version of “crack the whip” and the spin energy is carried off by the weights.
However, tethers have much greater potentials than just despinning satellites. They also hold the promise of allowing spacecraft to lower sensor packs from orbit into the Earth’s outer atmosphere, generate electricity by acting as an armature as they moves through the Earth’s magnetic sphere and even providing propulsion in the form of an electric solar sail.
The problem is, space tethers are a formidable engineering challenge. They need to be thin, light and flexible in order to fit inside of a spacecraft or launch vehicle, they need to be extremely long, they need to be strong enough to support a payload as well as their own weight, and, if they must transmit power or data, they need to maintain internal integrity. In other words, it doesn't help if the tether doesn’t break if the electric wire inside snaps. At the moment, experimental tethers haven’t had a great track record. About half fail to deploy, snap or are broken by micrometeoroids.
Researchers at the University of Helsinki believe that they’ve come a step closer to a solution with a new tether that is about half the diameter of a human hair. The 50-micrometer aluminum tether has a smaller 25-micrometer wire woven into it. The clever bit is that the Helsinki team used ultrasonic welding techniques from the microelectronics industry to cross connect several wires together every centimeter. These “subwires” allow electricity to flow through the wire even if the tether stretches, causing the wires to break. If only one subwire remains intact, the current still flows.
The tether will be tested aboard Estonia’s ESTCube-1. This 10 X 10 X 10 centimeter (3.9 in.) cubesat that weighs only 1.33 kilograms (2.86 lb) and was designed, built and operated by the students from a number of Estonian universities led by the University of Tartu with Tartu Observatory as part of ESA’s Plan for Cooperating States agreement with Estonia. Its twelve-month mission includes using the new tether as part of an electric solar sail or ”E-sail.”
The term “solar sail” is apt because the principle is exactly the same as terrestrial sailing with the space-faring sail being able to tack, run before the wind and carry out similar maneuvers. Conventional solar sails use gigantic spreads of gossamer-thin Mylar plastic with molecule-thick metal coatings to catch the solar winds to provide propulsion. Where the E-sail differs is that instead of plastic, it uses a sail made of an electric field to catch charged particles flowing from the Sun. The tether generates the electric field and acts something like a mast.
After it reaches orbit, the ESTCube-1 will deploy a ten-meter (32-ft) long, single-strand tether referred to as a “Heyther.” The change in the cubesat’s rotation and images from its camera will confirm this as well as measuring the amount of thrust caused by the solar winds. According to ESA, if successful, the tether and solar sail will be not only be able to propel space probes on missions of exploration, but could also be used to deorbit satellites cheaply at the end of their useful life.
A 100-meter (328 ft) E-sail tether be included in the Finnish CubeSat Aalto-1 next year.
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