Space elevator

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A space elevator would consist of a cable [4] anchored to the Earth's surface [6], reaching into space. By attaching a counterweight [3] at the end (or by further extending the cable for the same purpose), inertia ensures that the cable remains stretched taut, countering the gravitational pull on the lower sections, thus allowing the elevator to remain in geostationary orbit [1]. Once beyond the gravitational midpoint [2], carriage [5] would be accelerated further by the planet's rotation. (Diagram not to scale.)

A space elevator is a proposed megastructure designed to transport material from a celestial body's surface into space as a way of non-rocket spacelaunch. The term most often refers to a structure that reaches from the surface of the Earth to geosynchronous orbit (GSO) and a counter-mass beyond. The concept of a structure reaching to geosynchronous orbit was first conceived by Konstantin Tsiolkovsky,^[1] who proposed a compression structure, or "Tsiolkovsky tower." Most recent discussions focus on tensile structures (tethers) reaching from geosynchronous orbit to the ground. Space elevators have also sometimes been referred to as beanstalks, space bridges, space ladders, skyhooks, orbital towers, or orbital elevators.

The most common proposal is a tether, usually in the form of a cable or ribbon, spanning from the surface near the equator to a point beyond geosynchronous orbit. As the planet rotates, the inertia at the end of the tether counteracts gravity, and also keeps the cable taut. Vehicles can then climb the tether and reach orbit without the use of rocket propulsion. Such a structure could hypothetically permit delivery of cargo and people to orbit at a fraction of the cost of launching payloads by rocket.

Current technology is not capable of manufacturing materials that are sufficiently strong and light enough to build an Earth based space elevator as the total mass of conventional materials needed to construct such a structure would be far too great. Recent proposals for a space elevator are notable in their plans to use carbon nanotube-based materials as the tensile element in the tether design, since the theoretical strength of carbon nanotubes appears great enough to make this practical. Current technology may be able to support elevators in other locations in the solar system however, and other designs for space elevators exist that use current materials.

[edit] Geostationary orbital tethers

This concept, also called an orbital space elevator, geosynchronous orbital tether, or a beanstalk, is a subset of the skyhook concept, and is what people normally think of when the phrase 'Space elevator' is used (although there are variants).

Construction would be a vast project: a tether would have to be built of a material that could endure tremendous stress while also being light-weight, cost-effective, and manufacturable in great quantities. Today's materials technology does not meet these requirements, although carbon nanotube technology shows great promise. A considerable number of other novel engineering problems would also have to be solved to make a space elevator practical. Not all problems regarding feasibility have yet been addressed. Nevertheless, the LiftPort Group believes that the necessary technology might be developed as early as 2008^[2] and that by developing the technology, the first space elevator could be operational by 2014.^[3][4]

[edit] History

[edit] Early concepts

A ladder that reaches from earth to heaven is mentioned in Genesis 28:12. The angels of Elohim ascended and descended on it. This bible passage is cited as being a reference to a space elevator.^[5]

Konstantin Tsiolkovsky

The key concept of the space elevator appeared in 1895 when Russian scientist Konstantin Tsiolkovsky was inspired 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 above sea level (geostationary orbit). He noted that a "celestial castle" at the top of such a spindle-shaped cable would have the "castle" orbiting Earth in a geosynchronous orbit (i.e. the castle would remain over the same spot on Earth's surface).

Tsiolkovsky's tower would be able to launch objects into orbit without a rocket. 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 geosynchronous orbit. Unlike more recent concepts for space elevators, Tsiolkovsky's (conceptual) tower was a compression structure, rather than a tension (or "tether") structure.

[edit] Twentieth century

Building a compression structure from the ground up proved an unrealistic task; there was no material in existence with enough compressive strength to support its own weight under such conditions.^[6] In 1959 another Russian scientist, Yuri N. Artsutanov, suggested a more feasible proposal. Artsutanov suggested using a geosynchronous satellite as the base from which to deploy the structure downward. By using a counterweight, a cable would be lowered from geosynchronous 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 motionless relative to Earth. Artsutanov's idea was introduced to the Russian-speaking public in an interview published in the Sunday supplement of Komsomolskaya Pravda (usually named in English, "Young Person's Pravda") in 1960,^[7] but was not available in English until much later. He also proposed tapering the cable thickness so that the tension in the cable was constant—this gives a thin cable at ground level, thickening up towards GEO.

Making a cable over 35,000 kilometers 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.^[8] 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 required would be twice that of any existing material including graphite, quartz, and diamond.

In 1975 an American scientist, Jerome Pearson, reinvented the concept yet again, publishing his analysis in the journal Acta Astronautica. He designed^[9] a tapered cross section that would be better suited to building the elevator. The completed cable would be thickest at the geosynchronous 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 (almost half the distance to the Moon) 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 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 asteroidal or lunar ore.

In 1977, Hans Moravec published an article called "A Non-Synchronous Orbital Skyhook", in which he proposed an alternative space elevator concept, using a rotating cable,^[10] 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's novel, The Fountains of Paradise, 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, albeit moved south to the equator), and Charles Sheffield's first novel, The Web Between the Worlds, also featuring the building of a space elevator. Three years later, in Robert A. Heinlein's 1982 novel Friday the principal character makes use of the "Nairobi Beanstalk" in the course of her travels.

Kim Stanley Robinson's Mars trilogy chronicles the fictional settlement and terraforming of Mars and a space elevator is a focus point for one of the plotlines. In 1999, Larry Niven authored the book Rainbow Mars which contained a "Hanging Tree" - an organic 'Skyhook' which was capable of interstellar travel. The book skillfully discussed several merits/demerits of such an approach to the Beanstalk - the primary demerit being that the water necessary to sustain such an enormous 'tree' would require the drying up of all of its host planet's water bodies - which is used as a plot device to explain the drying up of Mars.

[edit] 21st century

After the development of carbon nanotubes in the 1990s, engineer David Smitherman of NASA/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, inviting many scientists and engineers to discuss concepts and compile plans for an elevator to turning the concept into a reality.^[11] The publication he edited compiling information from the workshop, "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium",^[12] provides an introduction to the state of the technology at the time, and summarizes the findings.

Another American scientist, Bradley C. Edwards, suggested creating a 100,000 km long paper-thin ribbon using nanotube fibers, suggesting that this structure would stand a greater chance of surviving impacts by meteoroids. Supported by the NASA Institute for Advanced Concepts, the work of Edwards was expanded to cover the deployment scenario, climber design, power delivery system, orbital debris 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.^[13][14] The largest holdup to Edwards' proposed design is the technological limits of the tether material. His calculations call for a fiber composed of epoxy-bonded carbon nanotubes with a minimal tensile strength of 130 GPa (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.^[15] Multi-walled carbon nanotubes have been measured with tensile strengths up to 63 GPa.^[16]

In order to speed development of space elevators, proponents are planning several competitions, similar to the Ansari X Prize, for relevant technologies.^[17][18] Among them are Elevator:2010 which will organize annual competitions for climbers, ribbons and power-beaming systems, the Robolympics Space Elevator Ribbon Climbing competition,^[19] as well as NASA's Centennial 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.^[20][21]

In 2005, "the LiftPort Group of space elevator companies has announced that it will be building a carbon nanotube manufacturing plant in Millville, New Jersey, 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 100,000 km (62,000 mile) space elevator, this move will allow it to make money in the short term and conduct research and development into new production methods."^[22] The group also announced that they had obtained permission from the Federal Aviation Administration to use airspace to conduct preliminary tests of its high altitude robotic lifters.^[23] The experiment was successful.

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 wide and 1 mm (approx. 6 sheets of paper) thick, lifted with balloons.^[24]

On August 24, 2006 the Japanese National Museum of Emerging Science and Technology in Tokyo has started to show the animation movie 'Space Elevator', based on ATA Space Elevator Project, also directed and edited by project leader, Dr. Serkan Anilir. This movie shows a possible image about the cities of future, placing the space elevator tower as a new infrastructure into the city planning, and aims to contribute children education. Currently, the movie is shown in all science museums in Japan.^[25]

The x-Tech Projects company has also been founded to pursue the prospect of a commercial Space Elevator.

In 2007, Elevator:2010 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.^[26] No teams won the competition, but a team from MIT entered the first 2-gram, 100% carbon nanotube entry into the competition.^[27]

[edit] Physics and structure

One concept for the space elevator has it tethered to a mobile seagoing platform.

There are a variety of tether designs. Almost every design includes a base station, a cable, climbers, and a counterweight.

[edit] Base station

The base station designs typically fall into two categories—mobile and stationary. Mobile stations are typically large oceangoing vessels,^[28] though airborne stations have been proposed as well.^{[citation needed]} Stationary platforms would generally be located in high-altitude locations, such as on top of mountains, or even potentially on high towers.^[6]

Mobile platforms have the advantage of being able to maneuver to avoid high winds, storms, and space debris. 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 (typically no more than a few kilometers), that can significantly reduce the minimal width of the cable at the center, and reduce the minimal length of cable reaching beyond geostationary orbit significantly.

[edit] Cable

The cable must be made of a material with a large tensile strength/density ratio. A space elevator can be made relatively economically feasible if a cable with a density similar to graphite and a tensile strength of ~65–120 GPa can be mass-produced at a reasonable price.

Carbon nanotubes would be a highly useful material for creating a space elevator

By comparison, most steel has a tensile strength of under 2 GPa, and the strongest steel resists no more than 5.5 GPa, but steel is dense. The much lighter material Kevlar has a tensile strength of 2.6–4.1 GPa, while quartz fiber^{[citation needed]} and carbon nanotubes^[29] can reach upwards of 20 GPa; the tensile strength of diamond filaments would theoretically be minimally higher.

Carbon nanotubes' theoretical tensile strength has been estimated between 140 and 177 GPa (depending on plane shape),^[29] and its observed tensile strength has been variously measured from 63 to 150 GPa, close to the requirements for space elevator structures.^[29][30] Even the strongest fiber made of nanotubes is likely to have notably less strength than its components.

Improving tensile strength depends on further research on purity and different types of nanotubes.

Designs call for single-walled carbon nanotubes. While multi-walled nanotubes are easier to produce and have similar tensile strengths, there is a concern that the interior tubes would not be sufficiently coupled to the outer tubes to help hold the tension. However, if the nanotubes are long enough, even weak Van der Waals forces will be sufficient to keep them from slipping, and the full strength of individual nanotubes (single or multiwalled) could be realized macroscopically by spinning them into a yarn. It has also been proposed to chemically interlink the nanotubes in some way, but it is likely that this would greatly compromise their strength. One such proposal is to take advantage of the high pressure interlinking properties of carbon nanotubes of a single variety.^[31] While this would cause the tubes to lose some tensile strength by the trading of sp² bond (graphite, nanotubes) for sp³ (diamond), it will enable them to be held together in a single fiber by more than the usual, weak Van der Waals force (VdW), and allow manufacturing of a fiber of any length.

A seagoing anchor station would incidentally act as a deep-water seaport.

The technology to spin regular VdW-bonded yarn from carbon nanotubes is just in its infancy: the first success in spinning a long yarn, as opposed to pieces of only a few centimeters, was reported in March 2004; but the strength/weight ratio was not as good as Kevlar due to the inconsistent quality and short length of the tubes being held together by VdW.

As of 2006, carbon nanotubes cost $25/gram, and even a space elevator that did not reach GEO would have a mass of 20,000 kg. However, this price is declining, and large-scale production could result in strong economies of scale.^[32]

Carbon nanotube fiber is an area of energetic worldwide research because the applications go much further than space elevators. Other suggested application areas include suspension bridges, new composite materials, lighter aircraft and rockets, armor technologies, and computer processor interconnects. This is good news for space elevator proponents because it is likely to push down the price of the cable material further.

[edit] Cable taper

Due to its enormous length a space elevator cable must be carefully designed to 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. In an ideal cable, the actual strength of the cable at any given point would equal to the required strength at that point (plus a safety margin). This implies a tapered design.

Using a model that takes into account the Earth's gravitational and "centrifugal" forces (and neglecting the smaller solar and lunar effects), it is possible to show^[9] that the optimal cross-sectional area of the cable as a function of height is given by:

Cable Taper Plot

where A(r) is the cross-sectional area as a function of distance r from the Earth's center.

The constants in the equation are:

• A₀ is the cross-sectional area of the cable on the earth's surface.
• ρ is the density of the material the cable is made out of.
• s is the tensile strength of the material.
• ω is the angular velocity of the Earth about its axis, 7.292 × 10⁻⁵ rad·s⁻¹.
• r₀ is the distance between the Earth's center and the base of the cable. It is approximately the Earth's equatorial radius, 6378 km.
• g₀ is the acceleration due to gravity at the cable's base, 9.780 m·s⁻².

This equation gives a shape where the cable thickness initially increases rapidly in an exponential fashion, but slows at an altitude a few times the Earth's radius, and then gradually becomes parallel when it finally reaches maximum thickness at geostationary orbit. The cable thickness then decreases again out from geosynchronous orbit. The relative thickness at all points is determined by the strength density ratio. This is shown in the figure to the right.

Thus the taper of the cable from base to GEO (r = 42,164 km),

Using the density and tensile strength of steel, and assuming a diameter of 1 cm at ground level, yields a diameter of several hundred kilometers at geostationary orbit height, showing that steel, and indeed all materials used in present day mechanical engineering, are unsuitable for building a space elevator.

The equation shows us that there are four ways of achieving a more reasonable thickness at geostationary orbit:

• Using a lower density material. Not much scope for improvement as the range of densities of most solids that come into question is rather narrow, somewhere between 1000 kg·m⁻³ and 5000 kg·m⁻³.
• Using a higher strength material. This is the area where most of the research is focused. Carbon nanotubes are tens of times stronger than the strongest types of steel, hugely reducing the cable's cross-sectional area at geostationary orbit.
• Increasing the height of a tip of the base station, where the base of cable is attached. If the cable is properly tapered, however (see next point) this will not make much difference unless a tower of the order of 1000 km is built.
• Making the cable as thin as possible at its base. It still has to be thick enough to carry a payload however, so the minimum thickness at base level also depends on tensile strength. A cable made of carbon nanotubes (a type of fullerene), would typically be just a millimeter wide at the base^{[citation needed]}.

[edit] Climbers

Most space elevator designs call for a climber to move autonomously along a stationary cable.

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. Usually, elevators are designed for climbers to move only upwards, because that is where most of the payload goes. For returning payloads, atmospheric reentry on a heat shield is a very competitive option, which also avoids the problem of docking to the elevator in space.

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.

[edit] 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.

Nuclear energy and solar power have been proposed, but generating enough energy to reach the top of the elevator in any reasonable time without weighing too much is not feasible.^[33]

The current method of favor is laser power beaming, using megawatt powered free electron or solid state lasers in combination with adaptive mirrors approximately 10 m wide and a photovoltaic array on the climber tuned to the laser frequency for efficiency.^[28] A major obstacle for any climber design is the dissipation of the substantial amount of waste heat generated due to the less than perfect efficiency of any of the power methods.

[edit] Counterweight

There have been several methods proposed for dealing with the counterweight need: a heavy object, such as a captured asteroid or a space station, positioned past geosynchronous orbit, or extending the cable itself well past geosynchronous orbit. The latter idea has gained more support in recent years due to the relative 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.

Additionally, Brad Edwards has proposed that initially elevators would be up-only, and that the elevator cars that are used to thicken up the cable could simply be parked at the top of the cable and act as a counterweight.

[edit] Angular momentum, speed and cable lean

As the car climbs, the elevator takes on a 1 degree lean, due to the top of the elevator traveling faster than the bottom around the Earth (Coriolis effect). This diagram is not to scale.

The horizontal speed of each part of the cable increases with altitude, proportional to distance from the center of the Earth, reaching orbital velocity at geosynchronous orbit. Therefore as a payload is lifted up a space elevator, it needs to gain not only altitude but angular momentum (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 effect) and thus the climber "drags" on the cable, carrying the cable with it very slightly to the west (and necessarily pulling the counterweight slightly to the west, shown as an offset of the counterweight in the diagram to right, slightly changing the motion of the counterweight). At a 200 km/h climb speed this generates a 1 degree lean on the lower portion of the cable. The horizontal component of the tension in the non-vertical cable applies a sideways pull on the payload, accelerating it eastward (see diagram) and this is the source of the speed that the climber needs. Conversely, the cable pulls westward on Earth's surface, insignificantly slowing the Earth, from Newton's 3rd law.

Meanwhile, 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. Provided that the Space Elevator is designed so that the center of weight always stays above geosynchronous orbit^[34] for the maximum climb speed of the climbers, the elevator cannot fall over. 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.

[edit] Launching into outer space

The velocities that might be attained at the end of Pearson's 144,000 km cable can be determined. The tangential velocity is 10.93 kilometers per second which is more than enough to escape Earth's gravitational field and send probes as far out as Saturn. If an object were allowed to slide freely along the upper part of the tower, a velocity high enough to escape the solar system entirely would be attained. This is accomplished by trading off overall angular momentum of the tower for velocity of the launched object, in much the same way one snaps a towel or throws a lacrosse ball. After such an operation a cable would be left with less angular momentum than required to keep its geostationary position. The rotation of the Earth would then pull on the cable increasing its angular velocity, leaving the cable swinging backwards and forwards about its starting point.

For higher velocities, the cargo can be electromagnetically accelerated, or the cable could be extended, although that would require additional strength in the cable.

[edit] Extraterrestrial elevators

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

A Martian 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 is much closer to the surface, and hence the elevator would be much shorter. Exotic materials might not be required to construct such an elevator. However, building a Martian elevator would be a unique challenge because the Martian moon Phobos is in a low orbit, and intersects the equator regularly (twice every orbital period of 11 h 6 min). A collision between the elevator and the 22.2 km diameter moon would have to be avoided through active steering of the elevator. One simpler way to resolve the problem of Phobos (1.1 degree orbital inclination) or Deimos (1.8 degree orbital inclination) interaction is to position the tether anchor perhaps five (5) degrees off the Martian equator. There would be a small payload penalty, but the tether would pass outside the orbital inclination of the two moons. Also, the tether would depart the Martian anchor at 5–10 degrees from vertical.

Conversely, a Venusian space elevator would need to be much longer. Although a tether placed at the stationary orbit of the slowly rotating Venus would intersect the Sun, one could be constructed that rotated with the fast-moving cloud decks of the planet which take only four Earth days to make a complete cycle. The cable would need to exceed 100,000 kilometers long but, counter-intuitively, would experience less stress due to the slightly smaller gravity exerted on the cable. Such an elevator could service aerostats or floating cities in the benign regions of the atmosphere.

Another Venusian design would require the anchor to be a mobile ground level platform that would circle Venus at the same rate that it rotates, 6.52 km/h. The counterweight on the other end would be hung toward the sun at all times, past the point where the Sun's and Venus's gravity cancel each other out, thereby keeping the tether balanced by the Sun's pull of gravity. This point is called a Lagrangian point, specifically L1. This is 1,000,000 km from Venus.

A lunar space elevator can possibly be built with currently available technology about 50,000 kilometers long extending though the Earth-moon L1 point from an anchor point near the center of the visible part of Earth's moon.

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. Alternatively, due to the lack of atmosphere on the Moon, a rotating tether could be used with its center of weight in orbit around the Moon with a counterweight (e.g. a space station) at the short end and a payload at the long end. The path of the payload would be an epicycloid around the Moon, touching down at some integer number of times per orbit. Thus, payloads are lifted off the surface of the Moon, and flung away at the high point of the orbit.

Rapidly spinning asteroids or moons could use cables to eject materials in order to move the materials to convenient points, such as Earth orbits; or conversely, to eject materials in order to send the bulk of the mass of the asteroid or moon to Earth orbit or a Lagrangian point. This was suggested by Russell Johnston in the 1980s. Freeman Dyson, 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^[35] and described very succinctly on the Island One website as a "Tapered Sling"^[36]

It may also be possible to construct space elevators at the three smaller gas giants, Saturn, Uranus and Neptune. These would all involve tapering several times greater than those of the inner solar system^{[citation needed]}. These outer space elevators could facilitate the exchange of supplies and helium-3 between floating mining colonies in the atmospheres and local moon settlements. However, difficulties such as the equatorially orbiting lower rings and moons of these giant planets would first need to be overcome.

Pluto could also have an elevator and tether. In recent years physicists have suggested that due to the Pluto and Charon dynamics, it could be possible to link the two planets by a single tether.^{[citation needed]}

[edit] Construction

The construction of a space elevator would be a vast project, requiring advances in engineering, manufacture and physical technology. David Smitherman of NASA has published a paper that identifies "Five Key Technologies for Future Space Elevator Development":^[37]

1. Material for cable (e.g. carbon nanotube and nanotechnology) and tower
2. Tether deployment and control
3. Tall tower construction
4. Electromagnetic propulsion (e.g. magnetic levitation)
5. Space infrastructure and the development of space-based industry and economy

Two different ways to deploy a space elevator have been proposed.

[edit] Traditional way

One early plan involved lifting the entire mass of the elevator into geosynchronous orbit, and simultaneously lowering one cable downwards towards the Earth's surface while another cable is deployed upwards directly away from the Earth's surface.

Tidal forces (gravity and centrifugal force) would naturally pull the cables directly towards and directly away from the Earth and keep the elevator balanced around geosynchronous orbit. As the cable is deployed, coriolis forces would pull the upper portion of the cable somewhat to the West and the lower portion of the cable somewhat to the East; this effect can be controlled by varying the deployment speed.

However, this approach requires lifting hundreds or even thousands of tons on conventional rockets, an expensive proposition. Hypothetically, such a plan could make extensive use of materials available in space to reduce costs, but this would require considerable space mining and space-based processing of materials, neither of which is currently practical using existing technology.

[edit] Brad Edwards' proposal

Bradley C. Edwards, former Director of Research for the Institute for Scientific Research (ISR), based in Fairmont, West Virginia proposed that, if nanotubes with sufficient strength could be made in bulk, a space elevator could be built in little more than a decade, rather than the far future. He proposed that a single hair-like 18-metric ton (20 short ton) 'seed' cable be deployed in the traditional way, giving a very lightweight elevator with very little lifting capacity. Then, progressively heavier cables would be pulled up from the ground along it, repeatedly strengthening it until the elevator reaches the required mass and strength. This is much the same technique used to build suspension bridges.

Although 18 tonnes for a seed cable may sound like a lot, it would actually be very lightweight — the proposed average mass is about 200 grams per kilometer. In comparison, conventional copper telephone wires running to consumer homes weigh about 4 kg/km.

[edit] Loop elevator design

This is a less well developed design, but offers some other possibilities.

If the cable provides a useful tensile strength of about 62.5 GPa or above, then it turns out that a constant width cable can reach beyond geosynchronous orbit without breaking under its own weight. The far end can then be turned around and passed back down to the Earth forming a constant width loop, which would be kept spinning to avoid tangling. The two sides of the loop are naturally kept apart by coriolis forces due to the rotation of the Earth and the loop. By increasing the thickness of the cable from the ground a very quick (exponential) build-up of a new elevator may be performed (it helps that no active climbers are needed, and power is applied mechanically.) However, because the loop runs at constant speed, joining and leaving the loop may be somewhat challenging, and the carrying capacity of such a loop is lower than a conventional tapered design.^[38]

[edit] Failure modes, safety issues and construction difficulties

As with any structure, there are a number of ways in which things could go wrong. A space elevator would present a considerable navigational hazard, both to aircraft and spacecraft. Aircraft could be dealt with by means of simple air-traffic control restrictions, but impacts by space objects (in particular, by meteoroids and micrometeorites) pose a more difficult problem.

[edit] Cable strength

The current strength/mass ratio of cables of any construction is inadequate to build a space elevator at the present time. Although carbon nanotubes embedded in the tether would give it enough strength to be practical, nanotubes of sufficient length have not yet been made.

Theoretical objections have been raised to manufacturing bulk carbon nanotube structures with strengths approaching that which simple models and microscopic strengths suggest. H. K. D. H. Bhadeshia argues that the presence of defects would significantly reduce the strength actually attainable.^[39]

[edit] Satellites

If nothing were done, essentially all satellites with perigees below the top of the elevator would eventually collide with the elevator cable. Twice per day, each orbital plane intersects the elevator, as the rotation of the Earth swings the cable around the equator. Usually the satellite and the cable will not line up. However, except for synchronized orbits, the elevator and satellite will eventually occupy the same place at the same time, almost certainly leading to structural failure of the space elevator and destruction of the satellite.

Most active satellites are capable of some degree of orbital maneuvering and could avoid these predictable collisions, but inactive satellites and other orbiting debris would need to be either preemptively removed from orbit by "garbage collectors" or would need to be closely watched and nudged whenever their orbit approaches the elevator. The impulses required would be small, and need be applied only very infrequently; a laser broom system may be sufficient to this task. In addition, Brad Edward's design actually allows the elevator to move out of the way, because the fixing point is at sea and mobile. However, such movements would excite transverse oscillations of the cable. Edwards claims that these oscillations could be controlled so as to ensure that the cable avoids satellites on known paths.

[edit] Meteoroids and micrometeorites

Meteoroids present a more difficult problem, since they would not be predictable and much less time would be available to detect and track them as they approach Earth. It is likely that a space elevator would still suffer impacts of some kind, no matter how carefully it is guarded. However, most space elevator designs call for the use of multiple parallel cables separated from each other by struts, with sufficient margin of safety that severing just one or two strands still allows the surviving strands to hold the elevator's entire weight while repairs are performed. If the strands are properly arranged, no single impact would be able to sever enough of them to overwhelm the surviving strands.

Far worse than meteoroids are micrometeorites; tiny high-speed particles found in high concentrations at certain altitudes. Avoiding micrometeorites is essentially impossible, and they will ensure that strands of the elevator are continuously being cut. Most methods designed to deal with this involve a design similar to a hoytether or to a network of strands in a cylindrical or planar arrangement with two or more helical strands. Constructing the cable as a mesh instead of a ribbon helps prevent collateral damage from each micrometeorite impact.

[edit] Failure cascade

It is not enough that other fibers be able to take over the load of a failed strand — the system must also survive the immediate, dynamical effects of fiber failure, which generates projectiles aimed at the cable itself. For example, if the cable has a working stress of 50 GPa and a Young's modulus of 1000 GPa, its strain will be 0.05 and its stored elastic energy will be 1/2 × 0.05 × 50 GPa = 1.25×10⁹ joules per cubic meter. Breaking a fiber will result in a pair of de-tensioning waves moving apart at the speed of sound in the fiber, with the fiber segments behind each wave moving at over 1,000 m/s (more than the muzzle velocity of a standard .223 caliber (5.56 mm) round fired from an M16 rifle). Unless these fast-moving projectiles can be stopped safely, they will break yet other fibers, initiating a failure cascade capable of severing the cable. The challenge of preventing fiber breakage from initiating a catastrophic failure cascade seems to be unaddressed in the current (January, 2005) literature on terrestrial space elevators. Problems of this sort would be easier to solve in lower-tension applications (e.g., lunar elevators).

[edit] Corrosion

Corrosion is a major risk to any thinly built tether (which most designs call for). In the upper atmosphere, atomic oxygen steadily eats away at most materials. A tether will consequently need to either be made from a corrosion-resistant material or have a corrosion-resistant coating, adding to weight. Gold and platinum have been shown to be practically immune to atomic oxygen; several far more common materials such as aluminum are damaged very slowly and could be repaired as needed.

Another potential solution to the corrosion problem is a continuous renewal of the tether surface (which could be done from standard, though possibly slower elevators). This process would depend on the tether composition and it could be done on the nanoscale (by replacing individual fibers) or in segments.

[edit] Radiation

The effectiveness of the magnetosphere to deflect radiation emanating from the sun decreases dramatically after rising several earth radii above the surface. This ionizing radiation may cause damage to materials within both the tether and climbers.

[edit] Material defects

Any structure as large as a space elevator will have massive numbers of tiny defects in the construction material. It has been suggested,^[40][41] that, because large structures have more defects than small structures, that large structures are inherently weaker than small, giving an estimated carbon nanotube strength of only 24 GPa down to only 1.7 GPa in millimetre-scale samples, the latter equivalent to many high-strength steels, which would be vastly less than that needed to build a space elevator for a reasonable cost.

[edit] Weather

In the atmosphere, the risk factors of wind and lightning come into play. The basic mitigation is location. As long as the tether's anchor remains within two degrees of the equator, it will remain in the quiet zone between the Earth's Hadley cells, where there is relatively little violent weather. Remaining storms could be avoided by moving a floating anchor platform. The lightning risk can be minimized by using a nonconductive fiber with a water-resistant coating to help prevent a conductive buildup from forming. The wind risk can be minimized by use of a fiber with a small cross-sectional area that can rotate with the wind to reduce resistance. Ice forming on the cable also presents a potential problem. It could add significantly to the cable's weight and affect the passage of elevator cars. Also, ice falling from the cable could damage elevator cars or the cable itself. To get rid of ice, special elevator cars could scrape the ice off.

[edit] Vibrational harmonics

A final risk of structural failure comes from the possibility of vibrational harmonics within the cable. Like the shorter and more familiar strings of stringed musical instruments, the cable of a space elevator has a natural resonant frequency. If the cable is excited at this frequency, for example by the travel of elevators up and down it, the vibrational energy could build up to dangerous levels and exceed the cable's tensile strength. This can be avoided by the use of suitable damping systems within the cable, and by scheduling travel up and down the cable keeping its resonant frequency in mind. It may be possible to dampen the resonant frequency against the Earth's magnetosphere.

[edit] In the event of failure

If despite all these precautions the elevator is severed anyway, the resulting scenario depends on where exactly the break occurred:

[edit] Cut near the anchor point

If the elevator is cut at its anchor point on Earth's surface, the outward force exerted by the counterweight would cause the entire elevator to rise upward into an unstable orbit.

The ultimate altitude of the severed lower end of the cable would depend on the details of the elevator's mass distribution. In theory, the loose end might be secured and fastened down again. This would be an extremely tricky operation, however, requiring careful adjustment of the cable's center of gravity to bring the cable back down to the surface again at just the right location. It may prove to be easier to build a new system in such a situation.

[edit] Cut up to about 25,000 km

If the break occurred at higher altitude, up to about 25,000 km, the lower portion of the elevator would descend to Earth and drape itself along the equator east of the anchor point, while the now unbalanced upper portion would rise to a higher orbit. Some authors (such as science fiction writers David Gerrold in Jumping off the Planet, Kim Stanley Robinson in Red Mars, and Ben Bova in Mercury) have suggested that such a failure would be catastrophic, with the thousands of kilometers of falling cable creating a swath of meteoric destruction along the planet's surface; however, in most cable designs, the upper portion of any cable that fell to Earth would burn up in the atmosphere. Additionally, because proposed initial cables have very low mass (roughly 1 kg per kilometer) and are flat, the bottom portion would likely settle to Earth with less force than a sheet of paper due to air resistance on the way down.^{[citation needed]}

If the break occurred at the counterweight side of the elevator, the lower portion, now including the "central station" of the elevator, would entirely fall down if not prevented by an early self-destruct of the cable shortly below it. Depending on the size, however, it would burn up on re-entry anyway. Simulations have shown that as the descending portion of the space elevator "wraps around" Earth, the stress on the remaining length of cable increases, resulting in its upper sections breaking off and being flung away. The details of how these pieces break and the trajectories they take are highly sensitive to initial conditions.^[42]

[edit] Elevator climbers

Any climbers on the falling section would also reenter Earth's atmosphere, but it is likely that the climbers will already have been designed to withstand such an event as an emergency measure. It is almost inevitable that some objects — climbers, structural members, repair crews, etc. — will accidentally fall off the elevator at some point. Their subsequent fate would depend upon their initial altitude. Except at geosynchronous altitude, an object on a space elevator is not in a stable orbit and so its trajectory will not remain parallel to it. The object will instead enter an elliptical orbit, the characteristics of which depend on where the object was on the elevator when it was released.

If the initial height of the object falling off of the elevator is less than 23,000 km, its orbit will have an apogee at the altitude where it was released from the elevator and a perigee within Earth's atmosphere — it will intersect the atmosphere within a few hours, and not complete an entire orbit. Above this critical altitude, the perigee is above the atmosphere and the object will be able to complete a full orbit to return to the altitude it started from. By then the elevator would be somewhere else, but a spacecraft could be dispatched to retrieve the object or otherwise remove it. The lower the altitude at which the object falls off, the greater the eccentricity of its orbit.

If the object falls off at the geostationary altitude itself, it will remain nearly motionless relative to the elevator just as in conventional orbital flight. At higher altitudes the object would again be in an elliptical orbit, this time with a perigee at the altitude the object was released from and an apogee somewhere higher than that. The eccentricity of the orbit would increase with the altitude from which the object is released.

Above 47,000 km, however, an object that falls off of the elevator would have a velocity greater than the local escape velocity of Earth. The object would head out into interplanetary space, and if there were any people present on board it might prove impossible to rescue them.

[edit] Van Allen Belts

Van Allen radiation belts

The space elevator would run through the Van Allen belts. This is not a problem for most freight, but the amount of time a climber spends in this region would cause radiation poisoning to any unshielded human or other living things.^[43][44] Some speculate that passengers would continue to travel by high-speed rocket, while space elevators haul bulk cargo. Research into lightweight shielding and techniques for clearing out the belts is underway.

More conventional and faster atmospheric reentry techniques such as aerobraking might be employed on the way down to minimize radiation exposure. De-orbit burns use relatively little fuel and are cheap.

An obvious option would be for the elevator to carry shielding to protect passengers, though this would reduce its overall capacity. The best radiation shielding is very mass-intensive for physical reasons. Alternatively, the shielding itself could in some cases consist of useful payload, for example food, water, fuel or construction/maintenance materials, and no additional shielding costs are incurred during ascent.

To shield passengers from the radiation in the Van Allen belt, perhaps counter-intuitively, material composed of light elements should be used, as opposed to lead shielding. In fact, high energy electrons in the Van Allen belts produce dangerous X-rays when they strike atoms of heavy elements. This is known as bremsstrahlung ("braking") radiation, and is the way X-rays are created for medical use (for example, dentistry). Materials containing great amounts of hydrogen, such as water or (lightweight) plastics such as polyethylene and lighter metals such as aluminium are better than heavier ones such as lead for preventing this secondary radiation. Such light-element shielding, if it were strong enough to protect against the Van Allen particle radiation, would also provide adequate protection against X-ray radiation coming from the sun during solar flares and coronal mass ejection events. Nevertheless the total mass required for radiation shielding is very high.

[edit] Economics

Main article: Space elevator economics

With a space elevator, materials might be sent into orbit at a fraction of the current cost. Conventional rocket designs give prices on the order of thousands of U.S. dollars per kilogram for transfer to low earth orbit,^[45] and roughly twenty thousand dollars per kilogram for transfer to geosynchronous orbit.^{[citation needed]} Even optimistic rocket proposals (such as the DH-1) only claim to bring prices down to about $400 per kilo.^{[citation needed]} For the first space elevator, the price could be as low as $220 per kilogram and would decrease as time went on.^[46]

Space elevators have high capital cost but low operating expenses, so they make the most economic sense in a situation where it would be used over a long period of time to handle very large amounts of payload. The current launch market may not be large enough to make a compelling case for a space elevator, but a dramatic drop in the price of launching material to orbit would likely result in new types of space activities becoming economically feasible. In this regard they share similarities with other transportation infrastructure projects such as highways or railroads.^{[citation needed]}

Development costs might be roughly equivalent, in modern dollars, to the cost of developing the shuttle system. A question subject to speculation is whether a space elevator would return the investment, or if it would be more beneficial to instead spend the money on developing rocketry further. If the elevator did indeed cost roughly the same as the shuttle program, recovering the development costs would take less than about a hundred thousand tons launched to low earth orbit or five thousand tons launched to geosynchronous orbit; but construction costs are predicted, to the extent that they can be predicted to be much higher than the development costs.^{[citation needed]}

[edit] Political issues

One potential problem with a space elevator would be the issue of ownership and control. Such an elevator would require significant investment (estimates start at about US$5 billion for a very primitive tether), and it could take at least a decade to recoup such expenses. At present, few entities in the space industry are able to spend at that magnitude.

Assuming a multi-national governmental effort was able to produce a working space elevator, many political issues would remain to be solved. Which countries would use the elevator and how often? Who would be responsible for its defense from terrorists or enemy states? A space elevator could potentially cause rifts between states over the military applications of the elevator. Furthermore, establishment of a space elevator would require removal of existing satellites if their orbit intersects the cable (unless the base station itself can move in order to make the elevator avoid satellites, as proposed by Edwards).

An initial elevator could be used in relatively short order to lift the materials to build more such elevators, but the owners of the first elevator might refuse to carry such materials in order to maintain their monopoly.

As space elevators (regardless of the design) are inherently fragile but militarily valuable structures, they would likely be targeted immediately in any major conflict with a state that controls one. Consequently, most militaries would elect to continue development of conventional rockets (or other similar launch technologies) to provide effective backup methods to access space.

The cost of the space elevator is not excessive^{[citation needed]} compared to other projects and it is conceivable that several countries or an international consortium could pursue the space elevator. Indeed, there are companies and agencies in a number of countries that have expressed interest in the concept. Generally, projects on the scale of a space elevator need to be either joint public-private partnership ventures or government ventures, and involve multiple partners.

The political motivation for a collaborative effort comes from the potential destabilizing nature of the space elevator^{[citation needed]}. The space elevator clearly has military applications, but more critically it would give a strong economic advantage for the controlling entity^{[citation needed]}. Information flowing through satellites, future energy from space, planets full of real estate and associated minerals, and basic military advantage could all potentially be controlled by the entity that controls access to space through the space elevator. An international collaboration could result in multiple elevators at various locations around the globe, since subsequent elevators would be significantly cheaper^{[citation needed]}, thus allowing general access to space and consequently eliminating the instabilities a single system might cause.

Arthur C. Clarke compared the space elevator project to Cyrus West Field's efforts to build the first transatlantic telegraph cable, "the Apollo Project of its age".^[47]

[edit] Alternatives to geostationary tether concepts

Many different types of structures ("space elevators") for accessing space have been suggested; many of which would appear to be buildable from currently available materials. It is possible that these may be used as preliminary waypoints in the development of a space elevator.

[edit] Compressive structures

The original concept envisioned by Tsiolkovski was a compression structure. The compressive concept is similar to an aerial mast. While such structures might reach the agreed altitude for space (100 km), they are unlikely to reach geostationary orbit (35,786 km). Due to the difference between sub-orbital and orbital spaceflights, a means of propulsion (such as a rocket) would be necessary to achieve orbital speed. Arthur C. Clarke proposed a compressive space tower made of diamond in his novel 2061: Odyssey Three. The towers have actually been built by 3001: The Final Odyssey. It has been proposed that the concept of a Tsiolkovski tower could be combined with that of a classic space elevator cable, so that the tower reaches upward from Earth and meets a cable extended downward.^[6]

[edit] Orbital ring

Main article: Orbital ring

An orbital ring would be a circular cable spinning in low earth orbit around the Earth, with stationary spokes hanging down to the ground, resting on superconducting magnetic bearings. Due to the short length of the cables, the materials issues are greatly eased, and it is thought that it could be built with today's materials. Vehicles could climb the spokes and then accelerate up to orbital speed along the cable or otherwise.

[edit] Space fountains

Main article: Space fountain

The space fountain concept fires pellets, with a mass driver, up from the ground through the center of a tower. These pellets then impart their kinetic energy to the tower structure via electromagnetic drag as they travel up and again as their direction was reversed by a magnetic field at the top. Thus the structure would not be supported by the compressive strength of its materials, and could be hundreds of kilometers high. Unlike tethered space elevators (which have to be placed near the equator), a space fountain could be located at any latitude. Space fountains would require a continuous supply of power to remain aloft. Some mechanism is needed at the top to launch objects into orbit.

[edit] Launch loops

Main article: Launch loop

A launch loop (Keith Lofstrom 1985)

A launch loop would be an iron ribbon, carried on magnetic bearings, running at 14 km/s within a stationary vacuum sheath around a very long (~2000 km) track. It would be constructed so that as the structure speeds up, the middle part of the loop raises up in an arc up to 80 km altitude and forms an acceleration track. Vehicles are electromagnetically accelerated along the structure using maglev with an acceleration of around 3 g. In this way, they are launched into an elliptic orbit, using only electrical power, at very low cost. It is thought that launch loops could be built with today's materials and technology. Many dozens of launches per hour could be achieved, more than with a geostationary space elevator.^[48]

Unlike a geostationary space elevator, launch loops would be intended for and suitable for launching human cargo.^[48]

[edit] Skyhooks

Main article: Skyhook (structure)

A tidal stabilized tether is called a "skyhook" since it appears to be "hooked onto the sky". This term was introduced by the Italian scientist Giuseppe Colombo. Skyhooks rotate precisely once per orbit and hence are always oriented the same way to the parent body. They are also called "hypersonic tethers" because the tip nearest the earth travels about Mach-12 in typical designs. Longer tethers would travel more slowly. An aircraft or sub-orbital vehicle transports cargo to one end of the skyhook. Skyhook designs typically require climbers to transport the cargo to the other end.

Robert Raymond Boyd and Dimitri David Thomas (with Lockheed Martin Corporation) patented the Skyhook idea in 2000 in a patent titled "Space elevator".^[49]

The company Tethers Unlimited Inc (founded by Dr. Robert Forward and Dr. Robert P. Hoyt) has called this approach "Tether Launch Assist".^[50]

[edit] Funding for alternatives

As of 2004, concepts using geostationary tethers seem to be the only space elevator concept that is the subject of active research and commercial interest in space.^[51]

[edit] See also

Wikimedia Commons has media related to:

Space elevators

Spaceflight Portal

• Lunar space elevator for the moon variant
• Space elevator in fiction
• Space elevator economics discusses capital and maintenance costs of a space elevator.
• Space fountain
• Tether propulsion
• Launch loop

[edit] References

[edit] Specific

[Isaa66] Isaacs, J. D., A. C. Vine, H. Bradner & G. E. Bachus (1966) ‘Satellite Elongation into a True “Sky-Hook”' Science 151: 682-683.

[edit] General

• Space Elevators Face Wobble Problem: New Scientist

[edit] External links

Listen to this article (info/dl)

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More spoken articles

• Space elevator - Google Patents
• Wikinomics Elevator to Space Article

[edit] Organizations

• The National Space Society Special Interest Chapter for the Space Elevator (NSECC)
• Elevator:2010 Space elevator prize competitions
• Space elevator, Institute for Scientific Research Last news item on web site dated July, 2004.
• The Space Elevator: 3rd Annual International Conference 28 June-30, 2004 in Washington, D.C.
• 3rd Annual International Conference Presentations
• 4th Annual International Conference Presentations
• LiftWatch.org - Space Elevator News
• Liftport Group - The Space Elevator Companies

[edit] Animations

• View space elevator animation Windows Media Video (WMV) file - Institute for Scientific Research
• Download space elevator animation Windows Media Video (WMV) file - Institute for Scientific Research
• Brief video (realmedia format) of the space elevator concept

[edit] Miscellaneous links

• Skyhook Bibliography
• Leaving The Planet By Space Elevator
• Kansas City Space Pirates
• Project of the Scientific Workgroup for Rocketry and Spaceflight(WARR) (German)
• The Space Elevator Reference
• California Engineering Company's Site Regarding Improvements to Current Designs
• Space Elevator Yahoo Group A discussion list for space elevator related topics
• A major Russian site about space elevators, by Y. Artsutanov and D. Tarabanov
• Some technical papers and a numerical/graphical tool for calculating ribbon properties and deployment scenarios.
• HowStuffWorks article on the space elevator
• The Space Elevator: A Brief Overview
• Space Elevator in 3D for Google Earth
• Space Elevator NOVA scienceNOW segment
• Arthur C. Clarke links & image archive
• Space Elevator Journal
• Space Elevator Search Engine
• Interview with Brad Edwards
• The Space Elevator Blog

[edit] Articles/Interviews

• October 2007 IEEE Spectrum interview with Arthur C Clarke mentions the space elevator
• To the Moon in a Space Elevator? (4 February 2003 Wired News)
• Liftoff (teenage education): Space Towers
• Audacious & Outrageous: Space Elevators
• Various thoughts on space elevators posted by Blaise Gassend
• Space elevator robot passes 1,000-foot mark
• The Economist: Waiting For The Space Elevator (June 8, 2006 - subscription required)
• CBC Radio Quirks and Quarks November 3, 2001 Riding the Space Elevator
• Space Elevators: A Future I can Envision...Part 1 (December 23, 2006) and Part 2 (January 15, 2007) from The Space Monitor