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In Space, tethers can also be used to attach spacecraft to each other, but this technology involves much more than simply tying things together. Solar powered "climber" machines, which are already under development, could use such a cable to haul cargo into orbit. Space tethers could also be used as a means of transportation, to swing from one place to another. In stories on Earth, Tarzan uses liana vines to swing from tree to tree, and there are many serious ideas for using tethers in space in a somewhat similar way: swinging satellites into another orbit, or even passing them from tether to tether all the way to the Moon and Mars!

Although this sounds like science fiction, many space missions using tethers have already flown.


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Most of them have been relatively small and experimental, but in the near future space tethers have the potential to revolutionize space flight. Skip to main content Skip to table of contents. Advertisement Hide. Space Tethers and Space Elevators. Front Matter Pages A better idea is to make the constellation spin around its own center of mass. This results in a centrifugal force that tries to make the satellites fly outward, and in reaction a centripetal, pulling force on the tethers physics purists say that the centripetal force is only an apparent force, not a real force, but we can ignore the mathematical niceties here.

The pull will keep the tethers taut, ensuring that the satellites all stay together. Imagine spinning around fast while in each hand holding a rope with a weight attached. As an interconnected constellation, the three of you will keep your relative positions with respect to each other. The downside of this method is that the group of satellites needs to rotate; their relative positions may remain fixed, but to the rest of the universe the satellites are constantly moving.

This is not practical for many formation- flying applications, such as observing a planet or a star. There are, however, other ways to push spacecraft away from each other that do not use propellant and that can be combined with tether interconnections. This will be explained later see Let's Stay Together, in Chapter 5. Safety Tethers If tethers can be employed to keep satellites together, it is not a far stretch to imagine their use in securing astronauts to prevent them from drifting away.

He was connected to the Gemini 4 spacecraft by a long tether. At the end of a spacewalk, astronauts pulled themselves along the tether back to their capsules. Nowadays, spacesuits have their own, completely independent systems included in an attached backpack, but cables are still used as safety lines that connect astronauts to their spacecraft or space station Fig.

The tethers used for this function are relatively short, on the order of several meters. However, researchers at the Massachusetts Institute of Technology MIT; Cambridge, MA have devised an application with long tethers that can help astronauts strolling across the surface of small asteroids without floating away. Asteroids have very little gravity, so walking on them is much more difficult than walking on a planet.

Tying a lightweight rope all the way around an asteroid could be a solution; astronauts could attach themselves to this safety line and maneuver or even walk along the surface. The MIT researchers envision that their system will be deployed by an astronaut or spacecraft unwinding a spool of rope while flying around the asteroid. The rope might cut into the soft, granular surface of an asteroid, but even then it could at least give spacewalkers something to hold onto. It is like being inside a falling elevator, but without the hard landing at the end. Astronauts inside the International Space Station ISS can simply float through its many modules, have dinner on a wall, and sleep on the ceiling.

Because of the microgravity, there is no gravity-defined up or down. This is fun for the astronauts, but the main use for space stations is that the microgravity conditions allow many kinds of experiments that are not possible on Earth. It is like turning off gravity. Fluids that float one on top of another on Earth can suddenly be mixed, as there is no gravity-induced separation based on differences in density. Larger and purer crystals can be grown, and the importance of gravity for the growth of cells and microbes can be studied. We can even do combustion experiments that are not possible on the ground.

A candle burning in space does not show the familiar elongated shape of the flame, but instead is spherical and almost transparent blue. The spherical shape is caused by the lack of convection, that is, heated air moving up, because that depends on gravity. The combustion in the hot bubble is very efficient because partly burned particles are not rapidly transported away by rising hot gases, but instead remain near the heat so they can burn up completely.

Microgravity allows us to study combustion processes in great detail, which teaches us how to optimize combustion, for example, in car engines on Earth, and thereby reduce pollution. However, if we want to fly people to Mars, microgravity is not so great. Blood normally pulled down into the legs now tends to move upward to the head, muscles are used much less and therefore deteriorate, and the body notices it do not need to maintain a strong skeleton anymore so it allows bones to weaken leading to a loss of bone mass.

Even though astronauts onboard the ISS typically exercise 2 hours a day to mitigate muscle atrophy, their strength inevitably weakens, especially in the legs. After half a year on the station, it may take an astronaut months to gain back his normal strength.

As a result, healthy space explorers who land on Mars after a flight of half a year may not even be able to walk anymore, despite the much lower gravity on the red planet. The centrifugal force that can be used to keep tethered satellite clusters together can also be employed to generate artificial gravity. Fill a bucket with water and swirl it around at the end of a length of rope.

If you do it fast enough, the water will not spill out. It is as if the liquid is pulled toward the now vertical bottom of the bucket. Another analogy is riding through a loop in a roller coaster without falling out. Even while upside down you are being pushed into your seat by the artificial gravity generated by the train's velocity in combination with the curved track.

In a similar way, putting crewed modules at the end of long beams and having the spacecraft rotate can generate artificial gravity for astronauts. That enables them to live more normally and prevents them from being affected by the harmful physiological changes resulting from exposure to microgravity.

In principle, we can attach such modules to each other with heavy structures and thus create a spinning wheel, with people living on the inner rim. This is the idea behind classical designs for wheel-shaped space stations such as seen in the movie A Space Odyssey and in many conceptual studies for space colonies Fig. The level of artificial gravity in such a system is determined by the length of the beams between the modules, the spacecraft center of rotation i.

The gravity level can be raised by lengthening the beams as well as by increasing the rotational speed. We can easily check this out with the earlier described bucket of water on a rope. Thus, a double beam length i. In theory, a gravity similar to that on Earth can be achieved with a spacecraft with short beams that rotates very fast, for example using a radius of 4 meters 13 feet and 15 rotations per minute. There is a large mirror orbiting above the wheel to reflect sunlight into the colony. Courtesy of NASA. Anything moving in a rotating system experiences sudden accelerations in directions perpendicular to the axis of rotation.

In other words, when you move your head away from or toward the center of a merry-go-round, you will feel as if you are speeding up or slowing down. The result is motion sickness. People can adapt to a maximum of six rotations per minute, but the number of rotations for comfortable living seems to be about two. Earth surface gravity simulation at two rotations per minute means a radius of meters feet , that is, a combined beam length or rotational diameter of meters feet. Mars surface gravity is 62 percent lower, but at two rotations per minute still requires beams that are 90 meters feet long Fig.

That means fairly heavy beams. Furthermore, because such long structures do not fit into a rocket, they would need to be launched in pieces and assembled in space. However, since these beams experience only tension forces, they could be replaced by tethers. These are much lighter and thinner, and can be rolled up for launch so that no assembly in orbit is required.

On a mission to Mars, the astronauts can deploy the system once they have entered the right transfer orbit, and wind up the tether when arriving at Mars to simplify both spacecraft maneuvering and the astronauts' moving from the habitation modules to the rest of the ship. Probe Towing Earth satellites generally orbit well above an altitude of km miles , because below that height the atmosphere is so dense that aerodynamic drag quickly slows a spacecraft, making it fall back to Earth. Gas-filled balloons, on the other hand, can reach altitudes up to about 50 km 30 miles and stay there for days, but can go no higher.

However, these rockets stay in the upper atmosphere for only a couple of minutes. At the moment there are no possibilities to reach the upper atmosphere and conduct experiments there for extended periods. A large satellite in a sufficiently high and thus virtually drag-free orbit could be used to tow a smaller probe on a long tether through the upper layers of the atmosphere. The aerodynamic drag on the probe and tether would start to slow down the mother satellite, but the higher its mass, the more difficult it is to decelerate. A rocket engine can be used to temporarily compensate for the drag, and once the scientific measurements of the atmosphere are done, the tether can be cut so that the large satellite can remain in orbit.

This concept can in principle be applied at all planets with an atmosphere: Earth, Venus, Mars, and the giant outer planets Jupiter, Saturn, Uranus, and Neptune. Small tethered probes could be lowered into an alien atmosphere to take gas samples or even to collect Mars dust that is blown up to high altitudes by the wind. Comet and Asteroid Sample Return Many comets and asteroids are so small and have so little gravity that it is possible for a spacecraft to hover over their surface, rather than orbit around them at high speed.

Such a spacecraft could shoot a tethered penetrator into the body's surface to collect a material sample, which could then be reeled in for onboard experiments or even for return to Earth. Using several of these harpoon-like penetrators, samples could be collected from various places on the same body, or from more than one individual comet or asteroid.

The penetrators could be launched by a spring system at a distance of several tens of meters from the surface, and then ignite simple solid propellant rockets to accelerate themselves to the impact point. A rotating turret housing multiple penetrators would allow the use of a single tether reel system. A major benefit over the use of a lander is the simplicity of the system: there is no need for an automated and careful, soft landing.

In case of a problem, the tether could be cut at the spacecraft's end. A disadvantage is the high-shock impact, which makes it necessary to use only very robust scientific experiments and equipment. However, the tether harpoon concept was discarded in favor of a sophisticated, small lander able to make detailed in-situ analysis of the surface composition.

Aerobraking A satellite with an atmospheric tether attached will slow down due to aerodynamic drag. For atmosphere probing this is a disadvantage, but it can be used to make an orbiting satellite return to Earth. A spacecraft could deploy a tether into the atmosphere so that the drag on the combination, however small, would eventually decelerate the satellite sufficiently for it to fall back to Earth.


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This can be used for capsules with microgravity experiments onboard that need to be examined in a laboratory on Earth, or to remove obsolete satellites from orbit in which case the intense heat that is generated when the spacecraft slams into the atmosphere is simply allowed to burn it up. Spacecraft currently returned to Earth without tethers need rocket propulsion systems and considerable amounts of propellant. Instead of decelerating a spacecraft so that it completely falls out of orbit, tethers can also be used to slow it down only a little bit.

Space probes flying to other planets using efficient Hohmann transfer orbits arrive at their destination with too much speed. Without slowing down, they would fly past instead of entering orbit around a planet. However, decelerating with rocket thrust uses a lot of propellant and thus adds considerable mass to a space probe early interplanetary probes such as Pioneer 10 and 11 and the two Voyagers were not equipped with sufficient propellant to slow down, and thus visited their targets for only brief periods during fast flybys.

Close to the target planet, an interplanetary spacecraft could roll out a long tether into the upper atmosphere. This would decelerate the spacecraft to a speed lower than the local escape velocity, so that it would no longer have the energy to fly away from the planet. Artificial Gravity Assist Interplanetary space probes often take advantage of the gravity pull of planets and large moons to alter course and gain speed.

It can be somewhat compared to a Ping-Pong ball hitting a revolving fan; the ball will bounce back at a much higher speed and in a different direction from which it was hit into the fan. The maneuver can also work the other way around: if a spacecraft flies by against the direction in which the planet moves around the Sun, the spacecraft will slow down.

There are many more asteroids than planets and moons, but they are all fairly small, and hence their gravity fields are too weak for gravity-assist maneuvers. However, instead of gravity a long tether may be used in an asteroid slingshot flyby. Imagine an interplanetary spacecraft on its way to Jupiter approaching an asteroid, somewhere beyond the orbit of Mars.

The spacecraft will whirl in an arc around the asteroid. At the right moment the tether is severed and the probe flies off in a different direction and with a different speed due to the asteroid's orbital velocity the asteroid has dragged the spacecraft on for a little while Fig.

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The force on the spacecraft is determined by the square of its flyby velocity divided by the length of the tether this is mathematically equivalent to the earlier described artificial gravity being a function of the length of the tether times the square of the rotational speed, because the rotational speed is equivalent to the tip velocity divided by the tether length. If the probe has Figure 1. Shooting a harpoon from a fast-flying space probe into an asteroid over a distance of tens of kilometers is extremely difficult, and the risk that the harpoon would slip out of the ground during the slingshot maneuver would be very high.

The idea is therefore interesting, but probably not very practical. Momentum Exchange The most often described space tethers are the momentum exchange tethers, so called because they allow the transfer of momentum and thus energy between two objects. The main benefit of these types of tethers is that they allow changing the orbits of spacecraft without using any rocket propulsion.

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For example, imagine two equal satellites connected by a long tether and circling Earth, one in a lower orbit than the other. Because the satellites are the same, the center of mass of the combination lies halfway between them. If we want to calculate the orbital speed of this satellite system, we could pretend we are dealing with a single spacecraft located at the combination's center of mass.

For the laws of orbital mechanics, it is irrelevant what the satellite system looks like; the whole contraption will stay in orbit as long as its center of mass has the right speed for the altitude at which it is circling. However, because the lower satellite is closer to Earth than is the center of mass, it is actually orbiting too slowly for its altitude. It will therefore try to fall back to Earth.

In contrast, the other satellite is moving too fast for the gravity at its orbital altitude, and will try to pull away. The result is that both satellites are pulling like dogs on a leash, so that the tether remains taut and forces the two to orbit like a single spacecraft system. Moreover, because one satellite is trying to fall back to Earth and the other is trying to pull away, the tether system automatically orientates itself into a stable, vertical position perpendicular to Earth's surface.

Since this effect is caused by the difference in gravity at different orbital altitudes, it is called gravity-gradient stabilization. It is as if the lower satellite is being dragged along by the higher one, and in turn the upper satellite is pulled back by the lower satellite. If the tether would go slack, each satellite would be able to move independently and go its own way, one moving down and one up until the tether was stretched tight and vertical again.

What happens if we cut the tether? Because it lacks the energy to stay at its original altitude, it will enter an elliptical orbit with a lower perigee than before. Its original orbital altitude becomes its apogee. If the new orbit intersects Earth's atmosphere, that is, if the perigee is too low, the lower satellite will actually reenter the atmosphere. Momentum exchange tethers can thus be used to return cargo capsules back to Earth or make obsolete satellites leave orbit and burn up in the atmosphere.

Its elliptical orbit will have its perigee at the altitude of the satellites original, forced orbit, but a higher apogee than before. The process is analogous to what happens when an Olympic hammer thrower spins around and then lets go of the heavy weight. The hammer will fly away, while in reaction the athlete is forced to step back we can think Figure 1. On Earth, cables can be used only to pull things with, because they go slack the moment you release the tension on them. However, if we push the lower satellite in our example, for example using a rocket motor, it goes into a higher orbit.

As a consequence, the upper satellite is given some leeway and allowed to increase its altitude as well, until blocked once more by the pull of the tether. As a result, the whole combination i. Forward he died in September of Tethers Unlimited, a company working on the development of advanced space tether concepts. Forward was also a famous science-fiction writer who featured space tethers in many of his books; the cable catapult appears in his novel Camelot 30K , but it is more than just fantasy.

The Cable Catapult System uses a long tether as a launch rail in combination with a so-called linear motor. A linear motor is a type of electric motor that makes use of electromagnetic forces without requiring any moving parts. On Earth, linear motors are for instance used in magnetic levitation maglev trains. The tether, which may be orbiting Earth or another planet, is extended in space and pointed in the right direction for the launch. Because of the lack of moving parts and because it is electromagnetically suspended and thus does not actually touch the tether, the linear motor and its cargo can reach very high speeds.

When the required velocity has been reached, the payload is released to fly to its destination. The motor is subsequently decelerated on a shorter section of the tether, so that the whole launch system can be used again its orbit will have changed, however, because as the payload accelerates and flies off it pushes the tether backward, following Newton's famous principle of action equals reaction.

The Basics 23 Figure 1. In Forward's novel, scientists launch themselves to the outer solar system using a cable catapult to investigate an alien civilization found there. The power supply for the catapult is a nuclear-thermal-electric system using the heat of a nuclear reactor to produce electrical power. The energy is used to generate a sustained burst of radiofrequency energy, which travels down the long conductive cable to be absorbed by the launching motor.

Just before the motor reaches the power supply, which is located at an optimum point along the cable, it releases the payload capsule. Then it slows down to a stop on the shorter length of the cable, on the other side of the power supply. Using a fixed power supply rather than one on the linear motor minimizes the mass that needs to be accelerated, and therefore the amount of energy required. Due to the electric resistance of the cable, part of the energy flowing through it is lost in the form of heat.

Electrodynamic Tethers Electrodynamic tethers are thin cables made of an electricity-conducting material, typically a metal. If we deploy such a tether from a satellite in a low Earth orbit, it will tend to orientate itself vertically due to the gravity gradient explained earlier see Momentum Exchange. It is not even really necessary to have another satellite on the other end of the tether; the tether itself has a mass and therefore experiences the same forces as the second satellite in the momentum exchange tether system example.

This voltage depends directly on the magnetic field strength, the velocity, and the length of the wire. Such a system can induce several hundreds of volts per kilometer of tether! To make this useful, we need to have an electric current, which means we need electrons going in at one end of the tether and getting out at the other end, being driven through the wire by the voltage.

You can think of a voltage as a height difference, such as between a mountaintop and a valley. Only when we add water will we get a stream flowing down the mountain i. Fortunately, there are plenty of free electrons available in the thin upper part of the atmosphere, called the ionosphere, where ultraviolet and x-ray radiation from the Sun knocks electrons from atmospheric gas molecules. The voltage along the tether will attract these free, negatively charged electrons at its positively charged end called the anode.

The electrons will then move through the cable to be expelled at the other end by a so-called plasma contactor also called the cathode. The conducting tether and the less conducting ionosphere thus together form a closed electrical circuit, making the flow of electricity possible. To facilitate the collection of electrons, the anode can be a large metal sphere. However, an uninsulated tether will be able to collect free electrons over a large part of its length instead of just at its tip.

It prevents the piling up of electrons in a small area, which would block the way for other electrons since electrons have the same electric charge, they repel each other and thus increases the tether's efficiency. The induced current flowing through a conductive tether will interact with Earth's magnetic field to cause what is called a Lorentz force.

The laws of electromagnetism state that this force will always have a direction that opposes the motion of a wire going through a magnetic field, which means it will try to slow it down and thus also decelerate the attached satellite. As the current flows through the tether, heat is generated because of the electrical resistance of the conductive tether material so-called ohmic heating. This heat is then dissipated into space as infrared radiation. The result is that the tether system loses kinetic energy, and thus slows down.

In principle, electrodynamic drag tethers can also be used to generate electrical power in space. A km-long mile-long wire in a low Earth orbit can produce up to 40 kilowatts of power, enough to run light bulbs of a watts each or, more suitably, a sizable space station. However, since the energy conversion means that the orbit of the satellite is lowered, it cannot be used for extensive periods of time without firing rocket thrusters to compensate for the electrodynamic drag force.

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In this case electrical power supplied by a set of solar arrays is used to run a current through the tether. If the direction of the current is opposite to the direction it would flow in case of an electrodynamic drag tether, the resulting Lorentz force will also work in the other direction and thus push the spacecraft rather than slow it down. It is similar to the winding in an electric motor pushing against the magnets of its armature, causing a torque force.

Such a tether can then be used to accelerate a satellite and send it into another orbit. Effectively, electrical energy from the Sun, in case solar arrays are used is added to the tether system and converted into kinetic movement energy, making the spacecraft go faster Fig. They can change the orbit of a satellite without the need for any propellant, which means important mass savings.

The orbit of a satellite also could be changed as many times as needed if there is no dependency on a limited amount of propellant. Furthermore, the propulsion version can be used to keep low-orbiting satellites, which are exposed to minute but continuous aerodynamic drag, at a proper altitude and thus prevent them from falling from the sky. These are potentially huge benefits, but electrodynamic tether systems only work when orbiting within a sufficiently strong magnetic field. Around the Moon, for example, they are useless because the Moon has virtually no magnetic field. Serious work on electrodynamic tethers has been going on since the s, when Mario Grossi of the Harvard-Smithsonian Center for Astrophysics and Giuseppe Colombo of the University of Padua in Italy first conducted scientific research on what was then a very novel technology.

Electrostatic Tethers Electrically conducting tethers can also be used for purposes other than changing orbits of satellites and electrical power production. One interesting concept is to use them to remove charged particles from the vicinity of Earth. The Sun sends out a continuous stream of electrons and ions. Ions are atoms that have an electrical charge because they contain too many or too few negatively charged electrons to compensate for the number of positively charged protons in the atom's nucleus.

Earth's magnetic field traps these charged particles like a magnet attracts iron dust, and keeps them locked up in the so-called Van Allen radiation belts. The particles in the Van Allen belts pose a serious threat to satellites and people venturing into them. They can upset and even destroy sensitive onboard electronics, degrade spacecraft materials, and cause biological damage in the cells of astronauts' bodies.

The Van Allen belts vary in intensity with latitude; they are thickest above the equator and diminish in the direction of the North and South Pole. Moreover, the intensity varies with altitude, so that spacecraft in the most useful orbits relatively close to Earth and satellites in high orbits such as GEOs are fortunately little affected. However, satellites transferring from low orbits to higher orbits such as GEOs or on their way to other planets necessarily have to fly through the belts.

However, low Earth orbit satellites are not completely safe. Earth's inner Van Allen belts are symmetrically aligned with the planet's magnetic axis. However, this axis is tilted with respect to Earth's rotational axis by about 11 degrees. In addition, the magnetic axis is offset from the rotational axis by some km miles. Due to this offset and tilt, the inner Van Allen belt is closest to Earth's surface over the southern Atlantic Ocean. The consequence is an increase in radiation levels in region off the east coast of South America.

This area is called the South Atlantic anomaly and can affect satellites in otherwise safe, low orbits between altitudes of about and km and miles Fig. Figure 1. The Basics 29 The explosion of an atomic bomb at high altitude could create an artificial and even more dangerous radiation belt.

The highly energetic particles ejected by such a nuclear explosion would threaten satellites that otherwise orbit in low-radiation, low Earth orbits. Such a weapon could thus be used to destroy military observation and communication satellites. In the process it would also damage other satellites, nonmilitary and those of neutral countries, in orbits affected by the artificially created radiation. Experiments performed in the s with nuclear bombs detonated in space showed that the lethal radiation belts can persist for many years the testing of nuclear weapons in space has since been prohibited by the United Nations Outer Space Treaty of It would be nice to be able to flush the Van Allen belts of charged particles, and it may be especially important to be able to get rid of any artificially created radiation belts as soon and as fast as possible.

This could be done by employing electrically conducting tethers of several tens of kilometers in length into orbits that bring them into the radiation belts. When these are charged to very high voltage levels, the electromagnetic fields thus generated can scatter the energetic radiation particles, over time sending many of them out of the radiation belts into the atmosphere or further into space and thereby lowering the radiation levels. Beanstalks All the tether applications described up to now work only in space.

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However, the most difficult and costly part of spaceflight is leaving Earth, and that is where the highest potential benefit of replacing conventional rockets with a tether system can be found. In the earlier explanations about how orbits work, satellites were launched with a hypothetical cannon on top of a tower that reaches all the way into space. In reality this would not work, because we are not able to build towers that are several hundred kilometers high. Moreover, the extreme acceleration needed to speed a normal satellite up to orbital velocity over the length of the barrel of a cannon would result into very flat and very damaged spacecraft.

Instead, we use rockets to carry satellites above the atmosphere and to relatively slowly increase their velocity. This requires a huge amount of propellant, and in addition large and heavy tanks to contain all that propellant plus all kinds of other rocket engine equipment. Every kilogram of satellite put in low Earth orbit requires about kg of propellant and rocket hardware. Without these additions, there is no way to bring the system back to Earth once its cargo has been delivered. Rockets are therefore mostly single-use, expendable systems.

The Space Shuttle is partly reusable, but the intensive maintenance it requires has led to its being the most expensive launch vehicle. A launch is estimated to cost around half a billion dollars, while a launch with an expendable rocket with comparable payload mass such as the Ariane 5 costs about a third of that. The development of more modern, more economical reusable launch vehicles continues to struggle with mass growth and cost problems, and no revolutionary launch vehicle technology solution is likely to be available in the foreseeable future see Chapter 2.

This seems to be a rather ludicrous idea, but considerable thinking by various space organizations has already been put into this concept. The general idea involves a cable of an incredibly strong material that directly connects an orbital space station with Earth's surface, kind of like a bridge or beanstalk into space. The center of mass of the system would have to be positioned in geostationary Earth orbit, where its orbital rotation is 24 hours and it thus remains above the same point on the equator as Earth rotates around its axis.

In this way, the tether could be connected to an earthbound base station located on a fixed position somewhere on the equator. With the center of mass in any other orbit, the system would move too fast or too slow with respect to Earth's surface, and the cable would break. To have the system's center of mass in geostationary Earth orbit, a countermass would be needed at a higher altitude to compensate for the mass of the cable going down to Earth; because the countermass would be moving too fast for its higher orbit, it would pull on the tether and keep it taut.

In other words, the centrifugal force on the part of the cable above the geostationary point would balance the gravity force pulling down the lower part Fig. If a space station would be put in the elevator system's center of mass, that is, in geostationary Earth orbit, it would be located at an altitude of 36, km 22, miles. Preliminary estimates suggest that an electromagnetically propelled elevator may take more than a day to reach such an altitude, and a mechanical climber may take more than a week. Elevator shuttles for people would thus have to be equipped with sleeping facilities, a kitchen, toilets, and so on, but for uncrewed spacecraft and cargo containers the long travel time would not matter.

The low acceleration levels and the lack of noisy, violently vibrating rocket engines would actually make the transfer much easier on satellites and other equipment, allowing them to be constructed less robustly than required for a rocket launch. This could result in less expensive spacecraft. The orbital station at the geostationary point could be made very large, as construction materials from Earth could be transported up easily and at low cost by the elevators. The station could incorporate satellite launch facilities, telecommunications equipment, astronomical observatories, Earth remote sensing instruments, and space tourist hotels with microgravity sports facilities and rooms with magnificent views of Earth and space.

Mass growth of the station would have no effect on the stability of the space elevator, because it is located in the geostationary point. That is where the system's center of mass needs to be, and where all the mass is effectively in orbit and thus does not pull on the cable in any direction. Without this, the space elevator would be pulled down by its own weight. Apart from offering an easily accessible microgravity space station at geostationary orbit altitude, and possibly stations below and above this point, the space elevator would greatly facilitate the launching of free-flying satellites.

To put a spacecraft in geostationary orbit, it could be transported up the cable with a cargo elevator, and then simply released from the center mass station. It would then immediately have the right velocity to stay in orbit, and require only small amounts of propellant to reach the intended geostationary orbit position which is fixed with respect to the surface of Earth.

When satellites are released above the geostationary point, their velocity will be too high for their altitude.

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The result will be an elliptical orbit, with the launch altitude being the perigee. Sufficiently high up the elevator, a spacecraft could even move off at interplanetary velocity and be sent on its way toward other planets. We could think of a space elevator as a giant slingshot hurling spacecraft into deep space.

The feasibility of the space elevator concept depends mostly on the availability of materials that can withstand the incredible forces on the cable. Steel is too heavy and by far not strong enough. Cables made from revolutionary materials based on so-called nanotubes are required. Carbon nanotubes are tiny cylindrical molecules of carbon, stiff as diamond yet a hundred times stronger than steel at one sixth the weight.

Unfortunately, the performance of cables we can currently construct out of carbon nanotubes is not yet up to space elevator standard see Cable Material in Chapter 7. The space elevator is the most exciting, most inspiring, and potentially most important application of space tethers, but also the most complex and most difficult to develop. Chapter 6 is devoted to this idea. Sustaining technology relies on incremental improvements to an already established technology.

A good example is the conventional petrol engine used in automobiles; the principle on which it works has not changed in over a hundred years, but a modern car engine is much more reliable and efficient than that of a model-T Ford. Disruptive technology, on the other hand, is a revolutionary technology that suddenly, and often unexpectedly, displaces an established technology.

Take, for instance, the rapid market takeover by digital cameras at the expense of long-proven and firmly established film-photography technology. According to Bower and Chris- tensen, disruptive technology initially often lacks refinement, appeals to a limited number of people, and may not yet have a proven practical application the idea is further developed in Christensen's best-selling book, The Innovator's Dilemma.

Tether technology has the potential to be a disruptive technology; if it works as advertised, it could radically change spaceflight and make conventional rocket propulsion systems largely obsolete. If so, it would not be the first time in history that an experimental technology totally replaced older, much better established means of transportation. They were fast, their rigging could cope with a variety of wind conditions, and their crews were trained to run the ships as best as possible.

Then came the steam engine. At first, ships powered by steam could not really compete with sailing, because the machines were tricky to run, the paddles initially used to propel the M. Many were skeptical of the need for this new technology and doubted that the potential advantages would outweigh the investments. Was it really worth the trouble of completely converting the established and trusted system of sailing ship transportation? However, as the technology matured, steam boats quickly made the old sailing vessels obsolete.

They became faster, were independent of the wind, and required much smaller crews, thus enabling rapid and predictable transportation at lower costs. Later, in the early 20th century, cars were initially also greeted with skepticism. The first automobiles were expensive, broke down frequently, and could not travel on rough roads like horses could. Nor were they able to carry the huge amounts of cargo transported at high speed by steam trains. Nevertheless, eventually the car made it possible to transport people and goods in quantities and at an efficiency that would be impossible to reach with horses.

Just think of how much food and stable space would be needed if everybody still used horses instead of cars, and how much slower local transportation would be. In addition, cars, unlike trains, are not limited to rails and can thus get to any village. They can be used by the driver and a few passengers, or even no passengers, while railroads are economical only if transporting large numbers of people or large amounts of cargo.

The modern, large-scale economies needed to support the billions of people now inhabiting our planet could never be supported by horsepower alone or by trains alone. The steamship and the car are just two examples of new technologies that dramatically disrupted the status quo, even when at the time their use did not seem to be required and their benefits seemed insufficient to warrant further investments. Space transportation now appears to be ready for a revolution as well. Even though conventional rocket propulsion can support all we do in space and has only relatively recently reached its maturity and become trusted, the limits of its performance capabilities and economic possibilities are already in sight.

For the launch, rockets with chemical propulsion engines are used because there is simply no other way to put anything in orbit.

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However, improvements in actual payload capability and launch cost have not been dramatic. Rocket engines have become somewhat more efficient, but the increases in thrust per amount of propellant have been relatively small. It appears we are currently reaching the limits of chemical rocket propulsion performance; any small increase in efficiency now requires such large amounts of development work, time, and money that it is often not worth the effort.

In addition, the structural masses the mass of the propellant tanks and support structures of rocket stages have not improved much over the last 50 years. The launchers in use today are very similar to those that were developed in the s to throw nuclear bombs at other countries. Modern rockets such as the American Atlas V and the Russian Soyuz are even direct descendants of early intercontinental ballistic missiles Fig. All in all, the total mass that a certain type and size of launcher can put into a certain orbit has not improved very much since the s.

Looking, for example, at the total satellite payload mass that launchers can put into a low Earth orbit LEO as a percentage of their total lift-off mass, it becomes clear that this number has been close to 3. For geosynchronous transfer orbit GTO payloads, that percentage has been a steady 1. A GTO is an elliptical orbit in which a geosynchronous orbit [GEO] satellite is initially launched in order to reach GEO altitude; at the GTO's apogee, a rocket motor is ignited to place it in a circular, geosynchronous orbit. For many modern launchers these percentages are even lower, depending on what the rocket is designed to do and how sophisticated it is Figs.

All launchers except the Space Shuttle are still of the expendable type, meaning they can be used only once. They drop off their empty stages along the way as a means of getting rid of dead weight. These stages then splash into the ocean or burn up in the atmosphere; outfitting them with retrieval equipment such as parachutes or deployable wings would make the launcher too heavy.

Once the satellite cargo has been put in orbit, no part of these expensive machines is left for reuse. Thus, this is an extremely expensive way to transport things. The launchers shown represent the best of their generation in terms of payload mass to LEO. Figure 2. The launchers shown represent the best of their generation in terms of payload mass to GTO. Improvements in how we develop, produce, and operate launchers can still decrease launch costs somewhat, as shown by SpaceX with its Falcon series of launchers, but we should not expect dramatically lower launch costs per kilogram satellite in orbit with any new expendable launchers.

An obvious way to lower launch costs is to reuse rockets. Instead of having to pay for a completely new one every time we need to put up a spacecraft, we would then only need to pay for the propellants, operations, and maintenance of the launch vehicle, such as with an airplane. Planes are not thrown away after each flight. The reason that expendable rockets are still the norm is that we have not had much success developing reusable systems.

To make a launch vehicle capable of being used again, it needs additional equipment to return to Earth. Heat shields, wings, parachutes, and additional propellant for landing make reusable systems relatively heavy. As the satellites on top of expendable launchers comprise only a few percent of the total launch mass, the mass available for useful payload is easily eaten up by additional components added to make the system reusable. The fact that rocket engine efficiency and structural mass reductions are already close to their achievable limits means that it is very hard to compensate the mass growth in a reusable launcher design.

Many concepts for reusable systems, therefore, would be capable only of launching and flying back the bare vehicle itself, without any mass allocation to spare for spacecraft cargo such as satellites and space station modules. Reusable launchers are also more difficult and therefore more expensive to develop than expendable launchers.

On top of the difficulty of developing something that can go into orbit, now the vehicle also needs to be designed to come back, which involves reentry into the atmosphere, a descent phase, and a soft landing. Furthermore, the use of such launchers requires not only a launch pad but also the development of additional infrastructure, such as a safe landing area, vehicle and engine maintenance buildings, and logistics facilities to store and manage the distribution of spare parts.

Instead of these recurring costs for a reusable system, every expendable launch involves a brand new vehicle, and therefore operations are limited to the launch preparations and the actual flight. In addition to this, however, a reusable launcher requires inspection and maintenance before each subsequent mission. The operations costs for reusable systems are therefore also higher than in the case of expendable rockets. Together with the maintenance hours on the three large Space Shuttle main engines and the two reused solid rocket boosters, this represents a huge amount of money that does not need to be spent when using expendable, single-use rockets.

Moreover, the Space Shuttle system is not fully reusable: the large brown external tank is discarded during each flight, so a new one is needed for every mission. The higher development, infrastructure, and maintenance costs mean that operating reusable launchers can result in lower launch prices only if they make many flights each year. It is just like with commercial airlines, which need to keep their planes in the air for as many hours as possible to keep costs down.

This requires short maintenance cycles; otherwise a large and therefore expensive fleet of vehicles would be needed. To justify launching many spaceflights, we also need a large number of customers who require the launch of many more payloads than is currently the case. The launch market will significantly increase in size only if launch prices drop dramatically, which in turn requires efficient reusable systems with little maintenance needs.

This is a really difficult catch situation: launches could become cheaper if there was a sufficiently large market, but this market will not grow until launch costs drop significantly. The maintenance of the partially reusable Space Shuttle turned out to be so time-consuming that the initial expectations of launching some 60 missions per year never became a reality; in a good year, the shuttle is launched about six times. The high maintenance and replacement costs and the low launch rate have resulted in very high launch costs.

The additional safety constraints put in place after the loss of Columbia probably put the current cost way over half a billion dollars per flight. This makes the Space Shuttle the most expensive launch vehicle, both in total launch price and in cost per kilogram payload put in orbit.

For the current relatively few satellite launches per year, it is cheaper to use expendable, one-shot rockets. The reason that the Space Shuttle is still in use despite its disadvantages is that it is the only vehicle the United States has available for human spaceflight. In fact, for the relatively few crewed missions it foresees to the Space Station in the s and to the Moon in the s, NASA has found that it will be less expensive to operate classic expendable rockets and capsules rather than some kind of reusable shuttle system.

Ares I will be topped with an Orion capsule that may be partly reusable, but is otherwise very similar in design to the Apollo Command and Service Module combination of 40 years ago Fig. For launching large cargoes such as lunar base modules, NASA will develop the Ares V, which will be a mostly expendable rocket only the solid rocket boosters derived from the Space Shuttle system may be reused Fig.

New versions of the Ariane, Atlas, Delta, and Soyuz rockets are also still being developed, and it does not look like these expendables will become obsolete and replaced by reusable launch vehicles anytime soon. Radically lowering launch prices for traditional rocket propulsion systems, even by means of reusable equipment, is extremely difficult. However, it is still a lot of money. Some private companies are developing reusable launch vehicles, having determined or hoping that with currently available technology it may yet be possible to develop cost-efficient reusable launch vehicles.

Smart concepts such as the suborbital hopper may still make lower launch costs possible. A hopper is a reusable vehicle that does not accelerate all the way up to orbital velocity but delivers payloads in space at almost orbital speeds. Such a launcher saves huge amounts of propellant by not having to boost its own mass into orbit; the mass of an additional booster stage required to give the cargo the bit of extra speed it needs is very limited in comparison.

If novel concepts and technology can drop launch prices to levels that make it affordable for smaller countries and organizations to launch satellites and people into space, the market may grow enough and launch rates may increase sufficiently to warrant the development of even better reusable systems.

However, a completely different new technology may be needed to radically lower the costs of access to space. Relatively large thrusters are used for trajectory adjustments, changing orbit altitude and inclination the angle of the orbit with respect to the planet's equator , and braking for getting into orbit around another planet when arriving there with too high velocity from an interplanetary transfer flight. Smaller thrusters, often on the order of a couple of tens of newton force, are used for attitude control and delicate maneuvering with 1 newton being equivalent to the force that gravity exerts on a gram [0.

The mass of the required propellant, rocket thrusters, tanks, pipes, and valves often comprises a large part of the total spacecraft mass. The Venus Express spacecraft of the ESA, for example, had a total mass of about kg pounds when it was sent on its way. No less than kg pounds of this was propellant, while the propulsion hardware had a mass of 60 kg pounds ; the propulsion subsystem thus accounted for about 50 percent of the total mass! Even when orbit adjustments are not needed, the attitude control stabilization and orbit maintenance of satellites requires a lot of propellant.

About 30 percent of the total mass of a typical geostationary communica- tions satellite with a lifetime of 15 years consists of propellant for so-called station keeping. Rather than ejecting hot gases that are products of a combustion process, electric propulsion systems eject charged particles using electromagnetic forces.

This makes ion engines much more efficient, giving the same impulse and therefore enabling the same increase in spacecraft orbital velocity or rotational velocity management in the case of attitude control for much less propellant. Ion engines are already widely used as attitude control thrusters for large communications satellites and are now also employed as a means of propulsion for interplanetary missions Fig.

However, electric propulsion also has disadvantages. Courtesy of ESA. An ion engine requires a strong magnetic field to charge and accelerate gas atoms, and for that it needs electricity. This is supplied by the Sun through the solar arrays, which is why the concept is usually referred to as solar electric propulsion.

It means that spacecraft with electric propulsion require larger and thus heavier solar arrays than similar probes with chemical propulsion systems. More serious is the problem that ion engines can produce only tiny thrust levels, on the order of the weight of a sheet of paper. Electric propulsion, therefore, can be used only in space; on Earth an ion engine is not even capable of lifting its own weight, let alone a spacecraft.

The extremely low thrust means that a spacecraft with ion engines can manage only very small acceleration levels, on the order of a fraction of a millimeter a hundredth of an inch per second every second. To increase the velocity by 1 meter per second can take more than an hour.