3 Challenges for Engineering A Space Elevator
Vincent Smith posted on June 22, 2017 |
Examining our best prospects for building materials, ensuring stability and combating orbital debris...
Artist's conception of a NASA space elevator.
Artist's conception of a NASA space elevator.
The notion of an elevator to space dates back more than a century. However, it’s only in the last few decades that the concept has received serious attention from scientists and engineers. Interest in the space elevator has been bolstered by the hope for cost-effective methods to put Earthly products and resources into orbit.

Despite igniting the imaginations of inventors and engineers alike, the space elevator is not without its critics.

This is to be expected as the concept represents one of the most audacious space-based projects since the Apollo Program. We are truly in uncharted territory when it comes to the sheer scale of such a structure.

So, can we build a space elevator?

To answer that question, let’s look at three of the most common criticisms levied against the idea.

1. Space Elevator Materials

Artist rendering of Space Elevator. (Image courtesy of Japan Space Elevator Association.)

Artist rendering of Space Elevator. (Image courtesy of Japan Space Elevator Association.)

The number one challenge quoted by most space elevator skeptics is our lack of materials that can meet the strength to density properties necessary to construct the primary tether. To give you an idea of the forces at play, a functional space elevator would have to have a center of mass at the point of geostationary orbit, 22,236 miles above the Earth’s equator. Hence, the cable would need to be capable of supporting the tension from the surface up to a counterweight far beyond the geostationary orbit.

The centrifugal force exerted on the elevator’s counterweight, past the point of geostationary orbit, is what 'holds up' the whole apparatus, keeping the cable taut. This constant tension on the cable raises crucial questions about its durability.

Combine this with the need to keep the cable’s mass as low as possible, and even our most cutting-edge materials fall short. Even carbon nanotubes (CNTs), the wonder material of the 21st century, fail to pass muster in this respect.

To make matters worse, researchers have yet to find a way to synthesize CNTs on a scale even remotely approaching what would be needed for the cable: so far, the longest nanotube, created by researchers at Tsinghua University in Beijing, is a whopping half meter long.

But even if we could find a way to mass produce carbon nanotubes, the same arrangement of carbon atoms that makes them so strong also poses a major barrier to applying them to the construction of a cable, as noted by long-time engineering writer Keith Henson at Gizmodo.

The problem is that when the covalent bonds that give nanotubes their strength get loaded to an extreme degree the hexagonal bonds become unstable. Even when converting to 5-to-7 member bonds, the bonds become unstable and begin to come apart at the seams, like a run in a stocking.

One alternative to conventional CNTs are so-called 'diamond nanothreads', which, like CNTs, are allotropes of carbon. With diamond nanothreads, the carbon atoms are arranged in a pyramid-shaped, tetrahedral formation which grants the structure incredible hardness.

John Badding, part of the team that initially constructed the nanothreads, explains:

Essentially, the structural “thickness” of the nanothread, even on the atomic level, would make it significantly less susceptible to unravelling when subjected to extreme forces.

A further benefit is that these threads address the mass-production problem. A team at Oak Ridge National Laboratory has already managed to produce them on a macroscopic scale by manipulating the pressures exerted upon a sample of liquid benzene.

As of this writing, it appears that diamond nanothreads are the best chance we have to develop a space elevator cable, but there are still unanswered questions.

For instance, although they are exceptionally strong and light, the threads are also supposedly stiffer than similar materials. That lack of flexibility could be a problem in the long run, resulting in less of a run in a stocking and more of a dead twig snapping.

This raises the next common criticism against space elevators.

2. Forces Acting on a Space Elevator

Building a space elevator is hard enough, but the difficulties don't end once we get it aloft.

The entire structure would have a whole host of forces to contend with, including gravitational tugs from the Sun and Moon and the aforementioned tension being exerted on the cable.

Weather patterns down here on Earth would only add to these issues, with storms and hurricanes buffeting the cable to-and-fro at lower altitudes. Indeed, the very operation of the elevator itself—i.e., the motion of the machinery used to convey payloads—has some skeptics worried about subsequent oscillations that could very well turn violent.

Since there are several concerns here, it's best to break down the issue into several parts to give each its due. The most readily solvable issue, according to a NASA feasibility report by Bradley Edwards, concerns hurricanes and other atmospheric phenomena.

The report acknowledges the damage that such weather patterns could cause and suggests simply building the base of the elevator in an area of the world where they are nearly nonexistent: namely, off the Pacific coast of Ecuador.

However, the report also acknowledges the presence of Murphy's Law in any engineering undertaking. In the event we to have to contend with wind-related damage, the report suggests altering the width to thickness ratio of the cable in order to cut down on wind resistance, reducing the forces' impact on the structure.

As the car climbs, the cable takes on a slight lean due to the Coriolis force. The top of the cable travels faster than the bottom. The climber is accelerated horizontally as it ascends by the Coriolis force which is imparted by angles of the cable. The lean-angle shown is exaggerated. (Image courtesy of Skyway/Wikimedia Commons.)
As the car climbs, the cable takes on a slight lean due to the Coriolis force. The top of the cable travels faster than the bottom. The climber is accelerated horizontally as it ascends by the Coriolis force which is imparted by angles of the cable. The lean-angle shown is exaggerated. (Image courtesy of Skyway/Wikimedia Commons.)
Given the previous suggestion of creating an anchor off of a coastline, another proposition for dealing with instability is to make the earth-bound anchor mobile. Proponents of this idea say that in order to compensate for the movement of the cable, the anchor could be moved in tandem with it.

However, this could create issues if the elevator's climbers were powered using a microwave laser, as NASA has previously suggested. The movement of the cable and anchor point could pose problems for reliably aligning a laser from ground zero to the photovoltaic cells on the climber over such a long distance.

The number of variables at play make such a power source difficult to implement, but Leo Golubovic and Steven Gnudsen at West Virginia University have generated an alternative. In their paper, the two suggest a climber that would avoid the alignment problem and use the cable’s instability to its advantage by actually harnessing the cable’s motion to power its own ascent.

Another alternative is a Rotating Space Elevator (RSE) is based on a slack cable that forms an ellipse-like shape. It would rotate in a quasi-periodic manner, using a combination of geosynchronous rotation around Earth and forces perpendicular to it in order to power its ascent. It’s an innovative potential alternative to dealing with problems of the traditional elevator design.

(Image courtesy of Leo Golubovic and Steven Gnudsen.)
(Image courtesy of Leo Golubovic and Steven Gnudsen.)
The final source of troublesome motion we'll discuss are the elevator climbers themselves.

Critics, such as Stephen Cohen and Arun Misra at McGill University, believe that the very operation of moving cargo could generate a dangerous whipping motion; enough to cause the elevator to tear itself to shreds or send it careening into other space debris. Since this remains a pertinent issue along the entire length of the cable, solutions aren't as easily forthcoming as those for problems confined to the extremes.

One solution that’s often brought up is the use of thrusters: small rockets attached along points of the cable that could help counterbalance and reposition it in the event of any untoward movement. It is not a remedy with many supporters, however. Additionally, adding thrusters to the elevators does somewhat defeat the purpose of the project.

The New Mexico Institute of Mining and Technology's Andre Jorgensen has said that the institution of thrusters would present a 'significant' annoyance in its operation, bringing with them the need for refueling, maintenance and all of the risks of weather and debris already present for the cable itself.

Edwards has also cautioned against their use, saying that the redesigns necessary to maneuver climbers around the thrusters as they ascend could prove unfeasible.

Ultimately, all of these precautions may prove unnecessary. According to the calculations in Section 10.8 Edwards' report, given proper counterweights, oscillations of the elevator cable would only move into hazardous territory if cargo ascending it approaches speeds in the realm of 10,000km (6,000mph). Still, given the scale of a space elevator and all the commensurate risks, it seems better to be safe than sorry.

3. Debris

(Image courtesy of NASA.)
(Image courtesy of NASA.)
There's a lot of junk orbiting Earth. Thousands of hours have been poured into previous NASA missions, ensuring the least possible contamination by even the tiniest motes of dust and dirt. The kinds of instrumentation that would monitor a space elevator would need to be similarly discerning. However, the fact that it would be a permanent fixture means that sooner or later, a space elevator would cross paths with meteors and even remnants of previous space missions left behind as space debris.

The extreme of this phenomenon even has a name: Kessler Syndrome, where the density of low earth debris becomes so large that nothing can pass it safely into outer space. This cascading problem of space debris collisions was featured in the film Gravity.

As Bullock and Clooney can tell you, this phenomenon could cause catastrophic damage to the overall structure (or knock it off balance, returning to our 'oscillation' concerns).

Edwards recognized this, and devoted an entire section of his report to addressing it. According to the report, part of dealing with this obstacle is recognizing and tracking low-earth orbit objects large enough to do damage to the structure.

According to Section 10.3 of the report, “A study was done at Johnson Space Center on the construction of a system that could track objects down to 1cm in size with 100m accuracy using effectively current technology. This is very close to the tracking network we would need for the space elevator.”

For situations in which avoidance is not always possible (the amount of low-earth orbit debris increases significantly from altitudes of approximately 300 to 1,000 miles), Edwards posits that increasing the thickness of the cable will make it robust enough to withstand all but the largest of objects, which could be tracked and avoided ahead of time using the systems previously mentioned.

The result of a hypervelocity impact, simulating the potential effect of space debris on orbital assest. (Image courtesy of European Space Agency.)
The result of a hypervelocity impact, simulating the potential effect of space debris on orbital assets. (Image courtesy of European Space Agency.)
Even for these exceptional pieces of debris, Edwards illustrates in a section simply labeled “Meteors” that only (i) direct impact by an object (ii) over 3cm in diameter, (iii) with enough force to stay on the initial plane of impact (as opposed to being deflected or redirected by contact with the elevator apparatus), would create the kind of catastrophic damage that we associate with a complete severing of the cable. Designing the cable with curvature and panels specifically for deflection has been proposed by both Edwards as well as several other survivability reports, including this one, put together for the 2010 International Space Elevator Consortium (ISEC). Definitive answers as to the effectiveness of these measures are hopefully forthcoming, but it's at least comforting to know that there are first, second, and third lines of defense prepared for just such occasions.

Will We Ever See a Space Elevator?

The Obayashi Corporation is aiming to build a sea-based space elevator by 2050. (Image courtesy of Obayashi Corporation.)
The Obayashi Corporation is aiming to build a sea-based space elevator by 2050. (Image courtesy of Obayashi Corporation.)
That there are a multitude of risks in building and operating a space elevator is uncontroversial. However, that sentiment applies to any engineering project that pushes the boundaries of our technological development. When considering the amount of obstacles in front of the elevator project (and there are many), it's important to take a step back and take a look at the situation in context.

What matters isn't necessarily how difficult the challenge is, but, in true engineering fashion, the question we must ask, “Are these obstacles greater than those of the alternatives?”

Is the return on investment (ROI) worth it?

Even if the answer is still “No,” it’s no longer an unqualified one. There are obstacles to building a space elevator, but overcoming obstacles is what engineering is all about.

What do you see as the biggest obstacle to building a space elevator? Share your thoughts in the comments below.

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