Rotating space tethers

~Here is a paste about space tethers~
In a related mission in 1994, a payload was left hanging at the end of a 20-kilometer tether to see how long the connection--as thick as a kite string--would survive collisions with micrometers and space debris. The expected lifetime of the tether, which could readily be cut by a particle the size of a sand grain traveling at high speed, was a meager 12 days. As things turned out, it was severed after only four.

The experiment demonstrated the need to make tethers out of many lines, separated so that they cannot all be cut by the same particle yet joined periodically so that when one line fails, the others take up the load. With that in mind, the Naval Research Laboratory (NRL) and the National Reconnaissance Office (NRO) fabricated a 2.5-millimeter-diameter hollow braid of Spectra fiber (a high-strength polymer used in fishing lines) loosely packed with yarn. A four-kilometer length linking two satellites that was launched in June 1996 has remained orbiting in space uncut for almost three years.

In a follow-up experiment last October, NRL and NRO tested a tether with a different design: a thin plastic tape three centimeters wide with strong fiber strands running along its length. The six-kilometer tether should survive for many years in space, but the tape makes it heavy. Our company, Tethers Unlimited in Clinton, Wash., is working with Culzean Fabrics and Flemings Textiles, both in Kilmarnock, Scotland, to fabricate multiline tethers with an open, fishnetlike pattern that will weigh less and should last in space for many decades.

Other tether demonstrations are scheduled. The Michigan Technic Corporation in Holland, Mich., has plans in 2000 for a shuttle to release two science packages joined by a two-kilometer tether.

In addition, the NASA Marshall Space Flight Center is investigating the use of electrodynamic tethers for propellantless space propulsion. In mid-2000 a mission will demonstrate that a conducting tether can lower the orbit of a Delta 2 upper stage. At Tethers Unlimited, we are developing a commercial version of the NASA concept: a small package that would be attached to a satellite or upper stage before launch. When the spacecraft completed its mission or malfunctioned, the conducting tether would unfurl and drag against Earth's magnetic field, causing the craft to lose altitude rapidly until it burned up in the upper atmosphere. We will test such a tether de-orbit device in late 2000 on an upper stage built by the Lavochkin Association of Russia.

NASA is also considering such electrodynamic tethers for upward propulsion. In the system, solar panels would supply a flow of electricity through the tether to push against Earth's magnetic field. The resulting force could haul payloads around Earth indefinitely. This approach might be used to keep the International Space Station in orbit without refueling.

How far can tethers take humankind in the future? We and others have analyzed a system of rapidly cartwheeling, orbiting tethers up to hundreds of kilometers long for delivering payloads to the moon and ever farther. The idea is simple--think of Tarzan swinging from one vine to the next. First, a low-Earth-orbit tether picks up a payload from a reusable launch vehicle and hands the delivery to another tether in a more distant elliptical-Earth orbit. The second tether then tosses the object to the moon, where it is caught by a Lunavator tether in orbit there.

The Lunavator would be cartwheeling around the moon at just the right velocity so that, after catching the payload, it could gently deposit the object onto the lunar surface a half-rotation later. Simultaneously, the tether could pick up a return load. No propellant would be required if the amount of mass being delivered and picked up were balanced. Such a transportation mechanism could become a highway to the moon that might make frequent lunar travel commonplace

~Present tether material allows tethers up to about 20 miles long which increase altitude by less than 40 miles speed only slightly and are suitable only for low mass payloads. When and if CNT is mass produced cheap that is space rated and meets optimistic projections, thousand mile rotating tethers can produce large altitude and speed increases of high mass pay loads. Please comment, refute and/or embellish. Neil~