Space elevator
A space elevator, also referred to as a space bridge, star ladder, and orbital lift, is a proposed type of planet-to-space transportation system, often depicted in science fiction. The main component would be a cable anchored to the surface and extending into space. An Earth-based space elevator would consist of a cable with one end attached to the surface near the equator and the other end attached to a counterweight in space beyond geostationary orbit. The competing forces of gravity, which is stronger at the lower end, and the upward centrifugal pseudo-force, which is stronger at the upper end, would result in the cable being held up, under tension, and stationary over a single position on Earth. With the tether deployed, climbers could repeatedly climb up and down the tether by mechanical means, releasing their cargo to and from orbit. The design would permit vehicles to travel directly between a planetary surface, such as the Earth's, and orbit, without the use of large rockets.
History
Early concept
The idea of the space elevator appears to have developed independently in different times and places. The earliest models originated with two Russian scientists in the late nineteenth century. In his 1895 collection Dreams of Earth and Sky, Konstantin Tsiolkovsky envisioned a massive sky ladder to reach the stars as a way to overcome gravity. Decades later, in 1960, Yuri Artsutanov independently developed the concept of a "Cosmic Railway", a space elevator tethered from an orbiting satellite to an anchor on the equator, aiming to provide a safer and more efficient alternative to rockets. In 1966, engineer and oceanographer John D. Isaacs and his colleagues developed the concept of the ' Sky-Hook', proposing a satellite in geostationary orbit with a cable extending to Earth.Innovations and designs
Space elevator research advanced further in America in 1975 when Jerome Pearson began studying the idea, inspired by Arthur C. Clarke's 1969 speech before Congress. After working as an engineer for NASA and the Air Force Research Laboratory, he developed a design for an "Orbital Tower", intended to harness Earth's rotational energy to transport supplies into low Earth orbit. In his publication in Acta Astronautica, the cable would be thickest at geostationary orbital altitude, where tension is greatest, and narrowest at the tips to minimize weight. He proposed extending a counterweight to 144,000 kilometers, as without a large counterweight, the upper cable would need to be longer due to the way gravitational and centrifugal forces change with distance from Earth. His analysis included the Moon's gravity, wind, and movable payloads as factors. Building the elevator would have required thousands of Space Shuttle trips, though material could be transported once a minimum strength strand reached the ground or could be manufactured in space from asteroidal or lunar ore. Pearson's findings, published in Acta Astronautica, caught Clarke's attention and led to technical consultations for Clarke's science fiction novel The Fountains of Paradise, which features a space elevator.The first gathering of multiple experts who wanted to investigate this alternative to space flight took place at the 1999 NASA conference 'Advanced Space Infrastructure Workshop on Geostationary Orbiting Tether Space Elevator Concepts' in Huntsville, Alabama. In August 2000, D.V. Smitherman, Jr., published the findings under the title Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium, concluding that the space elevator could not be built for at least another 50 years due to concerns about the cable's material, deployment, and upkeep.
B. C. Edwards suggested that a paper-thin ribbon of a carbon nanotube composite material could solve the tether issue, due to its high tensile strength and low weight. The proposed wide-thin ribbon-like cross-section shape, instead of earlier circular cross-section concepts, would increase survivability against meteoroid impacts. With support from the NASA Institute for Advanced Concepts, his work involved more than 20 institutions and 50 participants. The Space Elevator NIAC Phase II Final Report, in combination with the book The Space Elevator: A Revolutionary Earth-to-Space Transportation System summarized all effort to design a space elevator including deployment scenario, climber design, power delivery system, orbital debris avoidance, anchor system, surviving atomic oxygen, avoiding lightning and hurricanes by locating the anchor in the western equatorial Pacific, construction costs, construction schedule, and environmental hazards. Additionally, he researched the structural integrity and load-bearing capabilities of space elevator cables, emphasizing their need for high tensile strength and resilience. His space elevator concept never reached NIAC's third phase, which he attributed to submitting his final proposal during the week of the Space Shuttle Columbia disaster.
21st-century advancements
To speed space elevator development, proponents have organized several competitions, similar to the Ansari X Prize, for relevant technologies. Among them are Elevator:2010, which organized annual competitions for climbers, ribbons and power-beaming systems from 2005 to 2009, the Robogames Space Elevator Ribbon Climbing competition, as well as NASA's Centennial Challenges program, which, in March 2005, announced a partnership with the Spaceward Foundation, raising the total value of prizes to US$400,000.The first European Space Elevator Challenge to establish a climber structure took place in August 2011.
In 2005, "the LiftPort Group of space elevator companies announced that it will be building a carbon nanotube manufacturing plant in Millville, New Jersey, to supply various glass, plastic, and metal companies with these strong materials. Although LiftPort hopes to eventually use carbon nanotubes in the construction of a space elevator, this move will allow it to make money in the short term and conduct research and development into new production methods." Their announced goal was a space elevator launch in 2010. On 13 February 2006, the LiftPort Group announced that, earlier the same month, they had tested a mile of "space-elevator tether" made of carbon-fiber composite strings and fiberglass tape measuring wide and thick, lifted with balloons. In April 2019, Liftport CEO Michael Laine admitted little progress had been made on the company's lofty space elevator ambitions, even after receiving more than $200,000 in seed funding. The carbon nanotube manufacturing facility that Liftport announced in 2005 was never built.
In 2007, Elevator:2010 held the 2007 Space Elevator games, which featured US$500,000 awards for each of the two competitions, as well as an additional $4,000,000 to be awarded over the next five years for space elevator-related technologies. No teams won the competition, but a team from MIT entered the first 2-gram, 100-percent carbon nanotube entry into the competition. Japan held an international conference in November 2008 to draw up a timetable for building the elevator.
In 2012, the Obayashi Corporation announced that it could build a space elevator by 2050 using carbon nanotube technology. The design's passenger climber would be able to reach the level of geosynchronous equatorial orbit after an 8-day trip. Further details were published in 2016.
In 2013, the International Academy of Astronautics published a technological feasibility assessment, which concluded that the critical capability improvement needed was the tether material, which was projected to achieve the necessary specific strength within 20 years. The four-year-long study examined multiple facets of space elevator development, including missions, development schedules, financial investments, revenue flow, and benefits. It was reported that it would be possible to operationally survive smaller impacts and avoid larger impacts, with meteors and space debris, and that the estimated cost of lifting a kilogram of payload to GEO and beyond would be $500.
In 2014, Google X's Rapid Evaluation R&D team began designing a Space Elevator, eventually finding that no one had yet manufactured a perfectly formed carbon nanotube strand longer than a meter. They thus put the project in "deep freeze" and also keep tabs on any advances in the carbon nanotube field.
In 2018, researchers at Japan's Shizuoka University launched STARS-Me, two CubeSats connected by a tether, on which a mini-elevator will travel. The experiment was launched as a test bed for a larger structure.
In 2019, the International Academy of Astronautics published "Road to the Space Elevator Era", a study report summarizing the assessment of the space elevator as of summer 2018. The essence is that a broad group of space professionals gathered and assessed the status of the space elevator development, each contributing their expertise and coming to similar conclusions: Earth Space Elevators seem feasible, reinforcing the IAA 2013 study conclusion Space Elevator development initiation is nearer than most think. This last conclusion is based on a potential process for manufacturing macro-scale single crystal graphene with higher specific strength than carbon nanotubes.
Materials
One of the most significant challenges in manufacturing a space elevator for Earth is the strength of the materials utilized for its construction. Considering the structure must be designed to be sufficiently strong to hold up not only its own weight but also the payload it may carry, the strength-to-weight ratio, or specific strength, of the elevator's construction material needs to be extremely high.Since 1959, most ideas for space elevators have focused on purely tensile structures, with the weight of the system held up from above by centrifugal forces. In the tensile concepts, a space tether reaches from a large mass beyond geostationary orbit to the ground. This structure is held in tension between Earth and the counterweight like an upside-down plumb bob. The cable thickness is tapered based on tension; it has its maximum at a geostationary orbit and the minimum on the ground.
The concept is applicable to other planets and celestial bodies. For locations in the Solar System with weaker gravity than Earth's, the strength-to-density requirements for tether materials are not as problematic. Currently available materials are strong and light enough that they could be practical as the tether material for elevators there.
Currently available materials are not sufficiently strong and light enough to make the construction of an Earth space elevator practically feasible in the near future. Some sources expect that future advances in carbon nanotubes could lead to a practical design. Other sources believe that CNTs will never be strong enough. Possible future alternatives to carbon nanotubes include materials like boron nitride nanotubes, diamond nanothreads, and macro-scale single-crystal graphene.