3 Challenges for Engineering A Space Elevator

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

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 21^st 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:

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.

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.

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

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.