Would a very, very, very long piece of string reaching from Earth’s surface deep into space remain suspended?
• It could if the string were strong enough, and provided the force of gravity pulling it towards Earth was equal to the centrifugal force acting on the string as it spins around Earth, and that pulls it away. However, in reality, these opposing forces would be so great that the string would snap.
The discovery of superstrong carbon nanotubes (molecules made of carbon, which are atoms thick but really long in comparison) made a string with sufficient strength seem possible. But the longest carbon nanotube made so far is just 55 centimetres.
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Danielka Elliott, By email, no address supplied
• It depends on where the string is attached to Earth’s surface and what, if anything, is attached to its far end. In most cases, the string will wind itself around the rotating Earth, fall to the surface under the influence of gravity or simply break under the forces exerted on it.
The only stable case is where the string is attached to a point on the equator and the far end is rotating around Earth directly above that point.
Richard Parkins, Cambridge, UK
• The whole string must orbit Earth in exactly one day, so as to remain in a constant position relative to the ground. For this to happen, the string’s centre of mass must be positioned at the elevation of geostationary orbit (35,786 kilometres above Earth’s surface) where, for example, communication satellites are often placed.
The far end of the string either needs to be attached to a heavy counterweight, or you need the string there rolled up into a very big heavy ball. In fact, this is the concept of the space elevator, proposed as a way of transporting things into space without using rocket launches.
The big problem is that there is no known material that could withstand the accumulated weight of the long cables needed. But if you could invent a superstrong string that wouldn’t break under its own weight when reaching up such a great distance, then the idea would work – and you could earn a fortune.
Richard Swifte, Darmstadt, Germany
• In practice the string wouldn’t be very stable. Winds on Earth and dust in space would bend it, altering the position of its centre of mass. You would do better with a more rigid structure, which is the basis of the space elevator concept beloved of science-fiction writers: forget big rockets, just go up on the elevator to a geostationary platform and launch your spacecraft there.
Guy Cox, St Albans, New South Wales, Australia
For more on this subject, keep an eye out for our upcoming feature on space elevators – Ed
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