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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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Up above, the cable would be made from something that reflects much of light and have sharp edges. This would prevent someone (or birds) accidentaly flying into it. Sharp edges would prevent birds from sitting on it(unless they want to slice their feet).

Down below, the cable would be painted white(if thick enough), or have no paint at all, and instead be covered with multicolor LEDs. Since we have a flying thread, we might as well attract some tourists; you could emit various lights and make interesting shows(thread tetris, snake, some "avoid traffic" game...), colorful light shows, or paint the flag of the next country that gets hit by terrorism attack.

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  • $\begingroup$ How are birds going to sit on it? On which parts are you thinking? What are you going to paint all these light-shows and flags and such on? Are you thinking of the mobile objects that would move up and down along the structure? If you are thinking there could be constant effects like what you are talking about in the inner-sky portion of the elevator all the time, I cannot imagine how that works from the conventional view of a space elevator. $\endgroup$ – Loduwijk Apr 19 '17 at 17:51
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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 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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