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 and micrometeorites pose a more difficult problem for construction and operation of a space elevator.

Satellites

If nothing were done, essentially all satellites with perigees below the top of the elevator would eventually collide with the elevator cable.^[1] 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 be also be managed so as to damp-out transverse oscillations of the cable.

Meteoroids and micrometeorites

Meteoroids present another problem, they would not be predictable and much less time would be available to detect and track them as they approach Earth. It is likely that a space elevator would still suffer impacts of some kind, no matter how carefully it is guarded. However, most space elevator designs call for the use of multiple parallel cables separated from each other by struts, with sufficient margin of safety that severing just one or two strands still allows the surviving strands to hold the elevator's entire weight while repairs are performed. If the strands are properly arranged, no single impact would be able to sever enough of them to overwhelm the surviving strands.

Micrometeorites are tiny high-speed particles found in high concentrations at certain altitudes. Avoiding micrometeorites is essentially impossible, and they will ensure that strands of the elevator are continuously being cut. Most methods designed to deal with this involve a design similar to a hoytether or to a network of strands in a cylindrical or planar arrangement with two or more helical strands. Constructing the cable as a mesh instead of a ribbon helps prevent collateral damage from each micrometeorite impact.

Failure cascade

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.^[2]

Corrosion

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.^[3] 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.

Other analyses show atomic oxygen to be a non-problem in practice.^[4]

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.^[5][6] 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.^[7][8] 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.^[9]

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.

Some speculate that passengers would continue to travel by high-speed rocket, while space elevators haul bulk cargo. Research into lightweight shielding and techniques for clearing out the belts is underway. 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.^[10] · ^[11]

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.

Weather

In the atmosphere, the risk factors of wind and lightning come into play. The basic mitigation is location. As long as the tether's anchor remains within two degrees of the equator, it will remain in the quiet zone between the Earth's Hadley cells, where there is relatively little violent weather. Remaining storms could be avoided by moving a floating anchor platform. The lightning risk can be minimized by using a nonconductive fiber with a water-resistant coating to help prevent a conductive buildup from forming. The wind risk can be minimized by use of a fiber with a small cross-sectional area that can rotate with the wind to reduce resistance. Ice forming on the cable also presents a potential problem. It could add significantly to the cable's weight and affect the passage of elevator cars. Also, ice falling from the cable could damage elevator cars or the cable itself. To get rid of ice, special elevator cars could scrape the ice off.

Vibrational harmonics

A final risk of structural failure comes from the possibility of vibrational harmonics within the cable. Like the shorter and more familiar strings of stringed musical instruments, the cable of a space elevator has a natural resonant frequency. If the cable is excited at this frequency, for example by the travel of elevators up and down it, the vibrational energy could build up to dangerous levels and exceed the cable's tensile strength. This can be avoided by the use of suitable damping systems within the cable, and by scheduling travel up and down the cable keeping its resonant frequency in mind. It may be possible to dampen the resonant frequency against the Earth's magnetosphere.

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.^[12] 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.^[13] 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.^[13] The details of how these pieces break and the trajectories they take are highly sensitive to initial conditions.^[13]

Elevator climbers

It is almost inevitable that some objects — climbers, structural members, repair crews, etc. — will accidentally fall off the elevator at some point. Their subsequent fate would depend upon their initial altitude. Except at geostationary altitude, an object on a space elevator is not in a circular orbit, and so its trajectory will not remain parallel to that of the elevator. The object will instead enter an elliptical orbit, the characteristics of which depend on where the object was on the elevator when it was released.

If the initial height of the object falling off of the elevator is less than 23,000 km, its orbit will have an apogee at the altitude where it was released from the elevator and a perigee within Earth's atmosphere; it will intersect the atmosphere within a few hours, and not complete an entire orbit. Above this critical altitude, the perigee is above the atmosphere, and the object will be able to complete a full orbit, returning to the altitude that it started from. By then, the elevator would be somewhere else, but a spacecraft could be dispatched to retrieve the object or otherwise remove it. The lower the altitude at which the object falls off, the greater the eccentricity of its orbit.

If the object falls off at the geostationary altitude itself, it will remain nearly motionless relative to the elevator, just as in conventional orbital flight, and would thus be very easy to retrieve. At higher altitudes, the object would again be in an elliptical orbit, this time with a perigee at the altitude the object was released from and an apogee somewhere higher than that. The eccentricity of the object's orbit would increase with the altitude from which it was released.

Above 47,000 km, however, an object falling off of the elevator would have a velocity greater than the local escape velocity of Earth. The object would head out into interplanetary space, requiring on-board rocketry or an interception to retrieve.

A lightweight ballistic parachute system or escape module might be practical for lower-altitude release from the cable. For cases of higher-altitude release, maneuvering rockets and possibly heat shields might be feasible, though these would reduce the available payload capacity.

See also

Wikimedia Commons has media related to Space elevators.

• Lunar space elevator
• Non-rocket spacelaunch
• Space elevator construction
• Space elevator economics
• Space elevators in fiction

References

External links

• Elevator:2010 Space elevator prize competitions
• The Space Elevator Reference
• Audacious & Outrageous: Space Elevators