Space elevator safety

There are many safety issues associated with the construction and operation of a space elevator. A space elevator would present a considerable navigational hazard, both to aircraft and spacecraft. Aircraft could be dealt with by means of simple air-traffic control restrictions, but impacts by space objects (in particular, by meteoroids and micrometeorites) pose a more difficult problem.

Cable strength

The currentyear strength/mass ratio of cables of any construction is inadequate to build a space elevator at the present time.Fact|date=September 2008 Although carbon nanotubes embedded in the tether would give it enough strength to be practical, nanotubes of sufficient length have not yet been made.

Theoretical objections have been raised to manufacturing bulk carbon nanotube structures with strengths approaching that which simple models and microscopic strengths suggest. H. K. D. H. Bhadeshia argues that the presence of defects would significantly reduce the strength actually attainable. [ [http://www.msm.cam.ac.uk/phase-trans/2005/chunk.html 52nd Hatfield Memorial Lecture: Large Chunks of Very Strong Steel ] ]

atellites

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 manoeuvring 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 actually allows the elevator to move out of the way, because the fixing point is at sea and mobile.Fact|date=September 2008 However, such movements would excite transverse oscillations of the cable. Edwards claims that these oscillations could be controlled so as to ensure that the cable avoids satellites on known paths.

Meteoroids and micrometeorites

Meteoroids present a more difficult problem, since they would not be predictable and much less time would be available to detect and track them as they approach Earth. It is likelyFact|date=September 2008 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.Fact|date=September 2008

Far worse than meteoroids are micrometeorites; 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.Fact|date=September 2008

Failure cascade

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 (January, 2005)Updateneed literature on terrestrial space elevators. Problems of this sort would be easier to solve in lower-tension applications (e.g., lunar elevators).

Corrosion

Corrosion is a major risk to any thinly built tether (which most designs call for). In the upper atmosphere, atomic oxygen steadily eats away at most materials.Fact|date=September 2008 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 shownFact|date=September 2008 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

The effectiveness of the magnetosphere to deflect radiation emanating from the sun decreases dramatically after rising several earth radii above the surface. This ionizing radiation mayFact|date=September 2008 cause damage to materials within both the tether and climbers.

Material defects

Any structure as large as a space elevator will have massive numbers of tiny defects in the construction material. It has been suggested, [ [http://arxiv.org/abs/cond-mat/0601668 "ON THE STRENGTH OF THE CARBON NANOTUBE-BASED SPACE ELEVATOR CABLE: FROM NANO- TO MEGA-MECHANICS" Nicola M. Pugno] ] [ [http://www.msm.cam.ac.uk/phase-trans/2005/SWpaper/index.html "Bulk Nanocrystalline Steel" H. K. D. H. Bhadeshia] ] that, because large structures have more defects than small structures, that large structures are inherently weaker than small, giving an estimated carbon nanotube strength of only 24 GPa down to only 1.7 GPa in millimetre-scale samples, the latter equivalent to many high-strength steels, which would be vastly less than that needed to build a space elevator for a reasonable cost.

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.Fact|date=September 2008 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.Fact|date=September 2008

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 an unstable orbit.Fact|date=August 2008

The ultimate altitude of the severed lower end of the cable would depend on the details of the elevator's mass distribution. In theory, the loose end might be secured and fastened down again. This would be an extremely tricky operation, however, requiring careful adjustment of the cable's center of gravity to bring the cable back down to the surface again at just the right location. It may prove to be easier to build a new system in such a situation.Or|date=September 2008

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 west of the anchor point, while the now unbalanced upper portion would rise to a higher orbit.Fact|date=August 2008 Some authors (such as science fiction writers David Gerrold in "Jumping off the Planet", 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 fell to Earth would burn up in the atmosphere.Fact|date=August 2008 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.Fact|date=February 2008

If the break occurred at the counterweight side of the elevator, the lower portion, now including the "central station" of the elevator, would entirely fall down if not prevented by an early self-destruct of the cable shortly below it. Depending on the size, however, it would burn up on re-entry anyway. Simulations have shownFact|date=September 2008 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. [cite web| url=http://gassend.com/spaceelevator/breaks/index.html| title=Animation of a Broken Space Elevator| first=Blaise| last= Gassend| year=2004| accessdate=2007-01-14]

Elevator climbers

It is almost inevitable that some objects — climbers, structural members, repair crews, etc. — will accidentally fall off the elevator at some point.Fact|date=August 2008 Their subsequent fate would depend upon their initial altitude. Except at geostationary altitude, an object on a space elevator is not in a stable orbit and so its trajectory will not remain parallel to it. 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.Fact|date=August 2008

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 —Fact|date=August 2008 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 to return to the altitude 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.Fact|date=August 2008

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.Fact|date=August 2008 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 orbit would increase with the altitude from which the object is released.

Above 47,000 km, however, an object that falls off of the elevator would have a velocity greater than the local escape velocity of Earth.Fact|date=September 2008 The object would head out into interplanetary space, and if there were any people present on board it might prove impossible to rescue them.Fact|date=August 2008

Van Allen Belts

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. [cite news
title=Space elevators: "First floor, deadly radiation!"
date=2006-11-13
work=New Scientist
author=Kelly Young
url=http://space.newscientist.com/article/dn10520-space-elevators-first-floor-deadly-radiation.html] [cite journal
journal=Acta Astronautica
volume=60
issue=3
month=February | year=2007
pages=189–209
doi=10.1016/j.actaastro.2006.07.014
publisher=Elsevier Ltd.
title=Passive radiation shielding considerations for the proposed space elevator
author=A.M. Jorgensena, S.E. Patamiab, and B. Gassendc] SomeWho|date=September 2008 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.Fact|date=September 2008

More conventional and faster atmospheric reentry techniques such as aerobraking mightFact|date=September 2008 be employed on the way down to minimize radiation exposure. De-orbit burns use relatively little fuel and are cheap.Fact|date=September 2008

An obviousFact|date=September 2008 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.Fact|date=September 2008 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.Fact|date=September 2008

To shield passengers from the radiation in the Van Allen belt, perhaps counter-intuitively, material composed of light elements should be usedOr|date=September 2008Syn|date=September 2008, as opposed to lead shielding. In fact, high energy electrons in the Van Allen belts produce dangerous X-rays when they strike atoms of heavy elements.Fact|date=September 2008 This is known as bremsstrahlung ("braking radiation"), and is the method used to create X-rays for medical use (such as in dentistry). Materials containing large amounts of hydrogen, such as water or (lightweight) plastics such as polyethylene, and lighter metals such as aluminium are better than heavier ones such as lead for preventing this secondary radiation.Fact|date=September 2008 Such light-element shielding, if it were strong enough to protect against the Van Allen particle radiation, would also provide adequate protection against X-ray radiationFact|date=September 2008 coming from the sun during solar flares and coronal mass ejection events. Nevertheless the total mass required for radiation shielding is very high.

References

External links

* [http://www.elevator2010.org/ Elevator:2010] Space elevator prize competitions
* [http://www.spaceelevator.com/ The Space Elevator Reference]
* [http://science.nasa.gov/headlines/y2000/ast07sep_1.htm Audacious & Outrageous: Space Elevators]