International Space Elevator Consortium | Wikipedia audio article

International Space Elevator Consortium | Wikipedia audio article

A space elevator is a proposed type of planet-to-space
transportation system. The main component would be a cable (also called a tether) anchored
to the surface and extending into space. The design would permit vehicles to travel along
the cable from a planetary surface, such as the Earth’s, directly into space or orbit,
without the use of large rockets. An Earth-based space elevator would consist of a cable with
one end attached to the surface near the equator and the other end in space beyond geostationary
orbit (35,786 km altitude). The competing forces of gravity, which is stronger at the
lower end, and the outward/upward centrifugal force, which is stronger at the upper end,
would result in the cable being held up, under tension, and stationary over a single position
on Earth. With the tether deployed, climbers could repeatedly climb the tether to space
by mechanical means, releasing their cargo to orbit. Climbers could also descend the
tether to return cargo to the surface from orbit.The concept of a tower reaching geosynchronous
orbit was first published in 1895 by Konstantin Tsiolkovsky. His proposal was for a free-standing
tower reaching from the surface of Earth to the height of geostationary orbit. 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 by centrifugal forces. In the tensile
concepts, a space tether 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 an upside-down plumb bob. To construct a space elevator on Earth, the
cable material would need to be both stronger and lighter (have greater specific strength)
than any known material. Development of new materials that meet the demanding specific
strength requirement must happen before designs can progress beyond discussion stage. Carbon
nanotubes (CNTs) have been identified as possibly being able to meet the specific strength requirements
for an Earth space elevator. Other materials considered have been boron nitride nanotubes,
and diamond nanothreads, which were first constructed in 2014. In 2018 single-crystal
Graphene was also proposed as a potential material.
The concept is applicable to other planets and celestial bodies. For locations in the
solar system with weaker gravity than Earth’s (such as the Moon or Mars), the strength-to-density
requirements for tether materials are not as problematic. Currently available materials
(such as Kevlar) are strong and light enough that they could be practical as the tether
material for elevators there.==History=====
Early concepts===The key concept of the space elevator appeared
in 1895 when Russian scientist Konstantin Tsiolkovsky was inspired by the Eiffel Tower
in Paris. He considered a similar tower that reached all the way into space and was built
from the ground up to the altitude of 35,786 kilometers, the height of geostationary orbit.
He noted that the top of such a tower would be circling Earth as in a geostationary orbit.
Objects would attain horizontal velocity as they rode up the tower, and an object released
at the tower’s top would have enough horizontal velocity to remain there in geostationary
orbit. Tsiolkovsky’s conceptual tower was a compression structure, while modern concepts
call for a tensile structure (or “tether”).===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 as the base from which to deploy
the structure downward. By using a counterweight, a cable would be lowered from geostationary
orbit to the surface of Earth, while the counterweight was extended from the satellite away from
Earth, keeping the 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 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 gave a thinner cable at ground level that became thickest at the level
of geostationary orbit. Both the tower and cable ideas were proposed
in the quasi-humorous Ariadne column in New Scientist, December 24, 1964.
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. 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 area, and found that
the strength required would be twice that of any then-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 a cross-section-area altitude profile that tapered and 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
(89,000 miles), 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.
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 a space elevator 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 turn the concept into a reality.
In 2000, another American scientist, Bradley C. Edwards, suggested creating a 100,000 km
(62,000 mi) long paper-thin ribbon using a carbon nanotube composite material. He chose
the wide-thin ribbon-like cross-section shape rather than earlier circular cross-section
concepts because that shape would stand a greater chance of surviving impacts by meteoroids.
The ribbon cross-section shape also provided large surface area for climbers to climb with
simple rollers. Supported by the NASA Institute for Advanced Concepts, Edwards’ work 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.===21st century===
To speed space elevator development, proponents
have organized several competitions, similar to the Ansari X Prize, for relevant technologies.
Among them are Elevator:2010, which organized annual competitions for climbers, ribbons
and power-beaming systems from 2005 to 2009, the Robogames Space Elevator Ribbon Climbing
competition, 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. The first European Space Elevator Challenge
(EuSEC) to establish a climber structure took place in August 2011.In 2005, “the LiftPort
Group of space elevator companies 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 mi) space elevator, this move will allow it
to make money in the short term and conduct research and development into new production
methods.” Their announced 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 (2.0 in) wide and 1 mm (approx. 13 sheets of paper) thick, lifted with balloons.In 2007,
Elevator:2010 held the 2007 Space Elevator games, which featured US$500,000 awards for
each of the two competitions, ($1,000,000 total) as well as an additional $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-percent 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 led to Shuichi Ono, chairman of the Japan
Space Elevator Association, unveiling a space-elevator plan, putting forth what observers considered
an extremely low cost estimate of a trillion yen (£5 billion / $8 billion) to build one.In
2012, the Obayashi Corporation announced that in 38 years it could build a space elevator
using carbon nanotube technology. At 200 kilometers per hour, the design’s 30-passenger climber
would be able to reach the GEO level after a 7.5 day trip. No cost estimates, finance
plans, or other specifics were made. This, along with timing and other factors, hinted
that the announcement was made largely to provide publicity for the opening of one of
the company’s other projects in Tokyo.In 2013, the International Academy of Astronautics
published a technological feasibility assessment which concluded that the critical capability
improvement needed was the tether material, which was projected to achieve the necessary
strength-to-weight ratio within 20 years. The four-year long study looked into many
facets of space elevator development including missions, development schedules, financial
investments, revenue flow, and benefits. It was reported that it would be possible to
operationally survive smaller impacts and avoid larger impacts, with meteors and space
debris, and that the estimated cost of lifting a kilogram of payload to GEO and beyond would
be $500.In 2014, Google X’s Rapid Evaluation R&D team began the design of a Space Elevator,
eventually finding that no one had yet manufactured a perfectly formed carbon nanotube strand
longer than a meter. They thus decided to put the project in “deep freeze” and also
keep tabs on any advances in the carbon nanotube field.In 2018, researchers at Japan’s Shizuoka
University launched STARS-Me, two CubeSats connected by a tether, which a mini-elevator
will travel on. The experiment was launched as a test bed for a larger structure.==In fiction==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. In Kim Stanley Robinson’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’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.
The concept of a space elevator, called the Beanstalk, is also depicted in John Scalzi’s
2005 novel, Old Man’s War. In a biological version, Joan Slonczewski’s 2011 novel The
Highest Frontier depicts a college student ascending a space elevator constructed of
self-healing cables of anthrax bacilli. The engineered bacteria can regrow the cables
when severed by space debris.==Physics=====Apparent gravitational field===
An Earth space elevator cable rotates along with the rotation of the Earth. Therefore
the cable, and objects attached to it, would experience upward centrifugal force in the
direction opposing the downward gravitational force. The higher up the cable the object
is located, the less the gravitational pull of the Earth, and the stronger the upward
centrifugal force due to the rotation, so that more centrifugal force opposes less gravity.
The centrifugal force and the gravity are balanced at geosynchronous equatorial orbit
(GEO). Above GEO, the centrifugal force is stronger than gravity, causing objects attached
to the cable there to pull upward on it. The net force for objects attached to the
cable is called the apparent gravitational field. The apparent gravitational field for
attached objects is the (downward) gravity minus the (upward) centrifugal force. The
apparent gravity experienced by an object on the cable is zero at GEO, downward below
GEO, and upward above GEO. The apparent gravitational field can be represented
this way: where At some point up the cable, the two terms
(downward gravity and upward centrifugal force) are equal and opposite. Objects fixed to the
cable at that point put no weight on the cable. This altitude (r1) depends on the mass of
the planet and its rotation rate. Setting actual gravity equal to centrifugal acceleration
gives: On Earth, this distance is 35,786 km (22,236
mi) above the surface, the altitude of geostationary orbit.On the cable below geostationary orbit,
downward gravity would be greater than the upward centrifugal force, so the apparent
gravity would pull objects attached to the cable downward. Any object released from the
cable below that level would initially accelerate downward along the cable. Then gradually it
would deflect eastward from the cable. On the cable above the level of stationary orbit,
upward centrifugal force would be greater than downward gravity, so the apparent gravity
would pull objects attached to the cable upward. Any object released from the cable above the
geosynchronous level would initially accelerate upward along the cable. Then gradually it
would deflect westward from the cable.===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 given point. The greatest tension
on a space elevator cable is at the point of geostationary orbit, 35,786 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 its own weight from the surface up to 35,786 km
(22,236 mi). A cable which is thicker in cross section area at that height than at the surface
could better hold up its own weight over a longer length. How the cross section area
tapers from the maximum at 35,786 km (22,236 mi) to the minimum at the surface is therefore
an important design factor for a space elevator cable.
To maximize the usable excess strength for a given amount of cable material, the cable’s
cross section area would need to be designed for the most part in such a way that the stress
(i.e., the tension per unit of cross sectional area) is constant along the length of the
cable. The constant-stress criterion is a starting point in the design of the cable
cross section area as it changes with altitude. Other factors considered in more detailed
designs include thickening at altitudes where more space junk is present, consideration
of the point stresses imposed by climbers, and the use of varied materials. To account
for these and other factors, modern detailed designs seek to achieve the largest safety
margin possible, with as little variation over altitude and time as possible. In simple
starting-point designs, that equates to constant-stress. In the constant-stress case, the cross-section-area
can be described by the differential equation as: where The tower’s cross-section-area profile as
a function distance from Earth’s center can be solved with===
Cable materials===Using the above taper formula to solve for
the specific case of earth equatorial surface ( R
=6378 {\displaystyle R=6378}
km) and Earth geosynchronous orbit ( R g=
42164 {\displaystyle R_{g}=42164}
km), specific materials can be examined: A table of values for taper for various materials
are: The taper factor results in large increases
in cross-section-area unless the specific strength of the material used approaches 4.8×107
N·m/kg. Low specific strength materials require very large taper ratios which equates to large
(or astronomical) total mass of the cable with associated large or impossible costs.==Structure==There are a variety of space elevator designs
proposed for many planetary bodies. Almost every design includes a base station, a cable,
climbers, and a counterweight. For an Earth Space Elevator the Earth’s rotation creates
upward centrifugal force 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===
Modern concepts for the base station/anchor are typically mobile stations, large oceangoing
vessels or other mobile platforms. Mobile base stations would have the advantage over
the earlier stationary concepts (with land-based anchors) by being able to maneuver to avoid
high winds, storms, and space debris. Oceanic anchor points are also typically in international
waters, simplifying and reducing cost of negotiating territory use for the base station.Stationary
land based platforms would have simpler and less costly logistical access to the base.
They also would have an advantage of being able to be at high altitude, such as on top
of mountains. In an alternate concept, the base station could be a tower, forming a space
elevator which comprises both a compression tower close to the surface, and a tether structure
at higher altitudes. Combining a compression structure with a tension structure would reduce
loads from the atmosphere at the Earth end of the tether, and reduce the distance into
the Earth’s gravity field the cable needs to extend, and thus reduce the critical strength-to-density
requirements for the cable material, all other design factors being equal.===Cable===A space elevator cable would need to carry
its own weight as well as the additional weight of climbers. The required strength of the
cable would vary along its length. This is because at various points it would have to
carry the weight of the cable below, or provide a downward force to retain the cable and counterweight
above. Maximum tension on a space elevator cable would be at geosynchronous altitude
so the cable would have to be thickest there and taper carefully as it approaches Earth.
Any potential cable design 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 would
need to be made of a material with a large tensile strength/density ratio. For example,
the Edwards space elevator design assumes a cable material with a tensile strength of
at least 100 gigapascals. Since Edwards consistently assumed the density of his carbon nanotube
cable to be 1300 kg/m^3, that implies a specific strength of 77 megapascal/(kg/m^3). This value
takes into consideration the entire weight of the space elevator. An untapered space
elevator cable would need a material capable of sustaining a length of 4,960 kilometers
(3,080 mi) of its own weight at sea level to reach a geostationary altitude of 35,786
km (22,236 mi) without yielding. Therefore, a material with very high strength and lightness
is needed. For comparison, metals like titanium, steel
or aluminium alloys have breaking lengths of only 20–30 km. Modern fibre materials
such as kevlar, fibreglass and carbon/graphite fibre have breaking lengths of 100–400 km.
Nanoengineered materials such as carbon nanotubes and, more recently discovered, graphene 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.For a space elevator on Earth,
with its comparatively high gravity, the cable material would need to be stronger and lighter
than currently available materials. For this reason, there has been a focus on the development
of new materials that meet the demanding specific strength requirement. For high specific strength,
carbon has advantages because it is only the 6th element in the periodic table. Carbon
has comparatively few of the protons and neutrons which contribute most of the dead weight of
any material. Most of the interatomic bonding forces of any element are contributed by only
the outer few electrons. For carbon, the strength and stability of those bonds is high compared
to the mass of the atom. The challenge in using carbon nanotubes 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). As of 2014, carbon
nanotube technology allowed growing tubes up to a few tenths of meters.In 2014, diamond
nanothreads were first synthesized. Since they have strength properties similar to carbon
nanotubes, diamond nanothreads were quickly seen as candidate cable material as well.===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 at 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 would need to be paced at optimal timings so as to minimize cable stress and
oscillations and to maximize throughput. Lighter climbers could be sent up more often, with
several going up at the same time. This would increase throughput somewhat, but would lower
the mass of each individual payload. The horizontal speed, i.e. due to orbital
rotation, of each part of the cable increases with altitude, proportional to distance from
the center of the Earth, reaching low orbital speed at a point approximately 66 percent
of the height between the surface and geostationary orbit, or a height of about 23,400 km. A payload
released at this point would go into a highly eccentric elliptical orbit, staying just barely
clear from atmospheric reentry, with the periapsis at the same altitude as LEO and the apoapsis
at the release height. With increasing release height the orbit would become less eccentric
as both periapsis and apoapsis increase, becoming circular at geostationary level.
When the payload has reached GEO, the horizontal speed is exactly the speed of a circular orbit
at that level, so that if released, it would remain adjacent to that point on the cable.
The payload can also continue climbing further up the cable beyond GEO, allowing it to obtain
higher speed at jettison. If released from 100,000 km, the payload would have enough
speed to reach the asteroid belt.As a payload is lifted up a space elevator, it would gain
not only altitude, but horizontal speed (angular momentum) as well. The angular momentum is
taken from the Earth’s rotation. As the climber ascends, it is initially moving slower than
each successive part of cable it is moving on to. This is the Coriolis force: the climber
“drags” (westward) on the cable, as it climbs, and slightly decreases the Earth’s rotation
speed. The opposite process would occur for descending payloads: the cable is tilted eastward,
thus slightly increasing Earth’s rotation speed.
The overall effect of the centrifugal force acting on the cable would cause it to constantly
try to return to the energetically favorable vertical orientation, so after an object has
been lifted on the cable, the counterweight would swing back toward the vertical like
an inverted pendulum. Space elevators and their loads would 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 would need to be carefully planned so as to
keep the pendulum-like motion of the counterweight around the tether point under control.Climber
speed would be limited by the Coriolis force, available power, and by the need to ensure
the climber’s accelerating force does not break the cable. Climbers would also need
to maintain a minimum average speed in order to move material up and down economically
and expeditiously. At the speed of a very fast car or train of 300 km/h (190 mph) it
will take about 5 days to climb to geosynchronous orbit.===Powering climbers===
Both power and energy are significant issues for climbers—the climbers would 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 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 such as nuclear energy.
Solar power – After the first 40 km it is possible to use solar energy to power the
climberWireless 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 would need to be re-radiated
away with heat-dissipation systems, which add to weight.
Yoshio Aoki, a professor of precision machinery engineering at Nihon University and director
of the Japan Space Elevator Association, suggested including a second cable and using the conductivity
of carbon nanotubes to provide power.===Counterweight===Several solutions have been proposed to act
as a counterweight: a heavy, captured asteroid;
a space dock, space station or spaceport positioned past geostationary orbit
a further upward extension of the cable itself so that the net upward pull would be 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 just anything available that has mass.==
Launching into deep space==An object attached to a space elevator at
a radius of approximately 53,100 km would be at escape velocity when released. Transfer
orbits to the L1 and L2 Lagrangian points could be attained by release at 50,630 and
51,240 km, respectively, and transfer to lunar orbit from 50,960 km.At the end of Pearson’s
144,000 km (89,000 mi) cable, the tangential velocity is 10.93 kilometers per second (6.79
mi/s). That is more than enough to escape Earth’s gravitational field and send probes
at least as far out as Jupiter. Once at Jupiter, a gravitational assist maneuver could permit
solar escape velocity to be reached.==Extraterrestrial elevators==
A space elevator could also be constructed on other planets, asteroids and moons.
A Martian tether could be much shorter than one on Earth. Mars’ surface gravity is 38
percent of Earth’s, while it rotates around its axis in about the same time as Earth.
Because of this, Martian stationary orbit is much closer to the surface, and hence the
elevator could be much shorter. Current materials are already sufficiently strong to construct
such an elevator. Building a Martian elevator would be complicated by the Martian moon Phobos,
which is in a low orbit and intersects the Equator regularly (twice every orbital period
of 11 h 6 min). On the near side of the Moon, the strength-to-density
required of the tether of a lunar space elevator exists in currently available materials. A
lunar space elevator would be about 50,000 kilometers (31,000 mi) long. Since the Moon
does not rotate fast enough, there is no effective lunar-stationary orbit, but the Lagrangian
points could be used. The near side would extend through 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, could also 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 a portion of the mass of the asteroid or moon to Earth orbit or a Lagrangian point.
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.
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 90 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 would
need reduction of some 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 lower. Such follow-on space elevators would also benefit from the great
reduction in technical risk achieved by the construction of the first space elevator.Prior
to the work of Edwards in 2000 most concepts for constructing a space elevator had the
cable manufactured in space. That was thought to be necessary for such a large and long
object and for such a large counterweight. Manufacturing the cable in space would be
done in principle by using an asteroid or Near-Earth object for source material. These
earlier concepts for construction require a large preexisting space-faring infrastructure
to maneuver an asteroid into its needed orbit around Earth. They also required the development
of technologies for manufacture in space of large quantities of exacting materials.Since
2001, most work has focused on simpler methods of construction requiring much smaller space
infrastructures. They conceive the launch of a long cable on a large spool, followed
by deployment of it in space. The spool would be initially parked in a geostationary orbit
above the planned anchor point. A long cable would be dropped “downward” (toward Earth)
and would be balanced by a mass being dropped “upward” (away from Earth) for the whole system
to remain on the geosynchronous orbit. Earlier designs imagined the balancing mass to be
another cable (with counterweight) extending upward, with the main spool remaining at the
original geosynchronous orbit level. Most current designs elevate the spool itself as
the main cable is paid out, a simpler process. When the lower end of the cable is long enough
to reach the surface of the Earth (at the equator), it would be anchored. Once anchored,
the center of mass would be elevated more (by adding mass at the upper end or by paying
out more cable). This would add more tension to the whole cable, which could then be used
as an elevator cable. One plan for construction uses conventional
rockets to place a “minimum size” initial seed cable of only 19,800 kg. This first very
small ribbon would be adequate to support the first 619 kg climber. The first 207 climbers
would carry up and attach more cable to the original, increasing its cross section area
and widening the initial ribbon to about 160 mm wide at its widest point. The result would
be a 750-ton cable with a lift capacity of 20 tons per climber.===Safety issues and construction challenges
===For early systems, transit times from the
surface to the level of geosynchronous orbit would be about five days. On these early systems,
the time spent moving through the Van Allen radiation belts would be enough that passengers
would need to be protected from radiation by shielding, which would add mass to the
climber and decrease payload.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 below the maximum altitude of the cable that
are not synchronous with the cable would impact the cable eventually, unless avoiding action
is taken. 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$25,000 per kilogram (US$11,000 per pound) for transfer to geostationary orbit.
Current space elevator proposals envision payload prices starting as low as $220 per
kilogram ($100 per pound), similar to the $5–$300/kg estimates of the Launch loop,
but higher than the $310/ton to 500 km orbit quoted to Dr. Jerry Pournelle for an orbital
airship 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.”==
International Space Elevator Consortium (ISEC)==
The International Space Elevator Consortium (ISEC) is a US Non-Profit 501(c)(3) Corporation
formed to promote the development, construction, and operation of a space elevator as “a revolutionary
and efficient way to space for all humanity”. It was formed after the Space Elevator Conference
in Redmond, Washington in July 2008 and became an affiliate organization with the National
Space Society in August 2013. ISEC hosts an annual Space Elevator conference at the Seattle
Museum of Flight . ISEC coordinates with the two other major
societies focusing on space elevators: the Japanese Space Elevator Association and EuroSpaceward.
ISEC supports symposia and presentations at the International Academy of Astronautics
and the International Astronautical Federation Congress each year. The organization published
two issues of a peer-reviewed journal on space elevators called “CLIMB” and a magazine “Via
Ad Astra”. ISEC also conducts one-year studies focusing
on individual topics. The process involves experts for one year of discussions on the
topic of choice and culminates in a draft report that is presented and reviewed at the
ISEC Space Elevator conference workshop to allow input from space elevator enthusiasts
and other experts. Study Reports are usually published early the following year, to date
these are as follows : 2010 – Space Elevator Survivability, Space
Debris Mitigation 2012 – Space Elevator Concept of Operations
2013 – Design Consideration for Space Elevator Tether Climbers ,
2014 – Space Elevator Architectures and Roadmaps 2015 – Design Characteristics of a Space Elevator
Earth Port 2016 – Design Considerations for the Space
Elevator Apex Anchor and GEO Node 2017 – Design Considerations for a Software
Space Elevator Simulator 2018 – Design Considerations for the Multi-Stage
Space Elevator==
Related concepts==The conventional current concept of a “Space
Elevator” has evolved from a static compressive structure reaching to the level of GEO, to
the modern baseline idea of a static tensile structure anchored to the ground and extending
to well above the level of GEO. In the current usage by practitioners (and in this article),
a “Space Elevator” means the Tsiolkovsky-Artsutanov-Pearson type as considered by the International Space
Elevator Consortium. This conventional type is a static structure fixed to the ground
and extending into space high enough that cargo can climb the structure up from the
ground to a level where simple release will put the cargo into an orbit.Some concepts
related to this modern baseline are not usually termed a “Space Elevator”, but are similar
in some way and are sometimes termed “Space Elevator” by their proponents. For example,
Hans Moravec published an article in 1977 called “A Non-Synchronous Orbital Skyhook”
describing a concept using a rotating cable. The rotation speed would exactly match the
orbital speed in such a way that the tip velocity at the lowest point was zero compared to the
object to be “elevated”. It would dynamically grapple and then “elevate” high flying objects
to orbit or low orbiting objects to higher orbit.
The original concept envisioned by Tsiolkovsky was a compression structure, a concept similar
to an aerial mast. While such structures might reach 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 (reaching above the level of GEO) has been suggested. Other ideas use
very tall compressive towers to reduce the demands on launch vehicles. The vehicle is
“elevated” up the tower, which may extend as high as above the atmosphere, and is launched
from the top. Such a tall tower to access near-space altitudes of 20 km (12 mi) has
been proposed by various researchers.Other concepts for non-rocket spacelaunch related
to a space elevator (or parts of a space elevator) include an orbital ring, a pneumatic space
tower, a space fountain, a launch loop, a skyhook, a space tether, and a buoyant “SpaceShaft”.==Notes

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