Space tether

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Space_tether 

 

Artist's conception of satellite with a tether

Space tethers are long cables which can be used for propulsion, momentum exchange, stabilization and attitude control, or maintaining the relative positions of the components of a large dispersed satellite/spacecraft sensor system. Depending on the mission objectives and altitude, spaceflight using this form of spacecraft propulsion is theorized to be significantly less expensive than spaceflight using rocket engines.

Main techniques

Tether satellites might be used for various purposes, including research into tether propulsion, tidal stabilization and orbital plasma dynamics. Five main techniques for employing space tethers are in development:

Electrodynamic tethers

Electrodynamic tethers are primarily used for propulsion. These are conducting tethers that carry a current that can generate either thrust or drag from a planetary magnetic field, in much the same way as an electric motor does.

Momentum exchange tethers

Main article: Momentum exchange tether

These can be either rotating tethers, or non-rotating tethers, that capture an arriving spacecraft and then release it at a later time into a different orbit with a different velocity. Momentum exchange tethers can be used for orbital maneuvering, or as part of a planetary-surface-to-orbit / orbit-to-escape-velocity space transportation system.

Tethered formation flying

Main article: Tethered formation flying

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

Electric sail

Main article: Electric sail

A form of solar wind sail with electrically charged tethers that will be pushed by the momentum of solar wind ions.

Universal Orbital Support System

Main article: Universal Orbital Support System

A concept for suspending an object from a tether orbiting in space.

Many uses for space tethers have been proposed, including deployment as space elevators, as skyhooks, and for doing propellant-free orbital transfers.

History

Konstantin Tsiolkovsky (1857-1935) once proposed a tower so tall that it reached into space, so that it would be held there by the rotation of Earth. However, at the time, there was no realistic way to build it.

In 1960, another Russian, Yuri Artsutanov, wrote in greater detail about the idea of a tensile cable to be deployed from a geosynchronous satellite, downwards towards the ground, and upwards away, keeping the cable balanced. This is the space elevator idea, a type of synchronous tether that would rotate with the earth. However, given the materials technology of the time, this too was impractical on Earth.

In the 1970s, Jerome Pearson independently conceived the idea of a space elevator, sometimes referred to as a synchronous tether, and, in particular, analyzed a 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 and later Robert L. Forward investigated the physics of non-synchronous skyhooks, also known as rotating skyhooks, and performed detailed simulations of tapered rotating tethers that could pick objects off, and place objects onto, the Moon, Mars and other planets, with little loss, or even a net gain of energy.

In 1979, NASA examined the feasibility of the idea and gave direction to the study of tethered systems, especially tethered satellites.

In 1990, E. Sarmont proposed a non-rotating Orbiting Skyhook for an Earth-to-orbit / orbit-to-escape-velocity Space Transportation System in a paper titled "An Orbiting Skyhook: Affordable Access to Space". In this concept a suborbital launch vehicle would fly to the bottom end of a Skyhook, while spacecraft bound for higher orbit, or returning from higher orbit, would use the upper end.

In 2000, NASA and Boeing considered a HASTOL concept, where a rotating tether would take payloads from a hypersonic aircraft (at half of orbital velocity) to orbit.

Missions

Graphic of the US Naval Research Laboratory's TiPS tether satellite. Only a small part of the 4 km tether is shown deployed.

 

Main article: Space tether missions

A tether satellite is a satellite connected to another by a space tether. A number of satellites have been launched to test tether technologies, with varying degrees of success.

Types

There are many different (and overlapping) types of tether.

Momentum exchange tethers, rotating

Main article: Momentum exchange tether

Momentum exchange tethers are one of many applications for space tethers. Momentum exchange tethers come in two types; rotating and non-rotating. A rotating tether will create a controlled force on the end-masses of the system due to centrifugal acceleration. While the tether system rotates, the objects on either end of the tether will experience continuous acceleration; the magnitude of the acceleration depends on the length of the tether and the rotation rate. Momentum exchange occurs when an end body is released during the rotation. The transfer of momentum to the released object will cause the rotating tether to lose energy, and thus lose velocity and altitude. However, using electrodynamic tether thrusting, or ion propulsion the system can then re-boost itself with little or no expenditure of consumable reaction mass.

Skyhook

Main article: Skyhook (structure)

A rotating and a tidally stabilised skyhook in orbit

A skyhook is a theoretical class of orbiting tether propulsion intended to lift payloads to high altitudes and speeds. Proposals for skyhooks include designs that employ tethers spinning at hypersonic speed for catching high speed payloads or high altitude aircraft and placing them in orbit.

Electrodynamics

Medium close-up view, captured with a 70 mm camera, shows Tethered Satellite System deployment.

 

Main article: 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. Electric potential is generated across a conductive tether by its motion through the earth's magnetic field. The choice of the metal conductor to be used in an electrodynamic tether is determined by a variety of factors. Primary factors usually include high electrical conductivity and low density. Secondary factors, depending on the application, include cost, strength, and melting point.

An electrodynamic tether was profiled in the documentary film Orphans of Apollo as technology that was to be used to keep the Russian space station Mir in orbit.

Formation flying

Main article: Tethered formation flying

This is the use of a (typically) non-conductive tether to connect multiple spacecraft. A proposed 2011 experiment to study the technique is the Tethered Experiment for Mars inter-Planetary Operations (TEMPO³).

Universal Orbital Support System

Main article: Universal Orbital Support System

Example of a possible layout using the Universal Orbital Support System.

A theoretical type of non-rotating tethered satellite system, it is a concept for providing space-based support to things suspended above an astronomical object. The orbital system is a coupled mass system wherein the upper supporting mass (A) is placed in an orbit around a given celestial body such that it can support a suspended mass (B) at a specific height above the surface of the celestial body, but lower than (A).

Technical difficulties

Gravitational gradient stabilization

Description of the forces contributing towards maintaining a gravity gradient alignment in a tether system

 

Main article: Gravity-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 on the right 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) forces, 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

Further information: Atomic oxygen

Objects in low Earth orbit are subjected to noticeable erosion from atomic oxygen due to the high orbital speed with which the molecules strike as well as their high reactivity. This could quickly erode a tether.

Micrometeorites and space junk

Simple single-strand tethers are susceptible to micrometeoroids and space junk. Several systems have since been proposed and tested to improve debris resistance:

• The US Naval Research Laboratory has successfully flown a long term 6 km long, 2-3mm diameter tether with an outer layer of Spectra 1000 braid and a core of acrylic yarn. This satellite, the Tether Physics and Survivability Experiment (TiPS), was launched in June 1996 and remained in operation over 10 years, finally breaking in July 2006.
• Dr. Robert P. Hoyt 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. Hoytethers have theoretical lifetimes of decades.
• Researchers with JAXA have also proposed net-based tethers for their future missions.

Large pieces of junk would still cut most tethers, including the improved versions listed here, 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.

Radiation

Radiation, including UV radiation tend to degrade tether materials, and reduce lifespan. Tethers that repeatedly traverse the Van Allen belts can have markedly lower life than those that stay in low earth orbit or are kept outside Earth's magnetosphere.

Construction

Properties of useful materials

TSS-1R tether composition [NASA].

Tether properties and materials are dependent on the application. However, there are some common properties. To achieve maximum performance and low cost, tethers would 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, system designers will need to decide whether or not a protective coating is needed, including relative to UV and atomic oxygen. Research is being conducted to assess the probability of a collision that would damage the tether.

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, aramid or carbon fiber. A possible future material would be carbon nanotubes, which have an estimated tensile strength between 140 and 177 GPa (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 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 projected to be less than 65 newtons (15 lbf). 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).

There are design equations for certain applications that may be used to aid designers in identifying typical quantities that drive material selection.

Space elevator equations typically use a "characteristic length", L_c, which is also known as its "self-support length" and is the length of untapered cable it can support in a constant 1 g gravity field.

${\displaystyle L_{c}={\frac {\sigma }{\rho g}}}$,

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:

${\displaystyle V={\sqrt {\frac {\sigma }{\rho }}}}$

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,

${\displaystyle V_{c}={\sqrt {\frac {2\sigma }{\rho }}}}$

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

The inclusion of certain items in this list is currently being disputed. Please see the relevant discussion on the article's talk page. (April 2014)

Proposed materials include Kevlar, ultra high molecular weight polyethylene, carbon nanotubes and M5 fiber. M5 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. 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.

Potential tether / elevator materials

Material	Density ρ (kg/m³)	Stress limit σ (GPa)	Characteristic length Lc = σ/ρg (km)	Specific velocity Vs = √σ/ρ (km/s)	Char. velocity Vc = √2σ/ρ (km/s)
Single-wall carbon nanotubes (individual molecules measured)	2266	50	2200	4.7	6.6
Aramid, polybenzoxazole (PBO) fiber ("Zylon")	1340	5.9	450	2.1	3.0
Toray carbon fiber (T1000G)	1810	6.4	360	1.9	2.7
M5 fiber (planned values)	1700	9.5	570	2.4	3.3
M5 fiber (existing)	1700	5.7	340	1.8	2.6
Honeywell extended chain polyethylene fiber (Spectra 2000)	970	3.0	316	1.8	2.5
DuPont Aramid fiber (Kevlar 49)	1440	3.6	255	1.6	2.2
Silicon carbide	3000	5.9	199	1.4	2.0

Shape

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 reduce the excessive ratios of cable weight to payload weight. In lieu of tapering a modular staged tether system maybe used to achieve the same goal. Multiple tethers would be used between stages. The number of tethers would determine the strength of any given cross-section.

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:

${\displaystyle A(r)={\frac {Mv^{2}}{TR}}\mathrm {e} ^{{\frac {\delta }{T}}{\frac {v^{2}}{2}}\left(1-{\frac {r^{2}}{R^{2}}}\right)}}$

where R is the radius of tether, v is the velocity with respect to the centre, M is the tip mass, ${\displaystyle \delta }$ is the material density, and T is the design tensile strength (Young's modulus divided by safety factor).

Mass ratio

Graph of tether mass to payload ratio versus the tip speed in multiples of the characteristic speed of the material

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:

${\displaystyle {\frac {M}{m}}={\sqrt {\pi {\frac {\delta }{T}}{\frac {V^{2}}{2}}}}\mathrm {e} ^{\left({\frac {\delta }{T}}{\frac {V^{2}}{2}}\right)}\mathrm {erf} \left({\sqrt {{\frac {\delta }{T}}{\frac {V^{2}}{2}}}}\right)}$

where erf is the normal probability error function.

Let ${\displaystyle V_{r}=V/V_{c}\,}$,

${\displaystyle V_{c}={\sqrt {\frac {2T}{\delta }}}}$

then:

${\displaystyle {\frac {M}{m}}={\sqrt {\pi }}V_{r}\mathrm {e} ^{{V_{r}}^{2}}\mathrm {erf} ({V_{r}})}$

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; 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 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 (or hypersonic) 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. As of May 2013, all mechanical tethers (orbital and elevators) are on hold until stronger materials are available.

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 is difficult in a vacuum, so overheating may cause tether failures or damage.

Control and modelling

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 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 use 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 discharges have cut tethers (e.g. see Tethered Satellite System Reflight (TSS‑1R) on STS‑75), 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 and 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.

 

Space tether missions

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Space_tether_missions 

 

Main article: Space tether

 

Graphic of the US Naval Research Laboratory's TiPS tether satellite. Note that only a small part of the 4 km tether is shown deployed.

A number of space tethers have been deployed in space missions. Tether satellites can be used for various purposes including research into tether propulsion, tidal stabilisation and orbital plasma dynamics.

The missions have met with varying degrees of success; a few have been highly successful.

Description

Tethered satellites are composed of three parts: the base-satellite; tether; and sub-satellite. The base-satellite contains the sub-satellite and tether until deployment. Sometimes the base-satellite is another basic satellite, other times it could be a spacecraft, space station, or the Moon. The tether is what keeps the two satellites connected. The sub-satellite is released from the base assisted by a spring ejection system, centrifugal force or gravity gradient effects.

Tethers can be deployed for a range of applications, including electrodynamic propulsion, momentum exchange, artificial gravity, deployment of sensors or antennas etc. Tether deployment may be followed by a station-keeping phase (in particular if the target state is a vertical system orientation), and, sometimes, if the deployment system allows, a retraction.

The station-keeping phase and retraction phase need active control for stability, especially when atmospheric effects are taken into account. When there are no simplifying assumptions, the dynamics become overly difficult because they are then governed by a set of ordinary and partial nonlinear, non-autonomous and coupled differential equations. These conditions create a list of dynamical issues to consider:

• Three-dimensional rigid body dynamics (librational motion) of the station and subsatellite
• Swinging in-plane and out-of-plane motions of the tether of finite mass
• Offset of the tether attachment point from the base-satellite center of mass as well as controlled variations of the offset
• Transverse vibrations of the tether
• External forces

A NASA artist's rendering of a satellite tethered to the space shuttle.

Tether flights on human space missions

Gemini 11

Main article: Gemini 11

In 1966, Gemini 11 deployed a 30 m (98 ft) tether which was stabilized by a rotation which gave 0.00015 g.

Shuttle TSS missions

TSS-1 mission

Close-up view of the Tethered Satellite System (TSS-1) in orbit above the Space Shuttle Atlantis.

Tethered Satellite System-1 (TSS-1) was proposed by NASA and the Italian Space Agency (ASI) in the early 1970s by Mario Grossi, of the Smithsonian Astrophysical Observatory, and Giuseppe Colombo, of Padua University. It was a joint NASA-Italian Space Agency project, was flown in 1992, during STS-46 aboard the Space Shuttle Atlantis from 31 July to 8 August.

The purposes of the TSS-1 mission were to verify the tether concept of gravity gradient stabilization, and to provide a research facility for investigating space physics and plasma electrodynamics. This mission uncovered several aspects about the dynamics of the tethered system, although the satellite did not fully deploy. It stuck at 78 meters; after that snag was resolved its deployment continued to a length of 256 meters (840 ft) before sticking again, where the effort finally ended (the total proposed length was 20,000 meters (66,000 ft)). A protruding bolt due to a late-stage modification of the deployment reel system, jammed the deployment mechanism and prevented deployment to full extension. Despite this issue, the results showed that the basic concept of long gravity-gradient stabilized tethers was sound. It also settled several short deployment dynamics issues, reduced safety concerns, and clearly demonstrated the feasibility of deploying the satellite to long distances.

The voltage and current reached using the short tether length were too low for most of the experiments to be run. However, low-voltage measurements were made, along with recording the variations of tether-induced forces and currents. New information was gathered on the "return-tether" current. The mission was reflown in 1996 as TSS-1R.

TSS-1R mission

Four years later, as a follow-up mission to TSS-1, the TSS-1R satellite was released in latter February 1996 from the Space Shuttle Columbia on the STS-75 mission. The TSS-1R mission objective was to deploy the tether 20.7 km (12.9 mi) above the orbiter and remain there collecting data. The TSS-1R mission was to conduct exploratory experiments in space plasma physics. Projections indicated that the motion of the long conducting tether through the Earth's magnetic field would produce an EMF that would drive a current through the tether system.

TSS-1R was deployed (over a period of five hours) to 19.7 km (12.2 mi) when the tether broke. The break was attributed to an electrical discharge through a broken place in the insulation.

Despite the termination of the tether deployment before full extension, the extension achieved was long enough to verify numerous scientific speculations. These findings included the measurements of the motional EMF, the satellite potential, the orbiter potential, the current in the tether, the changing resistance in the tether, the charged particle distributions around a highly charged spherical satellite, and the ambient electric field. In addition, a significant finding concerns the current collection at different potentials on a spherical endmass. Measured currents on the tether far exceeded predictions of previous numerical models by up to a factor of three. A more descriptive explanation of these results can be found in Thompson, et al. Improvements have been made in modeling the electron charging of the shuttle and how it affects current collection, and in the interaction of bodies with surrounding plasma, as well as the production of electrical power.

A second mission, TSS-2, had been proposed to use the tether concept for upper atmospheric experimentation, but was never flown.

Tethers on satellite missions

Longer tether systems have also been used on satellite missions, both operationally (as yo-yo despin systems) and in missions designed to test tether concepts and dynamics.

Yo-yo despin

Main article: yo-yo de-spin

Short tether systems are commonly used on satellites and robotic space probes. Most notably, tethers are used in the "yo-yo de-spin" mechanism, often used in systems where a probe set spinning during a solid rocket injection motor firing, but needs the spin removed during flight. In this mechanism, weights on the end of long cables are deployed away from the body of the spinning satellite. When the cables are cut, much or all of the angular momentum of the spin is transferred to the discarded weights. As an example, the third stage of NASA's Dawn Mission utilized two weights with 1.44 kg (3.2 lb) each deployed on 12-meter (39 ft) cables.

NASA Small Expendable Deployer System experiments

In 1993 and 1994, NASA launched three missions using the "Small Expendable Deployer System" (SEDS), which deployed 20 km (12 mi) (SEDS-1 and SEDS-2) and 500-meter (1,600 ft) (PMG) tethers attached to a spent Delta-II second stage. The three experiments were the first successful flights of long tethers in orbit, and demonstrated both mechanical and electrodynamic tether operation.

SEDS-1

The first fully successful orbital flight test of a long tether system was SEDS-1, which tested the simple deploy-only Small Expendable Deployer System. The tether swung to the vertical and was cut after one orbit. This slung the payload and tether from Guam onto a reentry trajectory off the coast of Mexico. The reentry was accurate enough that a pre-positioned observer was able to videotape the payload re-entry and burnup.

SEDS-2

SEDS-2 was launched on a Delta (along with a GPS Block 2 satellite) on 9 March 1994. A feedback braking limited the swing after deployment to 4°. The payload returned data for 8 hours until its battery died; during this time tether torque spun it up to 4 rpm. The tether suffered a cut 3.7 days after deployment. The payload reentered (as expected) within hours, but the 7.2 km (4.5 mi) length at the Delta end survived with no further cuts until re-entry on 7 May 1994. The tether was an easy naked-eye object when lit by the sun and viewed against a dark sky.

In these experiments, tether models were verified, and the tests demonstrated that a reentry vehicle can be downwardly deployed into a reentry orbit using tethers.

PMG

A follow-on experiment, the Plasma Motor Generator (PMG), used the SEDS deployer to deploy a 500-m tether to demonstrate electrodynamic tether operation.

The PMG was planned to test the ability of a Hollow Cathode Assembly (HCA) to provide a low–impedance bipolar electric current between a spacecraft and the ionosphere. In addition, other expectations were to show that the mission configuration could function as an orbit-boosting motor as well as a generator, by converting orbital energy into electricity. The tether was a 500 m length of insulated 18 gauge copper wire.

The mission was launched on 26 June 1993, as the secondary payload on a Delta II rocket. The total experiment lasted approximately seven hours. In that time, the results demonstrated that current is fully reversible, and therefore was capable of generating power and orbit boosting modes. The hollow cathode was able to provide a low–power way of connecting the current to and from the ambient plasma. This means that the HC demonstrated its electron collection and emission capabilities.

NRL, TiPS, and ATEx experiments

TiPS

The Tether Physics and Survivability Experiment (TiPS) was launched in 1996 as a project of the US Naval Research Laboratory; it incorporated a 4,000 meter tether. The two tethered objects were called "Ralph" and "Norton". TiPS was visible from the ground with binoculars or a telescope and was occasionally accidentally spotted by amateur astronomers. The tether broke in July 2006. This long-term statistical data point is in line with debris models published by J. Carroll after the SEDS-2 mission, and ground tests by D. Sabath from TU Muenchen. Predictions of a maximum of two years survivability for TiPS based on some other ground tests have shown to be overly pessimistic (e.g. McBride/Taylor, Penson). The early cut of the SEDS-2 therewith must be considered an anomaly possibly related to the impact of upper stage debris.

ATEx

The Advanced Tether Experiment (ATEx), was a follow on to the TiPS experiment, designed and built by the Naval Center for Space Technology. ATEx flew as part of the STEX (Space Technology Experiment) mission. ATEx had two end masses connected by a polyethylene tether that was intended to deploy to a length of 6 km (3.7 mi), and was intended to test a new tether deployment scheme, new tether material, active control, and survivability. ATEx was deployed on 16 January 1999 and ended 18 minutes later after deploying only 22 m of tether. The jettison was triggered by an automatic protection system designed to save STEX if the tether began to stray from its expected departure angle, which was ultimately caused by excessive slack tether. As a result of the deployment failure, none of the desired ATEx goals were achieved.

Young Engineers' Satellite (YES)

Artist's conception of the deployment of the YES2 tether experiment and Fotino capsule from the Foton spacecraft

YES

In 1997, the European Space Agency launched the Young Engineers' Satellite (YES) of about 200 kg (440 lb) into GTO with a 35 km (22 mi) double-strand tether, and planned to deorbit a probe at near-interplanetary speed by swinging deployment of the tether system. The orbit achieved was not as initially planned for the tether experiment and, for safety considerations, the tether was not deployed.

YES2

The reconstructed deployment of the YES2 tether, i.e., the trajectory of the Fotino capsule in relationship to the Foton spacecraft. Orbital motion is to the left. The Earth is down. Mount Everest is shown several times for scale. The Fotino was released at the vertical, 32 km below Foton, about 240 km above the surface of the Earth, and made a re-entry towards Kazakhstan.

10 years after YES, its successor, the Young Engineers' Satellite 2 (YES2) was flown. The YES2 was a 36 kg student-built tether satellite, part of ESA's Foton-M3 microgravity mission. The YES2 satellite employed a 32 km tether to deorbit a small re-entry capsule, "Fotino." The YES2 satellite was launched on 14 September 2007 from Baikonur. The communications system on the capsule failed, and the capsule was lost, but deployment telemetry indicated that the tether deployed to full length and that the capsule presumably deorbited as planned. It has been calculated that Fotino was inserted into a trajectory towards a landing site in Kazakhstan, but no signal was received. The capsule was not recovered.

KITE Experiment

The Kounotori Integrated Tether Experiment (KITE) was a test of tether technology on the Japanese H-II Transfer Vehicle (HTV) 6 space station resupply vehicle, launched by the Japan Aerospace Exploration Agency (JAXA) in December 2016. After undocking from the International Space Station on 27 January 2016, it was intended to deploy a 700-meter (2,300 feet) electrodynamic tether, however, a failure resulted in the tether not deploying. The vehicle burned up in the atmosphere without deployment. The experiment did successfully demonstrate a carbon nanotube field-emission cathode.

CubeSat tether missions

CubeSats are small, low-cost satellites that are typically launched as secondary payloads on other missions, often built and operated as student projects. Several CubeSat missions have attempted to deploy tethers, so far without success.

MAST

The Multi-Application Survivable Tether (MAST) launched three 1-kg CubeSat modules with a 1-km tether. Two of the CubeSat modules ("Ted" and "Ralph") were intended as end-masses on the deployed tether, while the third ("Gadget") served as a climber that could move up and down the tether. The experiment used a multi-line "Hoytether" designed to be damage–resistant. The objectives of the MAST experiment were to obtain on-orbit data on the survivability of space tethers in the micrometeorite/debris orbital environment, to study the dynamics of tethered formations of spacecraft and rotating tether systems, and to demonstrate momentum-exchange tether concepts. The experiment hardware was designed under a NASA Small Business Technology Transfer (STTR) collaboration between Tethers Unlimited, Inc. and Stanford University, with TUI developing the tether, tether deployer, tether inspection subsystem, satellite avionics, and software, and Stanford students developing the satellite structures and assisting with the avionics design, as a part of the University CubeSat program.

In April 2007 the MAST was launched as a secondary payload on a Dnepr rocket into a 98°, 647 km × 782 km (402 mi × 486 mi) orbit. The experiment team made contact with the "Gadget" picosatellite, but not with "Ted", the tether-deployer picosatellite. While the system was designed so that the satellites would separate even if communications were not established to the tether deployer, the system did not fully deploy. Radar measurements show the tether deployed just 1 meter.

STARS, STARS-II, and STARS-C

Main article: STARS-II

The Space Tethered Autonomous Robotic Satellite (STARS or Kukai) mission, developed by the Kagawa Satellite Development Project at Kagawa University, Japan, was launched 23 January 2009 as a CubeSat secondary payload aboard H-IIA flight 15, which also launched GOSAT. After launch, the satellite was named KUKAI, and consisted of two subsatellites, "Ku" and "Kai," to be linked by a 5-meter (16 ft) tether. It was successfully separated from the rocket and transferred into the planned orbit, but the tether deployed only to a length of several centimeters, "due to the launch lock trouble of the tether reel mechanism."

A follow-on satellite, STARS-II, was a 9 kg (20 lb) satellite designed to fly a 300 m (980 ft) electrodynamic tether made from ultra-thin wires of stainless steel and aluminium. One objective of this program was to demonstrate possible technology for de-orbiting space debris. The mission launched on 27 February 2014 as a secondary payload aboard an H-2A rocket, and re-entered two months later, on 26 April 2014. The experiment was only partially successful, and tether deployment could not be confirmed. The orbit decayed from 350 km (220 mi) to 280 km (170 mi) in 50 days, considerably faster than the other CubeSats launched on the same mission, an indirect indication that its tether deployed, increasing the drag. However, telescopic photography of the satellite from the ground showed the satellite as a single point, rather than two objects. The experimenters suggest that this may have been due to the tether extending, but being tangled by rebound.

A third STARS mission, the STARS-C cubesat, was a 2U cubesat designed to deploy a 100 m (330 ft) aramid fiber tether with a diameter of 0.4 mm (0.016 in) between a mother satellite and a daughter satellite. The cubesat was designed by a team from Shizuoka University. The satellite has a mass of 2.66 kg (5.9 lb). It was launched on December 9, 2016 from the JEM Small Satellite Orbital Deployer on the International Space Station, and re-entered on March 2, 2018. However, the signal quality was intermittent, possibly due to failure of deployment of the solar panel, and data on tether deployment was not obtained. Estimates from orbital drag measurements suggest that the tether deployed to a length of about 30 meters.

ESTCube-1

ESTCube-1 was an Estonian mission to test an electric sail in orbit, launched in 2013. It was designed to deploy a tether using centrifugal deployment, but the tether failed to deploy.

TEPCE

Tether Electrodynamic Propulsion CubeSat Experiment (TEPCE) was a Naval Research Laboratory electrodynamic tether experiment based on a "triple CubeSat" configuration, which was built by 2012 and due to be launched in 2013, but eventually launched as a secondary payload as part of the STP-2 launch on a Falcon Heavy in June 2019. The tether deployed in November 2019 to detect electrodynamic force on the tether's orbit. TEPCE used two nearly identical endmasses with a STACER spring between them to start the deployment of a 1 km long braided-tape conducting tether. Passive braking was used to reduce speed and hence recoil at the end of deployment. The satellite was intended to drive an electrodynamic current in either direction. It was intended to be able to raise or lower the orbit by several kilometers per day, change libration state, change orbit plane, and actively maneuver. A large change in its decay rate on 17 November suggests the tether was deployed on that date, leading to its rapid reentry, which occurred on 1 February 2020.

MiTEE

The Miniature Tether Electrodynamics Experiment (MiTEE) from the University of Michigan is a cubesat experiment designed measure electrical current along a tether at different lengths between 10 and 30 meters (33 and 98 ft). It was to deploy a subsatellite of approximately 8 cm × 8 cm × 2 cm (3.15 in × 3.15 in × 0.79 in) from a 3U CubeSat to test satellite electrodynamics tethers in the space environment.

In 2015, NASA selected MiTEE as a University CubeSat Space Mission Candidate, and the project successfully delivered hardware for flight.

In January 2021, MiTEE-1 launched to space on Virgin Orbit's LauncherOne test flight.

Sounding rocket flights

CHARGE 2

The Cooperative High Altitude Rocket Gun Experiment (CHARGE) 2 was jointly developed by Japan and NASA, to observe the current collection along with other phenomena. The major objective was to measure the payload charging and return currents during periods of electron emission. Secondary objectives were related to plasma processes associated with direct current and pulsed firings of a low-power electron beam source. On 14 December 1985, the CHARGE mission was launched at White Sands Missile Range, New Mexico. The results indicated that it is possible to enhance the electron current collection capability of positively charged vehicles by means of deliberate neutral gas releases into an undisturbed space plasma. In addition, it was observed that the release of neutral gas or argon gas into the undisturbed plasma region surrounding a positively biased platform has been found to cause enhancements to electron current collection. This was due to the fact that a fraction of the gas was ionized, which increased the local plasma density, and therefore the level of return current.

OEDIPUS

OEDIPUS ("Observations of Electric-field Distribution in the Ionospheric Plasma — a Unique Strategy") consisted of two sounding rocket experiments that used spinning, conductive tethers as a double probe for measurements of weak electric fields in the aurora. They were launched using Black Brant 3-stage sounding rockets. OEDIPUS A launched on 30 January 1989 from Andøya in Norway. The tethered payload consisted of two spinning subpayloads with a mass of 84 and 131 kg, connected by a spinning tether. The flight established a record for the length of an electrodynamic tether in space at that time, 958 m (3,143 ft). The tether was a teflon-coated, stranded tin-copper wire of 0.85 mm (0.033 in) diameter and it was deployed from a spool-type reel located on the forward subpayload.

OEDIPUS C was launched on 6 November 1995 from the Poker Flat Research Range north of Fairbanks, Alaska on a Black Brant XII sounding rocket. The flight reached an apogee of 843 km (524 mi) and deployed a tether of the same type used in the OEDIPUS-A to a length of 1,174 m (3,852 ft). It included a Tether Dynamics Experiment to derive theory and develop simulation and animation software for analyses of multi–body dynamics and control of the spinning tether configuration, provide dynamics and control expertise for the suborbital tethered vehicle and for the science investigations, develop an attitude stabilization scheme for the payloads and support OEDIPUS C payload development, and acquire dynamics data during flight to compare with pre-flight simulation.

T-Rex

On 31 August 2010, an experiment by the Japan Aerospace Exploration Agency (JAXA) on space tether experiment called "Tether Technologies Rocket Experiment" (T-REX), sponsored by the Japanese Aerospace Exploration Agency (ISAS/JAXA), was launched on sounding rocket S-520-25 from Uchinoura Space Center, Japan, reaching a maximum altitude of 309 km (192 mi). T-Rex was developed by an international team led by the Kanagawa Institute of Technology/Nihon University to test a new type of electrodynamic tether (EDT). The 300 m (980 ft) tape tether deployed as scheduled and a video of deployment was transmitted to the ground. Successful tether deployment was verified, as was the fast ignition of a hollow cathode in the space environment.

The experiment demonstrated a "Foldaway Flat Tether Deployment System". The educational experiment featured the first bare tape tether deployment (i.e. without insulation, the tether itself acts as anode and collects electrons). 130 m (430 ft) of the total of 300 m (980 ft) of tether was deployed fire-hose style, purely driven by inertia and limited by friction, following a powerful, spring-initiated ejection. Accurate differential GPS data of the deployment was recorded, and video taken from the endmasses.

Proposed and future missions

ProSEDS

The use of a bare section of a space-borne electrodynamic tether for an electron-collection device has been suggested as a promising alternative to end-body electron collectors for certain electrodynamic tether applications. The bare-tether concept was to be tested first during NASA's Propulsive Small Expendable Deployer System (ProSEDS) mission. While the mission was canceled after NASA's space shuttle Columbia accident, the concept could potentially be undertaken in the future.

EDDE

ElectroDynamic Debris Eliminator (EDDE) was proposed in 2012 as an affordable system to deorbit or gather large orbital debris. The tether is flat for resistance to micromeroid impacts, and would carry large solar panels.

 

Lunar space elevator

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Lunar_space_elevator 

 

Diagram showing equatorial and polar Lunar space elevators running past L₁. An L₂ elevator would mirror this arrangement on the Lunar far side, and cargo dropped from its end would be flung outward into the solar system.

A lunar space elevator or lunar spacelift is a proposed transportation system for moving a mechanical climbing vehicle up and down a ribbon-shaped tethered cable that is set between the surface of the Moon "at the bottom" and a docking port suspended tens of thousands of kilometers above in space at the top.

It is similar in concept to the better known Earth-based space elevator idea, but since the Moon's surface gravity is much lower than the Earth's, the engineering requirements for constructing a lunar elevator system can be met using materials and technology already available. For a lunar elevator, the cable or tether extends considerably farther out from the lunar surface into space than one that would be used in an Earth-based system. However, the main function of a space elevator system is the same in either case; both allow for a reusable, controlled means of transporting payloads of cargo, or possibly people, between a base station at the bottom of a gravity well and a docking port in outer space.

A lunar elevator could significantly reduce the costs and improve reliability of soft-landing equipment on the lunar surface. For example, it would permit the use of mass-efficient (high specific impulse), low thrust drives such as ion drives which otherwise cannot land on the Moon. Since the docking port would be connected to the cable in a microgravity environment, these and other drives can reach the cable from low Earth orbit (LEO) with minimal launched fuel from Earth. With conventional rockets, the fuel needed to reach the lunar surface from LEO is many times the landed mass, thus the elevator can reduce launch costs for payloads bound for the lunar surface by a similar factor.

Location

There are two points in space where an elevator's docking port could maintain a stable, lunar-synchronous position: the Earth-Moon Lagrange points L₁ and L₂. The 0.055 eccentricity of the lunar orbit means that these points are not fixed relative to the lunar surface : the L₁ is 56,315 km +/- 3,183 km away from the Earth-facing side of the Moon (at the lunar equator) and L₂ is 62,851 km +/- 3,539 km from the center of the Moon's far side, in the opposite direction. At these points, the effect of the Moon's gravity and the effect of the centrifugal force resulting from the elevator system's synchronous, rigid body rotation cancel each other out. The Lagrangian points L₁ and L₂ are points of unstable gravitational equilibrium, meaning that small inertial adjustments will be needed to ensure any object positioned there can remain stationary relative to the lunar surface.

Both of these positions are substantially farther up than the 36,000 km from Earth to geostationary orbit. Furthermore, the weight of the limb of the cable system extending down to the Moon would have to be balanced by the cable extending further up, and the Moon's slow rotation means the upper limb would have to be much longer than for an Earth-based system, or be topped by a much more massive counterweight. To suspend a kilogram of cable or payload just above the surface of the Moon would require 1,000 kg of counterweight, 26,000 km beyond L₁. (A smaller counterweight on a longer cable, e.g., 100 kg at a distance of 230,000 km — more than halfway to Earth — would have the same balancing effect.) Without the Earth's gravity to attract it, an L₂ cable's lowest kilogram would require 1,000 kg of counterweight at a distance of 120,000 km from the Moon. The average Earth-Moon distance is 384,400 km.

The anchor point of a space elevator is normally considered to be at the equator. However, there are several possible cases to be made for locating a lunar base at one of the Moon's poles; a base on a peak of eternal light could take advantage of near-continuous solar power, for example, or small quantities of water and other volatiles may be trapped in permanently shaded crater bottoms. A space elevator could be anchored near a lunar pole, though not directly at it. A tramway could be used to bring the cable the rest of the way to the pole, with the Moon's low gravity allowing much taller support towers and wider spans between them than would be possible on Earth.

Fabrication

Because of the Moon's lower gravity and lack of atmosphere, a lunar elevator would have less stringent requirements for the tensile strength of the material making up its cable than an Earth-tethered cable. An Earth-based elevator would require high strength-to-weight materials that are theoretically possible, but not yet fabricated in practice (e.g., carbon nanotubes). A lunar elevator, however, could be constructed using commercially available mass-produced high-strength para-aramid fibres (such as Kevlar and M5) or ultra-high-molecular-weight polyethylene fibre.

Compared to an Earth space elevator, there would be fewer geographic and political restrictions on the location of the surface connection. The connection point of a lunar elevator would not necessarily have to be directly under its center of gravity, and could even be near the poles, where evidence suggests there might be frozen water in deep craters that never see sunlight; if so, this might be collected and converted into rocket fuel.

Cross-section profile

Space elevator designs for Earth typically have a taper of the tether that provides a uniform stress profile rather than a uniform cross-section. Because the strength requirement of a lunar space elevator is much lower than that of an Earth space elevator, a uniform cross-section is possible for the lunar space elevator. The study done for NASA's Institute of Advanced Concepts states "Current composites have characteristic heights of a few hundred kilometers, which would require taper ratios of about 6 for Mars, 4 for the Moon, and about 6000 for the Earth. The mass of the Moon is small enough that a uniform cross-section lunar space elevator could be constructed, without any taper at all." A uniform cross-section could make it possible for a lunar space elevator to be built in a double-tether pulley configuration. This configuration would greatly simplify repairs of a space elevator compared to a tapered elevator configuration. However a pulley configuration would require a strut at the counterweight hundreds of kilometers long to separate the up-tether from the down-tether and keep them from tangling. A pulley configuration might also allow the system capacity to be gradually expanded by stitching new tether material on at the Lagrange point as the tether rotated.

History

The idea of space elevators has been around since 1960 when Yuri Artsutanov wrote a Sunday supplement to Pravda on how to build such a structure and the utility of geosynchronous orbit. His article however, was not known in the West. Then in 1966, John Isaacs, a leader of a group of American Oceanographers at Scripps Institute, published an article in Science about the concept of using thin wires hanging from a geostationary satellite. In that concept, the wires were to be thin (thin wires/tethers are now understood to be more susceptible to micrometeoroid damage). Like Artsutanov, Isaacs’ article also was not well known to the aerospace community.

In 1972, James Cline submitted a paper to NASA describing a "mooncable" concept similar to a lunar elevator. NASA responded negatively to the idea citing technical risk and lack of funds.

In 1975, Jerome Pearson independently came up with the Space elevator concept and published it in Acta Astronautica. That made the aerospace community at large aware of the space elevator for the first time. His article inspired Sir Arthur Clarke to write the novel The Fountains of Paradise (published in 1979, almost simultaneously with Charles Sheffield's novel on the same topic, The Web Between the Worlds). In 1978 Pearson extended his theory to the moon and changed to using the Lagrangian points instead of having it in geostationary orbit.

In 1977, some papers of Soviet space pioneer Friedrich Zander were posthumously published, revealing that he conceived of a lunar space tower in 1910.

In 2005 Jerome Pearson completed a study for NASA Institute of Advanced Concepts which showed the concept is technically feasible within the prevailing state of the art using existing commercially available materials.

In October 2011 on the LiftPort website Michael Laine announced that LiftPort is pursuing a Lunar space elevator as an interim goal before attempting a terrestrial elevator. At the 2011 Annual Meeting of the Lunar Exploration Analysis Group (LEAG), LiftPort CTO Marshall Eubanks presented a paper on the prototype Lunar Elevator co-authored by Laine. In August 2012, Liftport announced that the project may actually start near 2020. In April 2019, LiftPort CEO Michael Laine reported no progress beyond the lunar elevator company's conceptualized design.

Materials

Unlike earth-anchored space elevators, the materials for lunar space elevators will not require a lot of strength. Lunar elevators can be made with materials available today. Carbon nanotubes aren’t required to build the structure. This would make it possible to build the elevator much sooner, since available carbon nanotube materials in sufficient quantities are still years away.

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 200, Dyneema (used on the YES2 spacecraft), or Zylon. All of these materials have breaking lengths of several hundred kilometers under 1g.

Potential lunar elevator materials

Material	Density ρ kg/m3	Stress Limit σ GPa	Breaking height (h = σ/ρg, km)
Single-wall carbon nanotubes (laboratory measurements)	2266	50	2200
Toray Carbon fiber (T1000G)	1810	6.4	361
Aramid, Ltd. polybenzoxazole fiber (Zylon PBO)	1560	5.8	379
Honeywell extended chain polyethylene fiber (Spectra 2000)	970	3.0	316
Magellan honeycomb polymer M5 (with planned values)	1700	5.7(9.5)	342(570)
DuPont Aramid fiber (Kevlar 49)	1440	3.6	255
Glass fibre (Ref Specific strength)	2600	3.4	133

The materials will be used to manufacture the ribbon-shaped, tethered cable which will connect from the L₁ or L₂ balance points to the surface of the moon. The climbing vehicles which will travel the length of these cables in a finished elevator system will not move very fast, thus simplifying some of the challenges of transferring cargo and maintaining structural integrity of the system. However, any small objects suspended in space for extended periods of time, like the tethered cables would be, are vulnerable to damage by micrometeoroids, so one possible method of improving their survivability would be to design a "multi-ribbon" system instead of just a single-tethered cable. Such a system would have interconnections at regular intervals, so that if one section of ribbon is damaged, parallel sections could carry the load until robotic vehicles could arrive to replace the severed ribbon. The interconnections would be spaced about 100 km apart, which is small enough to allow a robotic climber to carry the mass of the replacement 100 km of ribbon.

Climbing vehicles

One method of getting materials needed from the moon into orbit would be the use of robotic climbing vehicles. These vehicles would consist of two large wheels pressing against the ribbons of the elevator to provide enough friction for lift. The climbers could be set for horizontal or vertical ribbons.

The wheels would be driven by electric motors, which would obtain their power from solar energy or beamed energy. The power required to climb the ribbon would depend upon the lunar gravity field, which drops off the first few percent of the distance to L₁. The power that a climber would require to traverse the ribbon drops in proportion to proximity to the L₁ point. If a 540 kg climber traveled at a velocity of fifteen meters per second, by the time it was seven percent of the way to the L₁ point, the required power would drop to less than a hundred watts, versus 10 kilowatts at the surface.

One problem with using a solar powered vehicle is the lack of sunlight during some parts of the trip. For half of every month, the solar arrays on the lower part of the ribbon would be in the shade. One way to fix this problem would be to launch the vehicle at the base with a certain velocity then at the peak of the trajectory, attach it to the ribbon.

Possible uses

Materials from Earth may be sent into orbit and then down to the Moon to be used by lunar bases and installations.

Former U.S. President George W. Bush, in an address about his Vision for Space Exploration, suggested that the Moon may serve as a cost-effective construction, launching and fueling site for future space exploration missions. As President Bush noted, "(Lunar) soil contains raw materials that might be harvested and processed into rocket fuel or breathable air." For example, the proposed Ares V heavy-lift rocket system could cost-effectively deliver raw materials from Earth to a docking station, (connected to the lunar elevator as a counterweight,) where future spacecraft could be built and launched, while extracted lunar resources could be shipped up from a base on the Moon's surface, near the elevator's anchoring point. If the elevator was connected somehow to a lunar base built near the Moon's north pole, then workers could also mine the water ice which is known to exist there, providing an ample source of readily accessible water for the crew at the elevator's docking station. Also, since the total energy needed for transit between the Moon and Mars is considerably less than for between Earth and Mars, this concept could lower some of the engineering obstacles to sending humans to Mars.

The lunar elevator could also be used to transport supplies and materials from the surface of the moon into the Earth's orbit and vice versa. According to Jerome Pearson, many of the Moon's material resources can be extracted and sent into Earth orbit more easily than if they were launched from the Earth's surface. For example, lunar regolith itself could be used as massive material to shield space stations or crewed spacecraft on long missions from solar flares, Van Allen radiation, and other kinds of cosmic radiation. The Moon's naturally occurring metals and minerals could be mined and used for construction. Lunar deposits of silicon, which could be used to build solar panels for massive satellite solar power stations, seem particularly promising.

One disadvantage of the lunar elevator is that the speed of the climbing vehicles may be too slow to efficiently serve as a human transportation system. In contrast to an Earth-based elevator, the longer distance from the docking station to the lunar surface would mean that any "elevator car" would need to be able to sustain a crew for several days, even weeks, before it reached its destination.