Space elevator safety
There are risks associated with never-done-before technologies like the construction and operation of a space elevator. A space elevator would present a navigational hazard, both to aircraft and spacecraft. Aircraft could be dealt with by means of simple air-traffic control restrictions. Impacts by space objects such as meteoroids, satellites and micrometeorites pose a more difficult problem for construction and operation of a space elevator.
If nothing were done, essentially all satellites with perigees below the top of the elevator would eventually collide with the elevator cable. Twice per day, each orbital plane intersects the elevator, as the rotation of the Earth swings the cable around the equator. Usually the satellite and the cable will not line up. However, except for synchronized orbits, the elevator and satellite will eventually occupy the same place at the same time, almost certainly leading to structural failure of the space elevator and destruction of the satellite.
Most active satellites are capable of some degree of orbital maneuvering and could avoid these predictable collisions, but inactive satellites and other orbiting debris would need to be either pre-emptively removed from orbit by "garbage collectors" or would need to be closely watched and nudged whenever their orbit approaches the elevator. The impulses required would be small, and need be applied only very infrequently; a laser broom system may be sufficient for this task. In addition, Brad Edward's design allows the elevator to move out of the way, because the fixing point is at sea and mobile. Such movements would also be managed so as to damp-out transverse oscillations of the cable.
For stability, it is not enough that other fibers be able to take over the load of a failed strand — the system must also survive the immediate, dynamical effects of fiber failure, which generates projectiles aimed at the cable itself. For example, if the cable has a working stress of 50 GPa and a Young's modulus of 1000 GPa, its strain will be 0.05 and its stored elastic energy will be 1/2 × 0.05 × 50 GPa = 1.25×109 joules per cubic meter. Breaking a fiber will result in a pair of de-tensioning waves moving apart at the speed of sound in the fiber, with the fiber segments behind each wave moving at over 1,000 m/s (more than the muzzle velocity of a standard .223 caliber (5.56 mm) round fired from an M16 rifle). Unless these fast-moving projectiles can be stopped safely, they will break yet other fibers, initiating a failure cascade capable of severing the cable. The challenge of preventing fiber breakage from initiating a catastrophic failure cascade seems to be unaddressed in the current literature on terrestrial space elevators . Problems of this sort would be easier to solve in lower-tension applications (e.g., lunar elevators). This problem has been described by physicist Freeman Dyson.
Corrosion is thought by some to be a risk to any thinly built tether (which most designs call for). In the upper atmosphere, atomic oxygen steadily eats away at most materials. A tether will consequently need to either be made from a corrosion-resistant material or have a corrosion-resistant coating, adding to weight. Gold and platinum have been shown to be practically immune to atomic oxygen; several far more common materials such as aluminum are damaged very slowly and could be repaired as needed.
Another potential solution to the corrosion problem is a continuous renewal of the tether surface (which could be done from standard, though possibly slower elevators). This process would depend on the tether composition and it could be done on the nanoscale (by replacing individual fibers) or in segments.
Radiation and Van Allen belts
Most of the space elevator structure would lay inside the Van Allen radiation belt, and the space elevator would run through the Van Allen belts. This is not a problem for most freight, but the amount of time a climber spends in this region would cause radiation poisoning to any unshielded human or other living things. The inner belt would have to be crossed, where (behind a shield of 3 mm of aluminium) the dose rate can reach 465 mSv/h. The geostationary orbit (at 35,786 km) would still be inside the outer belt, with dose rates still in the 20-25 mSv/h range.
Furthermore, the effectiveness of the magnetosphere to deflect radiation emanating from the sun decreases dramatically after rising several earth radii above the surface. This ionising radiation may cause damage to materials within both the tether and climbers.
An obvious option would be for the elevator to carry shielding to protect passengers, though this would reduce its overall capacity. The best radiation shielding is very mass-intensive for physical reasons. Alternatively, the shielding itself could in some cases consist of useful payload, for example food, water, fuel or construction/maintenance materials, and no additional shielding costs are incurred during ascent.
For a space elevator to be used by human passengers, the Van Allen radiation belt must therefore be emptied of its charged particles. This has been proposed by the High Voltage Orbiting Long Tether project.
More conventional and faster atmospheric reentry techniques such as aerobraking might be employed on the way down to minimize radiation exposure. De-orbit burns use relatively little fuel and are cheap.
In the event of failure
If despite all these precautions the elevator is severed anyway, the resulting scenario depends on where exactly the break occurred:
Cut near the anchor point
If the elevator is cut at its anchor point on Earth's surface, the outward force exerted by the counterweight would cause the entire elevator to rise upward into a higher orbit, or escape Earth's gravity altogether. The ultimate altitude of the severed upper end of the cable would depend on the details of the elevator's mass distribution.
Cut up to about 25,000 km
If the break occurred at higher altitude, up to about 25,000 km, the lower portion of the elevator would descend to Earth and drape itself along the equator east of the anchor point, while the now unbalanced upper portion would rise to a higher orbit. Some authors (such as science fiction writers David Gerrold in Jumping off the Planet and Kim Stanley Robinson in Red Mars) have suggested that such a failure would be catastrophic, with the thousands of kilometers of falling cable creating a swath of meteoric destruction along the planet's surface. However, in most cable designs, the upper portion of any cable that falls to Earth would burn up in the atmosphere. Additionally, because proposed initial cables have very low mass (roughly 1 kg per kilometer) and are flat, the bottom portion would likely settle to Earth with less force than a sheet of paper due to air resistance on the way down.
Cut above 25,000 km
If the break occurred at the counterweight side of the elevator, the lower portion, now including the "central station" of the elevator, would begin to fall down and would continue down to reentry if no part of the cable below failed as well. Depending on the size, it would either burn up on re-entry or impact the surface. A mechanism to immediately sever the cable below the station would prevent reentry of the station and result in its continuation in a high and slightly modified orbit. Simulations have shown that as the descending portion of the space elevator "wraps around" Earth, the stress on the remaining length of cable increases, resulting in its upper sections breaking off and being flung away. The details of how these pieces break and the trajectories they take are highly sensitive to initial conditions.
|Wikimedia Commons has media related to Space elevators.|
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