Space elevator

A space elevator is a proposed way to travel between a planet and space. It is also called a space bridge, a star ladder, or an orbital lift, and it is often shown in science fiction stories.

The main part of the system would be a long cable, also called a tether, that is fixed to the ground and reaches out into space. For an elevator on Earth, one end of the cable would be attached near the equator. The other end would be attached to a counterweight far beyond geostationary orbit, which is about 35,786 km above the Earth.

The cable would stay up and hold still over one spot on Earth because two forces balance each other. Gravity pulls down on the lower end. The centrifugal force from the Earth's spinning pulls up on the upper end. These two pulls keep the cable stretched tight.

Once the tether was in place, climbers, sometimes called crawlers, could move up and down it using machines. They would carry cargo up to orbit and bring it back down.

The design would let vehicles travel straight between a planet's surface and orbit without the use of large rockets.

History

The idea of the space elevator seems to have grown separately in different places and times. The earliest ideas came from two Russian scientists in the late 1800s. In 1895, Konstantin Tsiolkovsky imagined a huge sky ladder to reach the stars. His idea was a tower standing up from the ground, not a cable hanging down from above like today's idea.

In 1960, Yuri Artsutanov described a "Cosmic Railway" that would connect an orbiting satellite to the equator. In 1966, John D. Isaacs and his team proposed the "Sky-Hook." In 1975, Jerome Pearson started studying the idea after a 1969 speech by Arthur C. Clarke. His "Orbital Tower" design was meant to carry supplies to low Earth orbit. In his paper in Acta Astronautica, the cable was thickest where the pull was strongest. He suggested a counterweight of 144,000 kilometers (89,000 miles). Building it would have needed thousands of Space Shuttle trips. Materials could later be sent up or made in space from asteroidal or lunar ore.

Pearson's work helped Clarke write his novel The Fountains of Paradise (1979). Later, Pearson found that a space elevator could also be built on the Moon, reaching Lagrange Points. In 2000, D.V. Smitherman, Jr., concluded that the elevator could not be built for at least 50 more years. Around this time, Bradley Edwards suggested using a thin ribbon of carbon nanotube material to solve the strength problem. He received support from the NASA Institute for Advanced Concepts. He also studied problems such as orbital debris and how the cable could survive atomic oxygen.

In the 21st century, supporters held several competitions, similar to the Ansari X Prize, to help speed up development. These included Elevator:2010 and NASA's Centennial Challenges program. In 2005, the LiftPort Group announced a carbon nanotube factory in Millville, New Jersey. The factory was never built, and by 2019 the company had made little progress. In 2007, a team from MIT entered a small carbon nanotube piece into a competition, but no team won. In 2012, the Obayashi Corporation said it could build a space elevator by 2050.

In 2013, the International Academy of Astronautics reported that a better tether material, with the needed specific strength, was the most important improvement still needed. In 2014, Google X began designing a space elevator but later paused the project. In 2019, the International Academy of Astronautics found that Earth space elevators seemed possible. It also said new materials, such as graphene, might help development move forward. Graphene has a higher specific strength than carbon nanotubes.

Materials

One of the biggest challenges in building a space elevator on Earth is finding strong materials. The structure has to support its own weight and the load it carries. So the specific strength, meaning how strong a material is compared to its weight, must be extremely high.

Since 1959, most space elevator ideas have used tensile structures. These are held up from above by centrifugal forces. A space tether would stretch from a large counterweight beyond geostationary orbit down to the ground, held tight like an upside-down plumb bob. The cable would be thickest at geostationary orbit and thinnest at the ground. The idea could also work on other planets and celestial bodies. On places with weaker gravity than Earth, such as the Moon or Mars, today's materials like Kevlar are strong and light enough for an elevator cable. On Earth, though, today's materials are not yet strong and light enough. Some experts think improvements to carbon nanotubes (CNTs) could make a practical design possible. Others believe CNTs will never be strong enough. Other possible materials include boron nitride nanotubes, diamond nanothreads, and large single-crystal graphene.

In fiction

In 1979, space elevators became known to more readers. Arthur C. Clarke published the novel The Fountains of Paradise. In it, engineers build a space elevator on a mountain peak in the made-up island country of "Taprobane," which is loosely based on Sri Lanka. That same year, Charles Sheffield published his first novel, The Web Between the Worlds, which also tells of building a space elevator. Three years later, Robert A. Heinlein's novel Friday mentions a disaster at the "Quito Sky Hook" and uses the "Nairobi Beanstalk" during her travels. Other books include Kim Stanley Robinson's Red Mars, where colonists build a space elevator on Mars, and Larry Niven's Rainbow Mars, which also describes one built on Mars. In David Gerrold's Jumping Off The Planet, a family trip up the Ecuador "beanstalk" becomes a frightening story about a child being taken away.

The space elevator, called the Beanstalk, also appears in John Scalzi's Old Man's War. In Joan Slonczewski's The Highest Frontier, a college student climbs an elevator built from self-healing cables made of anthrax bacilli. These cables can regrow when space debris cuts them. In Andy Weir's Project Hail Mary, the Eridians use a space elevator to reach orbit, since their planet's atmosphere makes regular launches and landings nearly impossible. The science fiction show Foundation features a space elevator called the "Star Bridge" on the home world of Trantor. Finally, the film The Wandering Earth 2 includes scenes where a working space elevator is attacked.

Physics

An Earth space elevator cable turns with the Earth. Because of this, the cable and objects on it feel an upward centrifugal force. This force pushes against the downward pull of gravity. Higher up the cable, gravity is weaker (Newton's law of universal gravitation) and the upward push is stronger. At geostationary orbit, about 35,786 km (22,236 mi) above Earth, the two forces balance. Objects there do not pull on the cable. Above that height, objects pull the cable upward. Below it, they pull the cable downward. The total force on objects attached to the cable is called the apparent gravitational field.

The cable must be strong enough to hold up its own weight. The greatest pull on it is at geostationary orbit, where a thicker cable is needed. So its width is an important design choice. Designers aim for a safety margin and an even stress along its length. Stronger materials make the cable much lighter. The International Space Elevator Consortium proposed a unit called the "Yuri," named after Yuri Artsutanov. It measures how strong a material is for its weight. A working space elevator would need a tether of 30–80 megaYuri. This measures the breaking force per length of cable, in newtons and kilograms. Materials that are weak for their weight would need a very wide cable at the top. That would make the cable extremely heavy and expensive.

Structure

Space elevators have been proposed for many planets. Most designs have a base station, a cable, climbers, and a counterweight. On Earth, the planet's spin pulls the counterweight upward with centrifugal force. The cable holds the counterweight down, and the counterweight keeps the cable up and tight. The base station anchors the system to the surface, and climbers carry cargo up and down the cable. A base station could be a moving ship or platform that can avoid high winds, storms, and space debris. Ocean anchor points are usually in international waters, which makes the area simpler to use. A land-based station is easier to supply and can be built high up. Another idea is a tower with a lower part that is pushed together and an upper part that is pulled tight, which would ease the strain on the cable.

The cable must hold its own weight and the weight of the climbers. The strength it needs changes along its length. It is thickest at geostationary altitude and thinner near the ground. The cable needs a very strong and light material with a high tensile strength/density ratio, also called breaking lengths. Materials such as carbon nanotubes and graphene are expected to be strong enough, while metals like steel are not. Carbon is useful because it is the sixth element in the periodic table, with few protons and neutrons. In 2014, diamond nanothreads were first made and were quickly seen as possible cable material. Climbers would move up a stationary cable, and they would need wireless energy transfer, such as laser power beaming, to power them. Climbers would also need to move at the right speed to keep the cable steady. A counterweight could be a captured asteroid, a space station, or a spaceport beyond geostationary orbit.

Applications

Pearson noted that a space elevator could also throw a spacecraft farther out, using centrifugal force from Earth's spinning. An object released at about 53,100 km would reach escape velocity. Releases at 50,630 and 51,240 km could reach the Lagrangian points L1 and L2, and a release at 50,960 km could reach lunar orbit. At the end of Pearson's 144,000 km cable, the speed would be enough to leave Earth and send probes at least as far as Jupiter. There, a gravitational assist could help them reach solar escape velocity. A space elevator could also be built on other worlds. On Mars, a cable could be much shorter than on Earth, because Mars has weaker gravity, and today's materials are already strong enough. The Martian moon Phobos would make building one harder, but it might supply useful resources, since it is thought to contain a lot of carbon.

Phobos is tide-locked to Mars, so one side always faces it. A 6,000 km cable from that side would end about 28 kilometers above the Martian surface, just outside the denser parts of the atmosphere of Mars. A similar cable on the opposite side would counterbalance it. The whole system would stay below areostationary orbit of Mars. A rocket would still be needed to reach the bottom of the elevator, needing about 0.52 km/s of Delta-v. The Earth's Moon is also a possible site for a Lunar space elevator, because the specific strength needed for the cable is low enough for materials available today. An elevator would pass through the Earth-Moon L1 point or the L2 point, depending on where it was anchored.

Rapidly spinning asteroids or moons could use cables to fling materials toward Earth orbit or a Lagrangian point. Freeman Dyson, a physicist and mathematician, suggested using such small systems as power generators far from the Sun, where solar power is not practical. According to Francis Graham of Kent State University, a space elevator using today's materials could connect worlds that are locked to each other, such as Pluto and Charon, or the parts of the double asteroid 90 Antiope, with no break at the end.

Building a space elevator would first need to reduce some technical risks, and some advances in engineering, manufacturing, and physical technology are needed. Once the first one is built, later space elevators could use it to help with construction, which would make them much cheaper. Early ideas had the cable made in space from an asteroid or a Near-Earth object, but this needed a large space-faring infrastructure. Since 2001, most work has focused on simpler methods. In these, a long cable is wound on a large spool, parked in orbit, and lowered toward Earth while a balancing mass moves away from it. When the cable reaches the equator, it is anchored, and more tension is added so it can be used as an elevator cable. One plan uses rockets to launch a small starter cable that climbers gradually thicken into a 750-ton cable able to lift 20 tons per climber.

Early systems would take about five days to reach the level of geosynchronous orbit. Passengers would need shielding from radiation in the Van Allen radiation belts, which would add weight. A space elevator could also be a hazard to aircraft and spacecraft, so air-traffic control might need to limit flights nearby. Objects whose perigee is below the cable's top height would eventually hit it unless they avoid it. One proposed solution is a movable anchor that lets the cable dodge large space debris. Small meteoroids and debris could also strike the cable, so it would need to move out of the way or absorb small impacts. Space elevators might also make sending materials into space far cheaper. As of 2022, conventional rockets cost about US$12,125 per kilogram (US$5,500 per pound) to reach geostationary orbit. Current proposals suggest prices as low as $220 per kilogram ($100 per pound). This is similar to the $5–$300/kg estimates for the Launch loop, but higher than the $310/ton to 500 km orbit quoted to Dr. Jerry Pournelle for an orbital airship system. Philip Ragan, co-author of the book Leaving the Planet by Space Elevator, states that "The first country to deploy a space elevator will have a 95 percent cost advantage and could potentially control all space activities."

International Space Elevator Consortium (ISEC)

The International Space Elevator Consortium (ISEC) is a non-profit organization in the United States. It was formed to help build and run a space elevator, which it describes as "a revolutionary and efficient way to space for all humanity". It was formed after the Space Elevator Conference in Redmond, Washington in July 2008. In August 2013, it became an affiliate organization of the National Space Society. ISEC holds a Space Elevator conference every year at the Seattle Museum of Flight.

ISEC works with two other major groups that focus on space elevators: the Japanese Space Elevator Association and EuroSpaceward. Each year, it also supports talks and presentations at the International Academy of Astronautics and the International Astronautical Federation Congress.

Related concepts

The idea of a "Space Elevator" has changed over time. Tsiolkovsky's first idea was a structure rising up from the ground, like an aerial mast. It might reach space, but probably not geostationary orbit.

Today's main idea is a tight, fixed structure anchored to the ground. It reaches well beyond geostationary orbit. Cargo could climb up it and be released into orbit. In 1977, Hans Moravec described a spinning cable called a Skyhook. It could catch fast-moving objects and lift them into orbit, or raise objects in low orbit higher.

Other ideas use very tall towers that may reach above the atmosphere. Vehicles could launch from the top and need less power. The aerovator was invented by a Yahoo Group and described in a 2009 book. It is a long ribbon held up by aerodynamic lift and spinning force. Aircraft pull on it, and payloads climb up and launch from its fast-moving tip. Other ideas for non-rocket spacelaunch include an orbital ring, a space fountain, a launch loop, a skyhook, a space tether, and a buoyant "SpaceShaft".