A space elevator is a proposed transportation system connecting the Earth's surface to space. The elevator would allow vehicles to travel to orbit or space without the use of rockets. While elevator travel wouldn't be faster than rocket travel, it would be much less expensive and could be used continuously to transport cargo and possibly passengers.
Konstantin Tsiolkovsky first described a space elevator in 1895. Tsiolkovksy proposed building a tower from the surface up to geostationary orbit, essentially making an incredibly tall building. The problem with his idea was that the structure would be crushed by all the weight above it. Modern concepts of space elevators are based on a different principle - tension. The elevator would be built using a cable attached at one end to the Earth's surface and to a massive counterweight at the other end, above geostationary orbit (35,786 km). Gravity would pull downward on the cable, while centrifugal force from the orbiting counterweight would pull upward. The opposing forces would reduce the stress on the elevator, compared with building a tower to space.
While a normal elevator uses moving cables to pull a platform up and down, the space elevator would rely on devices called crawlers, climbers, or lifters that travel along a stationary cable or ribbon. In other words, the elevator would move on the cable. Multiple climbers would need to be traveling in both directions to offset vibrations from the Coriolis force acting on their motion.
Parts of a Space Elevator
The setup for the elevator would be something like this: A massive station, captured asteroid, or group of climbers would be positioned higher than geostationary orbit. Because the tension on the cable would be at its maximum at the orbital position, the cable would be thickest there, tapering toward the Earth's surface. Most likely, the cable would either be deployed from space or constructed in multiple sections, moving down to Earth. Climbers would move up and down the cable on rollers, held in place by friction. Power could be supplied by existing technology, such as wireless energy transfer, solar power, and/or stored nuclear energy. The connection point at the surface could be a mobile platform in the ocean, offering security for the elevator and flexibility for avoiding obstacles.
Travel on a space elevator would not be fast! The travel time from one end to the other would be several days to a month. To put the distance in perspective, if the climber moved at 300 km/hr (190 mph), it would take five days to reach geosynchronous orbit. Because climbers have to work in concert with others on the cable to make it stable, it's likely progress would be much slower.
Challenges Yet to Be Overcome
The biggest obstacle to space elevator construction is the lack of a material with high enough tensile strength and elasticity and low enough density to build the cable or ribbon. So far, the strongest materials for the cable would be diamond nanothreads (first synthesized in 2014) or carbon nanotubules. These materials have yet to be synthesized to sufficient length or tensile strength to density ratio. The covalent chemical bonds connecting carbon atoms in carbon or diamond nanotubes can only withstand so much stress before unzipping or tearing apart. Scientists calculate the strain the bonds can support, confirming that while it might be possible to one day construct a ribbon long enough to stretch from the Earth to geostationary orbit, it wouldn't be able to sustain additional stress from the environment, vibrations, and climbers.
Vibrations and wobble are a serious consideration. The cable would be susceptible to pressure from the solar wind, harmonics (i.e., like a really long violin string), lightning strikes, and wobble from the Coriolis force. One solution would be to control the movement of crawlers to compensate for some of the effects.
Another problem is that the space between geostationary orbit and the Earth's surface is littered with space junk and debris. Solutions include cleaning up near-Earth space or making the orbital counterweight able to dodge obstacles.
Other issues include corrosion, micrometeorite impacts, and the effects of the Van Allen radiation belts (a problem for both materials and organisms).
The magnitude of the challenges coupled with the development of reusable rockets, like those developed by SpaceX, have diminished interest in space elevators, but that doesn't mean the elevator idea is dead.
Space Elevators Aren't Just for Earth
A suitable material for an Earth-based space elevator has yet to be developed, but existing materials are strong enough to support a space elevator on the Moon, other moons, Mars, or asteroids. Mars has about a third the gravity of Earth, yet rotates at about the same rate, so a Martian space elevator would be much shorter than one built on Earth. An elevator on Mars would have to address the low orbit of the moon Phobos, which intersects the Martian equator regularly. The complication for a lunar elevator, on the other hand, is that the Moon doesn't rotate quickly enough to offer a stationary orbit point. However, the Lagrangian points could be used instead. Even though a lunar elevator would be 50,000 km long on the near side of the Moon and even longer on its far side, the lower gravity makes construction feasible. A Martian elevator could provide ongoing transport outside of the planet's gravity well, while a lunar elevator could be used to send materials from the Moon to a location readily reached by Earth.
When Will a Space Elevator Be Built?
Numerous companies have proposed plans for space elevators. Feasibility studies indicate an elevator won't be built until (a) a material is discovered that can support the tension for an Earth elevator or (b) there's a need for an elevator on the Moon or Mars. While it's probable the conditions will be met in the 21st century, adding a space elevator ride to your bucket list might be premature.
- Landis, Geoffrey A. & Cafarelli, Craig (1999). Presented as paper IAF-95-V.4.07, 46th International Astronautics Federation Congress, Oslo Norway, October 2-6, 1995. "The Tsiolkovski Tower Reexamined". Journal of the British Interplanetary Society. 52: 175-180.
- Cohen, Stephen S.; Misra, Arun K. (2009). "The effect of climber transit on the space elevator dynamics". Acta Astronautica. 64 (5-6): 538-553.
- Fitzgerald, M., Swan, P., Penny, R. Swan, C. Space Elevator Architectures and Roadmaps, Lulu.com Publishers 2015