Space Elevators - sometimes known as orbital tethers or even beanstalks [1] - are structures that connect a ground station with a satellite in geostationary orbit. Hypothetically these offer vast advantages over rocketry for the ascent to Earth orbit, and have been independently reinvented by scientists and engineers numerous times, since at least the early 1960s. It is small wonder that they are a perennially recurring theme in science fiction.

 An idea whose time had come

In early 1979, two novels were published within a few months of one another. The first was Arthur C Clarke’s The Fountains of Paradise. In this vision of 2142 a leading bridge engineer, fresh from the challenge of bridging the Pillars of Hercules, decides to take on a space elevator. He is certain that the elevator is needed to ensure the future of human freight requirements, and that the technical challenges are surmountable using a superconducting crystalline carbon ‘hyperfibre’ produced in zero gravity and a structure gradually lowered along it towards the Earth’s surface. He designs extrusion mechanisms for the cable and a machine known as a spider to climb and descend it. But he faces a challenge in the form of an order of monks who inhabit the summit of the only mountain he considers suitable as the landing point for his tether, located on the island of Taprobane (a barely-disguised Sri Lanka). Clarke was living in Sri Lanka at the time, but still actively involved - as he had been for many years - in the British Interplanetary Society. This organisation has always aimed to promote human space flight and space utilisation and it’s small surprise that Clarke had been exposed to a great deal of the scientific and technical speculation regarding space elevators before he wrote the novel.

The second space elevator novel of early 1979 was The Web Between the Worlds by Charles Sheffield. Sheffield was a mathematician and physicist, and served as president of the American Astronautical Society (a society with similar remit to the BIS). By coincidence, the novel also features a bridge-building engineer recruited to build a space elevator using his custom Spider machine to spin the cable. Here the cable (known as the Beanstalk in honor of Jack's climb towards treasure) is described as a doped silicon compound with integrated drive rungs, linear motors and superconductor circuitry. The cable is constructed at the Earth-Moon L4 Lagrange point, and flown in to tether in a deep pit near Quito in a rather daunting sequence of microsecond-timed orbital manoeuvres. Again, the main purpose of the elevator is freight, with passenger capacity also provided. Given the similarity with Clarke’s Fountains of Paradise, the publishers, author and Clarke himself were at pains to point out that Web Between the Worlds was written and accepted for publication before Fountains of Paradise was published - the close parallels between the two novels were indicative of an idea whose time had come and the basic practicalities which had been discussed in the AAS, BIS and other space-utilisation societies for over a decade before the novels were written.

A vital tool for space resource utilisation

In both The Fountains of Paradise and The Web Between the Worlds the space elevator is shown as essential for the future of an Earth which is well advanced in the process of depleting its resources, and is beginning to depend on resources imported from the Moon or asteroid belt. As both Sheffield and Clarke noted, the passage of many rockets through the upper atmosphere would have devastating effects, both in terms of damage to the environment and in terms of the noise and pollution that would affect Earth below.

As well as their enormous environmental impact, rockets are fundamentally inefficient for mass freight transport. They require vast quantities of fuel, not just to lift matter but also to lift the fuel they need to complete the journey. They also require large amounts of fuel to lower matter from orbit to Earth’s surface in a controlled manner. Left to fall naturally, cargo from space would convert their gravitational energy to kinetic energy (speed), impact the surface at kilometres per second and cause enormous damage. Rockets must expend an equal or higher amount of energy to dissipate the kinetic energy of the cargo into light, heat, noise and exhaust gas velocity.

By contrast, the fundamental principle of a space elevator is that cargos descending the cable could drive dynamos that could convert their gravitational-kinetic energy into electrical power - basically the same principle that allows electric vehicles to recharge their batteries when braking. This recovered energy could then be expended to lift cargos ascending the cable - crucially, without needing to transport large amounts of fuel along with the payload. The centre of mass of such an elevator would be slightly above synchronous orbit to tension the cable, while a substantial extension beyond to the 60,000km, ending in a counterweight, would be in a higher orbit, making an ideal deep-space launch point with a negligible escape velocity. Hence space elevators are more energy efficient, less wasteful and - thus - make transportation to and from space far cheaper.

This cheap, efficient form of orbital lift and descent, it has long been argued, will be essential for effective human space-utilisation, particularly now it is apparent how much equipment, radiation shielding and other resources any off-world human settlement will need to source from Earth. Its primary use as a freight hauler is a principle carried forwards from the earliest space elevators in science fiction to more recent appearances.

Space Elevators are shown as the backdrop to a routine commercial exploitation of space in examples such as the Martian Trilogy of Kim Stanley Robinson, Sundiver by Robert Brin, in the Halo video game franchise and in the Warhammer 40,000 tabletop game universe (where they are described as orbital spires). Here the existence of the space elevator may be barely commented upon and the elevator itself often isn’t the main focus of the story (although it can become an important target).

In the Star Trek: Voyager episode “Rise” (TV, 1997), for example, a colony world inhabited by aliens called the Nezu is threatened by a series of asteroid impacts. Sent on a rescue mission to the surface, USS Voyager crew Tuvok and Neelix crash their shuttlecraft and, together with some of the settlers, must use a space elevator (described throughout as an orbital tether) to climb above the ionosphere in the hope of contacting Voyager for rescue. In terms of its drama and plot, this episode follows a classic pattern: placing a small subset of mutually-antithetic characters in a restricted space and requiring them to work together for survival. In this respect any other setting would have done as well as the space elevator, and its detailed physics is not explored. Of interest here though is the fact that orbital tethers are treated as relatively commonplace equipment for space-faring cultures, and are apparently a necessary requirement early in the colonization of a planet - a phase where much transport of equipment, people and goods between the surface and orbit is required [2].

Arthur C Clarke also returned to the use of freight space elevators to enable space utilisation several times in his writings. The Time Odyssey trilogy, co-authored with Stephen Baxter, for example, explores the idea that an ancient alien race may be trying to prevent humanity’s ascent to the stars - first through an enormous solar flare and then through the destruction of Earth by a quantum bomb. The second novel, Sunstorm (2005), concludes with the construction of a space elevator in Australia and the start of humankind’s effort to settle the Solar System. As the novel tells us:

“Of course the Elevator was just the beginning. The plans for the future were astonishing; with space opened up at last, asteroids would be mined for metals, minerals and even water, and solar power stations the size of Manhattan would be assembled in orbit. A new industrial revolution was about to begin, and with the flow of free energy up there in space the possibilities for the growth of civilization were unbounded.” (Del Ray books first edition, 2005, pg 322)

The final book in the trilogy, Firstborn (Clarke & Baxter, 2008), dwells on the relationship between Earth and its young colonies, and describes the Earth as being heavily dependent on raw materials from space. The novel features an extended sequence as the main character ascends a space elevator anchored at Cape Canaveral. Together with two companions, she is confined to a hijacked freight capsule for twelve days as it climbs a ribbon known as Jacob’s Ladder - the timescale giving a real sense of the vast physical scale of the elevator itself.

Areo Elevators

One of the most commonly discussed large-scale projects in the context of space elevators is the effective terraforming and human habitation of Mars [3]. In Clarke’s The Fountains of Paradise, early opposition to the elevator concept led to a serious suggestion that the young Republic of Mars would not only finance the project, but very much needed it. Although the first tower is eventually built on Earth, the second will be Martian.

The suitability of Mars for space elevator construction is striking. First and foremost, the planet’s relatively low gravity (a third of Earth’s) makes the demands on the cable less demanding. It can be far shorter than one on Earth (17,000 kilometres to areosynchronous orbit, as opposed to 42,000 km to geosynchronous). It also requires far less strength, both to support its own weight and to lift freight. In addition, Mars has fewer atmospheric disturbances (storms etc) and surface disturbances (earthquakes) to threaten the lower portion of the elevator ribbon. It also boasts some conveniently located and very tall mountains close to the equator, which provide perfect anchor points. And finally, it has a moon - Deimos - that is perfectly located to act both as a source of material for the construction and as a counterweight to tension the cable. While the orbit of the lower moon, Phobos, presents more difficulties, various methods have been suggested for how a Martian elevator would avoid collision.

Humans on Mars also have a clear need for substantial freight transport. Any attempt to terraform the planet, or to settle it on large scales, will require the import of substantial amounts of water ice, as well as other resources, and likely also easy access to orbit for the construction of solar reflectors (in order to control the temperature of the planet). As a result, it’s perhaps no wonder that a number of Mars-based human settlement narratives explore this idea.

Perhaps the most prominent example here is Kim Stanley Robinson’s epic Mars trilogy: Red Mars, Green Mars and Blue Mars. In this century-spanning narrative, the first space elevator is constructed on Mars in the 2040s, and its space station terminal is named Clarke, and a ground terminal on the Pavonis Mons volcano (also discussed in Clarke’s original The Fountains of Paradise). Eventually Earth is also fully equipped with a large number of elevators, but the Martian construct is an essential proof-of-concept.

[Image (right): A speculative triptych illustration of a Mars space elevator by Da Vinci - Mars Design, based on descriptions in Kim Stanley Robinson's Mars trilogy.]

A more spontaneous development of an Areo space elevator can be found in the short story "Atom Drive" by Charles L Fontenay (1956, in Worlds of If magazine). Here an independent contractor is trying to break into a monopolised trade route between Earth and Mars. Finding himself refused access to the "G-boat" atmospheric reentry craft required to unload and reload his space-travelling freighter, a captain improvises with a length of cable lowered from Phobos - eventually concluding that the method devised is actually more efficient. Since the cable in this case does not have a ground anchor, it is arguably more of a skyhook, although the final lines suggest an elevator in the near future.

The idea is also discussed in Larry Niven’s science fantasy novel Rainbow Mars. Here time travellers from a distant future travel back to collect examples of past life, without realising that their travels place them into fantasy contexts - including a Burroughs-esque Barsoomian Mars which hosts a literal beanstalk, a “hang-tree”, converted to a space elevator [4]. Providing evidence for the longevity and attraction of the idea, the Martian elevator concept actually forms the focus of Terry Pratchett and Stephen Baxter’s science fiction novel The Long Mars, in which a mission through numerous alternative realities is launched based on the logical premise that in at least one alternative history, intelligent life must have emerged on Mars and have constructed such an elevator in its ascent to space.

A Ladder to the Stars

As expected, the space elevator eventually located in The Long Mars is an ancient ruin, dating to the earlier, warmer and moister phase of Martian evolution. However it has evidently served its purpose: it allowed the evacuation of the planet when it gradually became uninhabitable. This use of space elevators - to permit the resettlement of large populations - is also a recurring feature in science fiction.

While early space elevators are often focussed on freight or on their use as tourist attractions (as discussed in various of the examples above and the recent animated series Thunderbirds Are Go episode “Chaos (part II)” [5]), space elevators are perhaps the only practicable method of moving large populations on and off of planetary surfaces - due to both its high energy efficiency and the relatively slight accelerations required, compared to a rocket launch. Clarke explored this use in his writings. In the closing sequence of The Fountains of Paradise, much of humanity has used the elevators to migrate to other planets, or to reach an orbital ring. This hypothetical megastructure is constructed by linking space elevator orbiting terminals and other habitats in synchronous orbit into a continuous torus (either rigidly or through flexible transport links): a ring that can provide habitable space for millions or billions of people and is connected to the ground by elevators like the spokes in a wheel.

Often, in this context, the evacuation of the planet’s surface is not entirely voluntary. In Clarke’s Fountains of Paradise, and again, discussed in more detail, in his 3001: The Final Odyssey, humanity has effectively abandoned the Earth’s surface for such a habitat due to a combination of environmental pressures (either climate change or the onset of a new ice age) and the desire to preserve what wildlife remains on the planet.

Similarly, in the narrative sections of Terry Pratchett’s The Science of Discworld, the inhabitants of Roundworld (our Earth) eventually leave the planet’s surface through a network of elevators. This book fails to be neatly categorized. Factual science chapters (written by Ian Stewart and Jack Cohen) alternate with fictional chapters (written by Terry Pratchett). The fiction is technically fantasy, with the main cast being wizards from Pratchett’s Discworld. However the plot is science fictional, with the wizards observing our world and its development in accelerated time. In fact, humanity’s tenure on the planet is too brief for the wizards even to observe, before the onset of an ice age and the threat of comet impact causes the evacuation to the stars. The point is made here that on geological timescales, the surface of any planet is vulnerable to impacts, climate change and other damage - and that space elevator-enabled evacuation may be a necessary stage in ensuring the continued existence of intelligent life.

A Material Problem

If space elevators solve so many problems, with such obvious advantages, the question has to be asked: why has such a device not been constructed?

The first problem is one of scale, with associated cost: a space elevator is a structure tens of thousands of kilometres long (much larger than the Earth, as shown above), designed to support and lift billions of tons of material, through a variety of environments from thick planetary atmosphere to zero gravity vacuum. An asteroid or other object of comparable mass must be captured for the counterweight that tensions the string. This is not going to be a cheap endeavour. Even setting aside the technical challenges, the cost of such a device would be well beyond any project humanity has yet undertaken. Even the optimistic early novels described a space elevator as consuming the planet’s entire gross domestic planet for several years, or the resources of the solar system’s richest man. Certainly the commitment would far exceed that of the Apollo Moon landing programme, or any current space effort, although most of the construction is theoretically possible at our current or near-future level of technology.

However the bigger problem, and the real reason why none of the speculative space elevator design studies have been able to progress is one of materials science. Like many other science-fictional constructs, a space elevator cable would require properties that place it firmly in the category of unobtanium - a material beyond our current knowledge.

The first requirement is tensile strength - the cable must support not only the weight of any cargo travelling along it, but also its own weight. Given that this cable must extend of order fifty thousand kilometres (for an Earth elevator), be thick enough to survive impacts with space debris and carry enough electronics for the power supply and energy recovery systems, the weight of the cable itself would be sufficient to snap any currently known material. The second requirement is shear strength - resistance to the horizontal forces caused by winds, ground movement, and the lateral forces that arise when materials are ascending vertically rather than travelling at orbital speeds. A third requirement is robustness - when such a cable is constructed, the stored energy is such that any snapping or distortion of the cables could release a devastating catastrophe. We can add to that the requirements for hardening the electronics against radiation damage and currents due to atmospheric friction or space weather, the suppression of resonant vibration modes (like those that famously snapped the Tacoma Narrows bridge), and the need for frictionless (e.g. magnetic levitation) tracks to enable the structure to be traversed at reasonable speeds and thus in a reasonable time. This combination is currently well beyond any material humanity can currently construct.

However it is not quite beyond any we can imagine. In his early effort Charles Sheffield imagined a silicon cable with integrated circuitry extruded in a continuous strand from the spinneret of a robot he termed a Spider. In Fountains of Paradise Clarke described a crystalline carbon hyperfibre which could only be manufactured in orbit. However the discovery of a new form of carbon, C-60 or buckminsterfullerene, in 1985 provided new hope for space elevator advocates. The class of materials to which C-60 belongs, known as fullerenes includes carbon nanotubes (hollow tubes of carbon) which can, in theory, be extended to any length as a single molecule. As a result it has the potential for generating enormously strong fibres, particularly in terms of tensile strength, which are also highly conductive and very light. However the manufacture of such cables is currently restricted to the microscopic scale - a far cry from the scales needed for an elevator cable - and their strength against shear is unclear.

This hasn’t stopped them being widely adopted in fiction. In Clarke’s Firstborn, as in the earliest space elevator stories, spider analogies are used, with a set of machines by that label constantly ascending and descending the ladder to add to its thickness and repair any damage. This narrative also addresses the changing knowledge of materials science between the 1970s and the 2000s. Asked about the ladder’s composition, one character updates us:

“Fullerines. Carbon nanotubes. Little cylinders of carbon atoms, spun into a thread. Immensely strong. The whole ribbon is under tension; the Earth’s spin is trying to fling the counterweight away, like a kid swinging a rock on a rope. No conventional substance would be strong enough. So the spiders go up and down, weaving on extra strips, and binding it all with adhesive tape.”
Mechanical spiders, endlessly weaving a web in the sky (Gollancz 2009 edition, pg 55-56)

 The novel also notes that the freight capsules themselves are seldom returned to Earth, instead being added to the orbiting counterweight which tensions the cable, balancing the extra weight added to the cable, and giving it more freight capacity with a view to future use.

However other authors are less sanguine about our ability to find a suitable material without assistance. Larry Niven’s Rainbow Mars fantasises about a living tree capable of constructing its own carbon structure which remains beyond understanding. Jason M Hough’s Dire Earth trilogy (starting with The Darwin Elevator, 2013) imagines space elevators as alien constructs beyond the capability of humans, indeed as unexpected and uninvited intrusions which form part of a plan that also involves disease and near-extinction for humanity. Similarly alien is the elevator in Terry Pratchett and Stephen Baxter’s The Long Mars (novel, 2014), the whole novel and the quest at its heart is motivated solely by the effort to find a fragment of the long-defunct Martian elevator and bring it back to Earth for materials analysis. As a character explains: 

“If you used fine-grade drawn steel wire, say, you’d only be able to raise your cable through thirty miles or so before it would pull itself apart like taffy. That’s a long way short of twenty thousand miles. In the old days there was much fancy talk of special materials with a much higher tensile strength - graphite whiskers and monomolecular filaments and nanotubes.” [...] “But if we can take this cable stuff home - learn its lessons, retro-engineer it to find out how it works, enhance its performance for Earth’s conditions - we’ll skip decades of development and investment.”
[Corgi UK edition, 2015, pg 351]

As intractable as the technical problems are though, it’s also impossible to overlook a more human issue: the potential vulnerability of space elevators to terrorism. The elevators in Robinson’s Mars Trilogy, Clarke & Baxter’s Firstborn, Thunderbirds Are Go and the television adaptation of Asimov’s Foundation, as well as other examples such as the Transformers IDW comics and games including Warhammer 40K, are all the subject of terrorist attacks, ranging from attacks on their ground or space terminals to efforts to sever the cable. In the case of single-cable systems, this risks sending the counterweight flying out of orbit, while the remnants of the cable could cause devastation if they fall to Earth. While most space elevators in fiction involve a dual or quad-cable system, and could likely be repaired, the force imbalances, and the sheer investment required, would make these a tempting target. For those opposed to human space utilisation, with political grievances or with religious inclinations that consider the elevators a modern-day Tower of Babel and an affront to God, they would make promising targets.

As they have been for the last half-century, space elevators remain tantalisingly close to the possibilities offered by our current technology, and yet still out of reach. They may well be a prerequisite to a true human utilisation of space, and a necessary tool in the settlement and terraforming of Mars. They remain a playground for technical speculation and the dreams of engineers. And perhaps, if new materials can be found and the political will gathers, one day they can carry those dreams beyond the confines of this world.