Shuttle to reel out satellite on 12-mile line
Steve Connor is the Science Editor of The Independent. He has won many awards for his journalism, including five-times winner of the prestigious British science writers’ award; the David Perlman Award of the American Geophysical Union; twice commended as specialist journalist of the year in the UK Press Awards; UK health journalist of the year and a special merit award of the European School of Oncology for his investigative journalism. He has a degree in zoology from the University of Oxford and has a special interest in genetics and medical science, human evolution and origins, climate change and the environment.
Sunday 19 July 1992
The first real practical test of his ideas will come later this month, when one of the most dangerous experiments in space begins with the launch of a space shuttle that will reel out a satellite on a thin line 12 1/2 miles long.
The experiment will test the feasibility of using extremely long tethers in space, but scientists at the US National Aeronautical and Space Administration are nervous about what may happen if the satellite and line start swinging out of control.
The science fiction author Arthur C Clarke, writing 70 years after Tsiolkovsky, suggested that space tethers could be used to form a ring of satellites around the planet, with 'space elevators' taking people for rides in space. 'The distinction between Earth and space would be abolished, though the advantages of either could still be retained,' he wrote.
But it was an Italian space engineer, Giuseppe Colombo, who gave the ideas of tethered objects in space a firm practical basis. Colombo, who died in 1984, moved from Padua University in the late 1970s to Nasa's Goddard Institute in New York, where he proposed that tethers could be used to build giant structures, such as space stations, in orbit.
It is largely Colombo's ideas that have led Nasa, in conjunction with the Italian space agency, to test its first tethered satellite system, TSS-1, to be launched on 31 July on board the shuttle Atlantis.
The seven-strong crew will reel out the tethered satellite into a higher orbit - considered safer than lowering it into the rarefied atmosphere of Earth below.
Explosive bolts and other safety measures are ready to jettison the tethered satellite should the 'skip-rope effect' become uncontrollable. 'It's a squishy subject,' said Dave Bowlan, a Nasa scientist. 'We don't really know what will happen until we try.'
Nasa's tether, a mixture of copper wire, nylon and carbon fibres, is only a tenth of an inch in diameter but it can support 400lb. Even so, in the near weightlessness of the shuttle's orbit it is expected to experience a tension of no more than 15lb, despite being connected to a 100-ton shuttle and its one-ton satellite.
Nevertheless, there are plans for a second TSS mission, which involves lowering a tethered satellite below a shuttle to sample the upper regions of Earth's atmosphere. At this altitude the air is extremely rarefied, and as a result is much too high for the air- breathing engines of conventional aircraft. However, because there is enough air to cause quite severe aerodynamic drag, it is too low for a satellite to keep orbiting.
Scientists envisage testing the aerodynamics of future space planes by towing models from a shuttle overhead.
Even further into the future, there are tentative proposals to tether large landing platforms - 'shuttle ports' - to space stations orbiting high above. Space elevators would take crew and cargo from one to the other, cutting down on fuel.
If the tether is spring-loaded, it would absorb the shock of shuttles landing and taking off from the platform and prevent the space station from being disturbed too much.
Tethers from the space station, or other shuttles, could also take communications satellites or weather satellites into very high orbits at a fraction of the cost of getting them there using rockets.
Fuel depots tethered high overhead could be refuelled from below with the fuel taken up via the elevators. Nasa also envisages that two tethered objects could be spun around 'much like a set of bolas' to create artificial gravity - another fantasy of science fiction writers.
A further reason for using tethers is that they can be used as giant antennas to transmit radio signals around the globe. The mission later this month will use its tether as a rudimentary radio transmitter to test the idea.
Perhaps the most interesting phenomenon the TSS-1 experiment will test is whether it is possible to generate electricity as the tethered satellite and shuttle move through the Earth's magnetic field at 17,000mph.
Just as a dynamo generates electricity by the movement of wires in a magnetic field, so the tether - with its copper wire - is expected to produce up to 4 kilowatts of power as it hurtles through the Earth's field.
The generation of electricity also means that electrons will flow from the satellite on the end of the 12-mile line down the tether and on to the shuttle, causing it to become negatively charged. An unanswered question is whether enough electrons will jump off the shuttle to return to the positively charged satellite through the charged ionosphere of space.
Electron guns, which fire negative charge from the shuttle and the tether into the ionosphere, are ready to help complete the electric circuit and prevent a build-up of static electricity on the shuttle. If the experiment proves successful, scientists believe it may be possible to use tethers as the electrical generators of future space missions.
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