Science Questions

How strong a cable could be used to stop the moon moving away from Earth?

Sun, 18th Dec 2011

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Hong asked:

Since the moon is moving away from the Earth, how strong a cable would we need to tie the moon down and stop it getting away?


Dominic -   One problem that you would have is the fact that the Moon goes around the Earth every 29 days and so, you'd need some quite good bearings on this string to carry it around the Moon’s orbit over the Earth.  That will be made especially difficult because the Moon doesn’t trace out the same path over the Earth each time it orbits.  So you'd need some kind of travelling system to hold on to the end of this piece of string.  Certainly, people have done the calculation for geosynchronous satellites to make space elevators and people have worked out that even spider’s thread which is this strongest  string that we know of, is not strong enough to support its own weight to travel up to a geosynchronous satellite.  To go out to the moon, you'd need something considerably stronger than that.

Chris -   Because the moon is actually going further away by a couple of centimetres every year and in the process, we’re giving the moon energy from the Earth’s spin.  So the moon is speeding up which is why it’s departing.  The moon slightly lags the Earth in terms of the Earth’s turn which is why we’re giving energy to the Moon as it heaps up water on the Earth’s surface.

Dominic -   That's right.  The Moon is triggering the tides on the Earth and in the process of triggering those tides, because the Earth is spinning, it’s putting a force back on the moon which is spinning up the Moon’s orbit and will eventually mean that the moon might leave the Earth’s orbit altogether.


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