Over the last hundred years, variations on the idea of space elevators have been suggested, explored and determined to be impossible with available technology over and over again. The early 21st century, is no different. People continue to propose the idea and we continue to have nothing remotely like the technology needed to make such an artifact. Nonetheless, belief in this chimera continues to siphon off funding, and talent from actually feasible plans. Meanwhile, boondoggles such as space elevators give all of us who desire a sustained human presence in space the reputation of wild-eyed dreamers with our heads in the clouds.
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Endeavors in space are often made difficult by a variety of circumstances here on the ground. These include technical and economic limitations, politics, and the changing and contradictory goals that different parts of the US government have for our space program. One of the things that has held both public and private efforts in space back is the perception that Space is nothing but pie-in-the-sky projects of little or no practical value. This perception is demonstratively incorrect, but as in the case of most stereotypes, it has a tiny kernel of truth to it. There are certain ideas for space exploration or space development that are repeatedly suggested and discarded and then continuously revisited. These ideas were usually popularized with some piece of classical science fiction, and consequently have a dedicated fan-base that is in love with them despite clear and easy proof that they can not work.
If space is going to fulfill our hopes then we are going to have to give up on these pie-in-the-sky ideas. They are intrinsically impractical, and will remain so for centuries and possibly forever. However, this does not stop them from drawing off intellectual focus and even funding from real viable projects. Further, as a result of guilt by association, they make ALL of us who believe in Space seem like crack-pots, and ALL space ideas to be nothing but pie-in-the-sky dreams that could never work.
Therefore, I have decided to explode some of the most egregious and common examples of these perennial boondoggles in the hopes that the intellectual focus, funding, and political good-will directed at space will not be diverted and wasted in these impractical distractions.
The Idea: Space Elevators.
The idea of the space elevator was first suggested by Russian scientists Konstantin Tsiolkovsky (1895) and and then later by Yuri N. Artsutanov(1959). As a concept, it was popularized by Science Fiction writer Arthur C. Clark in his 1976 novel The Fountains of Paradise. Since 1895, the idea has changed and evolved considerably; the modern version is this: A cable made of super-strong material would connect a small asteroid in orbit with the surface of the Earth. The center of mass of the cable + asteroid would be in geostationary orbit with the asteroid acting as a counter-weight to the mass of the elevator cable itself. The idea is that, once in place, cheap and easy access to space would be as simple as riding an elevator car that would climb up and down the cable.
In order to understand space elevators we need to understand a few very basic facts about orbits. A satellite that orbits close to the Earth moves around the Earth faster than a satellite that orbits higher. There is a certain altitude where the angular velocity of a satellite orbiting the Earth is equal to the angular velocity of the Earth's rotation about it's axis (360 degrees of rotation every 24 hours). This is called geostationary orbit, and is an orbit at an altitude of 35,786 km (22,236 mi). Satellites in a circular orbit around the Earth, at that altitude, in the plane of the equator, orbiting in the same direction as the rotation of the Earth remain stationary in the sky relative to an observer on the ground. The way most people are most directly exposed to the properties of geostationary orbit is satellite TV, you'll notice that the receiving dish always points at the same part of the sky. This is because the broadcasting satellite is in geostationary orbit.
Because the counter-weight asteroid of a space-elevator must orbit above geostationary orbit, and the vast majority of the cable is below geostationary orbit, the counter weight alone would orbit slower than the whole assembly, and the cable alone would orbit faster than the whole assembly. It is only the physical strength of the cable that prevents the whole assembly from tearing itself apart with the lower parts crashing into the Earth and the higher parts spinning out into space. The ability of a material to withstand pulling-tension like that is called "tensile strength". Others have calculated
that the cable of a space elevator, assuming it has a density similar to graphite (a good stand-in for carbon-nanotube based materials), must have a tensile strength of about 65-120 GPa. For a more complete description of a Space Elevator from the point of view of a proponent of the idea go here
Let me put 65-120 GPa in perspective. The strongest steel cables have a tensile strength of 5.5 GPa. The best carbon-nanotube fibers curently in existence have tensile strengths on the order of 20 GPa. Individual carbon-nanotubes have sufficient tensile strength, but they are microscopic. A cable produced of many of them is only as strong as the nanotube-nanotube bonds. There are simply no materials with a tensile strength remotely high enough to function as a space elevator cable. This is the first big problem that a space elevator technology must overcome: It requires a cable 3-6 times stronger than the strongest materials known to man. Unfortunately, this is not the last or the smallest problem for the idea of space elevators. NASA has created a centennial challenge to create stronger cable materials
The cable must meet other stringent requirements beyond being merely the strongest material ever invented. It must also be able to withstand the wear and tear of the elevator cars going up and down it. (The ability to resist abrasion is a separate quality from having a great tensile strength). The cable must be also able to withstand terrestrial weathering including but not limited to lightning-strikes, freezing-rain, salt-spray from the ocean, wind, sleet, snow, and hail. The cable must be able to withstand cosmic and solar radiation without losing its other material properties. It must also be able to survive impacts from micro-meteors and space-junk. The material must be strong enough to do all these things AND be 12-30 times stronger than steel!
Even if such a material existed, there are a number of failure modes that the space elevator could sustain. Impacts from large meteors, Earth-orbiting satellites, or aircraft could all lead to catastrophic failures of the cable's physical integrity. Electrical discharge along the length of the cable from the upper part of the ionosphere to the surface of the planet would make using the elevator hazardous and would have unforeseeable environmental effects. For this reason the cable must be electrically resistant. However, to withstand lightning strikes it must be conductive like a lightning rod. To avoid this contradiction, and to try and address some of the many other problems listed above, elevators are typically envisioned having a mobile anchor-point on the Earth's surface: a ship at sea. This solution does not provide as much as it seems however since the more the ship moves away from the equator, to avoid a thunder-storm for instance, the more stress is put on the cable.
Yet another potential problem is vibrations along the cable which will unavoidably vibrate like a guitar string. Like any string, it will have a resonant frequency. If something induces a vibration at that frequency or one of its harmonics, the resulting vibration could rip the entire elevator apart. Proponents of the elevator concept seem to think that vibrations could be avoided or wouldn't be very strong, but all they have to do is be strong enough to sink the ship at the Earth-side anchor point. Resonance and other oscillation effects have been responsible for the destruction
of massive structures before
, and are surprisingly difficult to avoid completely. Possible sources of vibration in a space elevator include but are not limited to: the climbing of cable-cars, tidal forces from the Moon's gravity, atmospheric wind, solar wind, expansion and contraction from solar heating, and electromagnetic effects. Some of these can be controlled or avoided, but most can't.
So far, I've only discussed the problems with making the cable itself, but other aspects of this idea have large problems as well. We have never maneuvered an asteroid into Earth orbit before. While I believe this could be done, it is just one example of how a space elevator is an engineering task outside the experience-base of any human achievement in history. Likewise, the cable-cars could not carry the fuel needed to power their assent, so energy would have to be transmitted to them from the ground or from the counterweight station at the top of the elevator. Wireless power transfer is in its infancy. There are people improving this capability
, and if this were the only technical stumbling block, I'd think that space elevators might eventually work. Sadly however, the transfer of megawatts of power over tens of thousands of miles (distances larger than any power-grid here on Earth) represents one of the easier challenges of building a space-elevator. Also, all the energy that is transmitted to the cable-car must eventually become waste-heat that the cable car must dissipate without roasting its cargo or damaging the cable.
This brings us to the most obvious problem with the idea of a space elevator: The sheer scale of such a project! Normally I approve of thinking big, especially for space projects. Large scale achievements, such as sending men to the Moon, elevate and inspire the entire human race. However, we are talking about building an object longer than the circumference of the Earth! The engineering challenges of such an endeavor are unpredictable at best. We literally have no way of guessing all the ways something could go wrong.
When something does go wrong, it could be disastrous for a substantial fraction of the population of the planet. If the cable were to break (everything breaks eventually) the lower portion will fall to Earth. When it comes down, it will come down hard... never mind the people/cargo on the elevator when it breaks, they were effectively lost the moment the break occurred. We're still talking about a 36,000 km rope that will mass many thousands of metric tons. The center of mass of the whole system before a cable-break will be in geostationary orbit (that's what makes the elevator idea conceivable in the first place). However, once the break occurs, the tension that was holding the elevator straight would be released. As a result, the lower portion of the cable would begin to wrap around the planet. Initially the impact of the cable will be minor. The parts that hit first won't have fallen for long and thus won't have that much speed/impact energy stored up. However, the more the cable wraps around the Earth, the faster it is going (since the Earth has turned more beneath it, and it has fallen further. Because the energy being released is not just potential energy turned into gravitational acceleration of the cable fragment but also the rotational energy stored as tension in the cable before the break, terminal velocity does not apply in the same way it would for other falling objects). This means that each subsequent mile of tether strikes with more force than the last... this in effect might become a traveling explosion that could circumnavigate the globe several times depending on how much of the cable was on the bottom portion after the initial break.
To be fair, the exact severity of such a cable fall is highly dependent upon the physical properties of the cable material. If it is a material similar to those available today, in order to make the elevator feasible it would have to be a tapered cable with very substancial mass at its thicker sections. Alternatively if the cable materials is one that doesn't yet exist, in the 120+ GPa range of tensile strength, then the cable fall is much less dangerouse since it will mass less. It will however still be moving at a phenomenal rate relative to the ground. That speed might cause it to burn-up in the atmosphere as it falls, or it might not depending on the heat-sensitivity of the cable material. The initial assumption is that the cable material would have to be extremely resistant to heat to prevent waste-heat from the cable cars from destroying it as they climb, but perhaps alternative solutions to that problem could be reached. The exact dynamics of how a cable-fall would happen are complex
, but suffice it to say, depending upon many variables, it has the potential to be a disaster of global proportions.
Meanwhile, the upper portion of the cable, including the counter weight will shift its orbital path dramatically as it's center of mass has effectively moved. This might swing it out of Earth orbit completely or turn it into a miles long whip of cable potentially swatting satellites out of the sky. If it is swung out of Earth orbit, there is the potential for it to cause a disaster later on as it will still have a very Earth-like orbit around the Sun and therefore might collide with the Earth at some later point.
To summarize, a space elevator requires materials and technologies well beyond anything we currently have. It is a project larger than anything humans have ever managed to accomplish. It has a large number of known failure modes, and almost certainly possesses ones we can't even imagine. If a catastrophic failure of the elevator does occur, the consequences could easily be global in nature. No matter how you slice it, this comes out as an incredibly poor ratio of risk to reward, and that's assuming it's possible in the first place, which with anything like current technology, it most certainly is not. Even people who think this could work are beginning to come down to Earth now that they are faced with the sheer scale of the problems that must be overcome to make a space elevator.
I do not feel that I can strike down the idea of the space elevator without suggesting a reasonable alternative in its place: In my opinion, the best such alternative is Electromagnetic Launch
. The idea of electromagnetically launching objects into orbit is another one that has been popularized by science fiction, but where a space elevator is fundamentally infeasible with current technology, electromagnetic launching requires no technology or materials that we don't already have and could launch payloads into orbit for about $100 a pound. That's about the same price it is estimated that an elevator could offer.
Electromagnetic launches would subject the payload to extreme stresses, so this is definitely not the way to launch people. However there are three very important classes of cargo that it could launch:
- Water. The International Space Station currently creates oxygen for its crew to breath by electrolyzing water. Resupply of that water constitutes a significant fraction of the mass that is shipped to the International Space Station each year. Water has no moving parts to break. No electric components to be fried by the intense fields created by launching this way. The tank would have to survive the trip, but it too need not have any moving parts. Making a hollow container that can survive acceleration is not intrinsically difficult, especially when one considers that it won't be launched empty, and water is fairly resistant to compression.
- Liquid or solid fuel. The same basic reasoning applies to fuel as applies to water. If you can do it with water, you can do it with kerosene. If you can do it with liquids, then doing it with solids is even easier.
- Steel I-Beams. Again, no moving parts, not electrical components, nothing fragile. In my opinion, it's the ability to launch large amounts of steel that could make this launch system transformative.
In conclusion, space elevators may never happen, and they certainly won't happen soon. However, they draw talent, funding, and public-support away from feasible ideas that CAN work.
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