* One of the conceptually simplest ideas for space propulsion is to simply winch a payload up in space on a cable. Despite the seeming absurdity of the idea, it is being taken seriously, and in fact has been tested in several space experiments. Such a "space tether" can in principle be used for a surprising range of applications, such as payload transfer, power generation, and orbital propulsion. This chapter discusses present space tether technology and its future possibilities.
* The idea of using tethers to link spacecraft goes back to the beginnings of space flight. In 1966, the NASA Gemini 11 and 12 manned space missions each performed an experiment where the capsule was tethered to an Agena rocket stage to investigate the mechanics of linking spacecraft with tethers, and their possible use in generating artificial gravity. This experiment was thinking small, however. In the 1970s and early 1980s Guiseppi Colombo, a professor at the University of Padua in Italy, conducted detailed studies of what could really be done with space tether technology.
Tethers can be used to connect multiple elements into a single assembly; for example, a space station might be linked by a 100 kilometer long tether to a "hangar" in a higher orbit, where spacecraft are assembled and then released to fly to higher orbits or interplanetary space.
It is important to remember in this context that, contrary to popular belief, a spacecraft in orbit around the Earth is not weightless. Gravity is pulling it down at all times, and the only reason the spacecraft doesn't fall to ground, is because it is moving forward so fast that the Earth curves out underneath as the spacecraft flies forward through its orbit. Gravity does weaken the farther the spacecraft gets from Earth, however, and so a spacecraft in a higher orbit doesn't have to fly as fast to stay at its altitude. Since the orbit gets bigger at the same time, as a spacecraft orbits farther from the Earth, the longer it takes to make one orbit.
If the hangar was not connected to the space station, it would orbit more slowly than the space station and fall behind. Since the two are linked, however, they both orbit with the same period, effectively as if they were one mass located at the mutual center of gravity of the two assemblies. The hangar is moving faster than it would if it were free-flying, and by the other side of the same coin the station is moving more slowly than it would if it were free-flying. The difference in gravitational force between the hangar and station, or "gravitational gradient", maintains a force on the tether that ensures the two spacecraft continue to orbit in synchronization.
The fact that the hangar is moving faster in its orbital path than it would be if it were free flying means that there is net outward force on the platform and everything in it, providing a small amount of artificial gravity. Similarly, the fact that the space station is moving slower in its orbital path means there is a net downward force on it, also providing a bit of artificial gravity. This might be desireable for spacecraft maintenance and assembly. An elevator could shuttle up and down the tether between the space station and the hanger to allow transfer of personnel, hardware, and supplies.
Tethers have also been considered for manned interplanetary missions. A manned Mars spacecraft, for example, could be linked by a long tether to the expended final stage of its launch vehicle. The two could then be set into a slow rotation, providing the crew of the spacecraft with artificial gravity.
That much had been known before Colombo investigated the issue, but he went beyond this scenario to envision how tethers could be used for space launch. Suppose a space shuttle is orbiting the Earth and wants to release a satellite to a higher orbit. It can do so by reeling it out on a long tether. As with the station and the hangar, the two spacecraft orbit as a single system, with the satellite at the far end of the tether moving faster than a free-flying object would move at that orbital altitude, and the shuttle at the near end of the tether moving slower than a free-flying object would at that orbital altitude. If the shuttle releases the satellite from the tether, the satellite retains its velocity and flies up to a higher orbit. Since we can't get something for nothing, the shuttle also retains its lower velocity and falls into a lower orbit. In a sense, however, we have got something for nothing, since if the shuttle is returning to Earth, it has saved fuel it would otherwise have been forced to burn to reduce its orbital velocity.
Colombo then went even farther, suggesting that a tether could be used to generate electricity. It is a fact of elementary physics that moving a conductor through a magnetic field generates an electrical current through that conductor. A satellite orbiting the Earth could generate power by unreeling two conductive tethers, each several kilometers long, with one extending upwards and the other extending downwards. Each tether ends in a terminal assembly that exhausts a plume of plasma to complete the electrical circuit through the Earth's tenuous upper atmosphere, the ionosphere. As the tether orbits through the Earth's magnetic field, it produces an electric current.
Similarly, a conductive tether could also be used to provide propulsion for orbital adjustment. It is another simple fact of physics that running a current through a conductor creates a magnetic field. If the satellite sends current generated by its solar arrays through the tether in one direction, the magnetic field produced by the tether will oppose the Earth's magnetic field, causing magnetic "drag" and degrading the satellite's orbit. If the satellite sends the current through the tether in the other direction, it will work with the Earth's magnetic field, and the satellite will rise.
Analyses show that the use of such "electrodynamic tethers (ETDs)" for orbital adjustment is far more efficient in terms of spacecraft mass requirements than chemical thrusters, though the orbital changes are slow. Current studies indicate that a 25-kilogram tether deployed by a 1,500-kilogram satellite in an 850-kilometer-high orbit can bring the satellite back to Earth in three months.
NASA researchers conducted studies of a tether system to keep the International Space Station (ISS) in orbit. The 200-kilogram ISS tether "reboost" system would use a tether ten kilometers long. It would use 5 kilowatts of electricity to produce a constant push of 0.5 newtons, about the same amount of force that one would use to pick up a cup on Earth, but still enough to keep the station in orbit.
NASA researchers have also considered using tethers with a Jupiter orbiter probe to provide power generation and propulsion, taking advantage of the giant planet's strong magnetic field. In fact, at a fairly close approach to Jupiter, a 10-kilometer-long tether would generate 20 amperes of current with a potential of 50 kilovolts across the tether, providing a megawatt of power. Since this would quickly fry the tether, the current flow through the tether would have to be "chopped", pulsing it on and off to throttle power delivery as needed. Along with electrical power, the tether would be used to brake the probe into Jupiter orbit; shift the probe's orbit to fly by Jupiter's moons; and, if needed, help give the probe a "kick" to help send it back to Earth.
BACK_TO_TOP* Following the two Gemini experiments in 1966, in 1992, to investigate more advanced uses of tethers, NASA flew a space shuttle mission that attempted to deploy the ASI (Italian Space Agency) "Tethered Satellite System (TSS)". The TSS was a 550-kilogram spacecraft that was to be wound out on a 20-kilometer-long conductive tether. One of the main objectives of the experiment was to investigate electrical power generation, and both the satellite and the shuttle carried plasma-generation gear to allow the circuit to be completed through the ionosphere. Unfortunately, the tether feed mechanism jammed during deployment, and the experiment was a failure.
In 1996, NASA and the ASI tried again. The TSS was successfully wound out from the space shuttle to its full 20-kilometer length, and generated 3,500 volts at ampere-level current. However, the tether's insulation was damaged, and an arc flashed between the tether and the shuttle's deployment boom, breaking the tether. That was a disappointment, but the experiment had achieved most of its goals before the mishap. There were also two secondary benefits. First, current levels were twice what had been expected. Second, when the satellite was released, it popped up to an orbit 140 kilometers above the shuttle, demonstrating the use of tethers for orbital insertion.
* Despite the troubles with TSS, other tether experiments have been conducted, and most have been successful:
A follow-up to SEDS, designated "Propulsive SEDS (ProSEDS)", was also to be deployed from a Delta II, but ProSEDS was to use a conductive tether, not a nonconductive tether, to allow electromagnetic orbital adjustment. The ProSEDS tether was to be 15 kilometers long, with ten kilometers insulated and the last five bare. The experiment was supposed to be flown along with a launch of a Global Positioning System satellite in the spring of 2003, but it was yanked at the last moment, due to concerns that the tether might collide with the International Space Station. For various reasons, ProSEDS was then canceled.
* A space startup company named "Tethers Unlimited Incorporated (TUI)" of Seattle, Washington, the brainchild of Robert P. Hoyt and the late "space visionary" Robert L. Forward -- more said about Forward in a later chapter -- has also worked on tether technologies. The company has developed a highly reliable tether called the "Hoytether", using synthetic fibers in a weblike arrangement.
TUI has worked on a tether-based satellite de-orbiting system called the "Terminator Tether" for commercial satellites. This is a small, lightweight module that contains a five-kilometer-long tether that is deployed when the satellite is to be de-orbited. It will eventually drag a satellite down from a high orbit to burn up in the Earth's atmosphere.
A particularly interesting proposal from TUI envisioned a robot "space tug", consisting of two solar-powered "shepherd" modules connected by a conductive tether. The tug would move from orbit to orbit, with the shepherd modules latching on to the hulks of dead satellites so the tug could then drag them down to orbits that send them into the atmosphere. This space "garbage collector" would prevent dead satellites from breaking up into clouds of space debris that would threaten active spacecraft. Interestingly, the tug could also be used to reboost active spacecraft, stealing momentum from hulks to send them down and using the momentum to lift a useful satellite up.
Tethers Unlimited flew a tether demonstration named the "Multi-Application Survivable Tether (MAST)" experiment as part of a set of a small spacecraft launched by a Russian Dnepr booster in 2007. MAST was based on three "CubeSats", which are small standardized satellites in the form of cubes 10 centimeters on a side, intended to provide low-cost access to space. MAST consisted of a 1-kilometer-long tether, made of three braided strands, with a CubeSat at each end and a third CubeSat, named "Gadget", moving up and down the tether to inspect it. All three satellites were fitted with GPS receivers for tracking. The system failed to deploy properly.
Since MAST, several further space tether experiments have been performed, including a number of CubeSats -- with very little success to date. Activity on space tethers is, for the time being, effectively stalled. Hopefully, things will improve in the future.
BACK_TO_TOP* Current tether experiments, as the previous section shows, have focused on using tethers for satellite deployment, power generation, and orbital adjustment. More exotic and ambitious applications of tethers have been considered.
For example, gravity assist trajectories are now often used to send space probes to distant planets, with the probes redirected by close flybys of Earth, Venus, Jupiter, or other worlds. Asteroids are very small and so cannot be used for gravity assist maneuvers, but some space researchers have envisioned a space probe firing a tether with a harpoon tip into an asteroid during a close flyby, swinging around it, and then releasing the tether once the probe is on the proper trajectory.
Tethers could also be used for sample return missions from asteroids, or planets with no atmosphere such as Mercury. An orbiting probe could release a sampling system connected to a tether as the probe makes a close approach to the object's surface. The sampling system would touch down for about ten seconds or so, take a sample, and then be reeled back up to the probe on the tether. The probe could use the same technique to sample several different sites before boosting back to Earth.
* One particularly ambitious concept envisions a "stepladder" of tethers that could be used to send payloads from Mars back to Earth. In this scheme, the payload is boosted off Mars into low orbit, and is then reeled out to a higher orbit on a 375-kilometer-long tether deployed from the launch vehicle. It is then released, and flies upward while the launch vehicle returns to Mars.
The payload reaches an altitude of 8,000 kilometers, where it is snagged by a 1,000-kilometer-long tether dropped down from a station on the Martian moon Phobos, orbiting Mars at 9,400 kilometers. The solar-powered station winches the payload up to Phobos on the tether, and winches out a second 1,000-kilometer-long tether that extends outward from the station on Phobos.
The payload is released and flies upward to be snagged by a 3,000-kilometer-long tether hanging down from a station on the Martian moon Deimos, orbiting Mars at 23,500 kilometers. The payload is winched up to Deimos by the station, and then winched out from Deimos on a tether that extends outward 6,000 kilometers. When released from this tether, the payload then coasts back to Earth on an interplanetary trajectory.
* Another related interesting concept, variously called the "rotating skyhook", "rotavator", or "bolo", envisions spinning tethers in orbit around the Earth that could be used to launch payloads to the Moon or beyond. A heavy central station could deploy a tether, say, 100 kilometers long with a docking module on the other end. The docking module rotates around the central station so that it approaches and then moves away from the Earth.
A payload could be launched from Earth to rendezvous with the docking module at its closest approach. The payload would be carried by the docking module as it swung outward on its "orbit" around the central station, to be released at the proper time to send it to its distant target, somewhat like throwing a rock off a sling. The tether system would lose momentum to the payload and so its orbit would fall, but it could also snag payloads coming in from beyond and transfer them to a lower orbit for return to Earth to regain momentum.
A bolo in low Earth orbit could snag a payload and send it up, to be snagged in turn by a bolo in geostationary orbit, which would then send it on into the deeps of space. Bolos in orbit around the Sun, instead of the Earth, could also snag payloads and send them off to different planetary destinations. NASA has conducted research on a 100-kilometer-long bolo concept under the "Momentum eXchange / Electrodynamic Reboost (MXER)" program, which as its name suggests envisions a bolo that uses electrodynamic propulsion to maintain its orbit, but there is no plan for flight at this time.
* A very large and heavy bolo, thousands of kilometers long, could be put into orbit around the Earth so that its tips brushed the upper atmosphere during its orbit. A Mach 3 carrier aircraft could haul a piggyback capsule to high altitude to release it for rendezvous with the docking module at the end of the bolo. The bolo would then haul the capsule into space and release it on a lunar trajectory.
On arrival at the Moon, the capsule would then be snagged by a lunar bolo, which would take it down close to the Moon's surface, stealing its momentum, and then releasing it for pickup by a tug spacecraft. If payloads were sent back to Earth in the reverse direction, the bolos would retain their net momentum and remain in orbit. A similar concept could be used for transferring payloads between Earth and Mars.
BACK_TO_TOP* Space tethers are a modern relative of a slightly older, much grander, and more speculative idea that goes back at least to the 1960s, known as the "beanstalk", or sometimes as the "(non-rotating) skyhook". A beanstalk is a simple idea, which is far from saying that it's easy. Suppose we have a space station in geostationary orbit. If we then string a 36,000-kilometer-long cable from the Earth to the space station, we would be able to take an elevator from the Earth into space and avoid the use of expensive rocket boosters.
Probably the first person to dream up the beanstalk was Yuri Artsutanov, a Soviet engineer and science writer. On learning about new materials that could be used to build long and strong cables, he did some calculations and wrote an article in 1960 describing what we would now call a beanstalk. However, he published it in a Sunday supplement of a youth newspaper, and so his idea attracted little attention.
Several other people rediscovered the beanstalk later, but the first person to attract attention was an aerospace engineer named Jerome Pearson, who published a detailed study of the concept in 1975. Pearson envisioned placing a cable-making station in geostationary orbit. In his scenario, the machine begins synthesizing cables both towards the Earth and away from the Earth, with the rate of growth of the two cables carefully controlled to ensure that forces on the system remain balanced and the station remains at GEO.
As the cable flows out of the station, its base grows thicker, or in other words it is "tapered". This allows the Earthbound cable to support its own weight as it grows longer, since the higher sections of the cable must support the weight of cumulatively longer sections of the cable. Centrifugal force similarly requires that the outbound cable be thicker closer to the station.
In completion, the Earthbound cable is 36,000 kilometers long, while the outbound cable is 110,000 kilometers long. Once the Earthbound cable reaches ground, it is solidly anchored to ensure its stability. A counterweight, such as an asteroid, would be attached to the tip of the outbound cable. The counterweight is orbiting the Earth every 24 hours but it is at a distance of 146,000 kilometers from the Earth, and so it is moving much faster than it would be if it were in free flight at that distance. This exerts a force on the tether and keeps it aligned.
Pearson thought that existing graphite fiber material could do the job in principle, since it is 20 times stronger than ordinary steel and has only a fourth the density, though nobody's ever built anything remotely as big as a beanstalk with graphite fibers.
Pearson's beanstalk cable would need a taper of about 10:1. Small cables could be laid down first, and then used to support deployment of more cables, until the beanstalk could bear large payloads. A completed beanstalk cable pair capable of handling 100 tonnes would weigh about 600,000 tonnes. This quantity of carbon could be obtained by moving a "carbonaceous chondrite" asteroid, which is more or less a big lump of soot, into geostationary orbit. The slag left over from processing the asteroid could be used as the counterweight.
The elevators would be electrically powered, likely using some form of magnetic levitation propulsion, at least once they left the Earth's atmosphere, floating up maglev rails attached to the beanstalk. The elevators would travel at thousands of KPH, and with maglev systems they would not need to make physical contact with the guide-rail system.
* Although a beanstalk seems definitely like a future technology, there is a faction that believes it can be done now. An American engineer named Bradley Carl Edwards, previously at Los Alamos, came up with a more minimal beanstalk that he estimated could be built for $10 billion USD -- not too bad a sum for an ambitious space project. Once the first one was built, the others would be much cheaper, like about $3 billion USD.
Since Pearson's speculations, chemists have learned how to manipulate carbon in amazing ways, folding sheets of hexagonal graphite rings into spherical "buckyballs" or cylindrical "buckytubes", also known as "carbon nanotubes". In principle, carbon nanotubes would be strong enough to be used for a beanstalk. The problem is that nobody's built really long carbon nanotubes just yet, though very strong composites of carbon nanotubes and conventional plastic are clearly possible.
Edwards envisions a beanstalk made up of a ribbon of carbon nanotube composite a meter wide, with a total weight of only 800 tonnes, not counting the 600 tonne counterweight at the remote end, 100,000 kilometers above the Earth. His beanstalk does not use maglev elevator cars, with the cars, or "climbers" as Edwards calls them, instead fitted with conventional traction wheels driven by DC electric motors. He chose a ribbon configuration for the beanstalk because it would survive impacts of space debris much more easily than would a cable configuration, and would also give more surface for the treads of the elevator cars to grab on to.
The climbers would move no faster than 190 KPH, which would get the elevator car to geostationary orbit in eight days. They would have a loaded weight of about 20 tonnes and include living quarters. They would be powered by photovoltaic cells, driven by lasers on the ground.
In the beginning, a 20-tonne spacecraft and two 20-tonne spools of ribbon would be put into orbit to lay down a lightweight beanstalk. The spacecraft would hook up with the ribbon spools and then use a solar electric propulsion system to increase its altitude to geostationary orbit for ribbon deployment, gradually rising higher as the ribbons were spooled out. Once deployed, the small climbers would crawl up the ribbons to haul up materials to build the full beanstalk. About 280 small climber loads would be adequate for the construction of the full beanstalk.
The Earth end could be on an equatorial floating platform, which would be able to move to avoid typhoons and other hazards, such as space debris, which would be tracked by a high-resolution radar system. Upper atmospheric sections would be coated with a thin film of gold or platinum as a shield against oxygen erosion. If the beanstalk did fall, the lightweight ribbon, with a weight less than paper, would simply burn up without causing any damage in the impact zone.
Several startup companies have worked on tether technologies, with NASA providing modest funding for studies. However, NASA has no commitment to actually implementing a beanstalk. The notion that a beanstalk might be ready for use in a few decades does seem like a bit of a stretch, and the promoters have some proving to do -- but it's such an attractive idea that it certainly merits further investigation.
* In the early 1980s, a group of space enthusiasts chatting on the internet reconsidered the beanstalk concept, and came up with an alternate approach. Instead of stringing a cable all the way up into the sky to carry payloads, they suggested that the payloads could ride up a stream of projectiles fired from an Earth station. The concept was named the "space fountain".
In the space fountain, the projectiles are actually fired up inside a hollow tower, but the tower is supported by electromagnetic braking of the projectiles. At the top of the tower, the projectiles are turned around and sent back down to the ground station, which then electromagnetically accelerates them right back up again.
One attractive feature of the space fountain is that it can be built from the ground up and eventually jacked up into orbit. The station would have to be sited so that if the electromagnetic systems used to control the projectiles failed, the projectiles would fall back into the Earth's atmosphere into the sea and not populated areas. The power required to get all the projectiles in motion is tremendous, but once they were in motion only incremental power would be needed to keep them there, though a hydropower dam or two would still be required.
A related idea, called the "launch loop", envisions firing the projectiles from one station into a partial orbit that terminates at a second station on a remote site on Earth. The projectiles would fly through a track and be used to accelerate payloads into space using electromagnetic interactions. An even more ambitious scheme, the "orbital ring", uses a constellation of space stations in low Earth orbit that fire projectiles at each other to keep them in stationary position in low orbit, at an altitude of a few hundred kilometers. The stations would have relatively short beanstalks made of Kevlar synthetic fiber to link them to the ground.
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