Space Elevator - Since the beginning of the space age on October 4^th, 1957 when Sputnik-1 launched on board a modified intercontinental ballistic missile with the designation R-7 space exploration has had grand successes, but it has also always suffered from the bottleneck of the launch by rocket. Rockets are inherently risky, violent vehicles that require huge resources in the form of propellants. The utilisation of rockets for space launch has two negative side effects: the payload is subjected to an extreme dynamic and thermal environment, and the launch is very costly. Launching one kilogram is between 10,000  € and 20,000 €, depending on the target orbit. The space transportation bottleneck has slowed space exploration after its initial successes and only large organizations can afford to be involved in space exploration.

In 1895 a Russian elementary school teacher by the name of Konstantin Tsiolkovsky concluded that if an equatorial tower would extend to an altitude of 35.786 km, a person climbing the full height of the tower would be in orbit when he reaches its top, because at that altitude the orbital period matches the Earth's rotation. Of course Tsiolkovsky knew that such a tower cannot be built out of any material that exists on Earth.

It was only in 1959 when the idea of a mechanical space launch system was picked up, again by a Russian scientist. Yuri Artsutanov proposed that instead of building a tower, a geosynchronous satellite could lower a cable all the way down to the Earth's surface. This way the material would not have to bear the immense compression forces, but tension instead. But again, the tension required to support the cable was too big to be born by existing materials. The strength/weight property of a cable can be expressed by its "rupture length". The rupture length is the maximum length to which a cable can be suspended in a constant gravity field of 1g, the force of gravity at the Earth's surface, before it breaks under its own weight. Because the Earth's gravity actually decreases with altitude, the required rupture length for constant cross-section elevator cable to the geostationary altitude of 35.786 km is 4.960 km [1].

Rupture lengths of existing materials are shorter by at least an order of magnitude: steel - 11.3 km, high-performance carbon fibre - 329 km.

It was the famous Arthur C. Clarke who introduced the space elevator to the popular culture in his science fiction novel "The Fountains of Paradise" in 1979. Since that time the elevator inspires young researchers and has been subject to a large number of scientific and engineering publications. So, when will we have a space elevator? Clarke once said: "We'll have the elevator 50 years after everybody has stopped laughing". He later corrected that figure to 10 years.

Basic Mechanical Properties of the Space Elevator
In 1975 Jerome Pearson found that a partial solution of the material problem could be to taper the cross-section of the elevator cable, so that it is thickest at the spot of highest tension: the geostationary altitude. At its ends the elevator cable would have zero tension and thus requires much less thickness. However, the required tapering is extreme for weak materials. A tapered steel cable would require a mass that exceeds the mass of the universe. Luckily the tapering becomes much more manageable for high-performance material. For example, if we had a material with a rupture length of 3.600 km (compared to 4.960 km required for a non-tapered cable), the cross-section at the geostationary altitude would have to be only four times the cross-section at the ends. The total mass of the cable with a payload capability of one hundred tonnes would also be on the order of a few tens of thousands of tons, not very different from the amount of material needed for cables of large suspension bridges. Pearson also showed that a cable could be constructed such that it is balanced by its own weight, a satellite is no longer required as counter weight. In this case the upper end of the cable would extend to an altitude of 144.000 km.

The "magic" of the space elevator comes from its mechanical connection to the Earth's surface. Any radial forces (up/down) are transferred to all of the cable via tension. This way, the cable as whole reacts to mechanical activities, for example a climbing cabin. A cabin that is connected to the lower end would slightly lower the altitude of the centre of mass from its ideal geo-synchronous point (120 km shift for a 100 t cabin on a 30,000 t cable). As a consequence the cable as a whole would start drifting in the direction of Earth rotation. This drift, in turn, would increase the centrifugal forces at the upper end, which would pull the whole structure up again. Of course all that would occur as a dynamic response, that is, there would be waves of transversal elongation migrating up and down the cable. Those waves would have to be managed together with the waves created by Coriolis forces of the climbing cabin by providing a lateral force at the anchoring point, for example by means of a sea-going platform that pushes the lower end of the cable along the equator.