The murky views which some scientists advocate as to the inevitable end of every living thing on Earth . . . should not be regarded as axiomatic. The finer part of mankind will, in all likelihood, never perish—they will migrate from sun to sun as they go out. And so there is no end to life, to intellect and the perfection of humanity. Its progress is everlasting.
Tsiolkovsky made that statement as a rebuttal to the dark future predicted for humankind by Thomas Malthus, a British clergyman who believed humankind was doomed to a future of misery because of overpopulation and the inadequacy of the food supply. The year was 1895, and Tsiolkovsky, considered by many the father of the space age, went to Paris, where he saw the Eiffel Tower and had a vision of a way to make space travel affordable. His idea was an elevator that would travel up a tower that would reach into space. With easy, affordable access to space and the other planets, it would be possible for humankind to spread out across the cosmos and avoid the catastrophe predicted by Malthus.
The tower Tsiolkovsky proposed was to be 35,786 kilometers (22,300 miles) tall. It needed to be that tall in order to reach the altitude of geostationary orbit , where the speed of orbit matches the rotational velocity of Earth. Anything less than that, and the people at the top of the tower would not be in orbit and spacecraft traveling to other planets would not be able to dock there to pick them up.
With such a tower, travel to other planets would become affordable to the mass of humanity, just as the steamships and transcontinental railroad made possible mass migration of Europeans to the United States. As the man who developed the mathematics for rocket-powered spaceflight, Tsiolkovsky knew that interplanetary migration would not be affordable if only rockets were used. Thus, the idea for his tower was born. Unfortunately, it is not possible to build Tsiolkovsky's tower even with today's materials.
The Earth Surface to Geo Space Elevator
Tsiolkovsky's tower has been studied and refined, and it has evolved into a more practical concept that involves a cable hanging both upward and downward from geostationary orbit. With this concept, the upper half of the cable and an asteroid counterweight are needed to balance the weight of the lower half of the cable that reaches down to the surface of Earth. This upper and lower cable combination centered on geostationary orbit, called an Earth Surface to Geo Space Elevator, was described by Arthur C. Clarke in 1978 in his book The Foundations of Paradise. Unfortunately, even this version of the tower is impossible to build. It was not until 1988 that an intermediate version of this concept that could be built with existing materials was conceived. It is called an Earth Orbiting Elevator.
The Earth Orbiting Elevator
The Earth Orbiting Elevator works by starting from a much lower-altitude orbit and hanging a cable down to just above Earth's atmosphere. Since the bottom end of that cable is traveling at less than orbital velocity for that altitude, it is possible for a high-speed aircraft to fly to the lower end of that cable without the need for stages and drop-off propellant tanks. This is possible because the speed of orbit decreases as one moves farther away from Earth. Since the altitude where the cable starts to be built is quite a bit higher than the altitude at the bottom of the cable, the bottom of the cable ends up moving at noticeably less than orbital velocity for its altitude. This means that an aircraft flying to the bottom end of the elevator does not have to go nearly as fast as a rocket going to orbit. As a result, the aircraft needs less propellant, does not need stages, and can carry a larger payload . Less propellant, no staging, and more payload means significantly lower launch costs.
The upward-pointing half of the Earth Orbiting Elevator is needed to counterbalance the lower half, but unlike the Earth Surface to Geo Space Elevator, it does not need an asteroid counterweight. Also, like the earlier elevator, the length of the upper half of the Earth Orbiting Elevator cable is selected so that a spacecraft arriving at or departing from the upper end of the cable is already traveling at Earth escape velocity. This is done to minimize the amount of propellant spacecraft need to carry to travel between planets, keeping the cost of travel affordable.
A Comparison of the Two Elevators
The differences between the two elevators can be visualized as variations of the Indian rope trick. Using this analogy, the Earth Surface to Geo Space Elevator is an Indian rope that hangs from a very high altitude cloud down to the ground and never moves. As a result, it is very easy to use. The Earth Orbiting Elevator does not reach all the way to the ground and moves across the sky as if it were a rope hanging from a low-altitude cloud on a windy day. Although it is obviously more difficult to use, the Earth Orbiting Elevator has the advantage of a significant reduction in the required cable length. The Earth Surface to Geo Space Elevator requires a cable over 47,000 kilometers (29,140 miles) long. The Earth Orbiting Elevator can be built with a cable as short as 1,500 kilometers (30 miles) but works better if one of 3,500 to 4,000 kilometers (2,170 to 2,480 miles) is used. The magnitude of the difference is obvious, and the fact that the Earth Orbiting Elevator can be built makes the choice obvious. The end result is that it is possible to build Tsiolkovsky's tower, and so the dark future predicted by Malthus is not inevitable.
Space Elevators for the Moon and Mars
As with all transportation systems, once enough people start making the journey, there is a need for more efficient transportation systems at the points of arrival. As a result, space elevators have been proposed for both the Moon and Mars. In this way, Earth, the Moon, Mars, the asteroids, and even the Earth-Moon LaGrange points known as L4 and L5 can all be tied together by an affordable transportation system that opens up the entire inner solar system to humankind.
see also Accessing Space (volume 1); Clarke, Arthur C. (volume 1); Getting to Space Cheaply (volume 1); Tsiolkovsky, Konstantin (volume 3).
Clarke, Arthur C. The Foundations of Paradise. New York: Harcourt Brace Jovanovich,1978.
Penzo, Paul A. "Tethers for Mars Space Operations." AAS Science & Technology Series,The Case for Mars II 62 (1984):445-465.
Sarmont, Eagle. "How an Earth Orbiting Tether Makes Possible an Affordable Earth-Moon Space Transportation System." SAE Technical Paper 942120.
Smitherman, David V. "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium." NASA/CP-2000-210429, Marshall Space Flight Center.
Sarmont, Eagle. "Affordable to the Individual Space Flight." <http://www.affordablespaceflight.com>.