10. The Space elevator

http://science.nasa.gov/headlines/y2000/ast07sep_1.htm [Pearson, Acta Astronautica 2 (1975) 785-

799]

Probably the wildest nanotechnology idea in addition to self-replicating nanomachines is that of the

space elevator.

Contrary to self-replicating nanomachines, which we just concluded are to be considered science

fiction for the foreseeable future, the space elevator concept is studied seriously and might according

to several respected scientists become reality in a century or so.

NASA has had a project to study it.

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So let us look a bit more into the concept.

- It originated with the famous Russian scientist Konstantin Tsiolkovsky (known for pioneering

rocketry ideas) who thought of a ”Celestial Castle” in geosynchronous Earth orbit attached to a

tower on the ground.

- Later a Leningrad engineer by the name of Yuri Artsutanov, wrote some of the first modern ideas

Introduction to Nanoscience, 2005 JJ J � I II × 2

about space elevators in 1960 in the Soviet newspaper Pravda. But this paper was the offifical

newspaper of the communist party and thus was of course not read by anyone, so the idea did not

gain wider recognition.

- The popularization of the idea started, though, with the 1975 paper by Pearson, who not only did

the basic strength calculation but also considered several complications and how it might be built

[Pearson, Acta Astronautica 2 (1975) 785-799]. Inspired by this in 1978 Arthur C. Clarke popularized it to a wider

audience in his 1978 science fiction novel, “Fountains of Paradise”.

- If realized, it would allow for putting up a passenger with baggage to space for something like 200

USD - so it really would revolutionize space travel

The elevator would consist of a tower (current idea is 50 km high) on top of which there is a cable.

It would be linked to a counterweigth (e.g. an asteroid) above geostationary orbit, which is at about

36000 km high. The counterweight would due to centrifugal forces balance the whole structure. In

principle it would also be possible to use the end part of the cable as a counterweight.

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The tower would for obvious reasons be at, or somewhere close, to the equator.

The elevator itself would be on four to six ”elevator tracks” extending up the sides of the tower and

cable, going to platforms at different levels.

Electromagnetic vehicles (maglevs) traveling along the cable could serve as a mass transportation

system for moving people, payloads, and power between Earth and space.

10.0.1. Space elevator cable material

[Above, also http://www.isr.us/Downloads/niac_pdf/chapter2.html]

For nanoscience the most interesting aspect is the cable material.

The cable would have its maximum stress at geosynchronous orbit, due both to the counterweight

but also due to its own mass

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- Bottom part of cable is pulled down by gravity, upper part pulled out by the counterweight (or

strictly speaking of course not outwards but in the orbital direction, remember the classic 1-st year

physics student nitpicking about centrifugal forces...)

- Hence it must be the thickest at the geosynchronous orbit.

- Thus one can for materials of known strength and density calculate how thick they need to be at

the midpoint compared to at the bottom and top. The ratio between these diameters is called the

taper factor.

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- For steel the taper factor is tens of thousands – clearly impossible.

- For diamond, the taper factor is 21.9 including a safety factor, but diamond is brittle

- What one would need is a very light material with a very high tensile strength

- The attendees of this course will now of course realize carbon nanotubes are in principle perfect

for this!

- For nanotubes the taper factor might, depending on design, actually be less than 2.

- Here is one proposed profile:

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(Note that in this design the cable is actually amazingly thin!)

- The cable would probably be some sort of a composite:

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- It is also best to have one cross-sectional dimension much larger than the other to reduce the

damage meteors can inflict on the cable.

- The desired tensile strength for the space elevator is about 62 GPa.

- Comparison with ch. 3b of this course shows that nanotubes can theoretically achieve up to maybe

300 GPa, and have experimentally already demonstrated to have 63 GPa.

- But this is single nanotubes; one would still need to achieve the same in nanotube ropes.

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- Ropes can be made:

[Xhang, Atkinson, Baughman, Science 306 (2004) 1359]

but they have strengths which are “only” of the order of 1 GPa

Still comparable to steel so even this is not so bad actually

- There are two basic reason to this weakness:

1. That the single tubes are relatively short

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2. The weak van der Waals interaction between the tubes

- Point 1 could be solved if it becomes possible to manufacture ropes from the recently made

cm-long nanotubes efficiently

- Point 2 might be solved by introducing covalent bonds between nanotubes:

- By chemical means: Fukushima and Aida have very recently developed ways to make

nanotube plastics 20-40 times stronger [article submitted, talk at NTNE 2005 conference]

- By electron or ion irradiation: have increased nanotube bending moduli by 1-2 orders

of magnitude (but this is not the same as the tensile strength, though) [Kis et al, Nature

Materials 3 (2004)]

Thus although no nanotube rope material strong enough for the space elevator yet exists, it is not

impossible one might be manufactured within a very close future!

10.0.2. The other challenges

[http://science.nasa.gov/headlines/y2000/ast07sep_1.htm]

The NASA report also list 4 other challenges for building the thing:

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2. “Continuation of tether technology development to gain experience in the deployment and

control of such long structures in space. ”

3. “The introduction of lightweight, composite structural materials to the general construction

industry for the development of taller towers and buildings”

- This is not so bad as it sounds: Buildings and towers can be constructed many

kilometers high already today with existing technology

4. “The development of high-speed, electromagnetic [maglev] propulsion for mass-transportation

systems, launch systems, launch assist systems and high-velocity launch rails. These are, basically,

higher speed versions of the trams now used at airports to carry passengers between terminals. “

5. “The development of transportation, utility and facility infrastructures to support space

construction and industrial development from Earth out to GEO. The high cost of constructing a

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space elevator can only be justified by high usage, by both passengers and payload, tourists and

space dwellers. “

There are also lots of possible erosion problems to be considered

if the cable could be constructed, summarized in the following:

• Lightning

• Meteors

• Space debris

• Low-Earth-orbit

• Wind

• Atomic oxygen

• Electromagnetic fields

• Radiation

• Erosion of cable by sulferic acid droplets in the upper atmosphere

So obviously it is exactly yet wise to start investing money into a space elevator company: the

challenges are huge and very likely there is some fatal flaw in the idea nobody has yet thought of!

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But comparing the wildest nanotech ideas, self-replicating fully human-designed machines and the

space elevator, the latter one seems to be more realistic at least in the foreseeable future.

And if the most optimistic visions of building a space elevator would be realized, then actually those

of us who live longest might still see it in operation!

Unless nanotech makes us all immortal and we all see it - haa haa

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