One of the treasured axioms of scientific inquiry is that science is incapable of proving something to be true; it can only prove that something is untrue. However, that statement does not mean that science is necessarily capable of proving that something is impossible. Scientists are fond of poking holes in what they perceive as the pipe dreams of technologists. Very often they use theory and fundamental physical laws, with a healthy dose of assumptions, to state their case. Unfortunately for vocal scientists, they’re not always right. In fact, when it comes to space flight, scientists were so wrong that they are still playing catch up.
Within the last few days, there have been two instances of what I consider to be the gross overstating of the impossibility of a task by scientists or science journalists.
First, there was an article in Scientific American several months ago followed on by a recent article in Discover (linked by Clark Lindsey here) positing that human space travel may never be possible because of the threat posed by high energy cosmic rays. Lindsey does a fine job of pointing out the fundamental problems with those two articles in his entry. Basically, the argument is that so much shielding will be necessary in order to reduce the damage caused by high energy charged particles from cosmic rays that spacflight will remain forever uneconomical. In addition to what Lindsey mentions, I’ll add that a hybrid of both mass shielding and energy shielding methods may be able to dramatically reduce shielding demands.
Betting Against the Elevator
Next, (also pointed out by the astute Lindsey) an article by Professor Nicola Pugno is highlighted in this news@nature.com article. Pugno’s article is set to appear in Journal of Physics: Condensed Matter, a peer-reviewed journal, but is also available here at arXiv. The news@nature.com story is, as most articles at nature.com, highly critical of human spaceflight (at least in editorial tone).
Pugno’s article argues that, based on modeling (and two big assumptions, which I will soon discuss), a carbon nanotube ribbon cannot be strong enough to meet the demands of a space elevator application. Basically, at minimum a space elevator ribbon must be able to support around 60 GPa of tensile stress. Theoretically, carbon nanotubes should be able to support in excess of 100 GPa. Pugno articles that due to 1) inherent defects in the carbon nanotube material and 2) the damaging influences of atomic oxygen and micrometeors, the maximum strength of a carbon nanotube ribbon will be something around 30 GPa. He comes to this conclusion after applying a number of theoretical and numerical models of defect and crack propagation to the proposed ribbon.
As mentioned in the news@nature.com study, Bradley Edwards argues that defects are not as important as Pugno argues because by intertwining many single-walled nanotubes, the weak attraction between the fibers will be enough to transfer loads from defective or weakened fibers to those that are much stronger. Pugno addresses this case in his article but does not do so with an eye towards optimizing the strength of such a cable. Instead, he assumes a certain defect density and applies his theories all the same.
Pugno concludes that a space elevator ribbon is impossible with today’s technology. This statement, in itself, is absolutely true. The longest nanotube cable that has been constructed is about 1.5 meters in length. So of course an elevator is impossible today. But an elevator will not be constructed today, it will be constructed tomorrow, or maybe the day after that. In the fifteen or so years (12, if you like to follow Lift Port’s countdown clock) between today and tomorrow, a lot is going to change.
Showstoppers?
Today, carbon nanotubes can be made with about 1 defect in every 1012 atoms, which equates to about 1 defect per 4 micrometers (according to the news@nature.com article). But, these nanotubes were constructed with bulk fabrication techniques (check out Wikipedia’s Nanotube page for more on manufacturing techniques) that are by no means optimal. Better fabrication techniques could greatly reduce defect densities. Additionally, nanotechnology may be capable of producing practically defect-free materials, including nanotubes. In fact, nanotubes may be the first such beneficiaries of applied nanotech manufacturing because of their great utility for a wide array of purposes. But, it is not necessary to produce nearly defect-free cables of thousands of kilometer length. It is only necessary to produce fibers on the order of a few centimeters long with no more than a few defects, according to Edward’s designs. This is a target that may be simple, or it may be difficult, but the likelihood of success will look a lot different in a few years.
Next, Pugno addresses the eroding effects of atomic oxygen and micrometeorites. Atomic oxygen is highly reactive, and tends to react quite readily with carbon nanotubes (at least we think). Layers of the atmosphere between 60-800 km, centered on 100 km altitude, are rich in atomic oxygen. The effect, as Pugno discusses it, is an erosion of the cable resulting in defect production and magnification. Micrometeorites pose a more significant danger because simply coating the cable cannot reduce their effects. They will be most intense between 500 and 1700 km. These two problems will cause headaches and constant need of repair. In Pugno’s view, their are showstoppers.
I am trained as a mechanical engineer, but only practice it from my armchair. Nevertheless, I can see a few rather simple ways of reducing these threats greatly. As mentioned, coating the cable in something that is far less reactive with atomic oxygen will be a low-weight and perhaps low-maintenance solution (although that depends on how strongly the coating bonds with the ribbon, there will be quite a bit of wear on the ribbon’s surface). But how can you shield the cable? How about a blanket of aerogel? It could be “zippered” to the ribbon such that it unzips and rezips as the climbers pass by. THe entire cable need not be protected, only that relatively short section in the greatest danger from micrometeorites. The blanket needn’t even be made of aerogel, even a few layers of traditional aluminized mylar shield might reduce the need for maintenance on any given section by several orders of magnitude.
The Bottom Line
Betting against the advance of technology is a bad proposition. Particularly if it’s being done by folks in the halls of academia on paper or computers. Science, believe it or not, is virtually incapable of declaring an activity impossible. Sure, you can make limited statements such as: “it’s impossible for a human to jump into space without additional supplies of energy.” But I, for one, don’t believe that scientists can even say for sure that information or material cannot travel faster than light. The nature of the universe is so poorly understood that everyone should refrain from overly-broad statements.
Now, Pugno’s criticism is well-made; steps must be taken to assure that defects are kept to a minimum and that damage is also greatly reduced. But these are engineering tasks, not fundamental scientific ones. Maybe a space elevator won’t be built by 2018, but I’m not going to put any money against it. Whatever you do, take predictions about what’s impossible with a good dose of salt. After all, prognostications about what the future holds are notoriously and often ludicrously wrong.
Paper Critiques Space Elevator and Scientists Overstate Their Case…
One of my favorite websites is Anthonares. The current essay responds to Nicola Pugno’s article claiming that Space Elevator’s are impossible with today’s technology (including carbon nanotubes). The author, Anthony Kendall, is n…
This is exactly the kind of article we need more of. The annals of spaceflight are littered with prognostications gone wrong, including the famous New York Times editorial ridiculing Robert Goddard, and as you say, many people of sound scientific background have provided hopelessly wrong estimates about the future of various kinds of technology. The one thing we ought to know by now is that the future will surprise us, deeply and irrevocably, and part of the surprise will be the number of ‘impossible’ things that will eventually become all but routine. Great article, Anthony.
Thanks Paul!
I hadn’t intended to break out of my blogging vacation until next week, but that article was just something I felt I needed to address.
As the world’s one-greatest purveyor of nanotube physical properties *wink* (http://www.pa.msu.edu/cmp/csc/ntproperties/) I can say that the 30GPa tensile strength estimate is pretty good for any kind of nanotube “rope”, i.e. not one continuous molecule (100GPa). His one long but defective nanotube must equate to that of a rope, essentially. That is unless my data is really out of date and they can make long ropes or bundles above 30GPa tensile strength that I am not aware of. Anyway, if you had one long molecule that was getting all sorts of damage done to it, 30GPa is not an unreasonable conclusion. So, I don’t know. This doesn’t mean that it’s impossible because nanotubes are being improved upon at such a fast rate that other technologies might make it possible.. particularly the ability to vapor-deposit coatings on the outside.
What I haven’t heard yet is how the nanotubes will be organized chirally… which makes a big difference in terms of electric conductivity. If they make it conductive, then one could send signals / power up and down the elevator, which could power the cabin or whatever. But, this thing is going to be the biggest tree on the golf course when it comes to weather since it’s going to be poking through the clouds. Do you know how this issue might be overcome?
Tom,
Good point about the fact that ropes are not as strong as single molecules, though I don’t know the latest science on what strengths materials scientists think that they will be able to achieve.
I don’t think that it would be a good idea to allow the nanotubes to be conducting, so I think the chirality should be chosen to make the nanotubes insulating. A nanotube ribbon would be one hell of a grounding conductor between the ionosphere and the Earth.
Great to have you back, Anthony - and with a very good article. I commented recently on the Discover article on the radiation threat and had read the piece on nanotube defects and skimmed the technical article. There is still much that is unknown about CNT’s but with so many applications from high-speed computer circuits to strong/light weight “whatever,” I’m sure the material issues will get plenty of attention in the next few years.
-Bruce
I think one point everyone seemed to have missed that will solve lot of problems is that the last 100 miles of the ribbon that would interact with storms,weather, atmosphere etc. need not be made of nanotubes at all. It could be made of kevlar, nylon and lots of other more traditional materials.
Note that the lower part of the cable will have the minimum load on it.
Also the ribbon can be planned on being put into a storm quite zone.
Here is a good online study from NIAC/NASA to read on this topic which points out solutions to some of the problems:
http://www.isr.us/Downloads/niac_pdf/contents.html
Just a little doomsday thought here:
IF one were to create a carbon ribbon through the ionosphere, there is a possibility that this would provide a short-circuit path to ground.
Discharging the ionosphere would destroy the Earth’s protection from solar wind and would probably make the planet uninhabitable very quickly.
Most likely, if this short-circuit did in fact form, the ribbon would burn out quickly. But the potential to create an ionized plasma along the path of the ribbon exists. The result would be the mother of all lightning strikes.
Even if the cable were an insulator such as Kevlar any contamination such as carbon or moisture would make it much more conductive than our atmosphere.
Let’s just say it would be a REALLY bad idea to try this on December 21, 2012.
For safety, any attempt to build this thing should place an electrical meter at the end of the ribbon. If high voltage potentials are read, the project should be aborted while the line is still very far from ground.