Space tether - AbsoluteAstronomy.com

Space tethers are cable

Cable

A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...

s, usually long and very strong, which can be used for propulsion, stabilization, or maintaining the formation of space systems by determining the trajectory of spacecraft

Spacecraft

A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

and payloads. Depending on the mission objectives and altitude, spaceflight

Spaceflight

Spaceflight is the act of travelling into or through outer space. Spaceflight can occur with spacecraft which may, or may not, have humans on board. Examples of human spaceflight include the Russian Soyuz program, the U.S. Space shuttle program, as well as the ongoing International Space Station...

using this form of spacecraft propulsion

Spacecraft propulsion

Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...

may be significantly less expensive than spaceflight using rocket engine

Rocket engine

A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...

s.

Three main techniques for employing space tethers are in development:

Electrodynamic tether
Electrodynamic tether
Electrodynamic tethers are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy...

This is a conductive tether that carries a current that can generate thrust

Thrust

Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

or drag from a planetary magnetic field, in much the same way as an electric motor

Electric motor

An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

Momentum exchange tether
Momentum exchange tether
Momentum Exchange Tethers is one of many applications for space tethers. This sub-set represents an entire area of research using a spinning conductive and/or non-conductive tether to throw spacecraft up or down in orbit , thereby transferring its momentum.Due to the centrifugal acceleration, the...

This is a rotating tether that would grab a spacecraft and then release it at later time. Doing this can transfer momentum and energy from the tether to and from the spacecraft with very little loss; this can be used for orbital maneuver

Orbital maneuver

In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft.For spacecraft far from Earth—for example those in orbits around the Sun—an orbital maneuver is called a deep-space maneuver .-delta-v:...

ing.

Tethered Formation Flying
Tethered Formation Flying
Tethered Formation Flying is one of many applications for space tethers. This sub-set represents an entire area of research using a non-conductive tether to connect multiple spacecraft....

This is typically a non-conductive tether that accurately maintains a set distance between space vehicles.

Electric Sail
Electric sail
An Electric sail is a proposed form of spacecraft propulsion using the dynamic pressure of the solar wind as a source of thrust. It uses an electric field for deflecting solar wind protons and extracting momentum from them...

A form of solar wind

Solar wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

sail with electrically charged tether

Tether

A tether is a cord, fixture, or signal that anchors something movable to a reference point which may be fixed or moving. There are a number of applications for tethers: balloons, kites, tethered wind-energy conversion systems, anchors, tethered water-flow energy conversion systems, towing, animal...

s that will be pushed by the momentum of solar wind ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

History

Tsiolkovsky once proposed a tower so tall that it reached into space, so that it would be held there by the rotation of the Earth. However, there was no realistic way to build it.

To try to solve the problems in Komsomolskaya Pravda

Komsomolskaya Pravda

Komsomolskaya Pravda is a daily Russian tabloid newspaper, founded on March 13th, 1925. It is published by "Izdatelsky Dom Komsomolskaya Pravda" .- History :...

(July 31, 1960), another Russian, Yuri Artsutanov

Yuri Artsutanov

Yuri N. Artsutanov is a Russian engineer born in Leningrad. A graduate of Leningrad Technological Institute, he is best known for being one of the pioneers of the idea of space elevator.In 1960 he wrote an article "V Kosmos na Electrovoze Yuri N. Artsutanov (born 1929) is a Russian engineer born...

, wrote in greater detail about the idea of a tensile cable to be deployed from a geosynchronous satellite

Geosynchronous satellite

A geosynchronous Satellite is a satellite whose orbit on the Earth repeats regularly over points on the Earth over time. If such a satellite's orbit lies over the equator, the orbit is circular and its angular velocity is the same as the earth's, then it is called a geostationary satellite...

; downwards towards the ground, and upwards away; keeping the cable balanced. This is the space elevator

Space elevator

A space elevator, also known as a geostationary orbital tether or a beanstalk, is a proposed non-rocket spacelaunch structure...

idea, a type of synchronous tether that would rotate with the Earth. However, given the materials, this too was impractical on Earth.

In the 1970s Jerome Pearson

Jerome Pearson

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

explored synchronous tethers further, and in particular analysed the lunar elevator that can go through the L1 and L2 points, and this was found to be possible with materials then existing.

In 1977 Hans Moravec

Hans Moravec

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

and later Robert L. Forward investigated the physics of synchronous and non synchronous skyhook

Skyhook (structure)

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

tethers, and performed detailed simulations of tapered tethers that could pick objects off and place objects onto the Moon

Moon

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

, Mars

Mars

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

and other planets, with little, or even a net gain of energy.

In 1979 NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

examined the feasibility of the idea and gave direction to the study of tethered systems, especially tethered satellites. In 2000, NASA and Boeing

Boeing

The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...

considered a HASTOL concept where a tether would take payloads from a hypersonic aircraft (at half of orbital velocity) to orbit

Orbit

In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

Missions

A tether satellite is a satellite

Satellite

In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

connected to another by a space tether

Space tether

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

.

Tether satellites can be used for various purposes including research into tether propulsion

Momentum exchange tether

Momentum Exchange Tethers is one of many applications for space tethers. This sub-set represents an entire area of research using a spinning conductive and/or non-conductive tether to throw spacecraft up or down in orbit , thereby transferring its momentum.Due to the centrifugal acceleration, the...

, tidal stabilisation

Gravity-gradient stabilization

Gravity-gradient stabilization is a method of stabilizing artificial satellites or space tethers in a fixed orientation using only the orbited body's mass distribution and the Earth's gravitational field. The main advantage over using active stabilization with propellants, gyroscopes or reaction...

and orbital plasma dynamics.

A number of tether satellites have been launched, with varying degrees of success.

Momentum exchange

Momentum Exchange Tethers is one of many applications for space tether

Space tether

s. This sub-set represents an entire area of research using a spinning conductive and/or non-conductive tether to throw spacecraft up or down in orbit (like a sling), thereby transferring (or taking) its momentum.

The act of spinning a long tether end-for-end creates a controlled acceleration on the end-masses of the system and a tension in the tether. This spin is manipulated by control of the angular frequency. From this, momentum exchange can occur if an endbody is released at the right point during the controlled rotation. The transfer in momentum to the released object will cause the tether system to lose (or gain) orbital energy, and lose (or gain) altitude (and may require reboosting) or change orbital planes; and the opposite to happen to the released mass.

When in a magnetic field, such as in low Earth orbit

Low Earth orbit

A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

, when using an electrodynamic tether

Electrodynamic tether

Electrodynamic tethers are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy...

it is possible to re-boost without the expenditure of consumables. Other schemes involve balancing the momentum flow (such as catching and releasing payloads at almost the same time), or using conventional rocket propulsion or ion drives.

Electrodynamic

Electrodynamic tethers are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators

Electrical generator

In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

, by converting their kinetic energy

Kinetic energy

The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

to electrical energy, or as motors

Electric motor

An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

, converting electrical energy to kinetic energy. Electric potential is generated across a conductive tether by its motion through the Earth's magnetic field. The choice of the metal conductor

Electrical conductor

In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

to be used in an electrodynamic tether is determined by a variety of factors. Primary factors usually include high electrical conductivity, and low density

Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

. Secondary factors, depending on the application, include cost, strength, and melting point.

Formation Flying

This is the use of a (typically) non-conductive tether to connect multiple spacecraft.

Gravitational gradient stabilization

Instead of rotating end for end, tethers can also be kept straight by the slight difference in the strength of gravity over their length.

A non-rotating tether system has a stable orientation that is aligned along the local vertical (of the Earth or other body.) This can be understood by inspection of the figure below where two spacecraft at two different altitudes have been connected by a tether. Normally, each spacecraft would have a balance of gravitational (e.g. F_g1) and centrifugal (e.g. F_c1), but when tied together by a tether these values begin to change with respect to one another. This phenomenon occurs because, without the tether, the higher altitude mass would travel slower than the lower mass. The system must move at a single speed, so the tether must therefore slow down the lower mass and speed up the upper one. The centrifugal force of the tethered upper body is increased while that of the lower altitude body is reduced. This results in the centrifugal force of the upper body and the gravitational force of the lower body being dominant. This difference in forces naturally aligns the system along the local vertical, as seen in the figure.

Atomic oxygen

Objects in low earth orbit are subjected to noticeable erosion from monomolecular oxygen, due to the high orbital speed

Orbital speed

The orbital speed of a body, generally a planet, a natural satellite, an artificial satellite, or a multiple star, is the speed at which it orbits around the barycenter of a system, usually around a more massive body...

with which the molecules strike as well as their high reactivity.

Micrometeorites and space junk

Simple tethers are quickly cut by micrometeoroids

Meteoroid

A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite...

and space junk. The lifetime of a simple, one-strand tether in space is on the order of five hours for a length of ten kilometers. This was originally a show stopper for the use of tethers.

Several systems have since been proposed and tested to improve debris resistance:

The US Naval Research Laboratory has successfully flown a long term four kilometer tether that used very fluffy yarn. This satellite (TiPS) was launched in June 1996 and remained in operation over 10 years, finally breaking in July 2006.
Dr. Robert P. Hoyt
Robert P. Hoyt
Dr. Robert P. Hoyt is a physicist and engineer who is famous for his invention of the Hoytether. He also originated the MXER Tether concept, which combines momentum-exchange techniques with electrodynamic reboost propulsion to enable a bolo tether system to serve as a fully reusable in-space upper...

patented an engineered circular net, such that a cut strand's strains would be redistributed automatically around the severed strand. This is called a Hoytether
Hoytether
The Hoytether is a trademarked name of for a novel topology for a space tether, consisting of a lattice of strands, arranged in a circular cross-section with redundancy to handle potential damage from space debris and micrometeoroids....

. Hoytethers have theoretical lifetimes of tens of years.
Researchers with JAXA have also proposed net-based tethers for their future missions.
Another proposal is to use a tape or cloth.

Large pieces of junk would still cut most tethers, including the improved versions listed above, but these are currently tracked on radar and have predictable orbits. A tether could be wiggled to dodge known pieces of junk, or thrusters used to change the orbit, avoiding a collision.

Properties of useful materials

Tether properties and materials are dependent on the application. However, there are some common properties. To achieve maximum performance and low cost, tethers need to be made of materials with the combination of high strength or electrical conductivity and low density. All space tethers are susceptible to space debris or micrometeroids. Therefore, mission designers need to decide whether or not a protective coating is needed, including if against UV

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

and atomic oxygen. Research is being conducted to assess the probability of a collision that would damage the tether MAST.

For applications that exert high tensile forces on the tether, the materials need to be strong and light. Some current tether designs use crystalline plastics such as ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene

Ultra-high-molecular-weight polyethylene , also known as high-modulus polyethylene or high-performance polyethylene , is a subset of the thermoplastic polyethylene. It has extremely long chains, with molecular weight numbering in the millions, usually between 2 and 6 million...

, aramid

Aramid

Aramid fibers are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic rated body armor fabric and ballistic composites, in bicycle tires, and as an asbestos substitute. The name is a portmanteau of "aromatic polyamide"...

or carbon fiber. A possible future material would be carbon nanotube

Carbon nanotube

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

s, which have an estimated tensile strength

Tensile strength

Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

between 140 and 177 GPa

Orders of magnitude (pressure)

This is a tabulated listing of the orders of magnitude in relation to pressure expressed in pascals....

(20.3-25.6 million psi), and a proven tensile strength in the range 50-60 GPa for some individual nanotubes. (A number of other materials obtain 10 to 20 GPa

Monocrystalline whisker

A monocrystalline whisker is a filament of material that is structured as a single, defect-free crystal. Typical whisker materials are graphite, alumina, iron, or silicon. Single-crystal whiskers of these materials are noted for having very high tensile strength...

in some samples on the nano scale, but translating such strengths to the macro scale has been challenging so far, with, as of 2011, CNT-based ropes being an order of magnitude less strong, not yet stronger than more conventional carbon fiber on that scale).

For some applications, the tensile force on the tether is less than 15 lbs (< 65 N) Material selection in this case depends on the purpose of the mission and design constraints. Electrodynamic tethers, such as the one used on TSS-1R, may use thin copper wires for high conductivity (see EDT

Electrodynamic tether

).

There are design equations for certain applications that can identify typical quantities that drive material selection.

Space elevator equations typically use a ‘characteristic length’ (L_c). L_c is also known as its 'self-support length' and is the length of untapered cable it can support in a constant 1g gravity field. L_c=σ/ρg, where σ is the stress limit (in pressure units) and ρ is the density of the material.

Hypersonic skyhook equations use the material’s ‘specific velocity’ which is equal to the maximum tangential velocity a spinning hoop can attain without breaking. V_s=√(σ/ρ).

For rotating tethers (rotovators) the value used is the material’s ‘characteristic velocity’ which is the maximum tip velocity a rotating untapered cable can attain without breaking. V_c=√(2σ/ρ). The characteristic velocity equals the specific velocity multiplied by the square root of two.

These values are used in equations similar to the rocket equation and are analogous to specific impulse or exhaust velocity. The higher these values are, the more efficient and lighter the tether can be in relation to the payloads that they can carry. Eventually however, the mass of the tether propulsion system will be limited at the low end by other factors such as momentum storage.

Practical materials

Materials proposed include Kevlar

Kevlar

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

, ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene

, carbon nanotubes, M5 fiber

M5 fiber

M5 fiber is a high-strength synthetic fiber first developed by Dr. Doetze Sikkema and his team at the Dutch chemical firm Akzo Nobel...

, and diamond

Diamond

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

.

One material that has great potential is M5 fiber. This is a synthetic fiber that is lighter than Kevlar or Spectra. According to Pearson, Levin, Oldson, and Wykes in their article "The Lunar Space Elevator," an M5 ribbon 30 mm wide and 0.023 mm thick, would be able to support 2000 kg on the lunar surface (2005). It would also be able to hold 100 cargo vehicles, each with a mass of 580 kg, evenly spaced along the length of the elevator. Other materials that could be used are T1000G carbon fiber, Spectra 2000, or Zylon. All of these materials have breaking lengths of several hundred kilometers under 1g (10 m/s²).

Potential tether/elevator materials
Material	Density ρ (kg/m³)	Stress Limit σ (GPa)	Char. length L_c=σ/ρg, (km)	Specific velocity V_s=√(σ/ρ), (km/s)	Char. velocity V_c=√(2σ/ρ), (km/s)
Single-wall carbon nanotubes (individual nanotubes in lab)	2266	50	2200	4.7	6.6
Aramid Aramid Aramid fibers are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic rated body armor fabric and ballistic composites, in bicycle tires, and as an asbestos substitute. The name is a portmanteau of "aromatic polyamide"... , Polybenzoxazole (PBO) fiber ("Zylon")	1340	5.9	450	2.1	3.0
Toray carbon fiber (T1000G)	1810	6.4	360	1.9	2.7
Magellan honeycomb polymer M5 (planned values)	1700	9.5	570	2.4	3.3
Magellan honeycomb polymer M5 (existing)	1700	5.7	340	1.8	2.6
Honeywell extended chain polyethylene fiber (Spectra Spectra Spectra are conditions or values that vary over a continuum.Spectra may also refer to:* Kia Spectra, a car developed by Kia Motors from 2000-present* Optare Spectra, a bus body built by Optare... 2000)	970	3.0	316	1.8	2.5
DuPont Aramid fiber (Kevlar 49)	1440	3.6	255	1.6	2.2
Specialty materials e.g. silicon carbide	3000	5.9	199	1.4	2.0
Aluminium Aluminium Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances.... (6061 T6)	2700	0.276	10.	0.32	0.45

Tapering

For gravity stabilised tethers, to exceed the self-support length the tether material can be tapered so that the cross-sectional area varies with the total load at each point along the length of the cable. In practice this means that the central tether structure needs to be thicker than the tips. Correct tapering ensures that the tensile stress at every point in the cable is exactly the same. For very demanding applications, such as an Earth Space Elevator, the tapering can result in excessive ratios of cable weight to payload weight.

Thickness

For rotating tethers not significantly affected by gravity, the thickness also varies, and it can be shown that the area, A, is given as a function of r (the distance from the centre) as follows:

where R is the radius of tether, v is the velocity with respect to the centre, M is the tip mass,

is the material density, and T is the design tensile strength (Young's modulus

Young's modulus

Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...

divided by safety factor).

Mass ratio

Integrating the area to give the volume and multiplying by the density and dividing by the payload mass gives a payload mass/tether mass ratio of:

where erf is the normal probability error function

Error function

In mathematics, the error function is a special function of sigmoid shape which occurs in probability, statistics and partial differential equations...

Let

then:

This equation can be compared with the rocket equation, which is proportional to a simple exponent on a velocity, rather than a velocity squared. This difference effectively limits the delta-v that can be obtained from a single tether.

Redundancy

In addition the cable shape must be constructed to withstand micrometeorites and space junk. This can be achieved with the use of redundant cables, such as the Hoytether

Hoytether

The Hoytether is a trademarked name of for a novel topology for a space tether, consisting of a lattice of strands, arranged in a circular cross-section with redundancy to handle potential damage from space debris and micrometeoroids....

; redundancy can ensure that it is very unlikely that multiple redundant cables would be damaged near the same point on the cable, and hence a very large amount of total damage can occur over different parts of the cable before failure occurs.

Material strength

Beanstalks and rotovators are currently limited by the strengths of available materials. Although ultra-high strength plastic fibers (Kevlar

Kevlar

and Spectra) permit rotovators to pluck masses from the surface of the Moon and Mars, a rotovator from these materials cannot lift from the surface of the Earth. In theory, high flying, supersonic

Supersonic

Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...

(or hypersonic

Hypersonic

In aerodynamics, a hypersonic speed is one that is highly supersonic. Since the 1970s, the term has generally been assumed to refer to speeds of Mach 5 and above...

) aircraft could deliver a payload to a rotovator that dipped into Earth's upper atmosphere briefly at predictable locations throughout the tropic (and temperate) zone of Earth.

Cargo capture

Cargo capture for rotovators is nontrivial, and failure to capture can cause problems. Several systems have been proposed, such as shooting nets at the cargo, but all add weight, complexity, and another failure mode. At least one lab scale demonstration of a working grapple system has been achieved however.

Life expectancy

Currently, the strongest materials in tension are plastics that require a coating for protection from UV radiation and (depending on the orbit) erosion by atomic oxygen. Disposal of waste heat

Waste heat

Waste heat sometimes called Secondary heat or Low-grade heat refers to heat produced by machines, electrical equipment and industrial processes for which no useful application is found. Energy is often produced by a heat engine, running on a source of high-temperature heat...

is difficult in a vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

, so overheating

Thermal shock

Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients...

may cause tether failures or damage.

Pendular motion instability

Electrodynamic tethers deployed along the local vertical ('hanging tethers') may suffer from dynamical instability. Pendular motion causes the tether vibration amplitude to build up under the action of electromagnetic interaction. As the mission time increases, this behavior can compromise the performance of the system. Over a few weeks, electrodynamic tethers in Earth orbit might build up vibrations in many modes, as their orbit interacts with irregularities in magnetic and gravitational fields.

One plan to control the vibrations is to actively vary the tether current to counteract the growth of the vibrations. Electrodynamic tethers can be stabilized by reducing their current when it would feed the oscillations, and increasing it when it opposes oscillations. Simulations have demonstrated that this can control tether vibration. This approach requires sensors to measure tether vibrations, which can either be an inertial navigation system

Inertial navigation system

An inertial navigation system is a navigation aid that uses a computer, motion sensors and rotation sensors to continuously calculate via dead reckoning the position, orientation, and velocity of a moving object without the need for external references...

on one end of the tether, or satellite navigation systems mounted on the tether, transmitting their positions to a receiver on the end.

Another proposed method is to utilise spinning electrodynamic tethers instead of hanging tethers. The gyroscopic effect provides passive stabilisation, avoiding the instability.

Surges

As mentioned earlier, conductive tethers have failed from unexpected current surges. Unexpected electrostatic discharge

Electrostatic discharge

Electrostatic discharge is a serious issue in solid state electronics, such as integrated circuits. Integrated circuits are made from semiconductor materials such as silicon and insulating materials such as silicon dioxide...

s have cut tethers (e.g. see Tethered Satellite System Reflight (TSS-1R) on STS-75

STS-75

STS-75 was a United States Space Shuttle mission, the 19th mission of the Columbia orbiter.-Crew:-Mission parameters:*Mass: payload*Perigee: *Apogee: *Inclination: 28.5°*Period: 90.5 min-Mission objective:...

), damaged electronics, and welded tether handling machinery. It may be that the Earth's magnetic field is not as homogeneous as some engineers have believed.

Vibrations

Computer models frequently show tethers can snap due to vibration.

Mechanical tether-handling equipment is often surprisingly heavy, with complex controls to damp vibrations. The one ton climber proposed by Dr. Brad Edwards for his Space Elevator may detect and suppress most vibrations by changing speed and direction. The climber can also repair or augment a tether by spinning more strands.

The vibration modes that may be a problem include skipping rope, transverse, longitudinal, and pendulum.

Tethers are nearly always tapered, and this can greatly amplify the movement at the thinnest tip in whip like ways.

Other issues

A tether is not a spherical object, and has significant extent. This means that as an extended object, it is not directly modelable as a point source, and this means that the center of mass

Center of mass

In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...

and center of gravity

Center of gravity

In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...

are not usually colocated. Thus the inverse square law does not apply except at large distances, to the overall behaviour of a tether. Hence the orbits are not completely Keplerian, and in some cases they are actually chaotic.

With bolus designs, rotation of the cable interacting with the non linear gravity fields found in elliptical orbits can cause exchange of orbital angular momentum and rotation angular momentum. This can make prediction and modelling extremely complex.

In fiction

The mechanics of tether propulsion are critical in resolving the climax of the book The Descent of Anansi
The Descent of Anansi
The Descent of Anansi is a 1982 science fiction novel by Steven Barnes and Larry Niven. -Plot setup:A space station-manufactury attempts to become commercially independent from its Government backers by exporting super-strong nanowire that can only be manufactured in free-fall.-Plot...

by Steven Barnes

Steven Barnes

Steven Barnes is an African American science fiction writer, lecturer, creative consultant, and human performance technician....

and Larry Niven

Larry Niven

Laurence van Cott Niven / ˈlæri ˈnɪvən/ is an American science fiction author. His best-known work is Ringworld , which received Hugo, Locus, Ditmar, and Nebula awards. His work is primarily hard science fiction, using big science concepts and theoretical physics...

External links

ProSEDS, a tether-based propulsion experiment
Special Projects Group
NASA Tether Overview
Tethers Unlimited Incorporated
"Tethers In Space Handbook" M.L. Cosmo and E.C. Lorenzini Third Edition December 1997
NASA IAC report on orbital systems
SpaceTethers.com, space tether simulator applet
Npr.org - Space Tethers: Slinging Objects in Orbit?
ESA - The YES2 project
ESA - Students test 'space postal service' during Foton mission
The Space Show #531 Robert P. Hoyt
Robert P. Hoyt
Dr. Robert P. Hoyt is a physicist and engineer who is famous for his invention of the Hoytether. He also originated the MXER Tether concept, which combines momentum-exchange techniques with electrodynamic reboost propulsion to enable a bolo tether system to serve as a fully reusable in-space upper...

discusses space tethers on The Space Show
The Space Show
The Space Show is a biweekly radio talk show, presented by Dr. David Livingston, about space commerce and exploration that is also available archived online as a podcast....

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NASA site on TSS-1R
NASA Tether Origami
New Scientist article
Tether Physics and Survivability Experiment
Tethers Unlimited: Publications
Tethers in Space Handbook (PDF)

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