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Consider a space elevator, a thread hanging from the heavens and being anchored to a spot in ground or sea barge at equator. As I understand it, it'd be very thin at ground and get thicker on the way up to GEO, and thinner again towards the counterweight station at the end. Construction material is basic pure carbon nanotube in this case.

Added details: I'm thinking of an early space elevator, with payload capacity in single digit metric tons. Hmm, how it is powered might radically affect how the whole thing looks like, but to avoid altering the question, it's ok to assume it does not.

What does it look like? What color is it? Would it glisten in sunlight, upper parts looking like a spear of light at early/late nighttime? Or would it be coal black, visible against blue sky as a black thread? Or something else?

Would the elevator need coating, paint, against UV light or water or anything else? Could it even be painted? I think not, weight would be too much. But if yes, perhaps atom- or molecule-thick layer of something that would stick, how would this change coloring options?

Edit: I'd prefer as hard-science answer as possible. For those suggesting paint and even lights, weight of the paint (this fortunately scales r²) vs. weight of the payload (this might scale r³, unless there are some factors I'm not aware of) would be important. Something to think of about this: the cable has carry the weight of the paint plus the extra weight of the cable below it needed because of the paint.

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    $\begingroup$ @Rek The paint still adds a lot of volume (proportionally) to the nanotube though, is that not an issue? $\endgroup$ – Bellerophon Sep 20 '16 at 20:32
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    $\begingroup$ Obviously it would be whatever color the advertising panels choose. It's too good a billboard to pass up. $\endgroup$ – WhatRoughBeast Sep 20 '16 at 22:07
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    $\begingroup$ @rek It needs to support it's own weight though, so wouldn't painting it call for much stronger thread? 15km of paint could weight a lot. $\endgroup$ – Borsunho Sep 20 '16 at 22:09
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    $\begingroup$ Right, adding weight does matter because the stress on the cable exists. The fact that the total weight is ballanced doesn’t negate that. $\endgroup$ – JDługosz Sep 21 '16 at 3:46
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    $\begingroup$ I really doubt they would bother painting it, or at least most of it. Maybe the Earthwardmost kilometer or so--what could be seen from ground. But consider that it's 35,786km (almost the circumference of Earth) just to get to geosynchronous orbit, and much much further to get to the top (bottom?). Even if machines do it, the cost to paint something that huge would be immense. $\endgroup$ – Devsman Sep 21 '16 at 12:42
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In Atmosphere

It would likely be painted or wrapped in high-contrast colours, such as alternating stripes or a checker board pattern of white and black or yellow and black reflective paint or material. Aircraft warning lights would be spaced around the diameter of the tube or shaft at 90°, every 100 metres between ground level and 15,000 metres altitude.

Aircraft warning light

In Orbit

Above 15 kilometres the paint scheme would continue but with wider stripes or larger checker boxes. The aircraft warning lights would be replaced with vacuum-safe lights, half anchored to the tube or shaft itself, half on arms extended out from the elevator and positioned to shine back on it, also spaced farther apart.

Paint

In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 gigapascals (9,100,000 psi). (For illustration, this translates into the ability to endure tension of a weight equivalent to 6,422 kilograms-force (62,980 N; 14,160 lbf) on a cable with cross-section of 1 square millimetre (0.0016 sq in). (Source)

The weight of paint coating can be calculated as area x thickness x density.

Geostationary orbit is achieved just shy of 36,000 km, meaning the minimal paintable area is 36,000 km x 3.54 mm (the circumference of a circle with a cross sectional area of 1mm$^2$): 127,440 m$^2$. (The elevator climber won't be going up a cable this thin, but the exact dimensions haven't been provided yet.)

For paint I'm going to assume a state-of-the-art aerogel coating, which can be as thin as 1 µm (0.001 mm) and comes in a variety of colours (including transparent, for the black). Silica aerogel has a density of 1,000 g/m$^3$ and aerographene has a density of 160 g/m$^3$ but is transparent (the carbon black will show through), so half and half.

White: 0.50 x Total paint volume (127,440m$^2$ x 0.001 mm = 0.12744 m$^3$) x aerogel density (1,000 g/m$^3$)

plus

Black: 0.50 x Total paint volume (127,440m$^2$ x 0.001 mm = 0.12744 m$^3$) x aerogel density (160 g/m$^3$)

= 73.9152 grams

Not even 100 grams of paint weight would be added, for every 1 mm cross section of carbon nanotube under geostationary altitude. Well within the tensile strength tolerance.

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    $\begingroup$ @Bellerophon It needs to be visible under all sorts of lighting conditions and against the full range of day and night skies. $\endgroup$ – rek Sep 20 '16 at 20:51
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    $\begingroup$ @hyde Mercedes have the characteristic grey color because they had a car that was too heavy, so they scraped off the paint, exposing the metallic color, if the weight difference on a car can have such an impact, i think it is a really good question on such a large structure. $\endgroup$ – Magic-Mouse Sep 21 '16 at 7:09
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    $\begingroup$ Also if the lights are strong enough to see from further away than a few meters, thousands of them and the means of getting electricity to them are also going to weigh a lot. $\endgroup$ – RemcoGerlich Sep 21 '16 at 7:23
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    $\begingroup$ I would think that the world's first space elevator is surrounded by a LARGE no-fly-zone (likely enforced by military), negating the need for aircraft warning lights. $\endgroup$ – Sanchises Sep 21 '16 at 8:18
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    $\begingroup$ There's really no point in painting or lighting it above the operational altitude for aircraft. By the time a spacecraft could see it with the naked eye, it would be much too late to take any evasive action. $\endgroup$ – Mike Scott Sep 21 '16 at 10:12
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Members of the fullerene structural family (which includes carbon nanotubes) are usually black when solid. I don't think that the nanotube needs an anti-UV coating and nanotubes usually shed water naturally so no coating is needed to prevent water vapour building up.

Hydrogen does react with diamond so there may need to be something to stop hydrogen reacting with the nanotube (not sure if this reaction still happens with nanotubes) but this coating could probably be colourless. I would avoid adding paint just for show due to its added volume and mass so your elevator is likely to be dull grey/black.

Edit According to @Rek, extra mass from the paint isn't a problem. In that case the colour is entirely up to you as paint can be almost any colour.

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    $\begingroup$ ah, i didn't think of the added mass of painting it for my answer. good call $\endgroup$ – Madcow Sep 20 '16 at 20:20
  • $\begingroup$ Usually? Well, if it supports metal-like conduction it might be silver instead! Or it might be white. The electronic properties are tunable or vary all over the place. $\endgroup$ – JDługosz Sep 21 '16 at 3:48
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    $\begingroup$ Weight does matter. $\endgroup$ – JDługosz Sep 21 '16 at 3:49
  • $\begingroup$ From some early article about fullerene from 20 years ago I remember sample of C60 being bold yellow. $\endgroup$ – Agent_L Sep 21 '16 at 15:02
  • $\begingroup$ @Angent-L As liquids fullerenes have more colour variation so maybe that was it? $\endgroup$ – Bellerophon Sep 21 '16 at 16:59
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I know you are looking for the color of the material it is made out of, but unfortunately it might be painted neon-yellow or neon-orange like a traffic sign. Some color to make it stand out when you are looking at a night sky or clear blue sky background. Just to help plans and space ships from accidentally hitting it.

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  • $\begingroup$ Planes and spaceships don't navigate by looking out the windows. I can't imagine there not being a no-fly zone for a large distance around the cable, and military installations to take care of rogues. $\endgroup$ – Innovine Dec 14 '19 at 9:09
  • $\begingroup$ en.wikipedia.org/wiki/Aircraft_warning_lights Lights are added to radio towers and building just for this purpose. $\endgroup$ – Madcow Dec 15 '19 at 15:32
  • $\begingroup$ Yes, because they are not valuable assets, and because they do not go very high. The lights are to warn off vfr planes. A space cable will have a no-fly zone around it, like military installations and nuclear power facilities. The difference is that in the case of buildings and radio towers, the intent of warning lights is to protect the plane. In a space elevator, the intent will be to protect the elevator. A space elevator will have ground-to-air missiles to prevent collisions, not a red light. $\endgroup$ – Innovine Dec 16 '19 at 7:57
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Others have answered that it would need color to prevent planes and spaceships from flying into it. However I'd propose that a no-fly zone would be put in place around the elevator to prevent any danger of accidents and reduce any terror threat. Taking into account the paint weight issue, it would therefore be its natural black color.

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  • $\begingroup$ How do you know the natural color is black? $\endgroup$ – hyde Sep 21 '16 at 11:06
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    $\begingroup$ bellerephon pointed this out in his answer. $\endgroup$ – Jonathan van de Veen Sep 21 '16 at 11:50
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    $\begingroup$ Is there anything new in your answer? $\endgroup$ – Mołot Sep 21 '16 at 15:07
  • $\begingroup$ @Mołot it is a shame no one mention no-fly zone. Paint is totally useless for purposes. This kinda strategic object, even not taking price in to account, should be guarded better then area51. $\endgroup$ – MolbOrg Sep 21 '16 at 16:05
  • $\begingroup$ @MolbOrg A no-fly zone was mentioned in a comment several hours before this answer was posted. Arguably though a no-fly zone is outside the domain of the answer, as it doesn't contribute "colour" to the structure. $\endgroup$ – rek Sep 21 '16 at 16:40
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It’s worth noting that even fairly dark objects will reflect some light. Unless the space elevator is completely absorbent (which is possible, as Vantablack is made of carbon nanotubes, but unlikely, as those are specially configured — vertically aligned, while surely a space elevator cable will be made of laterally aligned nanotubes), enough light will reflect to make it a streak of ribbon in the sky.

It would probably stand out more on a cloudless night than it would during the day. Just as we cannot see Mercury as it traverses the sun, or a fly sitting on a car headlight, so a relatively thin object, no matter what colour or how reflective, will not be easily discerned in a bright sky.

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  • $\begingroup$ 1. Vantablack is not completely absorbent, just one of the darkest known substances. 2. VANTAs (Vertically aligned carbon nanotubes) are EXACTLY the microstructure of carbon nanotubes theorized to be used in high stress applications such as a space elevator ncbi.nlm.nih.gov/pubmed/22966627 $\endgroup$ – Andrew Mellor Dec 12 '19 at 17:54
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    $\begingroup$ @AndrewMellor In VANTAblack, the tubules are viewed end on, hence their extremely low reflectivity. The cable's tubes are parallel to the cable length, so you're looking at the sides of the tubes. Not at all the same. $\endgroup$ – Zeiss Ikon Dec 12 '19 at 18:33
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Assuming you have a solar power station at the top of the structure (and why wouldn't you -- it's free power, at least to run the elevator), the cable can be any color you like, including flashing multi-color with chasing patterns along its length ("Sir, are you claiming you failed to see a cable flashing multiple high-visibility colors in a virtual barber pole stripe pattern? How did a blind man get a pilot's license, anyway?").

All that's needed to make it this way is to use the cable itself as the conductors to bring the power from the GEO station or Top End down to Earth -- and build LEDs into the cable nanotube structure.

The nanotubes have to be bonded together periodically along their length, to make the cable act like, well, a cable, instead of so much spun glass holiday decoration. The bonding points can be integral LEDs, and assuming they're close together (better for cable handling and strength) when lit they'll make the cable look like a line of light. Make them in equal numbers of red, green, blue, and you can make the cable lighting any color you like -- and because there are billions of these tiny LEDs, with some computing power added it can display any pattern the cable's shape will permit. It could even act as a display screen through a window in elevator car, sliding up or down as the car moves.

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A Chain of Blinking Lights

The big problem with a space elevator is that unless you have an extremely strong tensile material the cable will have an extreme taper to it. The maximum tensile stress on the cable occurs at geostationary orbit, and that's where it would be thickest.

Theoretically, carbon nanotubes could require a taper ratio of only 1.6, but we can't make 40,000 km long single nanotubes. Kevlar would require a taper of 2.5 X 10^8.

In either case, the tension is lowest at the ground, and so it would be important to make the cable as thin as possible at the bottom. Let's assume we found a material that only required a taper ratio of 1000. In that case, every foot in diameter at the bottom would require the geostationary portion to be 1000 feet larger in circumference.

If a cable were to go from a 10 ft diameter to a 10,000 ft diameter at geostationary orbit, then even by the time it reached airliner levels (say, 10 miles up) it would still only be 40 ft in diameter.

Wikipedia has a good article on this showing the taper ratios of various materials:

Wikipedia - Space Elevators

So, if you were standing near it, the thing would basically look like a radio tower extending to infinity. It would only be visible at all from a very close distance - a mile or two maybe. Therefore, it would be a navigation hazard and would be likely to have beacon lights attached to it every 500 ft or so.

The FAA requires any tower over 750 ft to be marked with two red lights and one high intensity white light every few hundred feet. These would be visible from a much farther distance than the cable itself, so from any distance more than a mile or two away you'd see what looked like an unsupported string of lights in the sky, and once you got close you'd see something like a radio tower that climbs to infinity.

Color won't be important - when you look at a radio tower do you notice the color of the material? The color will be the natural color of the material, except for the bottom few hundred feet where again the FAA has a rule - alternating colors including aviation orange.

Since we don't have a material that be used to make a space elevator, we don't know what natural color it would be. A good guess would be black, charcoal, grey, or a dull metallic color of some kind. But you wouldn't really see it unless you walked right up to the thing.

FAA Tower and Obstruction Marking Guidelines

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White

the ISS is testing "paint" AKA an electrically conductive coating to prevent static buildup, the radiation in space can cause static build up that can fry electronics. presumably the crawler would have electronics. I have no idea what color it is though.

mirror reflective or white is the favored color for space equipment just to minimize thermal transfer from light from the sun, the drastic difference in heating can cause strain due to drastic differences in thermal expansion. White has an advantage as mirrored reflective coatings usually requires a heavier metallic paint si white is the most common. So whatever portion of the cable that is beyond the thermal inertia of the atmosphere will be white.

For what to make the paint out of rek's answer about aerogel based coatings is excelent both for weight but also because it is an excelent themral insulator.

https://www.forbes.com/sites/quora/2017/12/27/why-are-spaceships-painted-if-its-just-extra-weight/#3bdb395b69ca

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  • $\begingroup$ Could useable power be drawn from such static charges? Won't electrical currents in the cable cause it to move in the planets magnetic field? $\endgroup$ – Innovine Dec 14 '19 at 9:13
  • $\begingroup$ A standing static charge is unaffected by a magnetic field, and generating usable power would require adding a lot of weight in terms of conducting wires. $\endgroup$ – John Dec 14 '19 at 15:45

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