As humanity slowly ventures out into the cosmos, we are struggling to overcome the challenge of getting all our stuff into space. Rockets are impractical for the long term as they're prohibitively expensive, they require a ridiculous amount of fuel, and they're an environmental menace. The panacea, we've been told, is the much heralded space elevator — a massive Jack-and-the-Beanstalk-like structure that would reach up into space from Earth's surface.
Problem solved, right? Well, not exactly. Unfortunately, there are enough technological, logistical, and political hurdles in play that just might make the entire venture impossible — or if not impossible, extremely impractical. Here's why we'll probably never build a space elevator.
Image of space elevator by Kenn Brown of Mondolithic Studios.
Just to quickly recap, a space elevator would consist of a 22,000 mile (35,400 km) cable extending from the surface of Earth to geosynchronous orbit. Once anchored and counterbalanced, laser-powered climbers would ascend the cable, bringing their precious cargo into space.
It's an incredibly elegant solution, but as several researchers and experts have suggested, there are some potential deal-breakers that need to be addressed.
This is the big one. And I could probably leave it at this (but there are other factors to consider as well).
To better understand this particular challenge, I contacted Keith Henson, a technologist and engineer who has written about space colonies and related space engineering subjects for nearly four decades. In 1975, he co-founded the L5 Society, now known as the National Space Society. Henson, despite his enthusiasm for space colonization, is skeptical that a space elevator will ever get off the ground.
"No current material exists with sufficiently high tensile strength and sufficiently low density out of which we could construct the cable," he told me. "There's nothing in sight that's strong enough to do it — not even carbon nanotubes."
Indeed, this is the handy piece of evidence that's conveniently touted as the wonder-material that will make space elevators a reality. No doubt, these structures are the strongest and stiffest materials yet discovered in terms of tensile strength and elasticity — a strength that results from the covalent sp2 bonds formed between the individual carbon atoms.
"The best that theorists can do right now is come up with a material that's about two-thirds the strength needed to make a practical elevator," Henson told me. "And that's a very, very short tiny tube."
The problem, says Henson, is that when the carbon bonds get loaded to such an extreme extent, the hexagonal bonds that exist in carbon nanotubes become unstable when converting to 5-to-7 member bonds."
"It's not unlike a run in a lady's stocking," he says.
Henson worries that the cable, when exposed to such a tremendous strain, will simply unzip. Based on some preliminary models, the strain on the tether could exceed 100,000 kN/(kg/m) — so the material will have to have an extraordinarily large tensile strength/density ratio. Even with nanotechnology, he argues, it may not be possible to build material that's strong enough for the job. "It's not immediately obvious what can be done about this," he added.
"The bond strengths are known and you have a very limited number of bond strengths you can use around carbon," he says. "You can go outside of carbon and use boron nitride — it doesn't save you anything in weight — but it would conceivably be more resistant to this unzipping thing." He notes that no one has made nanotubes out of boron nitride.
"So, while it may be theoretically possible to get the material, it still looks pretty unlikely owing to the strengths of the bonds involved... the strength just seems inadequate."
It's worth noting that not everyone agrees with Henson. According to Bradley Edwards, a former Los Alamos physicist who has started several elevator-related companies in recent years, carbon nanotubes are up to the task. Science writer David Appell explains:
The discovery of carbon nanotubes breathed new life into the space-elevator idea, moving it from science fiction to high-level engineering studies. Being only 30% denser than water, and 32 times stronger than steel, carbon nanotubes have a theoretical breaking length of more than 10,000 km...
...Carbon nanotubes are microscopic: a pile of them looks like fine, black soot. The tensile strength of an individual tube with a single cylindrical wall has been measured as high as 120 GPa (1.2 × 1011 Pa) but in theory it could be up to 300 GPa. Bradley Edwards...thinks that about 130 GPa would be needed for a safe orbital tether.
But how could you make a 100,000 km-long structure from carbon nanotubes? Unfortunately, no-one knows, or at least not yet.
As a final note on this, the longest carbon nanotube that has ever been constructed is only a few inches long and a nanometer in width. Assuming that Henson is wrong, and that this material can in fact sustain such tremendous weights, it will be some time yet — if ever — before engineers can scale it up to something exceeding several thousand kilometers in length.
So until someone figures out the cable issue, the space elevator is propped-up on nothing more than highly conceptual vaporware and some convenient handwaving.
Another serious problem is that of radical cable movement and the potential for whipping action and vibrations.
Lubos Perek of the Astronomical Institute at the Czech Academy of Sciences has warned that gravitational tugs from the Moon and Sun, along with pressure from gusts of solar wind, will shake the cable. These unpredictable — and potentially violent motions — could veer the tether into satellite traffic or space debris (another serious problem we'll get into in just a bit).
Perek says the lack of resistance against buckling or bending will have a profound impact on the elevator's stability, both in its initial phase as a geostationary (GEO) satellite as well as in its operational phase as a "sling."
As a possible remedy, thrusters could be attached to the cable to compensate for any movement, but as Anders Jorgensen of the New Mexico Institute of Mining and Technology told New Scientist, that would be a "serious annoyance":
If it turns out that thrusters are needed on the cables, he says they could pose a serious challenge to building a space elevator. "I am sure that having thrusters hanging off the cable at regular intervals is going to be a serious annoyance in terms of maintenance, refuelling, and simply the logistics of attaching them and having the elevator bypass them."
Similarly, Bradley Edwards, who authored a detailed 2003 space elevator concept study for NASA, cautions against the idea of attaching thrusters to the tether, noting that the complexity of operating climbers with hardware on the ribbon is serious.
Lastly, vibrational harmonics may pose another problem. The cable will have a natural resonant frequency, and if excited (say, by the climbers), the vibrational energy could exceed tolerances. This could be remedied by using dampening systems — adding to a growing list of unwieldy and complex prescriptions.
The climbers themselves create another problem: Wobble. Thanks to the Coriolis force, which influences objects in a rotating system, the climber — and by consequence the cable itself — would be forced in the opposite direction of the Earth's rotation.
According to mechanical engineers Arun Misra of McGill University in Montreal, Canada, and Stephen Cohen, at MDA Space, this would pull the elevator away from its vertical resting position, causing it to oscillate back and forth like a pendulum.
As a consequence, even the smallest of deviations would cause a wobble, resulting in an end-point far removed from the intended orbit. The tether's swing would also boost or reduce the velocity of any spacecraft or object exiting the elevator. The engineers say this could put objects off track (either too high or too low) by as much as ten or more kilometers.
The solution: An agonizingly slow ascent by the crawlers, or the careful orchestration of multiple crawlers. In fact, scientists are estimating trips lasting almost a month.
There's also the hazardous stuff in orbit to consider.
"Even if you solve those problems you still have another problem to deal with — and that's all the space junk and active satellites," says Henson. "You've got to find it all and clean it up — and then you have to install dodging capabilities in all the existing satellites, except for the ones in geosynchronous orbit."
Henson says violent impacts with the cable would be a regular occurrence, and that most satellites and junk would be fast enough to "vaporize six or eight feet of the elevator."
Creating satellites with dodging capabilities is not a big problem, he says, it's just that every pre-existing one would have to be retired or re-configured.
"But you've got to clean all the old junk out as well because you can't move the cable around in any practical sense of the term — and there's 6,000 tons of junk up there," he says.
Cleaning up all that stuff is possible, argues Henson, especially if you build a big laser in space.
"If you're going to build a laser in space, however, you'll be using so much energy that you should just go ahead and raise the stuff via hydrogen rockets in place of an elevator," he adds.
There are non-technical aspects to consider as well. While these are not deal-breakers per se, they do present challenges for the planning and eventual construction of a space elevator.
It's quite possible, for example, that a space elevator could be the target of a terrorist attack. A successful operation would be extremely costly and result in tremendous damage. Defense measures and 24/7 surveillance would likely have to run in tandem with each elevator.
Environmentalists may also object to the space elevator on account of unknown consequences.
"If you're exporting more stuff than you're importing along the cable," says Henson, "you've got this rigid section between the center of the Earth and where the cable is anchored. As a result, the cable leans back in the sky dragging on the Earth." This, he says, will slow the Earth down, giving opportunity to an environmental group to mount a campaign saying that we need to "conserve angular momentum."
Henson is only half-joking. Even though the effect could only be measured in nanoseconds, he suspects someone will raise a stink about it.
The five problems listed here are the most serious, but they're not the only ones. Other concerns include:
- Meteoroids and micrometeorites
- Radiation and resulting ionization
- Journeys through the Van Allen belts (not human-friendly on account of dangerous radiation)
Now, all this said, the space elevator shouldn't be ruled out — at least not on Earth. Henson points to another option, one that was devised by the American engineer and space scientist Jerome Pearson.
"It turns out that, while it doesn't make a lot of sense for the Earth, the moon is a different situation entirely," Henson told me. "You'd think that the Moon doesn't rotate fast enough, but you can build a different kind of elevator — one that you could anchor in the Earth's gravity field."
In this scenario, a cable would be run from the moon and out through the L1 Lagrangian point. From there it would be dangled down into Earth's gravity field where it would have a large counterweight attached to its end.
"So, to lift a thousand tons per day off the lunar surface, it would take less than 100,000 tons of elevator to do it — which means it pays back its own mass in just 100 days, or somewhere between three and four times its own mass per year — which is not a bad rate of return."
And as for the materials required, the moon is a different matter. "You don't need nanotubes and very, very high strength materials. But the higher the strength, the more of the ratio you can get for hauling stuff on the moon," he says.
Henson imagines a configuration in which an endless loop of cable would be run from the lunar surface and back. "Then you just clamp payloads onto the thing and a 15 megawatt power plant will pull up around 1,000 tons per day. It should be 190,000 km long, because it turns out this distance is at the top end of the Hohmann Transfer Orbit."
More about space elevators on the moon here.
Needless to say, space elevators make the best kind of sense for building something on the moon. So it doesn't really solve our payload problem here on Earth.
Images: Miriam Maslo/Science Photo Library, Lifeport.