I have considered rotating space habitats of various forms for my worldbuilding projects (i.e. cylinders, rings/torii, spheres) and recently discovered this intriguing design by physicist Pekka Janhunen https://arxiv.org/abs/1806.09808

To summarize, the habitat is a squat cylinder that is passively stable in its rotation. It is fairly large (10 km in diameter) to optimize for a number of variables such as structural and shielding requirements, and material expenditure per inhabitant. The cylinder is open on both ends; its inner volume is exposed to space, with two low-g docking ports and tubes that connect them to the cylinder. The equatorial region has substantial volume for "urban block space", which I imagine would be structured as a sort of arcology.

The landscaped part of the habitat housing its biosphere is under a transparent roof 50 meters high, to save on the amount of nitrogen required for the atmosphere. But this layer is itself enclosed within the "light channel", the most novel aspect of the design. The outer layer of the cylinder, half of which will be facing the sun at any moment, is a vast array of parabolic concentrators which funnel light into the channel. The inner surface of the channel is composed of a highly reflective material, such that light fills the volume, even the half that is anti-sunward. Semi-toroidal mirrors at the equator deflect the light by 180 degrees so that it fills the volume directly "above" the rural, landscaped part of the habitat.

Here are a couple cutaway diagrams from the paper to help visualize the habitat as a whole:

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The amount of light that reaches the surface is regulated by an array of blinders positioned along the transparent roof, so that diurnal and seasonal cycles can be simulated. The actual design of blinders used is not specified in the paper. This is where my problem arises, and it is a problem I've run into with other habitat designs as well, when mirrors are used to redirect and scatter sunlight.


Essentially, I'm having difficulty visualizing what the sky would look like to a person standing in this rural space, and the ways this could be adjusted using various types of blinding mechanisms and/or different transparent materials for the ceiling 50 meters overhead. The theoretical builders of such a habitat would want to provide a proxy to normal skies for the psychological well-being of inhabitants, and particularly in this case where the roof is only 50 meters high, they would want to prevent a sense of claustrophobia. I know the sky would not have a point source of light as we do with the sun, so I imagine all light would be diffuse, like an overcast day but perhaps much brighter?

I am also unsure as to the scale of blinders that would be preferable to the builders—they could be meters across, or centimeters, or even smaller, perhaps built into the transparent roof itself [in theoretical settings where space habitats are being built I always assume that material science has advanced significantly]. If they were too large I'd suspect that inhabitants could perceive them visually, giving the sensation of being inside a large room, unless the intensity of light was great enough that you couldn't look at the sky directly, which is something I think would be avoided. Small blinders might allow for variations in brightness that feel more natural, like clouds passing overhead, but depending on the size used you're left with anywhere from hundreds of billions to hundreds of trillions of individual blinders which require monitoring and maintenance. In any case, my problem is that I don't have an intuitive sense as to the behavior of light when it is being reflected and scattered in this context.

Hopefully that didn't come across as directionless rambling; I wanted to provide some of my thoughts. To boil it down to the question itself: What would the sky look like in the habitat described above, particularly with regards to illumination, and how do these conditions change when you adjust the size of blinders, or method of blinding? [Note: this is my first question posted on this site. If necessary I will edit it, or add more information]

  • $\begingroup$ Just a note: in the Latin language, the plural of torus is tori (note the one single -i at the end). In the English language, the plural is either toruses (preferred) or tori (allowed, but rather strange in our days when a classical education is becoming rare). Neither Latin nor English know what is a **torii with double i. P.S. I suppose that what the question is about is the aspect of the ceiling, not of the sky. People with normal vision would very obviously see that there is a ceiling above them. $\endgroup$
    – AlexP
    May 20, 2023 at 22:58
  • $\begingroup$ @AlexP Thanks for commenting. The second "i" was a typo. I was unaware that "toruses" is preferred, and can adjust that if necessary. And you're correct; I suppose I meant "sky" in the sense that one is being simulated. Observers would be looking at a ceiling, though it would be transparent, with the reflective wall of the light channel some distance above that. But if enough light is coming from above to simulate daylight, would this not mask how low the ceiling is somewhat? $\endgroup$ May 21, 2023 at 0:17
  • $\begingroup$ It isn't at all obvious what the benefit of your configuration is, to be honest. But I can suggest investing a little time in learning how to use some kind of 3D rendering software, because it'll give you a much better idea of what this stuff looks like, and the chance of someone making an answer here that does it for you is slim. $\endgroup$ May 22, 2023 at 8:22
  • $\begingroup$ @StarfishPrime Janhunen's habitat is a sort of counterpoint to the O'neill cylinder, avoiding large components such as external mirrors, which require large mechanical joints that could become single failure points. Here only the blinders are required to move, the habitable area is entirely enclosed in thick radiation shielding and micrometeorite protection (i.e. no large windows), and no active stabilization is needed. Other solutions might be favored by future habitat builders. I have zero experience with 3D rendering, but it would prove very useful in imagining large artificial spaces. $\endgroup$ May 23, 2023 at 5:22

2 Answers 2


If the interior of the radiator wall is a diffuse reflector, then it will just look white / gray. The image of the sun will be thoroughly scrambled by the outer-hull concentrator array (its job, after all, is to let light in and not let it out, not to maintain any sort of image), and diffuse white light light will then be projected at a shallow angle onto the ceiling to be dispersed into the living area below. If the ceiling is colored something other than white, then... it'll look like that, but matte white would be the optimal finish.

If, on the other hand, the ceiling on the interior of the radiator wall is a specular reflector--i.e., a mirror--then the sky will mostly look like a reflection of the ground, except in a narraow band where you actually see the reflection of the toroidal reflectors that direct light into the "rural" zone, with most of the illumination in the habitable area being indirect. This seems like a suboptimal option.

How this changes if you change the size of blinders or method of blinding depends on exactly what method of blinding you change to! You could make it look like pretty much anything you want with appropriate choice of light-interrupting options--a perfectly uniform reduction in ambient light levels, a band of night travelling radially, a band of night rotating around the cylinder, ambient dimming caused by distinct visible dark patches on the ceiling, etc.

  • $\begingroup$ I had not considered that using a true mirror would cause the ground to be reflected in the sky. A diffuse reflector would be the optimal choice. In that case, with blinders fully open, would the sky appear as if overcast, but brighter? For the blinders themselves, I suppose I was asking how their size/design initially (i.e. during construction) influenced the perception of the sky, if at all. I may have not clarified this enough, in which case I can edit the question. My thought is that they'd be like mechanical tiles, tilting open and shut variably for day/night. $\endgroup$ May 21, 2023 at 3:51
  • 1
    $\begingroup$ @sevenempest: "Would the sky appear as if overcast, but brighter?" That depends on the absorption properties of the diffuser. Without any further processing, sunlight has a much lower color temperature ("warmer white", more red-yellow) than the light filtered through the clouds (high color temperature, "cooler white", more blue). This is why an overcast sky has a blueish tint and why photocameras have distinct white balance settings for "sunlight" and "overcast". $\endgroup$
    – AlexP
    May 21, 2023 at 11:02
  • $\begingroup$ @AlexP Interesting, so this may be more complex than I originally thought. If I understand correctly, color temperature is independent of brightness. I've drawn comparison to overcast skies because that's the only condition I can think of where a large environment is illuminated by diffuse light only. It's difficult for me to visualize an environment where the intensity of this diffuse light is greater. If ambient light levels are similar to daylight, but there is no point source of light, would the sky be overwhelming to look at, or similar to the brightness of clear skies on Earth? $\endgroup$ May 22, 2023 at 3:02
  • $\begingroup$ @sevenempest Similar to daylight when and where? The brightness of ambient daylight varies enormously with the altitude of the Sun. I am admittedly weird, with a congenital abnormality in retinal development, but there are circumstances in which I find it painful to look at a clear sky away from the Sun, or even at the ground in full sunlight, so it seems obvious that you could make the ceiling painfully bright for at least some people. If I were designing the colony, I would simply not collect that much light! $\endgroup$ May 22, 2023 at 3:48
  • 2
    $\begingroup$ @sevenempest: Indoors "confortable level" of artifical illuminance is about 200 to 300 lux. Indoors "working area" artificial illumination is about 750 lux. Outdoors with overcast but not heavy overcast or rainy sky the illuminance (summer, midday) is about 1500 lux. Outdoors in summer at midday in the shade (the sun completely masked, light coming only from the blue sky) is about 20,000 lux. Outdoors in summer in direct sunlight at midday is about 50,000 to 100,000 lux. Growing crops need 15,000 to 20,000 lux. $\endgroup$
    – AlexP
    May 22, 2023 at 6:00

The sky is the limit

Or: Most results are possible with enough creative engineering

Collecting light in a tunnel for uniform spreading is a great idea. The parabolic concentrators use the directionality of the incoming sunlight to create a one way barrier, much like a diode in electronics or a valve in fluid dynamics.

From the inside of the tunnel the light will be received as a field of small dots or as stripes depending on whether the parabolic mirrors are circular or linear respectively. In the tunnel light can be directed with mirrors or scattered with diffuse materials like panels with small crystals.

With some creativity any light pattern can be created, but what can't be done is create a single very bright sun like object in the sky. Once light is scattered it cannot be 'unscattered'. But early in the reflective chain some local directionality is still available and that can be used to reflect the light to photovoltaic panels at night or further down the tunnel at day.

One question I have with this design is why the collector part of the cylinder needs to rotate with the habitat part? They do not need to be structurally connected. Only the light needs to have a path to follow. That also reduces the need to put parabolic concentrators 360 deg around, only put them at the sun facing side and rotate it once a year. This also eliminates a lot of structure, because now all those mirrors are effectively weightless.

Some practical options:

  1. The blinders could also be LCD's like we tend to put on some buildings to keep intense daylight out.
  2. The 'ceiling' can be made of small round tubes in a honeycomb like structure. This means that light going straight down passes unaltered. And light at an angle must reflect at the sides of the holes. Those sides can reflect, absorb or scatter the light. This will create a sky that is bright above, but darkens at an angle.
  3. With a grid of small rotatable LED's that project light at a tight angle a sun like object can be created. Nothing in intensity like the real sun, but enough to cast a shade. Our eyes like some shade effect to sharply define objects. A totally cloudy day always feels a bit disorienting. This assumes enough solar panels elsewhere to generate the required electricity.
  4. Some transparent materials can are also transparent in the infrared and/or UV spectrum. Like Magnesium Oxide and Magnesium Fluoride and many more. With mirrors from these materials larger parts of the spectrum can be directed inwards. At your discretion. Infrared will add warmth.

Note that every structure that does not reflect 100% of the light will heat up and needs to be cooled. This might add to much detail to the story, but is a very real thing if you want to build it in RL.

  • $\begingroup$ I do think some UV transparency would be desired for the well-being of inhabitants. I wonder if MgF2 could be produced in adequate quantities for the transparent ceiling though. LCD's are an interesting option for the blinders. I imagine they'd open a lot of possibilities. They also might have the added benefit of requiring less electricity than motors on mechanical blinders. $\endgroup$ May 24, 2023 at 22:29
  • $\begingroup$ To answer your question, the physicist who designed it seemed to prioritize passive stability. You're correct, the concentrator array experiencing centrifugal force equivalent to 1.1 g would require additional support structure, but I believe his goal was for the robust structure to double as radiation shielding. Rotating and non rotating components being unconnected, or connected only by some sort of magnetic rail, could well be a non-issue for a society building space habitats. For whatever reason, I don't particularly like the aesthetics of large external mirrors at 45° $\endgroup$ May 24, 2023 at 22:49

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