Futuristic frontier cities become less far-fetched.

Don’t sell your house just yet, but the technology that would allow humans to live in space or under the sea is taking shape.

One reason to move to the depths would be the pursuit of resources to meet the world’s ever-expanding development needs. Rare-earth metals — used in powerful magnets and a host of electronic devices — are believed to exist in abundance in parts of the seabed.

Japanese general contractor Shimizu is one company trying to make the dream of an underwater city a reality. In November last year, Shimizu floated the concept of a deep-sea colony called Ocean Spiral. The plan has three parts: a residential structure near the surface; a spiral-shaped tunnel descending to the seafloor; and a factory at the bottom, some 3,000 to 4,000 meters underwater.

Subway to the deep

The living space would be a sphere, 500 meters in diameter and capable of accommodating up to 5,000 people — 4,000 residents and 1,000 visitors. It would house hotels and commercial facilities

Think of the tunnel as a vertical subway line, through which humans and materials would travel up and down. The tunnel would be a critical channel for transporting oxygen and fresh water to the factory at the bottom. This plant would mine rare earths as well as develop technology for converting carbon dioxide emitted on the surface into methane gas. Resources developed on the seafloor could be put to use in the residential space; eventually, the entire operation might become self-sufficient.

Showa Denko has suggested using special concrete to construct the residential sphere. Known as resin concrete, this material is made with synthetic resin in the hardening agent, in place of cement. This makes the concrete lighter and impermeable to water. The downside? It is more expensive than regular concrete. Though it is already employed in some marine engineering work, the high cost of using it to build a large structure is a big hurdle.

Building Ocean Spiral could require several hundred workers. Since it would be impractical for the workers to continually travel to and from the offshore construction site, marine accommodations would need to be built nearby.

Unless, of course, human workers were left out of the process: Shimizu is contemplating a process similar to 3-D printing to build the residential sphere — an automated method of pouring concrete layer by layer from the bottom up.

Ocean Spiral is just one blueprint; Shimizu might opt to pursue a smaller deep-sea endeavor. The company says it is targeting construction costs of up to several trillion yen. It hopes to create a marine hub for cutting-edge industry sometime between 2030 and 2050.

As Shimizu looks beneath the waves, others are gazing upward.

Creating a livable colony on another planet is still considered a long way off. But the idea of connecting Earth to a space station via an elevator is starting to gain traction.

The space elevator concept has been around for a long time. It can be traced to the late-19th century Russian scientist Konstantin Tsiolkovsky, who looked into the possibility of building a tower to the heavens. Today, researchers are considering not a tower but a cable rising into the sky. This cable would guide a vehicle called a climber up and down, carrying people and materials.

The problem with the plan was always the lack of materials capable of withstanding tremendous opposing forces — gravity, which pulls objects toward Earth, and the outward centrifugal force created by our planet’s rotation. Steel would promptly snap.

Late in the 20th century, a potential answer emerged: carbon nanotubes, which have a tube-shaped molecular structure measuring around 1 nanometer in diameter. Despite weighing one-fifth as much as steel, they are reportedly more than 10 times as strong and do not break when pulled.

A carbon nanotube cable would be just 1.38mm thick and 18 to 48mm wide, but 96,000km long. When viewed from space, it would appear to be piercing Earth’s surface.

Because rockets must overcome gravity, they require massive amounts of fuel. Launches are also risky. Theoretically, a space elevator would be safer and require less fuel, easing the burden on the environment. It could also bring Mars and other celestial bodies within easier reach, since the journey would begin not from Earth but in outer space.

According to the Japan Space Elevator Association — an organization dedicated to realizing the concept — it costs 1.05 million yen ($8,690) to transport 1kg of material into space aboard Japan’s H-IIA rocket. Sending a spaceship into high orbit on an elevator, then launching it from there, would reduce the cost to 10,000 yen per kilogram, the association estimates.

Japanese construction company Obayashi is studying plans to build a space station that could be connected to Earth via an elevator. In stationary orbit — where centrifugal force and gravitational pull offset each other — a state of weightlessness occurs. This makes it “easier to create large structures,” said Yoji Ishikawa, who belongs to Obayashi’s space elevator component technology team.

Akebono Brake Industry says its technology could be used in the climber. High-performance materials maker Teijin is providing its fibers for use as cables in a space elevator competition in Japan. The contestants send payloads up a tether held aloft by balloons.

Researchers still need to devise the technology for assembling carbon nanotubes to a length of some 100,000km. Obayashi expects this to take until around 2050 and cost around 10 trillion yen.

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