Imagine a tether made out of super-tensile material that reaches from the surface of Earth to a station in geostationary orbit — aka. geosynchronous equatorial orbit (GEO). Transportation to and from orbit would be provided by high-speed vehicles instead of rockets, ensuring regular access to space for pennies on the dollar!
This fabulous idea is known as a Space Elevator, a proposed Earth-to-orbit transport system that could revolutionize space travel and exploration as we know it. While creating this structure would be a herculean feat of engineering and very expensive and time-consuming to boot, it would also allow for unprecedented long-term benefits.
The most notable benefit of such a structure is how it would significantly reduce the cost of sending payloads and crews to space. Even with the benefit of reusable rockets, it is still mighty expensive to send anything into orbit. The reason has to do with Earth’s gravity, which weighs in at a robust 9.8 m/s2 (32.174 ft/s2).
Because of this, a rocket has to achieve a velocity of 11,186 m/s (40,270 km/h; 25,020 mph) to escape Earth’s gravity (this is known as “escape velocity”). The amount of fuel needed to generate this velocity is considerable, which means huge rockets and massive propellant tanks are needed, which means more mass, which means more fuel is needed, etc.
In short, there’s a reason people say “space is hard.” Like many ambitious ideas that have the potential to revolutionize space exploration and life here on Earth, the idea of some type of space elevator is not new. In fact, one can trace its roots back to the late 19th century and Konstantin Tsiolkovsky — known by many as the “father of rocketry.”
Similarly, it’s only since the dawn of the Space Age that attempts have been made to develop and refine the idea. While progress has been made in several respects, the concept still hovers on the edge of possibility. For some scientists and engineers, the enduring challenges are enough to conclude that a Space Elevator will never be built (at least here on Earth).
For others, the possibilities that it will allow for are reason enough to keep the concept alive. On top of all that, people are working hard and taking risks to make it a reality. So what exactly is the deal with the Space Elevator? What makes it so appealing, challenging, and controversial as a concept? And if it is technically feasible, when can we expect one?
Space elevators 101
NASA defined the concept as follows in a 2000 report titled, “Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium“:
“A space elevator is a physical connection from the surface of the Earth to a geostationary Earth orbit (GEO) above the Earth ≈35,786 km in altitude. Its center of mass is at the geostationary point such that it has a 24-hr orbit and stays over the same point above the equator as the Earth rotates on its axis.”
Arthur C. Clarke offered another helpful definition during the address he made to the 30th International Astronautical Congress (1979), titled “The Space Elevator: ‘Thought Experiment,’ or key to the Universe?“:
“The space elevator (alias Sky Hook, Heavenly Ladder, Orbital Tower, or Cosmic Funicular) is a structure linking a point on the equator to a satellite in the geostationary orbit directly above it. By providing a ‘vertical railroad’ it would permit orders-of-magnitude reduction in the cost of space operations.
“The net energy requirements would be almost zero, as in principle all the energy of returning payloads could be recaptured; indeed, by continuing the structure beyond the geostationary point (necessary in any event for reasons of stability) payloads could be given escape velocity merely by utilizing the ‘sling’ effect of the Earth’s rotation.“
While many proposed versions of the Space Elevator exist, the basic structural elements are almost always the same. These include a Base (or Anchor), a Cable (or Tether), Climbers, Power Systems, and a Counterweight in space. In terms of the Base, the proposals alternate between mobile seagoing platforms or stationary land-based platforms.
The Cable (or tether) is a tensile structure that would carry the climbers from Earth to space, and the strength and thickness would need to vary along its length. Since the tension is greatest near the end of the cable (at GEO altitude), it needs to be thickest at this point and taper downwards closer to the surface.
Because of the stresses involved, the cable would need to have a very high tensile strength/density ratio. Based on various assessments, the material involved would need to have a strength of at least 100 gigapascals (GPa). Before the development of nanoengineered materials like carbon nanotubes and graphene, no known material could fulfill these requirements.
The Climbers are the cable cars that would be responsible for transporting crews and payloads to orbit. The climbers’ design depends on the number of cars on the cable at any given time, the cable design, and other requirements. Proposed Power Systems for these cars include solar power, nuclear reactors, and wireless or direct energy transfer.
The Counterweight could take the form of a captured asteroid or a spaceport positioned beyond geostationary orbit or a combination thereof. In fact, many proposals envision a space platform placed at GEO and the tether reaching beyond this, where it connects to the counterweight.
Like so many other high-minded space concepts, the first recorded mention of a Space Elevator was made by Russian/Soviet rocket scientist Konstantin Tsiolkovsky. In 1895, after witnessing the Eiffel Tower in Paris, he described a similar tower that reached an altitude of 22,370 mi (36,000 km) — the height of GEO:
“On the tower, as one climbed higher and higher up it, gravity would decrease gradually; and if it were constructed on the Earth’s equator and, therefore, rapidly rotated together with the earth, the gravitation would disappear not only because of the distance from the center of the planet, but also from the centrifugal force that is increasing proportionately to that distance.
“The gravitational force drops. . .
but the centrifugal force operating in the reverse direction increases. On the earth the gravity is finally eliminated at the top of the tower, at an elevation of 5.5 radii of the earth (36,000 km).”
However, Tsiolkovsky’s concept called for a compression structure, whereas modern concepts call for a tensile structure (or “tether”). The earliest recorded example of such a structure was made by Soviet/Russian engineer Yuri N. Artsutanov in 1959, who suggested deploying a tether from a geostationary satellite down into Earth’s atmosphere and anchoring it to the surface.
Artsutanov described his idea during an interview in 1960, which appeared in the Sunday supplement of Komsomolskaya Pravda, titled “To the Cosmos by Electric Train.” Comparing the idea to a rope tied around a stone, Artsutanov claimed that a similar effect could be achieved by attaching a “rope” to Earth’s equator.
Provided the rope was long enough and connected to a counterweight of sufficient mass, centrifugal forces would hold it taught. He even provided some rough figures for his readers:
“Thus it can be shown how long our “rope” to the cosmos must be — fifty or maybe sixty thousand kilometers! Yes, and the “load” suspended on it should be rather large — indeed, centrifugal force should equal the weight of the long cable at almost 40 thousand kilometers! But if this is done, there will emerge a direct cable away from Earth into the cosmos.”
In 1966, a team of American engineers led by John D. Isaacs of the Scripps Institute of Oceanography conducted a study titled “Satellite Elongation into a True ‘Sky-Hook.'” While they claimed that the concept was feasible in theory, their calculations showed that the tether would need to have a tensile strength twice that of any known material (including diamond).
In 1975, Jerome Pearson (an American engineer and space scientist) went further in a study where he described an “Orbital Tower” that could launch spacecraft using Earth’s rotational energy. Pearson’s reinvented version of the Space Elevator called for a counterweight extended out to a distance of 89,000 mi (144,000 km) and a tether that tapered upwards, being the thickest at GEO.
His analysis also considered the influence of the Moon’s gravity, Earth’s wind, and the stresses caused by moving payloads up and down the cable. However, these and other concepts continued to falter whenever the tether’s material was involved. Simply put, no known material was strong enough to handle the stresses and weight.
In 1999, NASA engineers Geoffery A. Landis and Craig Cafarelli made a presentation during the 46th International Astronautics Federation Congress (IAFC), titled “The Tsiolkovski Tower Re-Examined.” They proposed a compression tower paired with a tether to reduce the overall stress and tensile strength required.
In 1979, the famous scientist and sci-fi novelist Arthur C. Clarke introduced the idea of space elevators to a broader audience with the release of The Fountains of Paradise. In this story, engineers construct a space elevator on top of a mountain peak in the fictional island country of “Taprobane,” located south of the Equator.
In that same year, science fiction author Charles Sheffield released his first novel, The Web Between the Worlds, which also featured a space elevator. In 1982, Robert A. Heinlein released Friday, a novel that features two space elevators that are located in the nations of Equator (the “Quito Sky Hook”) and Kenya (the “Nairobi Beanstalk”).
Kim Stanley Robinson includes space elevators in both his Mars Trilogy, where Martian colonists build one to facilitate the arrival of people and payloads from Earth, and 2312, where planet Earth has several elevators reaching into orbit. John Scalzi’s 2005 novel, Old Man’s War, also depicts a “Beanstalk.”
In a biological version, Joan Slonczewski‘s 2011 novel The Highest Frontier depicts a college student ascending a space elevator constructed of self-healing cables composed of anthrax bacilli. The engineered bacteria can regrow the cables when severed by space debris. The Analemma Tower is a study for an inhabitable variant of a space elevator, proposed as the ‘tallest building in the world.’
Research and development
With the development of carbon nanotubes in the 1990s, engineers began to reconsider the Space Elevator as a viable concept. In 2000, David Smitherman of NASA’s Advanced Projects Office (APO) hosted a workshop at the Marshall Space Flight Center with o
ther scientists and engineers to discuss how carbon nanotubes could be used to realize a Space Elevator.
Their findings were published in a report titled “Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium.” The report addresses all related issues to building and maintaining such a structure and a consideration of both the benefits and challenges, with the ultimate determination that it is feasible.
With the support of the NASA Institute for Advanced Concepts (NIAC), Bradley expanded the scope of this study and considered other factors. In his final report, “The Space Elevator,” he offered an assessment on possible deployment scenarios, climber design, power requirements, and delivery, anchor systems, and how orbital debris and natural hazards could be mitigated.
Since that time, numerous attempts have been made by space agencies and the commercial space sector to develop the necessary technologies. In March of 2005, NASA’s Centennial Challenges program merged with the Spaceward Foundation to offer prizes of up to $400,000 for innovative ideas that would speed this development.
From 2005 to 2009, an annual competition known as Elevator:2010 was held that focused on climbers, ribbons, and power-beaming systems. In 2008, the first Space Elevator Conference was hosted in Redmond, Washington, which led to the formation of the International Space Elevator Consortium (ISEC), an affiliate of the National Space Society (NSS).
In 2012, the Obayashi Corporation in Japan announced their intention to build a space elevator by 2050 using carbon nanotube technology. Their overall plan called for a 59,650 mi (96,000 km) long tether that could accommodate a 30-passenger climber that would travel at ~125 mph (200 km/h) and reach GSO after a 7.5-day trip.
Since 2013, the International Academy of Astronautics (IAA) has issued reports highlighting the need for a Space Elevator, culminating in 2018 with the release of the “Road to the Space Elevator Era.” The report concluded that the ability to mass-produce single-crystal graphene (stronger than carbon nanotubes) means that a Space Elevator will be feasible soon.
In addition to being a bold feat of engineering, a Space Elevator offers a wide range of benefits. For starters, the savings it would offer over conventional rockets would expedite plans to commercial and even populate Low Earth Orbit (LEO), establish human settlements to Mars, and shave billions off of missions destined for deep-space destinations.
Between 1970 and 2000, launch costs remained fairly steady at an average of ~$8,400 per lb ($18,500 per kg). Today, thanks to reusable rockets, that price has dropped to between $640 and $1,235 per lb ($1,410 and $2,719 per kg). Based on various estimates, the cost of sending payloads to space using a Space Elevator could be as little as $113 per lb ($250 per kg).
In their “Sky-Hook” study, Isaacs and his colleagues summarized what these additional benefits were:
“In addition to their use for launching materials into space, such installations could support laboratories for observation of conditions in space at high altitudes; they could resupply energy or materials to satellites or spacecraft, collect energy or materials from space and the high atmosphere, support very tall structures on the earth’s surface, and others. There is no immediate limit to the total mass that could be retained near the l-day orbit by such a cable.”
Another major benefit is the way they would allow for the cheap deployment of things like space-based solar arrays. Solar power collection is not subject to intermittency in space because in space it is not affected by weather and the diurnal cycle, as on Earth. Solar energy collected in space could then be beamed to stations on Earth using microwave lasers or beams.
It would also speed the commercialization and industrialization of Low Earth Orbit, which would mean placing habitats, refueling stations, and manufacturing in orbit. Spacecraft could be manufactured cheaply in orbit and could launch to deep-space destinations with the need to launch materials or components into space.
Unfortunately, we cannot reap any of these benefits until we resolve a slew of challenges, and not all of them have to do with engineering (though they are legion!) Historically, the greatest challenge has been how to keep the tether taught while also ensuring that the mass is kept below a certain threshold.
While progress has been made in this area (thanks to the discovery of carbon nanotubes), there are still several stumbling blocks. For starters, researchers still can’t create nanotubes that are particular
ly long and have high tensile strength. The current record for single tubes still stands at just under 20 inches (50 cm) and 5.5 inches (14 cm) for “forests” of them.
Second, the very thing that makes carbon nanotubes so strong (their hexagonal covalent bonds) also poses a major problem in constructing a tether. When loaded to an extreme degree, these bonds become unstable and come apart, which would cause the tether to fray in the same way a stocking would.
Graphene and diamond nanothreads are a possible solution since their structure is not susceptible to fraying, and they present less of a problem when it comes to mass production. However, producing enough to compose a tether that reaches an altitude of about 22,236 mi (35,786 km) or beyond would be a very costly venture.
Second, there’s also the matter of the titanic forces the structure will have to deal with, like wind shear, storms, and hurricanes at lower altitudes, and micrometeoroids, and the Sun and Moon’s gravitational influence at higher altitudes. Add to that the stress of regularly sending pod cars up and down the tether, and the result could be oscillations that eventually turn violent.
Third, there’s the matter of orbital debris, which is already a considerable problem. The ESA estimates that there are currently 34,000 objects bigger than ~4 inches (10 cm) in diameter, 900,000 that measure between 0.4 to 4 inches (1 cm to 10 cm), and 128 million objects 0.4 to 0.04 inches (1 mm to 1 cm). Since objects in Earth’s orbit move at a velocity of ~4.8 mi/s (7.8 km/s) – 17,000 mph; 28,000 km/h — even the tiniest bit of debris can pose a high risk.
And of course, there’s the issue of cost, which is beyond the capacity of any one nation to build. The only way we could afford to create a space elevator in the foreseeable future is if the wealthiest nations of the world came together and committed to a multi-generational effort to build one and could agree on a common framework for administering and using it.
This raises the equally sticky issue of national sovereignty. The elevator would need to be built in neutral territory and protected at all times by an international body to protect it from terrorists attempting to sabotage it while also preventing anyone from seizing control or having exclusive access to it.
Like the structure itself, the challenges of building and operating a space elevator are massive! But considering the pay-offs of having one, it’s understandable why it continues to attract advocates and adherents. And as the requisite technologies continue to improve, we may find ourselves inching closer and closer to its realization.
And if building one for Earth is not possible, there is the possibility of building them on other celestial bodies where the gravity is lower (the Moon, Mars, etc.) But that’s a story for another day!