Visibility of a solar shade from a planetary surface

[Shaira]

Visibility of a "solar shade" from a planetary surface

Hello everyone,

I'm a fiction writer in the process of publishing my first science-fiction novel, and have a question which I'm having trouble answering, relating to the visibility of a "solar shade" placed between a star and one of its planets.

The planet is roughly 150 million km from its star, and roughly 20000 km in diameter. A solar shade 80000 km in diameter has been placed between the planet and its star at the L1 Lagrange point a distance of some 3 million kilometres from the planet, and filters insolation by about 30%.

Now, my numbers may be off in the above para, but my question is: would you be able to see the shade from the planet's surface? My gut feeling is no: the shade is effectively eclipsing the sun and placing the planet in the area of totality, and therefore you can't see any penumbra or any telltale signs that there is a big circular parasol between you and the sun. However, I'm unsure my reasoning is right.

Could anybody (pardon the pun) please shed any light?

Many thanks in advance!



Sarah Newton

 

[xts]

1. It is impossible (well, sci-fi...) to make such shade not scattering any light. Even small amount of scattered light must be seen, especially just after sunset and just before sunrise.
2. The shade must glow with a kind of aurora borealis, caused by solar wind. This glow will be visible after sunset/before sunrise.
3. Your shade will obscure bright stars visible low above horizon just after sunset /before sunrise
4. If your planet has Moon(s), your shade will produce Moon eclipses.

Good luck!

 

[Shaira]

Hello xts!

That's awesome - many thanks indeed for the information. This sounds cooler than I'd imagined! :)

Just to clarify some of your points:

1. It is impossible (well, sci-fi...) to make such shade not scattering any light. Even small amount of scattered light must be seen, especially just after sunset and just before sunrise.

So this scattered light would appear around the sun itself, or the edges of the shade? ie would the sun seem to have a kind of glowing halo, or would there be a "sky bow" where the shade would be? Would you be able to trace the edges of the shade visually, or would it "simply" be a change to the visual appearance of the sun?

2. The shade must glow with a kind of aurora borealis, caused by solar wind. This glow will be visible after sunset/before sunrise.

This sounds quite spectacular: again, would this be in a kind of "bow-shape", tracing the arc of the edge of the shade across the sky after sunset / before sunrise? Would it be any particular colour, assuming a star like Earth's sun and a hot planetary surface with a breathable atmosphere with large quantities of particulates and relatively high CO2?

3. Your shade will obscure bright stars visible low above horizon just after sunset /before sunrise

Understood - so this would be coupled with the aurora effect, I guess? A dark "arc" in the sky.

4. If your planet has Moon(s), your shade will produce Moon eclipses.

Of course!

Thank you so much for your answers - the points above have helped me realize the visual effects of the shade would be much more spectacular and far reaching than I'd realized.



Sarah

 

[xts]

1. Halo of more or less uniform luminance. It would be easy to reduce scattering to such level that you won't spot it on a sunny day, but seems impossible to make it not visible when the Sun itself is below horizon.

2. Again - t will be the whole disc glowing. There are large scale irregularities of solar wind, so it won't be a uniform glowing - rather patterns changing in timescale of minutes or hours (and, of course, changing dramatically in longer time, with the solar activity cycle.) You'll also have sudden flashes, lasting for seconds, minutes up to hours. Aurora borealis is a good example of such phenomena. Book your Christmas holiday at Tromsø!

3. Not dark arc - deep blue colour of the sky won't be darkened. But stars visually passing shade edge will suddenly got darker or brighter. However, if your planet is located in some Galaxy position similar to ours, the Milky Way strip will get shaded if seen through your shade.You may construct your shade in two ways:
first is a 'solar glasses' approach - using some semi-transparent material.
second is Durchschlag (colander) - reflecting alu-foil with holes. In this case you won't have visible 'aurora' effects (all such radiation will be in X-ray region), but - depending on geometry of holes you have some kind of diffraction grate, which may produce even more spectacular visual effects.

 

[DaveC426913]

You know what you might try? Do some experiments.

grab a flashlight and some materials (say, some door screening, and some silk)
stretch each material over a frame or hoop
observe the effects

OK, that won't cover the borealis effect...

xts, is that a given? Or are you speculating?

 

[xts]

xts, is that a given? Or are you speculating?

"that"?

If "that" = spot aurora during Christmas vacation in Tromsø
- it is almost sure, we are now in a maximum of solar activity cycle, so if you have opportunity to spend Christmas somewhere in arctic region - this Christmas are best choice.
Tromsø offers nice (not too cold) weather but at the cost of higher probability of cloudy weather. If you are brave enough to stand -40C - continental arctic regions are better.

If "that" = "aurora-like effects in such shade"
- that's given. Virtually all transparent materials, from glass to plastics, have some kinds of fluorescence if hit by medium energy particles.
And Cherenkov's radiation (lots of it in visual spectrum) must be emitted when charged particles pass through them.

 

[DaveC426913]

"that"?

The second one.

 

[I like Serena]

Nice!
It's like I'm reading a science fiction story right here in this thread!

 

[Shaira]

Wow, this is seriously awesome xts - thanks *very* much! I really hadn't anticipated it would look so spectacular. I'm going to get on with some rewrites to incorporate this really cool detail.

The shade in question is more of the "solar glasses" version rather than the "colander", so your descriptions sound like they'll work fine.

Thank you for the excellent advice!



Sarah

 

[I like Serena]

Wait! I want more!

 

[xts]

My pleasure! Give me a draft of the book!

Two clarifications:

1. Halo (scattering, glow) differs from the one you see around Moon on misty night.
Here the 'halo' is a disc with sharp edges, bigger than Sun, its centre is not quite aligned with the centre of the Sun. That's not a one-minute-to-answer problem to say how big this misalignment must be. So, just after sunset, you may see part of this disc still above horizon. But the most apparent difference is that the 'halo' has a sharp edge.

2. DaveC, I admit, I lied!
Majority of Cherenkov's radiation would be emitted at angles much wider than pointing to the planet. So visible Cherenkov's glow would be quite small.
But we still have lots of fluorescence and scintillation effects, producing visible light. The colours depend on chemical composition of the material. Unfortunately it is not so colourful like for Cherenkov's based aurora borealis (the fluorescent colours are rather stable, maybe just two or three colours responding in slightly different proportions to different excitation) .
But we would still have changing patterns, flowing spots, etc. due to irregularities of solar wind flow.

I must think if other inelastic scattering mechanisms (e.g. Raman's) may produce visible effects...

 

[xts]

Just one more issue...

Maybe some student of mathematics/theoretical mechanics would like to play with it?

Our shade is pretty big - too big to be treated as a "material point". So we can't say that "it is placed at Lagrange point". Its centre of mass may be placed there, but it spans on the pretty large distance from it. Let's assume its mass is uniformly distributed. What will be the tension forces caused by that span? And what will be a dynamic tension due to elliptical orbital movement?
To make it a bit harder: the shade is pushed by pressure of light. So it should be placed a bit closer (how much - depends on its mass per surface unit density) to the Sun, than Lagrange point.

 

[Shaira]

Hello all,

With regard to the mathematics / mechanical aspect, the shade is itself an 'artificial intelligence', and is able to regulate itself (within reason) to accommodate the effects of solar wind, etc, on its positioning and orbit. That much is handwavium. Regarding the required tensile properties of the constituent material(s), however, I have no idea what this might be other than obviously very strong! I have played with bonded graphene crystal-type descriptions, etc, but frankly the maths/mechanics is beyond me. If there's anyone out there who might like to take a stab at what such an object might be made of (especially given its translucent properties) and how it might maintain position, I'd be extremely interested.

Also, xts - you mention it might be displaced from the L1 Lagrange point proper. Would you say that displacement would be very considerable, ie a sizable percentage of the distance?

One thing I haven't been able to work out myself yet is how large the 'halo' or 'aurora arc' might appear in the sky - how much it would subtend to an observer on the ground. If the diameter was 80000 km and the separation was 3 million or so km from a 20000km diameter world, how many minutes of arc would the halo subtend against the pre-sunrise / post-sunset sky? Many times larger than the sun, or just a little?

As an aside, the science-fiction part of the story has the shade having an artificial gravity field. However, I've been wondering if the shade might naturally possesses its own gravity - given that it's a 'thin' plane of material 80000 km in diameter, I haven't the first idea of how to start calculating that!

Many thanks again xts and everyone for your fascinating analysis and comments!



Sarah

 

[xts]

My understanding of your idea is that the shade may make minor corrections to its orbit (like TV broadcasting satellites do), but generally should obey physics and move as inertial body.

Your shade is rather thin (comparing to its size). So it must be formed not flat, but shaped such, that stress forces will be close to zero, but still stretching it. The problem to solve (well, it requires some calculations - that's why I called for a student to do hard work with lagrangians...) is to find such shape, if such shape exists at all. If such shape doesn't exist - you'll have to use more of sci-fi-hand-waving.

Other related issue is how that shape would wave and displace due to tidal forces caused by non-circular planet orbit. As for the previous issue - I have no simple intuition here - it must be calculated.

Displacement due to pressure of light:
Let's take solar system conditions. Solar light pressure at Earth orbit is around 5 µPa. At the distance or 3 million km Earth gravity imposes the same force to a construction of the weight of about 100g/m². So light construction would have to be displaced dramatically (like twice further from the Earth). For very heavy constructions (I am too lazy to calculate it more accurate than 1st order approximation...) the displacement is equal to $2r\frac{100g/m^2}{\hbox{density}}$. (r - distance to the Earth) Without calculations I can't say how big may be such displacement out of Lagrange point before the orbit becomes unstable.

Halo size:
80,000km from 3,000,000 km makes 1°32' in diameter - around 3 times bigger than Sun (Sun angular size is about 30'). Of course - your sun may be a bit smaller or bigger than ours, but don't exaggerate, unless you want to bake or freeze your planet!
BTW - tidal librations of its orbit would (for my intuition, we need a student to make calculations...) cause the shede must be larger than that in order to always keep the planet in full shade.

Artificial gravity:
the simplest solution for that is to make it rotating. It also solves the problem of folding forces in it. But on other hand it would cause pretty strong stretching tension. I am afraid - without sci-fi-extremely-strong-materials it could be hard to achieve... On the rotating shade the "gravity" would be stronger at the edge and none in the centre, but I believe you may cope with it (or even utilize it for your story).

EDIT: Oooops! The formula for displacement was wrong (reversed). => Corrected.

 

[DaveC426913]

Artificial gravity:
the simplest solution for that is to make it rotating. It also solves the problem of folding forces in it. But on other hand it would cause pretty strong stretching tension. I am afraid - without sci-fi-extremely-strong-materials it could be hard to achieve... On the rotating shade the "gravity" would be stronger at the edge and none in the centre, but I believe you may cope with it (or even utilize it for your story).

It doesn't have to be strong materials as long as the forces are low.

And it doesn't have to have centrifugal forces all the way to the centre to keep it taut. Despite having no significant centrifugal forces near the centre, as a pseudo-rigid structure, the outer edges will pull on the inner material to keep it taut.