From MICHAEL MORTON
It appears that Annabel Macleod and John Allsop believe that those who
have been working on space tethers are ignorant of the law of conservation
of energy (Letters, 5 September).
Of course a satellite using a tether to generate electricity would slowly
lose altitude as energy was extracted from its orbital motion. Conversely,
a satellite could raise its orbital altitude by allowing power to flow into
the tether.
If a space shuttle was equipped with a tether to produce the 40 kilowatts
of electricity it consumes during flight, its orbit would drop by about
3 kilometres per day. As the maximum flight duration is around eight days
this would not be important.
A permanently orbiting satellite would use a tether as a store rather
than a source of electricity. Current satellites must carry banks of nickel
cadmium or nickel hydrogen cells. Charged by the solar cells, they provide
power when the satellite is in the Earth’s shadow. A satellite with a tether
could use it to pump up its orbit when in sunlight and then produce electricity
when its orbit takes it into the Earth’s shadow. Effectively, the orbit
is being used as a battery.
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Also a tether could be used to make adjustments to the satellite’s orbit
without using its thrusters. This is very useful as the finite supply of
thruster fuel is usually what limits a satellite’s lifetime.
Michael Morton Taunton, Somerset
