What is a Space Elevator?

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A space elevator is a proposed non-rocket space launch structure (a structure designed to transport material from a celestial body's surface into space). Non-rocket space launch (NRS) is the idea of reaching outer space specifically from the Earth's surface predominantly without the use of conventional chemical rockets, which currently is the only method in use. Many elevator variants have been suggested, all of which involve travelling along a fixed structure instead of using rocket-powered space launch, most often a cable that reaches from the surface of the Earth on or near the equator to geostationary orbit (GSO) and a counterweight outside of the geostationary orbit. A geostationary orbit (or Geostationary Earth Orbit - GEO) is a geosynchronous orbit (orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period) directly above the Earth's equator (0 degree latitude), with a period equal to the Earth's rotational period and an orbital eccentricity (orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle of approximately zero).

While some variants of the space elevator concept are technologically feasible, current technology is not capable of producing practical engineering materials that are sufficiently strong and light to build an Earth-based space elevator of the geostationary orbital tether type. Recent concepts for a space elevator are notable for their plans to use carbon nanotube or boron nitride nanotube based materials.

What is the History of Space Elevators?

In 1895, the primary concept of the space elevator appeared, when Russian scientist Konstantin Tsiolkovsky was enthused by the Eiffel Tower in Paris to consider a tower that reached all the way into space, built from the ground up to an altitude of 35,790 kilometers (22,238 mi) above sea level (geostationary orbit). Unlike more recent concepts for space elevators, Tsiolkovsky's (conceptual) tower was a compression structure, rather than a tension (or "tether") structure. In 1959 another Russian scientist, Yuri N. Artsutanov, suggested a more feasible proposal. Artsutanov suggested using a geostationary satellite as the base from which to deploy the structure downward. By using a counterweight, a cable would be lowered from geostationary orbit to the surface of Earth, while the counterweight was extended from the satellite away from Earth, keeping the center of gravity of the cable constant relative to Earth.

Making a cable over 35,000 kilometers (22,000 miles) long is a difficult task. In 1966, Isaacs, Vine, Bradner and Bachus, four American engineers, reinvented the concept, naming it a "Sky-Hook," and published their analysis in the journal Science. After the development of carbon nanotubes in the 1990s, engineer David Smitherman of NASA/Marshall's Advanced Projects published a book which provides an introduction to the state of the technology at the time, and summarizes the findings. In 2008 the book "Leaving the Planet by Space Elevator", by Dr. Brad Edwards and Philip Ragan, was published in Japanese and entered the Japanese best seller list. This has led to a Japanese announcement of intent to build a space Elevator at a projected price tag of 5 billion dollars.

What is a Geostationary orbital tether?

This notion, also known an orbital space elevator, geostationary orbital tether, or a beanstalk, is a division of the skyhook concept. Skyhooks are a theoretical class of cable based techniques intended to lift payloads to high altitudes and speeds. Construction would be a large project: the minimum length of an Earth-based space elevator is well over 38,000 km (24,000 mi) long. The tether would have to be built of a material that could bear tremendous stress while also being light-weight, cost-effective, and manufacturable in great quantities. Materials available at present do not meet these requirements, although carbon nanotube technology shows great guarantee.

What is the Physics behind space elevators?

• Apparent gravitational field: The space elevator cable rotates along with the rotation of the Earth. Objects fastened to the cable will experience upward centrifugal force that opposes some, all, or more than the downward gravitational force at that point. Along the length of the cable, the actual (downward) gravity minus the (upward) centrifugal force is called the apparent gravitational field.
• Cable section: The cable material combined with its design must be strong enough to support 35000 km (22,000 mi) itself. By making any cable larger in cross section at the top compared to the bottom, it can hold up a longer length of itself. For a space elevator cable, an important design factor in addition to the material is how the cross section area tapers down from the maximum at 22,000 miles to the minimum at the surface. To maximize strength to weight of the cable, the cross section area will need to be designed in such a way that at any given point, it is proportional to the force it has to withstand.

What is the Principle behind Space elevators?

The centrifugal force of earth's rotation is the main principle behind the elevator. As the earth rotates, the centrifugal force tends to align the nanotube in a stretched manner. There are a variety of tether designs. Almost every design includes a base station, a cable, climbers, and a counterweight.

• Base station: The base station designs typically fall into two categories—mobile and stationary. Mobile stations are basically large oceangoing vessels which are able to maneuver in order to avoid high winds, storms. Stationary platforms would generally be located in high-altitude locations, such as on top of mountains, or even potentially on high towers and they typically would have access to cheaper and more reliable power sources, and require a shorter cable.
• Cable: Carbon nanotubes are one of the candidates for a cable material. A space elevator cable must carry its own weight as well as the (smaller) weight of climbers. The required strength of the cable will vary along its length, since at various points it has to carry the weight of the cable below, or provide a centripetal force to retain the cable and counterweight above. The cable must be made of a material with a large tensile strength/mass ratio.

• Climbers: While various designs employing moving cables have been proposed, most cable designs call for the "elevator" to climb up a stationary cable. Climbers cover a wide range of designs. On elevator designs whose cables are planar ribbons, most propose to use pairs of rollers to hold the cable with friction. Climbers must be paced at optimal timings so as to minimize cable stress and oscillations and to maximize throughput. Lighter climbers can be sent up more often, with several going up at the same time. This increases throughput somewhat, but lowers the mass of each individual payload. Both power and energy are significant issues for climbers—the climbers need to gain a large amount of potential energy as quickly as possible to clear the cable for the next payload. The proposed method is laser power beaming, using megawatt powered free electron or solid state lasers in combination with adaptive mirrors approximately 10 m (33 ft) wide and a photovoltaic array on the climber tuned to the laser frequency for efficiency.

What are the difficulties in constructing the Space elevator?

The construction of a space elevator would be a vast project requiring advances in engineering, manufacturing, and physical technology.

• A space elevator would present a navigational hazard, both to aircraft and spacecraft.
• Aircraft could be diverted by air-traffic control restrictions.
• Impacts by space objects such as meteoroids, micrometeorites and orbiting man-made debris, pose a more difficult problem.
• With a space elevator, materials might be sent into orbit at a fraction of the current cost. As of 2000, conventional rocket designs cost about $11,000 per pound ($25,000 per kilogram) for transfer to geostationary orbit. Current proposals envision payload prices starting as low as $100 per pound ($220 per kilogram).

What are Extra terrestrial elevators?

A space elevator could also be constructed on other planets, asteroids and moons. 

Mars' surface gravity is 38% of Earth's, while it rotates around its axis in about the same time as Earth. Current materials are already sufficiently strong to construct such an elevator. A lunar space elevator can possibly be built with currently available technology about 50,000 kilometers (31,000 miles) long extending through the Earth-Moon L₁ (the L₁ point lies on the line defined by the two large masses M₁ and M₂, and between them) point from an anchor point near the center of the visible part of Earth's moon. However, the lack of an atmosphere allows for other, perhaps better, alternatives to rockets. A lunar space elevator is a proposed cable running from the surface of the Moon into space.

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