Space Elevator An Impossible Dream? 448
bj8rn writes "Three months ago, the dreams of a space elevator finally seemed to be coming true after a successful test. An article in Nature, however, suggests that there's reason to be pessimistic. Ever since carbon nanotubes were discovered, many have been hoping that this discovery would turn the dream into reality. Pugno, however, argues that inevitable defects in the nanotubes mean that such a cable simply wouldn't be strong enough. Even if flawless nanotubes could be made for the space elevator, damage from micrometeorites and even erosion by oxygen atoms would render them weak. It would seem that sci-fi will never be anything other than what it is: a fiction."
Re:Damaged by Oxygen? (Score:3, Informative)
http://www.sciencedaily.com/releases/2005/11/0511
Re:Wireless Elevators (Score:4, Informative)
The acceleration would kill you. That's the nice thing about the elevator, it could be a very mild ride.
Re:Is that the only problem? (Score:3, Informative)
The considerations you listed aren't considered problems because there are fairly obvious solutions for each of them:
For more information on the engineering involved in building a space elevator, check out this book [amazon.com] -- it goes into detailed explanations about your objections, as well as many others.
In short, there are big problems to be solved before you can build a space elevator, but those aren't them.
Re:Never say never (Score:3, Informative)
http://www.isracast.com/tech_news/271204_tech.htm [isracast.com]
Steve
Liftport already responded to this (Score:5, Informative)
I've discussed the article with a couple of CNT researchers, and they say that they're not convinced by the paper. My attitude is that we have to wait and see what really happens, because there's a lot about carbon nanotubes that we don't know yet.
Despite anyone's predictions, we won't know what the material will be like until it's made. There's a LOT of other work that needs to be done on SE development regardless of what the material winds up being. And in the "worst" case, you can still build a space elevator on the moon with near-term materials.
One thing to remember is that, even if bulk CNT were limited to 30 GPa, we could still build the space elevator. It would just become limited by finances. That's because, with a density of 1300kg/m^3 and a strength of 30GPa, the mass of a seed ribbon (using the same assumptions as in my November article - safety factor of 2, and 1,000kg capacity) would be roughly 3,440 tonnes (i.e., 3.44*10^6 kg), or roughly 170 rocket launches (using current medium-lift rockets) to loft it (i.e., ~80 times as massive as in the 2002 NIAC report). The expense and logistics of creating a seed ribbon at that point (assuming you're launching from Earth) becomes much more daunting, but not impossible.
and for people raising other concerns, which I see in several places here:
Breaking is a minor issue. Most of it would fall up. The base station doesn't support the elevator, it holds it down. The Earth's rotation keeps it up. People tend to forget the scale we're dealing with here. The bits that fall down would burn up, land as ash.
Space debris is well mapped. We can avoid it, for the most part. Small adjustments made from either end of the elevator can be used to shift the bulk of the thing. Remember, serious plans for it call for building it on a floating platform, which can move, and rockets can be used to adjust the space end of things.
Storms, well, like I said, we can move the thing. Also bear in mind that storms only affect the part of it in the lower atmosphere. Resonance is an issue which is being seriously considered, as well as induced current.
Any more problems you'd like to raise? Read the wikipedia article [wikipedia.org].
Impossible? (Score:1, Informative)
Re:Never? (Score:4, Informative)
Not necessarily - Planck length [wikipedia.org] may be a minimum [blogspot.com] unit of distance in the universe, making the set of possible states potentially not merely countable but (along with the other Planck units) finite.
Re:Damaged by Oxygen? (Score:5, Informative)
And as we are not really able to produce material that would be strong enough and light enough to support the space lift even in perfect conditions (there are really nice Internet-available articles and research papeers on this issue), producing a practical model is still much more thing of fiction, than of science. Therefore any coating or protection from whatever may be hazardous for our lift needs also to be developed and is a topic for the future. But may be in far future...
Oh, and there was extensive research done on many different earth-to-orbit propulsion systems, some more possible than the others. My biggest enthusiasm got the nuclear-engine, but for obvious reasons research in this area is right now strongly inhibited (if there is any at all).
Re:Never? (Score:3, Informative)
Let x be a real number, such that 4 > x > 3.
Let y be a real number such that y = ((4 - x) / 2) + x
By basic algebra, ((4 - x) / 2) + x x
and y != x
By our definition, x > 3, and transitively, ((4 - x) / 2) + x > 3
Therefore 3 ((4 - x) / 2) + x 4, and thusly 3 y 4
Simply put, for every x as defined above, you can always create another number between 3 and 4, simply by adding (((4 - x) / 2) + x) to it.
I'm sure this proof wouldn't get me a passing grade were it a descrete math assignment, but it's good enough for slashdot. I'm also sure there's a better proof in every descrete math textbook on the planet, but mine's 60 miles away at the moment
Re:Never? (Score:4, Informative)
Here's a simpler, more general way to state it:
For every two real numbers A, B where A < B, there exists a number x = (A+B)/2.
Since A < x < B, you can repeat the existence postulate for A, x and x, B
This is true for A,B = 3,4.
Re:Oxygen!! What about lightning!? (Score:3, Informative)
Yes, lightning is a definite hazard for a space elevator.
The solution [www.isr.us]: locate the space elevator in a lightning-free area.
Re:Is that the only problem? (Score:5, Informative)
We are talking a device ~60,000 miles long, feet wide, and paper-thin. So...
I am thinking of storm type winds blowing it off balance
The atmosphere extends up a few tens of miles at most. The Space Elevator is 60,000 miles long.
making it resonate
Compute the resonance frequency of a device 60,000 miles long.
Even to the extent it's a problem, it's not like it's hard to react to; you've got all day.
the danger to aeroplanes,
What danger to airplanes? Are you envisioning something that's going to randomly and rapidly maraud across the surface of the Earth or something?
It's way, way, way easier to dodge a stationary space elevator than all the other constantly moving planes in the sky.
the disastrous consequences of breakage
You're just assuming. Somebody beat me to pointing out this is false, but I want to point out you're assuming based on your everyday experience. It works poorly in this domain.
For instance, what you probably think happens if there is a cut near the ground is the exact opposite of what happens, because your intuition is not set up for these kinds of problems.
You need to turn to the math on this. Other people have worked out the issues. Most of what you consider the "real problems" aren't, and I don't mean that as a comment on your particular post, I mean it in general. Other things that you might never think about are, such as the concern raised in TFA, which I think are valid but aren't necessarily stoppers, and the ever-present question of whether we'll ever be able to turn out 60,000 miles of cable of any kind.
Your intuition is worthless. Nothing personal; mine is too. Having studied the topics involved I can say I understand some of this stuff intellectually, but I can't say I understand it in my gut. But I do know not to trust my gut in this domain.
(For what it's worth, similar concerns apply w.r.t. nanotechnology. Your intuition about how things work does not do very well at that scale. Our brains function at the in-between scale we all live and work in, and does not do well outside of that domain.)
(60,000 mile note: I'm assuming the elevator design that extends in both directions from geosync, as I like the "throwing" ability it exhibits over the counter-weight-just-outside-of-geosync model. Other distances are possible but don't fundamentally change the results.)
Re:Damaged by Oxygen? (Score:5, Informative)
Economics (Score:3, Informative)
Of course, the only reason anyone would built such a bridge is as a prototype demonstration to scare up investors. The potential ROI for a space elevator is pretty spectacular, not so much for a bridge... and buckytube isn't cheap.
Re:Another way? (Score:3, Informative)
Can we enhance current CNT methods to not produce any defects? Probably not. CNTs typically have irregular balls of carbon at the center or the ends because this is what they develop from. The strongest SWNT ever measured was, if I recall correctly, 61 GPa tensile strength. Way too weak for all but a high taper factor elevator, which would cost a fortune and have a low payload. You really need >100GPa to make it economicall realistic; >120GPa makes it reasonable present-day.
Even if you can produce perfect individual CNTs, that's not all of what you need. You need *very long* individual CNTs if you want VdW and pi bonding to hold your ropes together. Not only that, but you need nice, neat ropes. Normal ropes simply don't get you a strength that approaches the strength of the CNTs themselves. If you can't get ropes made of CNTs that are a dozen centimeters in length, you'll need to do intertube bonding (pressure-induced bonding is already possible), but trading the sp2 bonds for sp3 will weaken the ropes' tensile strength.
Even if you can get perfect ropes, that's *still* not everything. You need long, continuous, affordable ropes. And you need to be able to bond coatings to it and all sorts of other things.
If an earth elevator is possible, we're nowhere close to it. It's nice to just assume that we can do anything we want. Sadly, we can't. Perhaps some day an Earth elevator will be realistic, but no day soon.
Quality control at the nanoscale. (Score:2, Informative)
Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load.
SCIENCE, VOL 287, p. 637-640, 28 JANUARY 2000
http://bucky-central.mech.northwestern.edu/Ruoffs
This report showed actual measured tensile strengths up to 150 GPa:
Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes.
B.G. Demczyk et al.
Materials Science and Engineering A334 (2002), 174, 173-178.
http://www.glue.umd.edu/~cumings/PDF%20Publicatio
Both of these studies were done on multiwalled tubes since they are larger and it's easier to make attachments with them.
In the earlier study in Science, the authors from SEM imaging noted that it was actually the outer single-walled nanotube that broke first therefore it was carrying the load. This would make sense from the way the attachments were formed which could only form a bond with the outer surface of the multiwalled tube. Therefore the numbers quoted were for the strength of this outer single-walled nanotube using as thickness only that of this single-walled nanotube.
However, in the later study in Materials Science and Engineering, the authors believed the attachments were made to all the layers of the multi-layered nanotube, which would explain their higher measured strength.
The prevailing theory is that the range of strengths is due to the number of imperfections in the nanotubes. So we should be able to look at the nanotubes at the nanoscale using SEM,'s, STM's or AFM's and find which ones have the least imperfections. These should be the strongest tubes.
In the Science study, 1 out of 21 of them, 5%, have the best strength, 63 GPa. At a production of millions of tubes at a time this should still be feasible economically and technically.
The lengths of the nanotubes in these studies were however, were at the micron scale though. Nanotubes have been created at the centimeter length scale, but as far as I know the strength of these have not been tested.
Note that the reported strengths of centimeter long or longer "fibers" made of nanotubes being less than 1 GPA are not measuring the strength of individual nanotubes at these lengths. This is because the fibers are composed of the nanotubes stuck together end to end by weaker Van der Waals forces, rather than the much stronger carbon-carbon bonds that prevail in individual nanotubes.
Here is one study that detects, characterizes defects in the nanotubes at the nanoscale:
Resonant Electron Scattering by Defects in Single-Walled Carbon Nanotubes.
Science 12 January 2001, Vol. 291. no. 5502, pp. 283 - 285.
http://www.sciencemag.org/cgi/content/abstract/29
Methods such as this might make it possible to find the nanotubes with the least defects beforehand and therefore automatically select those of the highest strengths.
Bob Clark