With one end of a steel cable hovering in Earth's orbit and the other end somewhere in outer space, the concept of a futuristic floating "space elevator" promises to amplify humans' ability to explore the universe — and scientists engineering an improved take on the 19th-century idea say the one-time fantasy is close to becoming a reality.
"Technical-wise, it's kind of ready," said George Zhu, a professor of mechanical engineering at York University and a coauthor of a new study on the idea. "It just has small engineering [adjustments], and there's no fundamental difficulty to do that."
Zhu's paper, published March 17 in Acta Astronautica, refines his foundational design for a mechanically feasible space elevator, which has several applications for space-related missions. It suggests that instead of previous concepts employing only one cable, or tether, there should be two attached, which can maintain opposite forces while transporting cargo in parallel.
Such a mechanism addresses the potential risk of the cable wiggling, or experiencing libration, from Earth's rotational force, the Coriolis force.
"Our idea is, when we put two tethers together, one cargo will be moving up and the other cargo will be moving down, so the forces will cancel each other," Zhu said. "They stabilize, and it's more efficient, because you have a loop. You can go up and down at the same time."
Scientists have already built elevator prototypes, and the researchers behind the new paper sent a version into space last year.
Proposed as a thought experiment in 1895 by a Russian aviation engineer, Konstantin Tsiolkovski, space elevators were initially imagined as much more akin to their name. The bottom would be fixed on Earth, the top in space, and the elevator would directly transport things into the universe.
But a major problem remained with the original vision.
"It required a supermaterial," Zhu explained. "That material doesn't exist yet, so that idea is currently not engineering-feasible."
Tsiolkovski's concept involved placing the tether on the ground, but Earth's gravity would create extreme tension in the cable, causing it to break or tumble. Overall, its existence was improbable.
However, Zhu and his team have been capitalizing on a much newer idea for a space elevator, proposed 40 or 50 years ago: one that is floating. The Earth-based end would be in the planet's orbit, where gravity would no longer pose a threat.
The proposed tether can be made with titanium and other types of steel already used in rocket science, and the length of the elevator could range from a few meters to several thousand kilometers long.
"Because both ends are floating in space, the tension within the tether is limited," Zhu said. "Current material can support that tension."
In fact, many companies have already entertained the idea of a functional space elevator, including a private organization called Tethers Unlimited and the Japan Aerospace Exploration Agency. China even plans to build one by 2045.
In the mid-1960s, NASA and an Italian space agency revisited the concept, according to Zhu. He said that today, "If NASA says they're willing to do it, it's doable."
Once engineered, such a development could make it significantly easier to travel to the moon and transport supplies to astronauts. It even has potential to address the growing issue of space debris accumulation.
Zhu explained that at present, when a rocket is launched directly into space from Earth, 98% of its capacity is spoken for by the fuel that's needed to make the trip. That leaves only 2% for supplies or people.
Expensive and inefficient, it's the only method humans have — for now, at least.
"If you use the space elevator, you can use the same rocket and double or triple — maybe even 10 times more — the payload sent to space," Zhu said.
In this scenario, the rocket would theoretically attach to the space tether using a system very similar to one NASA uses today to connect smaller rockets to the International Space Station when sending supplies every three months.
Once the rocket is successfully mounted on the tether, the tether would be able to push it along through space, limiting the amount of fuel required primarily to what is needed to get to Earth's orbit, where the tether's end is located. (Space is only about 62 miles above sea level.)
The team published a paper in December explaining how this idea could be applied for astronauts headed to the moon. In that case, the other end of the tether would be in the moon's orbit.
However, Zhu noted one limitation for the tether system — which is also a point of concern for NASA.
"If [space debris] is too close to the International Space Station, that station needs to do some maneuvering to avoid the direct impact of the debris," he said. "For the space elevator, if you have the several thousand meter-long tether, it would be difficult to do maneuvering to avoid the debris."
In response, the team proposes using the tether system to combat space debris, as well. It's a timely plan, as nearly 60% of Earth's 6,000 satellites are non-operational, and scientists are seeking innovative ways to clean up the planet's orbit.
"We study a special kind of space tether called an electrodynamic tether," Zhu said. "That can convert the satellite's kinetic energy into electrical energy, and like a motor moving in a magnetic field, the line is moving in Earth's magnetic field. It will generate a force that can accelerate the de-orbit of the spacecraft."
In November, Zhu's team launched a small satellite to validate the idea for an electrodynamic tether. While they're still running tests, they don't expect it to have any impediments.
In the long term, the researchers believe this tether could be revolutionary.
"More forward-thinking is when people consider space mining," Zhu said. "We can mine on the moon, and take those materials back to Earth."
The paper, "Libration suppression of partial space elevator by controlling climber attitude using reaction wheel," published March 17 in Acta Astronautica, was authored by Gefei Shi, Sun Yat-sen University; and George (Zheng) H. Zhu and Gangqiang Li, York University.
Correction: A previous version of this article incorrectly described Konstantin Tsiolkovski's profession. The error has been corrected.