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Space Elevator Fundamentals

Space Elevator Fundamentals

A space elevator is a long thin cable, anchored on the Earth end and held in place on the space end by a counterweight of ballast. The central balance point of the cable is located at a geo-synchronous orbit level, keeping the cable steady over the anchor point. The weight of the cable (and ballast) out in space beyond the central balance point is the same as the weight of cable below the central point. The space end wants to hurl itself farther out into space and the inner end wants to fall with gravity down to the surface of the Earth. The two forces balance each other, making the assembly stable.

Once the elevator cable is in place and stable, elevator cars or “climbers/lifters” can use the cable as a guide, along with some form of climbing motor that moves the climbing car up the cable. Using solar energy to power electric motors is a primary consideration. Instead of needing to meet a high escape velocity using a massive chemical rocket, the elevator car can move slowly and steadily up into space, requiring much less energy to lift the same amount of payload into space

A Geo-Synchronous (GS) orbit is an orbit that matches the rotation of the Earth below, allowing an object in such an orbit to remain stationary over the same point on Earth. The standard altitude for a GS orbit is 35,786 km, or 22,236 miles above sea level. This can also be expressed as a radius from the center of Earth where r = 42,164 km, or 26,199 miles. Variations in GS orbits are due to: solar wind, radiation pressure, variations in Earths gravity, gravity from Earth and Moon, and for satellites it is possible to use elliptical versions of a GS orbit.

In order to accomplish the task as described above, the cable will need to be roughly 100,000 kilometers (62,000 miles) in length and will need to be strong enough to support its own weight along with the weight of climbing cars and the payloads they carry. The length of the cable will depend upon the functional requirements that are selected, and how any counter balance is designed. With no counter balance in a single block, the cable will have to supply all the counter balance and is likely to be longer. With a large single block counter balance, the cable can be shorter. Until recently, no available material substances reached the strength requirements. Now, carbon nanotubes are being considered, but they are not yet being produced in great lengths or production levels.

Recent news stories announced the concept of nano-threads made of diamond, and suggested that this substance would enable the construction of space elevators. Diamond nano-threads do not exist yet and carbon nanotubes are only being made in short lengths. This makes it difficult to describe the exact nature of a space elevator cable, however, many features can be predicted with some level of reliability. The tensile strength of the cable must be able to support its own weight plus the weight of the climbing car and any payload. Current design ideas describe a cable or “tether” that is about one meter wide, paper thin, and curved to ensure survivability against strikes from small objects. The width and thickness may vary in different sections of the cable/tether according to altitude, weather, and considerations like tension and oscillations on the cable.

The Earth based “anchor station” should be near the equator to minimize the danger from strong circular storms caused by coriolis force. While the anchors can be either land based or water based, putting them on ships will make them easier to move to avoid collisions with satellites and orbital debris. The cable/tether will be in a state of balanced equilibrium, so the anchor station does not have to exert a strong downward force, just keep the tether from drifting laterally and twisting.

It’s interesting to note that while the downward force from gravity will make objects fall toward Earth from the lower portion of the tether, the outward centrifugal force will make objects fall upward (away from Earth), from the top portion of the tether above the GS platform at 35,786 km, or 22,236 miles above sea level. An object released just above the platform will slowly drift upward and one released a large distance above the platform will fly away quickly. With a tether length of 100,000 km, or 62,000 miles, the top portion will be long enough to launch objects from its end out to the orbit of Jupiter, simply by releasing them.

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