Space elevator
Encyclopedia
A space elevator, also known as a geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...

al tether
or a beanstalk, is a proposed non-rocket spacelaunch
Non-rocket spacelaunch
Non-rocket space launch is a launch into space where some or all needed speed and altitude is provided by non-rocket means, rather than simply using conventional chemical rockets from the ground. A number of alternatives to rockets have been proposed...

structure (a structure designed to transport material from a celestial body
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...

's surface
Surface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...

into space). Many elevator variants have been suggested, all of which involve travelling along a fixed structure instead of using rocket-powered space launch
Space launch
Space launch is the earliest part of a flight that reaches space. Space launch involves liftoff, when a rocket or other space launch vehicle leaves the ground at the start of a flight...

, most often a cable that reaches from the surface of the Earth on or near the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

to geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...

(GSO) and a counterweight outside of the geostationary orbit.

Discussion of a space elevator dates back to 1895 when Konstantin Tsiolkovsky
Konstantin Tsiolkovsky
Konstantin Eduardovich Tsiolkovsky was an Imperial Russian and Soviet rocket scientist and pioneer of the astronautic theory. Along with his followers the German Hermann Oberth and the American Robert H. Goddard, he is considered to be one of the founding fathers of rocketry and astronautics...

proposed a free-standing "Tsiolkovsky Tower" reaching from the surface of Earth to geostationary orbit 35786 km (22,236 mi) up. Like all buildings, Tsiolkovsky's structure would be under compression, supporting its weight from below. Since 1959, most ideas for space elevators have focused on purely tensile structures, with the weight of the system held up from above. In the tensile concepts, a space tether
Space tether
Space tethers are cables, usually long and very strong, which can be used for propulsion, stabilization, or maintaining the formation of space systems by determining the trajectory of spacecraft and payloads...

reaches from a large mass (the counterweight) beyond geostationary orbit to the ground. This structure is held in tension between Earth and the counterweight like a guitar string held taut. Space elevators have also sometimes been referred to as beanstalks, space bridges, space lifts, space ladders, skyhooks, orbital towers, or orbital elevators.

While some variants of the space elevator concept are technologically feasible, current technology is not capable of manufacturing tether materials that are sufficiently strong and light
Specific strength
The specific strength is a material's strength divided by its density. It is also known as the strength-to-weight ratio or strength/weight ratio. In fiber or textile applications, tenacity is the usual measure of specific strength...

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
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...

or boron nitride nanotube based materials as the tensile element in the tether design, since the measured strength of carbon nanotubes appears great enough to make this possible. Technology as of 1978 could produce elevators for locations in the solar system with weaker gravitational field
Gravitational field
The gravitational field is a model used in physics to explain the existence of gravity. In its original concept, gravity was a force between point masses...

s, such as the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

or Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

.

For human riders on an Earth-based elevator, adequate protection against radiation would likely need to be provided, depending on the transit time through the Van Allen belts. At the transit times expected for early systems, radiation due to the Van Allen belts would, if unshielded, give a dose well above permitted levels.

## Geostationary orbital tethers

The orbital space elevator, or geostationary orbital tether, is a subset of the skyhook
Skyhook (structure)
Skyhooks are a theoretical class of cable based techniques intended to lift payloads to high altitudes and speeds. The name skyhook is a reference to an imaginary hook that hangs from the sky....

concept, and is what people normally think of when the phrase 'space elevator' is used (although there are variants).

Construction would be a large project: the minimum length of an Earth-based space elevator is well over 36,000 km (22,369 mi) long. The tether would have to be built of a material that could endure tremendous stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...

while also being light-weight, cost-effective, and manufacturable in great quantities. Materials currently available do not meet these requirements, although carbon nanotube
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...

technology shows great promise. As with all leading-edge engineering projects, other novel engineering problems would also have to be solved to make a space elevator practical, and there are problems regarding feasibility that have yet to be addressed.

### Early concepts

The key concept of the space elevator appeared in 1895 when Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

n scientist Konstantin Tsiolkovsky
Konstantin Tsiolkovsky
Konstantin Eduardovich Tsiolkovsky was an Imperial Russian and Soviet rocket scientist and pioneer of the astronautic theory. Along with his followers the German Hermann Oberth and the American Robert H. Goddard, he is considered to be one of the founding fathers of rocketry and astronautics...

was inspired by the Eiffel Tower
Eiffel Tower
The Eiffel Tower is a puddle iron lattice tower located on the Champ de Mars in Paris. Built in 1889, it has become both a global icon of France and one of the most recognizable structures in the world...

in Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

to consider a tower that reached all the way into space, built from the ground up to an altitude of 35,790 kilometers
1 E7 m
To help compare different orders of magnitude, this page lists lengths starting at 107 metres .Distances shorter than 107 metres- Conversions :10 megametres is* 6,215 miles....

(22,238 mi) above sea level (geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...

). He noted that a "celestial castle" at the top of such a spindle-shaped cable would have the "castle" orbiting Earth in a geostationary orbit (i.e. the castle would remain over the same spot on Earth's surface).

Since the elevator would attain orbital velocity as it rode up the cable, an object released at the tower's top would also have the orbital velocity necessary to remain in geostationary orbit. Unlike more recent concepts for space elevators, Tsiolkovsky's (conceptual) tower was a compression structure, rather than a tension (or "tether") structure.

### 20th century

Building a compression structure from the ground up proved an unrealistic task as there was no material in existence with enough compressive strength to support its own weight under such conditions. In 1959 another Russian scientist, Yuri N. Artsutanov, suggested a more feasible proposal. Artsutanov suggested using a geostationary satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

as the base from which to deploy the structure downward. By using a counterweight
Counterweight
A counterweight is an equivalent counterbalancing weight that balances a load.-Uses:A counterweight is often used in traction lifts , cranes and funfair rides...

, 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 cable constantly over the same spot on the surface of the Earth. Artsutanov's idea was introduced to the Russian-speaking public in an interview published in the Sunday supplement of Komsomolskaya Pravda
Komsomolskaya Pravda
Komsomolskaya Pravda is a daily Russian tabloid newspaper, founded on March 13th, 1925. It is published by "Izdatelsky Dom Komsomolskaya Pravda" .- History :...

in 1960, but was not available in English until much later. He also proposed tapering the cable thickness so that the stress in the cable was constant—this gives a thin cable at ground level, thickening up towards GSO.

Both the tower and cable ideas were proposed in the quasi-humorous Ariadne column
Daedalus is a fictional inventor created by David E. H. Jones for his Ariadne column in the New Scientist and The Guardian, and which is currently featured in Nature.Daedalus's imaginary inventions are solidly grounded in science —...

in New Scientist
New Scientist
New Scientist is a weekly non-peer-reviewed English-language international science magazine, which since 1996 has also run a website, covering recent developments in science and technology for a general audience. Founded in 1956, it is published by Reed Business Information Ltd, a subsidiary of...

, 24 December 1964.

In 1966, Isaacs, Vine, Bradner and Bachus, four American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

engineers, reinvented the concept, naming it a "Sky-Hook," and published their analysis in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

. They decided to determine what type of material would be required to build a space elevator, assuming it would be a straight cable with no variations in its cross section, and found that the strength
Specific strength
The specific strength is a material's strength divided by its density. It is also known as the strength-to-weight ratio or strength/weight ratio. In fiber or textile applications, tenacity is the usual measure of specific strength...

required would be twice that of any existing material including graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

, quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

, and diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

.

In 1975 an American scientist, Jerome Pearson
Jerome Pearson
Jerome Pearson is an American engineer and space scientist best known for his work on space elevators and lunar space elevator. He is president of STAR, Inc., and has developed aircraft and spacecraft technology for the United States Air Force, DARPA, and NASA...

, reinvented the concept yet again, publishing his analysis in the journal Acta Astronautica. He designed a tapered cross section that would be better suited to building the elevator. The completed cable would be thickest at the geostationary orbit, where the tension was greatest, and would be narrowest at the tips to reduce the amount of weight per unit area of cross section that any point on the cable would have to bear. He suggested using a counterweight that would be slowly extended out to 144,000 kilometers (90,000 miles, almost half the distance to the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

) as the lower section of the elevator was built. Without a large counterweight, the upper portion of the cable would have to be longer than the lower due to the way gravitational and centrifugal forces change with distance from Earth. His analysis included disturbances such as the gravitation of the Moon, wind and moving payloads up and down the cable. The weight of the material needed to build the elevator would have required thousands of Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

trips, although part of the material could be transported up the elevator when a minimum strength strand reached the ground or be manufactured in space from asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

al or lunar ore.

In 1977, Hans Moravec
Hans Moravec
Hans Moravec is an adjunct faculty member at the Robotics Institute of Carnegie Mellon University. He is known for his work on robotics, artificial intelligence, and writings on the impact of technology. Moravec also is a futurist with many of his publications and predictions focusing on...

published an article called "A Non-Synchronous Orbital Skyhook", in which he proposed an alternative space elevator concept, using a rotating cable, in which the rotation speed exactly matches the orbital speed in such a way that the instantaneous velocity at the point where the cable was at the closest point to the Earth was zero. This concept is an early version of a space tether transportation system.

In 1979, space elevators were introduced to a broader audience with the simultaneous publication of Arthur C. Clarke
Arthur C. Clarke
Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...

's novel, The Fountains of Paradise
The Fountains of Paradise is a Hugo and Nebula Award–winning 1979 novel by Arthur C. Clarke. Set in the 22nd century, it describes the construction of a space elevator. This "orbital tower" is a giant structure rising from the ground and linking with a satellite in geostationary orbit at the...

, in which engineers construct a space elevator on top of a mountain peak in the fictional island country of Taprobane (loosely based on Sri Lanka
Sri Lanka
Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...

, albeit moved south to the Equator), and Charles Sheffield
Charles Sheffield
Charles Sheffield , was an English-born mathematician, physicist and science fiction author. He had been a President of the Science Fiction and Fantasy Writers of America and of the American Astronautical Society....

's first novel, The Web Between the Worlds, also featuring the building of a space elevator. Three years later, in Robert A. Heinlein
Robert A. Heinlein
Robert Anson Heinlein was an American science fiction writer. Often called the "dean of science fiction writers", he was one of the most influential and controversial authors of the genre. He set a standard for science and engineering plausibility and helped to raise the genre's standards of...

's 1982 novel Friday
Friday (novel)
Friday is a 1982 science fiction novel by Robert A. Heinlein. It is the story of a female "artificial person," the titular character, genetically engineered to be stronger, faster, smarter, and generally better than normal humans...

the principal character makes use of the "Nairobi Beanstalk" in the course of her travels. In Kim Stanley Robinson
Kim Stanley Robinson
Kim Stanley Robinson is an American science fiction writer known for his award-winning Mars trilogy. His work delves into ecological and sociological themes regularly, and many of his novels appear to be the direct result of his own scientific fascinations, such as the fifteen years of research...

's 1993 novel Red Mars, colonists build a space elevator on Mars that allows both for more colonists to arrive and also for natural resources mined there to be able to leave for Earth. In David Gerrold
David Gerrold
Jerrold David Friedman , better known by his pen name David Gerrold, is an American science fiction author who started his career in 1966 while a college student by submitting an unsolicited story outline for the television series Star Trek. He was invited to submit several premises, and the one...

's 2000 novel, Jumping Off The Planet, a family excursion up the Ecuador "beanstalk" is actually a child-custody kidnapping. Gerrold's book also examines some of the industrial applications of a mature elevator technology.

### 21st century

After the development of carbon nanotubes in the 1990s, engineer David Smitherman of NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

/Marshall's Advanced Projects Office realized that the high strength of these materials might make the concept of an orbital skyhook feasible, and put together a workshop at the Marshall Space Flight Center
Marshall Space Flight Center
The George C. Marshall Space Flight Center is the U.S. government's civilian rocketry and spacecraft propulsion research center. The largest center of NASA, MSFC's first mission was developing the Saturn launch vehicles for the Apollo moon program...

, inviting many scientists and engineers to discuss concepts and compile plans for an elevator to turn the concept into a reality. The publication he edited, compiling information from the workshop, "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium", provides an introduction to the state of the technology at the time, and summarizes the findings.

Another American scientist, Bradley C. Edwards
In space engineering, Bradley C. Edwards is a researcher who is notable for having worked on Space elevators. He received funding from NIAC from 2001 to 2003 to write a paper proposing a way in which one could be built...

, suggested creating a 100000 km (62,137.3 mi) long paper-thin ribbon using a carbon nanotube composite material. He chose a ribbon type structure rather than a cable because that structure might stand a greater chance of surviving impacts by meteoroids. Supported by the NASA Institute for Advanced Concepts
right|200pxNASA Institute for Advanced Concepts was a NASA-funded program that was operated by the Universities Space Research Association for NASA from 1998 until its closure on 31 August 2007. NIAC sought proposals for revolutionary aeronautics and space concepts that could dramatically impact...

, Edwards' work was expanded to cover the deployment scenario, climber design, power delivery system, orbital debris
Space debris
Space debris, also known as orbital debris, space junk, and space waste, is the collection of objects in orbit around Earth that were created by humans but no longer serve any useful purpose. These objects consist of everything from spent rocket stages and defunct satellites to erosion, explosion...

avoidance, anchor system, surviving atomic oxygen, avoiding lightning and hurricanes by locating the anchor in the western equatorial Pacific, construction costs, construction schedule, and environmental hazards. The largest holdup to Edwards' proposed design is the technological limit of the tether material. His calculations call for a fiber composed of epoxy-bonded carbon nanotubes with a minimal tensile strength of 130 GPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

(19 million psi
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...

) (including a safety factor of 2); however, tests in 2000 of individual single-walled carbon nanotubes (SWCNTs), which should be notably stronger than an epoxy-bonded rope, indicated the strongest measured as 52 GPa (7.5 million psi). Multi-walled carbon nanotubes have been measured with tensile strengths up to 63 GPa (9 million psi).

To speed space elevator development, proponents are planning several competitions, similar to the Ansari X Prize
Ansari X Prize
The Ansari X Prize was a space competition in which the X Prize Foundation offered a US\$10,000,000 prize for the first non-government organization to launch a reusable manned spacecraft into space twice within two weeks...

, for relevant technologies. Among them are Elevator:2010
Elevator:2010
Elevator:2010 is a competition with monetary prizes similar to the Ansari X Prize, but with the purpose of developing space elevator and space elevator-related technologies...

, which will organize annual competitions for climbers, ribbons and power-beaming systems, the Robogames Space Elevator Ribbon Climbing competition, as well as NASA's Centennial Challenges
Centennial Challenges
The Centennial Challenges are NASA space competition prize contests for non-government-funded technological achievements by American teams.-Current Challenges:...

program, which, in March 2005, announced a partnership with the Spaceward Foundation (the operator of Elevator:2010), raising the total value of prizes to US\$400,000.
The first European Space Elevator Challenge (EuSEC) to establish a climber structure took place in August 2011.

In 2005, "the LiftPort Group
LiftPort Group
LiftPort Group is a privately held Washington State corporation. It was founded in April, 2003 by Michael J. Laine and is dedicated to creating cheap, reliable, and safe access to space. The company is focused on the construction of a space elevator using carbon nanotubes...

of space elevator companies announced that it will be building a carbon nanotube manufacturing plant in Millville, New Jersey
Millville, New Jersey
Millville is a city in Cumberland County, New Jersey, United States. As of the 2000 United States Census, the city population was 26,847. Millville, Bridgeton and Vineland are the three principal New Jersey cities of the Vineland-Millville-Bridgeton Primary Metropolitan Statistical Area which...

, to supply various glass, plastic and metal companies with these strong materials. Although LiftPort hopes to eventually use carbon nanotubes in the construction of a 100000 km (62,137.3 mi) space elevator, this move will allow it to make money in the short term and conduct research and development into new production methods. The goal was a space elevator launch in 2010." On February 13, 2006 the LiftPort Group announced that, earlier the same month, they had tested a mile of "space-elevator tether" made of carbon-fiber composite strings and fiberglass tape measuring 5 cm (2 in) wide and 1 mm (approx. 6 sheets of paper) thick, lifted with balloons.

In 2007, Elevator:2010
Elevator:2010
Elevator:2010 is a competition with monetary prizes similar to the Ansari X Prize, but with the purpose of developing space elevator and space elevator-related technologies...

held the 2007 Space Elevator games, which featured US\$500,000 awards for each of the two competitions, (US\$1,000,000 total) as well as an additional US\$4,000,000 to be awarded over the next five years for space elevator related technologies. No teams won the competition, but a team from MIT entered the first 2-gram (0.07 oz), 100% carbon nanotube entry into the competition. Japan held an international conference in November 2008 to draw up a timetable for building the elevator.

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 a trillion yen (£5 billion/ \$8 billion). In a report by Leo Lewis, Tokyo correspondent of The Times newspaper in England, plans by Shuichi Ono, chairman of the Japan Space Elevator Association, are unveiled. Lewis says: "Japan is increasingly confident that its sprawling academic and industrial base can solve those [construction] issues, and has even put the astonishingly low price tag of a trillion yen (£5 billion/ \$8 billion) on building the elevator. Japan is renowned as a global leader in the precision engineering and high-quality material production without which the idea could never be possible."
Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...

was revealed to be working on plans for a space elevator at its secretive Google X Lab
Google X Lab, sometimes referred to simply as Google X, is a secret facility run by Google thought to be located somewhere in the Bay Area of Nothern California....

location.

### Apparent gravitational field

A 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. The higher up the cable, the stronger is the upward centrifugal force and the more it opposes the downward gravity. Eventually it becomes stronger than gravity above the geosynchronous level. Along the length of the cable, this (downward) actual gravity minus the (upward) centrifugal force is called the apparent gravitational field.

The apparent gravitational field can be represented this way:
The downward force of actual gravity
Newton's law of universal gravitation
Newton's law of universal gravitation states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them...

decreases with height:
Newton's law of universal gravitation
Newton's law of universal gravitation states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them...

The upward centrifugal force
Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...

due to the planet's rotation increases with height:
Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...

Together, the apparent gravitational field is the sum of the two:

where
g is the acceleration of actual gravity or apparent gravity down (negative) or up (positive) along the vertical cable (m s−2),
a is the centrifugal acceleration up (positive) along the vertical cable (m s−2),
G is the gravitational constant
Gravitational constant
The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...

(m3 s−2 kg−1)
M is the mass of the Earth (kg)
r is the distance from that point to Earth's center (m),
ω is Earth's rotation speed (radian/s).

At some point up the cable, the two terms (downward gravity and upward centrifugal force) equal each other, objects fixed to the cable there have no weight on the cable. This occurs at the level of the stationary orbit. This level (r1) depends on the mass of the planet and its rotation rate. Setting actual gravity and centrifugal acceleration equal to each other gives:

On Earth, this level is 35786 km (22,236 mi) above the surface, the level of geostationary orbit.

Seen from a geosynchronous station, any object dropped off the tether from a point closer to Earth will initially accelerate downward. If dropped from any point above a geosynchronous station, the object would initially accelerate up toward space.

### Cable section

Historically, the main technical problem has been considered the ability of the cable to hold up, with tension, the weight of itself below any particular point. The vertical point with the greatest tension on a space elevator cable is at the level of geostationary orbit, 35786 km (22,236 mi) above the Earth's equator. This means that the cable material combined with its design must be strong enough to hold up the weight of its own mass from the surface up to 35,786 km. By making any cable larger in cross section at this level compared to at the surface, it can better 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 35,786 km to the minimum at the surface. To maximize strength of the cable compared to its weight, 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. For such an idealized design without climbers attached, without thickening at high space-junk altitudes, etc., the cross-section will follow this differential equation:,
where
g is the acceleration along the radius (m·s−2),
S is the cross-area of the cable at any given point r, (m2) and dS its variation (m2 as well),
ρ is the density of the material used for the cable (kg·m−3).
σ is the stress the cross-section area can bear without yielding
Yield (engineering)
The yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed...

(N·m−2=kg·m−1·s−2), its elastic limit.

The value of g is given by the first equation, which yields:,

the variation being taken between r1 (geostationary) and r0 (ground).

It turns out that between these two points, this quantity can be expressed simply as:
, or
where is the ratio between the centrifugal force on the equator and the gravitational force.

### Cable material

The second technical problem is that the g0 r0 factor is quite large. Since its influence on the maximal cross-section is exponential, one needs to find materials where σ will be large enough to cancel our gravity. On Earth, we have: (or Joules per kg) for most solid materials, so that σ needs to be:

This corresponds to a cable capable of sustaining 30 tons with a cross-section of one square millimeter, under Earth's gravity.

The free breaking length can be used to compare materials: it is the length of a un-tapered cylindrical cable at which it will break under its own weight under constant gravity. For a given material, that length is σ/ρ/g0. The free breaking length needed is given by the equation, where
If one does not take into account the x factor (which reduces the strength needed by about 30%), this equation also says that the section ratio equals e (exponential one) when:
If the material can support a free breaking length of only one tenth this, the section needed at a geosynchronous orbit will be e
E (mathematical constant)
The mathematical constant ' is the unique real number such that the value of the derivative of the function at the point is equal to 1. The function so defined is called the exponential function, and its inverse is the natural logarithm, or logarithm to base...

10 times the ground section, which is more than a hundredfold in diameter.

## Structure

There are a variety of space elevator designs. Almost every design includes a base station, a cable, climbers, and a counterweight. Earth's rotation creates upward centrifugal force
Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...

on the counterweight. The counterweight is held down by the cable while the cable is held up and taut by the counterweight. The base station anchors the whole system to the surface of the Earth. Climbers climb up and down the cable with cargo.

### Base station

The base station designs typically fall into two categories—mobile and stationary. Mobile stations are typically large oceangoing vessels. Stationary platforms would generally be located in high-altitude locations, such as on top of mountains, or even potentially on high towers.

Mobile platforms have the advantage of being able to maneuver to avoid high winds, storms, and space debris
Space debris
Space debris, also known as orbital debris, space junk, and space waste, is the collection of objects in orbit around Earth that were created by humans but no longer serve any useful purpose. These objects consist of everything from spent rocket stages and defunct satellites to erosion, explosion...

. While stationary platforms don't have these advantages, they typically would have access to cheaper and more reliable power sources, and require a shorter cable. While the decrease in cable length may seem minimal (no more than a few kilometers), the cable thickness could be reduced over its entire length, significantly reducing the total weight.

### Cable

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
Centripetal force
Centripetal force is a force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path. The mathematical description was derived in 1659 by Dutch physicist Christiaan Huygens...

to retain the cable and counterweight above. In a 1998 report, NASA researchers noted that "maximum stress [on a space elevator cable] is at geosynchronous altitude so the cable must be thickest there and taper exponentially as it approaches Earth. Any potential material may be characterized by the taper factor – the ratio between the cable's radius at geosynchronous altitude and at the Earth's surface."

The cable must be made of a material with a large tensile strength/density ratio
Specific strength
The specific strength is a material's strength divided by its density. It is also known as the strength-to-weight ratio or strength/weight ratio. In fiber or textile applications, tenacity is the usual measure of specific strength...

. For example, the Edwards space elevator design assumes a cable material with a specific strength of at least 100,000 kN/(kg/m). This value takes into consideration the entire weight of the space elevator. A space elevator would need a material capable of sustaining a length of 4960 kilometres (3,082 mi) of its own weight at sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

to reach a geostationary altitude of 35786 km (22,236 mi) without tapering and without breaking. Therefore, a material with very high strength and lightness is needed.

For comparison, metals like titanium, steel or aluminium alloys have breaking lengths
Specific strength
The specific strength is a material's strength divided by its density. It is also known as the strength-to-weight ratio or strength/weight ratio. In fiber or textile applications, tenacity is the usual measure of specific strength...

of only 20–30 km. Modern fibre materials such as kevlar
Kevlar
Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids such as Nomex and Technora. Developed at DuPont in 1965, this high strength material was first commercially used in the early 1970s as a replacement for steel in racing tires...

, fibreglass and carbon/graphite fibre
Carbon fiber
Carbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fiber...

have breaking lengths of 100–400 km. Quartz fibers have an advantage that they can be drawn to a length of hundreds of kilometers even with the present-day technology. Nanoengineered materials such as carbon nanotubes and, more recently discovered, graphene
Graphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...

ribbons (perfect two-dimensional sheets of carbon) are expected to have breaking lengths of 5000–6000 km at sea level, and also are able to conduct electrical power.

Carbon is such a good candidate material (for high specific strength) because, as only the 6th element in the periodic table
Periodic table
The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...

, it has very few of the nucleons which contribute most of the dead weight of any material (whereas most of the interatomic bonding forces
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...

are contributed by only the outer few
Valence electron
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...

electrons); the challenge now remains to extend to macroscopic sizes the production of such material that are still perfect on the microscopic scale (as microscopic defects are most responsible for material weakness). The current (2009) carbon nanotube technology allows growing tubes up to a few tens of centimeters only.

### Climbers

A space elevator cannot be an elevator in the typical sense (with moving cables) due to the need for the cable to be significantly wider at the center than the tips. 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.
The horizontal speed of each part of the cable increases with altitude, proportional to distance from the center of the Earth, reaching orbital velocity
Orbital velocity
Orbital velocity can refer to the following:* The orbital speed of a body in a gravitational field.* The velocity of particles due to wave motion, in particular in wind waves....

at geostationary orbit. Therefore as a payload is lifted up a space elevator, it needs to gain not only altitude but angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

(horizontal speed) as well. This angular momentum is taken from the Earth's own rotation. As the climber ascends it is initially moving slightly more slowly than the cable that it moves onto (Coriolis force) and thus the climber "drags" on the cable.

The overall effect of the centrifugal force acting on the cable causes it to constantly try to return to the energetically favourable vertical orientation, so after an object has been lifted on the cable the counterweight will swing back towards the vertical like an inverted pendulum. Space elevators and their loads will be designed so that the center of mass is always well-enough above the level of geostationary orbit to hold up the whole system. Lift and descent operations must be carefully planned so as to keep the pendulum-like motion of the counterweight around the tether point under control.

By the time the payload has reached GEO the angular momentum (horizontal speed) is enough that the payload is in orbit.

The opposite process would occur for payloads descending the elevator, tilting the cable eastwards and insignificantly increasing Earth's rotation speed.

It has also been proposed to use a second cable attached to a platform to lift payload up the main cable, since the lifting device would not have to deal with its own weight against Earth's gravity. Out of the many proposed theories, powering any lifting device also continues to present a challenge.

Another design constraint will be the ascending speed of the climber. As geosynchronous orbit is at 35786 km (22,236 mi), assuming the climber can reach the speed of a very fast car or train of 300 km/h (180 mph) it will take 5 days to climb to geosynchronous orbit.

### Powering climbers

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.

Various methods have been proposed to get that energy to the climber:
• Transfer the energy to the climber through wireless energy transfer
Wireless energy transfer
Wireless energy transfer or wireless power is the transmission of electrical energy from a power source to an electrical load without artificial interconnecting conductors. Wireless transmission is useful in cases where interconnecting wires are inconvenient, hazardous, or impossible...

while it is climbing.
• Transfer the energy to the climber through some material structure while it is climbing.
• Store the energy in the climber before it starts – requires an extremely high specific energy
Specific energy
Specific energy is defined as the energy per unit mass. Common metric units are J/kg. It is an intensive property. Contrast this with energy, which is an extensive property. There are two main types of specific energy: potential energy and specific kinetic energy. Others are the gray and sievert,...

such as nuclear energy.
• Solar power – power compared to the weight of panels limits the speed of climb.

Wireless energy transfer such as laser power beaming is currently considered the most likely method. 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. For climber designs powered by power beaming, this efficiency is an important design goal. Unused energy must be re-radiated away with heat-dissipation systems, which add to weight.

Yoshio Aoki, a professor of precision machinery engineering at Nihon University
Nihon University
Nihon University is the largest university in Japan. Akiyoshi Yamada, the minister of justice, founded Nihon Law School in October 1889....

and director of the Japan Space Elevator Association, suggested including a second cable and using the conductivity of carbon nanotubes to provide power.

Various mechanical means of applying power have also been proposed; such as moving, looped or vibrating cables.

### Counterweight

Several solutions have been proposed to act as a counterweight:
• a heavy, captured asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

;
• a space dock, space station
Space station
A space station is a spacecraft capable of supporting a crew which is designed to remain in space for an extended period of time, and to which other spacecraft can dock. A space station is distinguished from other spacecraft used for human spaceflight by its lack of major propulsion or landing...

or spaceport
Spaceport
A spaceport or cosmodrome is a site for launching spacecraft, by analogy with seaport for ships or airport for aircraft. The word spaceport, and even more so cosmodrome, has traditionally been used for sites capable of launching spacecraft into orbit around Earth or on interplanetary trajectories...

positioned past geostationary orbit; or
• a further upward extension of the cable itself so that the net upward pull is the same as an equivalent counterweight;
• parked spent climbers that had been used to thicken the cable during construction, other junk, and material lifted up the cable for the purpose of increasing the counterweight.

Extending the cable has the advantage of some simplicity of the task and the fact that a payload that went to the end of the counterweight-cable would acquire considerable velocity relative to the Earth, allowing it to be launched into interplanetary space. Its disadvantage is the need to produce greater amounts of cable material as opposed to using anything that has mass.

## Alternative concepts

The original concept envisioned by Tsiolkovsky was a compression structure, a concept similar to an aerial mast
Radio masts and towers are, typically, tall structures designed to support antennas for telecommunications and broadcasting, including television. They are among the tallest man-made structures...

. While such structures might reach the agreed altitude for space
Karman line
The Kármán line lies at an altitude of above the Earth's sea level, and is commonly used to define the boundary between the Earth's atmosphere and outer space...

(100 km—62 mi), they are unlikely to reach geostationary orbit. The concept of a Tsiolkovsky tower combined with a classic space elevator cable has been suggested.

A mini version of the Space Elevator to access near-space altitudes of 20 km (12.4 mi) has been proposed by Canadian researchers. The structure would be pneumatically supported and free standing with control systems guiding the structure's center of mass. Proposed uses include tourism and commerce, communications, wind generation and low-cost space launch.

Other alternatives to a space elevator include an orbital ring
Orbital ring
An Orbital Ring is a concept for a space elevator that consists of a ring in low earth orbit that rotates at above orbital speed, that has fixed tethers hanging down to the ground.The structure is intended to be used for space launch....

, a pneumatic space tower, a space fountain
Space fountain
A space fountain is a proposed form of space elevator that does not require the structure to be in geostationary orbit, and does not rely on tensile strength for support. In contrast to the original space elevator design , a space fountain is a tremendously tall tower extending up from the ground...

, a launch loop
Launch loop
A launch loop or Lofstrom loop is a proposed system for launching objects into space orbit using a moving cable-like system attached to the earth at two ends and suspended above the atmosphere in the middle...

, a Skyhook
Skyhook (structure)
Skyhooks are a theoretical class of cable based techniques intended to lift payloads to high altitudes and speeds. The name skyhook is a reference to an imaginary hook that hangs from the sky....

, a space tether
Space tether
Space tethers are cables, usually long and very strong, which can be used for propulsion, stabilization, or maintaining the formation of space systems by determining the trajectory of spacecraft and payloads...

, a space hoist and the SpaceShaft
SpaceShaft
A SpaceShaft is a proposed atmospherically levitating structure that would serve as an elevator system to near-space altitudes. It will support multiple platforms distributed at several elevations that would provide habitation facilities for long term human operations throughout the mid-atmosphere...

.

## Launching into deep space

An object attached to a space elevator at a radius of approximately 53,100 km will be at escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...

when released. Transfer orbits to the L1 and L2 Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

s can be attained by release at 50,630 and 51,240 km, respectively, and transfer to lunar orbit from 50,960 km.

The velocities that might be attained at the end of Pearson's 144000 km (89,477.7 mi) cable can be determined. The tangential velocity is 10.93 kilometers per second (6.79 mi/s), which is more than enough to escape
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...

Earth's gravitational field and send probes at least as far out as Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

. Once at Jupiter, a gravitational assist maneuver permits solar escape velocity to be reached.

## Extraterrestrial elevators

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

A Martian
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

tether could be much shorter than one on Earth. Mars' surface gravity is 38% of Earth's, while it rotates around its axis in about the same time as Earth.
Because of this, Martian areostationary orbit
Areostationary orbit
An areostationary orbit is a circular areo­synchronous orbit in the Martian equatorial plane about above the surface, any point on which revolves about Mars in the same direction and with the same period as the Martian surface...

is much closer to the surface, and hence the elevator would be much shorter. Current materials are already sufficiently strong to construct such an elevator. However, building a Martian elevator would be complicated by the Martian moon Phobos
Phobos (moon)
Phobos is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. With a mean radius of , Phobos is 7.24 times as massive as Deimos...

, which is in a low orbit and intersects the Equator regularly (twice every orbital period of 11 h 6 min).

A lunar space elevator
Lunar space elevator
A lunar space elevator is a proposed cable running from the surface of the Moon into space.It is similar in concept to the better known Earth space elevator idea...

can possibly be built with currently available technology about 50000 kilometres (31,068.6 mi) long extending through the Earth-Moon L1 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, such as mass driver
Mass driver
A mass driver or electromagnetic catapult is a proposed method of non-rocket spacelaunch which would use a linear motor to accelerate and catapult payloads up to high speeds. All existing and contemplated mass drivers use coils of wire energized by electricity to make electromagnets. Sequential...

systems.

On the far side of the moon, a lunar space elevator would need to be very long (more than twice the length of an Earth elevator) but due to the low gravity of the Moon, can be made of existing engineering materials.

Rapidly spinning asteroids or moons could use cables to eject materials to convenient points, such as Earth orbits; or conversely, to eject materials to send the bulk of the mass of the asteroid or moon to Earth orbit or a Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

. Freeman Dyson
Freeman Dyson
Freeman John Dyson FRS is a British-born American theoretical physicist and mathematician, famous for his work in quantum field theory, solid-state physics, astronomy and nuclear engineering. Dyson is a member of the Board of Sponsors of the Bulletin of the Atomic Scientists...

, a physicist and mathematician, has suggested using such smaller systems as power generators at points distant from the Sun where solar power is uneconomical. For the purpose of mass ejection, it is not necessary to rely on the asteroid or moon to be rapidly spinning. Instead of attaching the tether to the equator of a rotating body, it can be attached to a rotating hub on the surface. This was suggested in 1980 as a "Rotary Rocket" by Pearson and described very succinctly on the Island One website as a "Tapered Sling".

A space elevator using presently available engineering materials could be constructed between mutually tidally locked worlds, such as Pluto and Charon or the components of binary asteroid Antiope, with no terminus disconnect, according to Francis Graham of Kent State University. However, spooled variable lengths of cable must be used due to ellipticity of the orbits.

## Construction

The construction of a space elevator is considered to be a large project. Like other historical large projects it entails technical risk: some advances in engineering, manufacturing and physical technology are required. Once a first space elevator is built, the second one and all others would have the use of the previous ones to assist in construction, making their costs considerably cheaper. Such follow-on space elevators would also benefit from the great reduction in technical risk achieved by the construction of the first space elevator.

Construction is conceived as the deployment of a long cable from a large spool. The spool is initially parked in a geostationary orbit above the planned anchor point. When a long cable is dropped "down" (toward Earth), it must be balanced by balancing mass being dropped "up" (away from Earth) for the whole system to remain on the geosynchronous orbit. Some designs imagine the balancing mass being another cable (with counterweight) extending upward, other designs elevate the spool itself as the main cable is paid out. When the lower end of the cable is so long as to reach the Earth, it can be anchored at some place. Once anchored, the center of mass is elevated upward more (by adding mass at the upper end or by paying out more cable). This adds more tension to the whole cable, which can then be used as an elevator cable.

### Safety issues and construction challenges

Depending on transit times through the Van Allen radiation belts passengers will need to be protected from radiation by shielding, which adds mass to the climber and decreases payload. For early systems, transit times are expected to be long enough where, if unshielded, total exposure would be above levels considered safe.

A space elevator would present a navigational hazard, both to aircraft and spacecraft. Aircraft could be diverted by air-traffic control restrictions. All objects in stable orbits that have perigee
Perigee
Perigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...

below the maximum altitude of the cable that are not synchronous with the cable will impact the cable eventually, unless avoiding action is taken. For spacecraft one potential solution proposed by Edwards is to use a movable anchor (a sea anchor) to allow the tether to "dodge" any space debris large enough to track.

Impacts by space objects such as meteoroids, micrometeorites and orbiting man-made debris, pose another design constraint on the cable. A cable would need to be designed to maneuver out of the way of debris, or absorb impacts of small debris without breaking.

### Economics

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 US\$11,000 per pound
Pound (mass)
The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...

(US\$25,000 per kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

) for transfer to geostationary orbit. Current proposals envision payload prices starting as low as \$100 per pound
Pound (mass)
The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...

(\$220 per kilogram), similar to the \$5–\$300/kg estimates of the Launch loop
Launch loop
A launch loop or Lofstrom loop is a proposed system for launching objects into space orbit using a moving cable-like system attached to the earth at two ends and suspended above the atmosphere in the middle...

, but higher than the \$310/ton to 500 km orbit quoted to Dr. Jerry Pournelle
Jerry Pournelle
Jerry Eugene Pournelle is an American science fiction writer, essayist and journalist who contributed for many years to the computer magazine Byte and has since 1998 been maintaining his own website/blog....

for an orbital airship
Orbital airship
The orbital airship, also called the space blimp, is a proposed space transportation system that carries payloads to and from low Earth orbit...

system.

Philip Ragan, co-author of the book "Leaving the Planet by Space Elevator", states that "The first country to deploy a space elevator will have a 95 percent cost advantage and could potentially control all space activities."

• Elevator:2010
Elevator:2010
Elevator:2010 is a competition with monetary prizes similar to the Ansari X Prize, but with the purpose of developing space elevator and space elevator-related technologies...

– a space elevator prize competitions
• Lunar space elevator
Lunar space elevator
A lunar space elevator is a proposed cable running from the surface of the Moon into space.It is similar in concept to the better known Earth space elevator idea...

for the moon variant
• Space elevator construction
Space elevator construction
The construction of a space elevator is considered to be a large project. Like other historical large projects it entails technical risk: some advances in engineering, manufacture and physical technology are required...

discusses alternative construction methods of a space elevator.
• Space elevator economics
Space elevator economics
Space elevator economics compares the cost of sending a payload into Earth orbit via a space elevator with the cost of doing so with alternatives, like rockets.-Costs of current systems :...

discusses capital and maintenance costs of a space elevator.
• Space elevator safety
Space elevator safety
There are risks associated with never-before-done technologies like the construction and operation of a space elevator. A space elevator would present a navigational hazard, both to aircraft and spacecraft. Aircraft could be dealt with by means of simple air-traffic control restrictions...

discusses safety aspects of space elevator construction and operation.
• Space elevators in fiction
Space elevators in fiction
This is a list of occurrences of space elevators in fiction. Some depictions were made before the space elevator concept became fully established.-Novels and Fairy tales:* 2061: Odyssey Three, novel by Arthur C. Clarke...

• Tether propulsion – for other transportation methods using long cables

• Non-rocket spacelaunch
Non-rocket spacelaunch
Non-rocket space launch is a launch into space where some or all needed speed and altitude is provided by non-rocket means, rather than simply using conventional chemical rockets from the ground. A number of alternatives to rockets have been proposed...

:
• Launch loop
Launch loop
A launch loop or Lofstrom loop is a proposed system for launching objects into space orbit using a moving cable-like system attached to the earth at two ends and suspended above the atmosphere in the middle...

– a hypervelocity belt system that forms a launch track at 80 km
• Lightcraft
Lightcraft
A lightcraft is a space- or air-vehicle driven by laser propulsion. Laser propulsion is currently in early stages of development. Lightcraft use an external source of laser or maser energy to provide power for producing thrust. The laser/maser energy is focused to a high intensity in order to...

– an alternative method for moving materials or people
• Space gun
Space gun
A space gun is a method of launching an object into outer space using a large gun, or cannon. It provides a method of non-rocket spacelaunch‎.In the HARP Project a U.S...

or StarTram
StarTram
StarTram is a proposal for a maglev space launch system. The initial Generation 1 facility would be cargo only, launching from a mountain peak at 3 km to 7 km altitude with an evacuated tube staying at local surface level, raising ≈150,000 tons to orbit annually...

– among methods for launching materials
• Space fountain
Space fountain
A space fountain is a proposed form of space elevator that does not require the structure to be in geostationary orbit, and does not rely on tensile strength for support. In contrast to the original space elevator design , a space fountain is a tremendously tall tower extending up from the ground...

– very tall structures using fast moving masses to hold it up
• SpaceShaft
SpaceShaft
A SpaceShaft is a proposed atmospherically levitating structure that would serve as an elevator system to near-space altitudes. It will support multiple platforms distributed at several elevations that would provide habitation facilities for long term human operations throughout the mid-atmosphere...

– A atmospherically buoyant spar that could reach up to LEO and provide super-heavy lifting capacity.

• Edwards BC, Ragan P. "Leaving The Planet By Space Elevator" Seattle, USA: Lulu; 2006. ISBN 978-1-4303-0006-9 See Leaving The Planet
• Edwards BC, Westling EA. The Space Elevator: A Revolutionary Earth-to-Space Transportation System. San Francisco, USA: Spageo Inc.; 2002. ISBN 0-9726045-0-2.
• Space Elevators – An Advanced Earth-Space Infrastructure for the New Millennium [PDF]. A conference publication based on findings from the Advanced Space Infrastructure Workshop on Geostationary Orbiting Tether "Space Elevator" Concepts, held in 1999 at the NASA Marshall Space Flight Center, Huntsville, Alabama. Compiled by D.V. Smitherman, Jr., published August 2000.
• "The Political Economy of Very Large Space Projects" HTML PDF, John Hickman, Ph.D. Journal of Evolution and Technology
Journal of Evolution and Technology
The Journal of Evolution and Technology is the peer-reviewed, electronic, academic journal of the Institute for Ethics and Emerging Technologies, which publishes contemporary foresight into long-term developments in science, technology and philosophy....

Vol. 4 – November 1999.
• The Space Elevator NIAC
right|200pxNASA Institute for Advanced Concepts was a NASA-funded program that was operated by the Universities Space Research Association for NASA from 1998 until its closure on 31 August 2007. NIAC sought proposals for revolutionary aeronautics and space concepts that could dramatically impact...

report by Dr. Bradley C. Edwards
• A Hoist to the Heavens By Bradley Carl Edwards
• Ziemelis K. (2001) "Going up". In New Scientist
New Scientist
New Scientist is a weekly non-peer-reviewed English-language international science magazine, which since 1996 has also run a website, covering recent developments in science and technology for a general audience. Founded in 1956, it is published by Reed Business Information Ltd, a subsidiary of...

2289: 24–27. Republished in SpaceRef. Title page: "The great space elevator: the dream machine that will turn us all into astronauts."
• The Space Elevator Comes Closer to Reality. An overview by Leonard David of space.com, published 27 March 2002.
• Krishnaswamy, Sridhar. Stress Analysis — The Orbital Tower (PDF)
• Space Elevators Face Wobble Problem: New Scientist

• Peter Swan & Cathy Swan, "Space Elevator Systems Architecture." Lulu.com 2007. isbn 978-1-4303-1405-9 See ref. 555344 at www.lulu.com