Because of the rotation of the Earth, the Space Elevator can be described (to Earth dwellers) as a tether that hangs from the ground and falls into the sky. (Take a moment to visualize it. It might help to cock your neck all the way sideways until your head is upside-down.)
˙˙˙ ʍou dn ʞɔɐq dıןɟ ǝsɐǝןd - ƃuıppıʞ ʇsnɾ - ʞo
The tether is very long and slender, is under tension, and extends through many environments: a storm-laden atmosphere, space-junk filled Low-Earth-Orbit, the Van-Allen belts... So of course it is very natural to ask "what happens if the tether breaks"?
Science fiction stories have depicted the collapse of a Space Elevator as a planet-wide catastrophe. This might make for an exciting storyline, but reality is quite different and a lot more mundane...
In fact, if the Space Elevator tether were ever to break, us Earth dwellers will be hard pressed to even know that it did.
- First and foremost, if the tether breaks, everything above the break-point will "fall" upwards, escaping into space. Since most of the dangerous environments are near the bottom of the tether, only a short bit will collapse back down to Earth.
- Remember that other than the atmospheric portion of the tether (the bottom 50 km) the rest is fashioned like a thin ribbon - closer to Saran Wrap than to a round tether - hardly a harbinger of doom. As it tries to go through the upper atmosphere, the ribbon will break into small pieces which will flutter to the water and then slowly sink.
- The tether falls along a miles-wide corridor pointing Eastwards of the anchor station. The nominal (20 ton) Space Elevator tether weighs about 10 grams (a third of an ounce) per meter, and so over any square kilometer, the amount of material will be miniscule.
- Perhaps the closest analog to a tether break is a small cargo ship sinking. While we do not like the idea of letting material loose in the ocean, as an aftermath of an extremely rare potential accident, it is an acceptable outcome.
- Carbon Nanotubes, to the best of our present knowledge, are best categorized as "irritants", and the amount of material involved does not pose a significant risk to anyone. Remember that there's a 100 miles "no fly" exclusion zone around the base of the Space Elevator anyway.
- Finally, there will 4-5 climbers on the tether. While the top ones may be able to remain in orbit, the rest will start falling towards the Earth. Manned climbers will of course have the ability to soft-land, and cargo climber might be allowed to simply splash down.
The biggest problem we'll face if the tether breaks is that the Space Elevator will be gone, and we'll have to build another one! For this reason, one of the first things that a Space Elevator will do is lift up a spare elevator, safely store away in orbit, and ready to be deployed in case the main one breaks.
A Space Elevator GEO Station/Hotel
(Artist: Alan Chan)
A Space Elevator Climber near LEO
(Artist: Alan Chan)
But will it break?
Of course, we'd like the tether to never break, or at the very least, to barely ever ever ever break....
This is how we ensure it will stay intact:
- Small Orbital Debris: Our #1 Enemy, small space junk is composed of anything from nuts and bolts to flecks of paint, traveling through low earth orbit at 8 km/sec and obviously eventually striking the tether. It is mostly because of this "orbital pollution" that the tether must be fashioned as a ribbon, so that small hits translate into small holes rather than catastrophic damage.
- Large Orbital Debris: The are larger pieces of space junk that can damage more than 10% of the ribbon. Luckily for us, these objects are fewer and farther between, and we are able to both track them and avoid them, primarily by moving the anchor point well in advance.
- Wear and Tear: The tether will see all sorts of gradual damage throughout its life: mechanical wear and tear, solar radiation damage, atomic oxygen degradation, micro-meteorite damage... As in all transportation systems, the infrastructure will be continuously inspected (by the climbers) and when necessary, bad segments can be replaced.
- Heat of re-entry: Spacecraft, when entering the atmosphere, travel at many miles per second, and so heat up to very high temperatures. The Space Elevator tether, on the other hand, is moving along with the Earth and atmosphere, so the only air velocity is that produced by wind.
- Storms: While the tether will be able to resist wind rather well, thunderstorms can prove fatal. This is another reason to move that anchor point - ship can move faster than storms, and so with the aid of weather maps, we can always remain in blue sections of the ocean. Hurricanes, of course, do not form at the equator.
- Bad People: One of the reasons the anchor of the Space Elevator is located at sea is that we can enforce a 100 mile no-fly no-boat not-even-swim zone around it, which makes it awfully hard for anyone to harm the ribbon from the ground.
- Stray Airplanes: Even at jetliner speeds, it takes 10 minutes to cross 100 miles. This is plenty of time to communicate with an airplane that crossed into the no-fly zone and is heading towards its center.
- Electric Fields and Potential Differences: True, there is a high voltage (potential difference) between the ground and the upper atmosphere, and also between the ground and the Ionosphere. The short answer here is that what matters is the voltage gradient, or the voltage per meter length - the high voltage is handled well by the very large resistance of the tether.
- Magnetic Fields and Induced Voltages Many tether experiments in orbit failed due to arcing that resulted from electrical induction - a phenomenon that occurs when a conductive wire moves through a magnetic field. In our case, there is no relative motion between the tether and the magnetic field, and so this problem will not occur.
- Oscillations: Even though we draw the tether as a straight line, it will in fact oscillate, much like a guitar string. These oscillations, however, will be very slow, and will not jeopardize the stability of the tether.
- Exploding Climbers: Our one Achilles heel. Climbers do not carry fuel, but if an on-board bomb will pierce the skin and send shrapnel in the direction of the ribbon, it has a good chance of downing the ribbon.
So we think we have most bases covered, but given what we know of the best-laid plans of mice and men, it is not inconceivable that a tether will break. The point to keep in mind is that we don't stop flying when an airplane crashes, or stop driving when a bridge collapses. If a Space Elevator tether ever does snap, we will simply be ready with a replacement one.