Critique of Earth based Space Tethers Forum: SSI-List
Thread: Critique of Earth based Space Tethers
Hi Tangoman,
Here is my critique of Earth based space tethers. I'll bet someone else has done a better job somewhere so treat this as the arm chair scientist version. 1) There is a lot of radiation in the form of protons and electrons travelling at relativistic energies in the Earths Van Allen belts. It is not steady and fluctuates quite a bit especially during solar flares. A space tether from Earth to GSO would act like a giant antenna and experience charge differentials of unknown intensity because there has never been a means directly of testing them before. NASA did one experiment to see if a very short tether would generate sufficient electricity to use as experiments. It ended as I remember with the cable essentially shorting out due to much current. This means to me that at times a space tether that goes through these regions is going to "short" them out leading to charge densities that may exceed the current carrying capacity of the tether. To mitigate this the design of the tether would have to be of a consistent diameter and very conductive (probably higher than the known current conductivity of carbon nanotubes). 2) A stress analysis of a space tether indicates that it should be thickest at the point of highest tension at GSO and thinnest at the ends. This is the opposite of the shape to minimize electrical problems (see #1) and means that the fibers will be much closer to their maximum stress capacity at GSO to support all that extra tether weight. There are several things in space that will cause erosion of the surface (ultraviolet radiation, micrometeorites,highenergy plasma). This means that a thin ribbon design is out of the question oryou would loose strength toofast if anything unusualhappened. Armor is outof thequestion since it's weight would quickly exceed the strength of the tether. That leave's the option of a round tether that isharder to climb and weathers unevenlyespecially in the Van Allen belts. Once a century a comet does pass close and lots of fragments pass by the earth. If one of these hits the tether (which is a mathematical probability over so much surface area) you run the risk of a substantial sudden weakening of the tether. Sub total Summary: Thin light tapered tether out - heavy round tether in. Unpredictability of properties of space over hundreds of years is sizeable. 3) Getting the political will together to allow a structure 30,000+ kilometers built with the risk of catastrophic failure would be an impressive feat. Any country underneath this structure would face the prospect of an enormous mass comming down on the atmostphere above which would produce shock waves and probably a very large noise. This means your risk/reward analysis is in rather bad shape since virtually all your costs are up front and your return is dependent on how long your tether stays in place. I personally have no expectations that this would be a perminent structure and convincing people that a failure is impossible would be extremely difficult let alone explaining the dangers of an impact. Grand total summary: These three issues (shorting, erosion/impacts, and financing) make me think the idea of cheap access to space via a long carbon thread seem rather idealistic. I have my bias that space is a hostile place and most of the time this may not hold true. In the event of a super flair from the sun aimed right at the earth or a comet trail that sweeps across, then I would likely be vindicated. We have barely 100 years of electromagentic observations and more like 50 for the behaviors of space local to the Earth. This is not enough to encourage me that there are not 200 year or 1000 year events that would doom a project like this. I like my gambles to have a bit more reliability. Cheers, Thor
> Of course, Thor Olson has pointed out (in personal correpsondence)
> that long tethers (whether for elevators or not) have major
> practical problems, e.g. electrical discharges, and might never work
> in the size range of tens of kilometres.
I'd be most appreciative if Thor would share with us his thoughts on
problems that tethers will likely encounter.
TangoMan