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Comment by WhompingWindows | Hacker News Reader

Can we discuss space sunshades? Assuming the cost of getting freight to orbit goes down substantially as companies like SpaceX ramp up launches, what's the limiting parameter on some sort of sunshade over the north and/or south pole? Is it indeed the cost of launching the sunshade, or are there other factors at play which make the space sunshade unrealistic? What materials would be reflective and resilient and cost-effective to block the sun? Would polar blockers be enough to cool the planet as a whole and stop warming, or would multiple sunshades spread around the globe be better?

Also, generally, what about increasing cloud cover in general? Wouldn't this reap a huge reward for cooling the planet? I know it doesn't remove gasses and prevent ocean acidification, however it may help with heat-related issues.

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My personal preference is an inflatable sun shield method like these 10,000 kilometer-wide objects: "Self-deployed, extremely large low mass space structures" http://diyhpl.us/~bryan/papers2/space/Self-deployed%20extrem...

I don't think people realize how a lot of these problems are solvable for only a few billion dollars... It's an engineering problem, mainly. Someone just spends the money and goes and fixes the problem.

That's one great thing about sunshades- they can be implemented by just convincing a spacefaring billionaire or two instead of having to change policy / steer nation-states.

Unfortunately, you still need to convince the billionaire that the project will make him into a 10-billionaire, otherwise he’ll just keep it in the bank, or bathe in it or whatever rich people do with all the vast piles of money they are sitting on.

Perhaps there's the chance that they're interested in their legacy being saving the human race. There will probably be statues of them, schools named after them, and who knows what else if the plan succeeded.

How would this affect plant life and agriculture?

Since this would be over the poles, agriculture would not be impacted. There would likely be a substantial impact to ecology of these regions, depending on size of the shade of course.

I'm skeptical -

You've shown a proposal, not a reality. (and the proposal didn't even mention a "shield" that I saw, but I may have skimmed past it). I didn't see a price tag, nor a timeline. Money can boost research speed, but not remove the need entirely.

Second, If the cross section of the earth is 1.2x10^14 m^2, one of these would...well, we don't know, because the link mentioned volumes and length (the 10k km is a potential "span length" and wasn't tied to being "one" of anything). I failed to find the cargo volume for a Falcon 9 or other rocket (everything is in mass) but I think it's safe to say that you're talking a lot of launches which clearly aren't trivial to do.

Third, and most importantly, you've just decreased the amount of energy coming to earth. You've not solved the problem, you've changed it.

I love tech, but I think it's worth noticing the ratio of times someone says "It's [just] an engineering problem" versus the number of issues that have been actually been solved in this way (seeing the problem, spending a boatload of money, seeing problem solved). Most of our industries are based on the fact that we KNOW they can grow into more, but figuring out how and the complications thereof are literally the work of countless lifetimes.

> Third, and most importantly, you've just decreased the amount of energy coming to earth. You've not solved the problem, you've changed it.

Probably the most important advantage sunshades have over other kinds of geoengineering megaprojects is that they can be trivially (compared to their construction costs, that is...) turned off. If they turn out to be very harmful in some previously unforeseen way, we can choose to not have them anymore.

That is not so for, for example, seeding lots of nutrients for algae over the continental shelf.

As others have noted, solar radiation management doesn't do anything to counteract ocean acidification. I still think that SRM of some sort may be necessary to stop runaway feedback loops like loss of soil carbon from thawing permafrost.

The sunshade approach to SRM looks like it may have been devised like other big speculative space projects: "start by assuming that big projects in space are the solution, then find a problem to motivate that solution." (See also: mining helium 3 from the Moon.)

Solar radiation management with reflective aerosols instead of space sunshades looks simpler and easier to me. Most importantly, SRM based on aerosols is incrementally scalable from small low initial investment/scope. The sunshade approach, like many envisioned large space projects, appears to require large "lumpy" investments before delivering any detectable benefit.

On the other hand, we've got a pretty bad track history with adding gases to the atmosphere.

And, as mentioned, it's pretty easy to 'turn off' (fold or destroy) a sunshade. Not so easy to remove a bunch of aerosol in the stratosphere. I'd prefer more expensive but reversible action, personally.

On the other hand, a slow approach may negate that aspect, and just about anything is better than the nothing we've got now.

Depending on how high in the atmosphere the aerosols are added, they remain aloft for a few weeks to a few years. It's not a long-term commitment to altering the atmosphere's composition like burning fossils has been. This is in fact one of the arguments raised against solar radiation management: that the temperature rebound could be terrifyingly rapid if SRM is stopped. But that danger applies to space sunshades too.

>Third, and most importantly, you've just decreased the amount of energy coming to earth. You've not solved the problem, you've changed it.

There's a lot of EM that neither chlorophyll nor existing PV can use. Hopefully a sunshade would be chosen that selectively works on those less-useful bands. I hope that someone is looking into aerogels for this purpose; they can cover a lot of area per unit mass and have tunable optical properties.

>> for only a few billion dollars.

Really? This isn't just a matter of putting a big ball of tin foil out there letting it go. What about the station keeping? How is this big piece of tinfoil going to stay in place? At these sizes, solar pressure and the buildup of static charges become huge issues. It is going to drift. It will need engines, power and fuel. Ion thrust would seem an obvious answer, but electrical/static issues may make a web of ion drives difficult. It will need either new tech or, at least, extensive testing of current tech before any attempt. = a great many billions of dollars. I'd rather see that money spent on solar panels.

Anything you can do in space would be cheaper on earth. So rather than shading the pole by spreading ~100,000 sq. km of material in orbit, instead ask why we wouldn't spread the same shade cloth / reflective mylar etc. on the surface of the earth. Then you realize how much of a colossal project it is and how completely impractical is is.

Earth is a closed system, so the shade cloth on Earth would simply warm up and reradiate the absorbed energy as infrared. Reflective mylar would see a portion of the reflected sunlight (which would have warmed the atmosphere upon entry) trapped by the greenhouse effect.

This is not necessarily true as demonstrated by Steven Chu's 'Paint the Earth White' campaign.

Earth is is not even close to a closed system. There's a giant thermonuclear explosion factory in space that pumps insane amounts of energy into our system. If earth was a closed system it could not harbor life as entropy increased.

That's why you set up orbital infrastructure so that you don't have to bring up everything from Earth and can just mine stuff in space.

It's way too late for that. Recent UN report basically says we're hosed by 2040.

We do not have decades to develop a mature space-mining and construction industry.

Anything you build on earth is inside the atmosphere. Keeping heat away from greenhouse gasses is the whole point.

Objects in space are generally also subject to less weather, and as such they don't materially degrade. You could make a shield out of opaque film that would tear to shreds at the slightest breeze (see JWST sun shield)

The easiest surfaces to make more reflecting should be the oceans. Some proposed carbon capture methods involve algae. Maybe algae could be synthesized that are white/reflective as well as capture carbon? Patches of algae can be huge without the need for any construction.

I don't know about that; land is expensive and inhabited by plants and animals, and you have to deal with wind. In space, there are other problems but at least you're probably not getting in anyone's way. You might also be able to use a much smaller shade, as it can be parked in L1 where it's always shading the Earth, and you don't have to deal with some of the reflected light diffusing through the atmosphere and not actually making it back out into space.

That said, I think the usual objection to the space mirror approach is that you can get the same result much cheaper by spreading a cloud of tiny reflective particles into the upper atmosphere. (I've heard of proposals that would just mandate some additive to the fuel used by large passenger aircraft.)

Making the sky 1% hazier is kind of uninspiring (especially when compared to giant orbital mirrors, which could also act as death-rays if aligned and focused properly), but if it's cheap and is likely to work, then maybe it's worth a try.

(Unfortunately none of these options is that they don't do anything about ocean acidification.)

I think Nathan Myhrvold [1] proposed something like this. There's a summary of it on one of the Freakonomics podcasts, but the summary is basically that it acts like an erupting volcano to cool the earth slightly.

"MYHRVOLD: So, climate change is a 1-percent effect. Now all we have to do is make the sun 1 percent dimmer. Now I don’t literally mean changing the sun. But there are a variety of things that bounce sunlight back into space. Clouds are one of those things: white clouds bounce white light back up into space. It turns out that volcanoes throw ash and particles, if it’s a big volcano, very high in the atmosphere. That reflects some of that light. And in fact this happened in 1991 when Mount Pinatubo went off. It cooled worldwide temperatures by a degree, degree-and-a-half-Fahrenheit for 12 to 18 months. Well, my company has come up with some very practical and cost-effective ways of deliberately putting particles into the upper atmosphere. And on paper, it works out that you could nullify all of global warming that way." [2]

[1] http://www.nathanmyhrvold.com/ [2] http://freakonomics.com/podcast/save-the-planet/

"My company" referring to Intellectual Ventures, which has such an illustrious reputation with patents?

Because the earth is effectively a black body it cools down at T in kelvin^4. This means you need to block a huge amount of energy ~2% of the sun to get around global warming. Sticking something at L1 looks elegant, but you cant just toss junk there it needs serious station keeping making it far more expensive than many assume.

PS: Unfortunately, as a side effect you reduce the energy of all plant life by ~2% solar panels would also be effected. While this is probably no where near as bad as global warming it is something to look into.

The launch volume looks infeasibly large in the foreseeable future, but I don't think the station keeping problem is that bad. Any sun shade is basically going to be a solar sail already and steering via LCD panels like IKAROS did should be sufficient.

https://en.wikipedia.org/wiki/IKAROS

The problem is you now need some actual structure rather than just some thin sheets / inflatable structure or whatever. Further applying even small amounts of force across huge sheets adds up while as you say it's acting like a solar sail.

That is to say it's clearly still possible, just not as cheap as people are assuming.

IKAROS handled that by spinning very slowly to remain rigid with centrifugal force. To be practical you really can't have one single structure but a swarm of medium sized shades would work much better.

If by medium size your thinking ~1km^2 then sure. But, much smaller than that you you start talking about billions of these things. Keeping them properly aligned and not shading each other would be a non trivial challenge.

My proposal is to send up a robot, capture a smallish asteroid, and have it use the material to weave/spin out a shield. The robot sits at the edge and spins and weaves away. More robots would be employed as the shield grows. Honestly, I've not worked out all the details. I'd love to hear what others think. Is this just pie(sheet) in the sky? Or is this the salvation of humankind.

Earth-Sol L1 is probably a bad place for a solar shade. That's where a solar shade of any given size is the most efficient, but the cost of getting it there is much more than the cost of having more, less efficient ones.

A better choice is something akin to a semi-sun-synchronous Molniya-style-orbit. That is, orbit that is somewhat highly elliptical (so a satellite on it spends most of it's time near apogee, and zips through the perigee relatively quickly), and which precesses so that the apogee is always between the earth and the sun.

A satellite on such an orbit can easily spend >70% of it's time between the earth and the sun, and you could send many satellites there for a similar cost to putting just one to Earth-Sun L1.

LEO to L1 is not that bad using solar sails or ionic propulsion.

There are going to be real trade-offs to any orbit, but I think the minimum cost is going to be above X mass in L1 means X mass into LEO as you can't use less mass than that in any other orbit.

But then you have to put solar sails or ionic propulsion into the shade.

BFR could deploy a completely passive sail directly into such a sun-synchronous elliptical orbit, return to land, and launch again after a few hours.

Anything that blocks that much light is going to be a huge solar sail and need a lot of station keeping.

The only reason ionic propulsion might be considered is the they are already going to be solar panels. If you make a large chunk of them from solar panels you then have crazy energy for space based industry and plenty of energy for ionic propulsion.

Also, with that much mass in orbit you are going to want some way do move dammaged segments.

Supposing you can re-orient parts of the "huge solar sail" then wouldn't it become rather like the sail of a ship or the wing of a plane, and be usable as a control surface for maneuvering?

I'd imagine that in the neighborhood of L1 you would try to find some equilibrium point a little closer in to the sun so that the force of the solar wind matches gravitational pull to the sun. If you start drifting in to the sun, you expose more surface area, and if you're drifting towards the Earth, you re-orient the sail to absorb less solar wind.

That's the idea I was going with. You let the sun counteract most of the force from your solar sail, but that requires you to constantly to make minor adjustments because unlike a normal orbit it's not even close to stable over time.

But, while not necessarily difficult it does prevent a lot of the more simple designs as you can't just toss out a bunch of reflective beach balls with very low pressure or other 'simple' means of blocking sunlight.

How are you going to use a solar sail to go towards L1 from LEO?

You don't have a keel, but you can use the suns gravity with a similar goal. The sun's gravity is much stronger than a sail's thrust so you can't hover, but you do end up in a higher orbit by aiming it at the sun. However, this averages over an orbit so you don't gain energy over time.

But, aim the sail off-center say 45 degrees from the sun and you can pick if that's adding or subtracting angular momentum from your orbit over time. Reduce angular momentum and gravity pulls you into a lower orbit and thus more orbits/year. Reverse that and your gain angular momentum and get fewer orbits per year. L1 is a special case as it's balanced between the sun and earth so you have the same number of orbits per year.

Now, rather than just orbiting the sun your also orbiting the earth. But, similar manovers work just fine.

PS: It's even more complected as you end up in elliptical orbits if you don't keep things balanced etc, but that's the basic idea.

Is L1 the most efficient for a fixed size? The difference is ~6000 km for LEO versus 1.5 million km for L1 to the Earth's center and the r^2 law means you need to put a lot more area in L1 than in LEO, even after discounting for the fact that in LEO it'll be on the wrong side of the Earth half the time. Filling up appreciable fractions of LEO might have it's own problems.

r^2 is not that relevant. The moon is 384,400 km from the earth and can cause a total solar eclipse despite only being 3,474 km wide where the earth is 12,742 km wide.

The geometry works out more or less linearly with distance. So 1.5 / 0.384 = 1.065. Which means and object 1.065x as wide as the earth would also provide a total solar eclipse. Making sunshades more than 2x effective L1 vs LEO.

PS: Only 1/2 the time they are on the sun facing side, but they are not perpendicular with sunlight for that entire half. At a 45 degree angle a sunshade blocks ~70% as much sunlight vs a 0 degree angle.

That falls under a number of strategies for Solar Radiation Management [https://en.wikipedia.org/wiki/Solar_radiation_management], which, as you say, doesn't address the core CO2 concentration problem, but it might buy us time.

Personally, I like the idea of Marine Cloud Brightening. Getting 1 or 2 thousand ships spraying seawater into the air seems tractable, if not trivial.

Sunshades don't address other attendant issues stemming from increased CO2, such as ocean acidification. The ocean biosphere wholesale is threatened by acidification. (Among other things, bio-carbonate generation and stability are undermined. And thus, the entire food chain above organisms that depend upon this.)

How would you keep a sunshade over one pole? Keeping anything over a single spot requires a geostationary orbit, which by definition is only possible on the equator.

I suppose you could have a bunch of shades in a geosynchronous polar orbit, but I suspect they'd still be more effective over the equator, just because it gets more sunlight.

Seconded. It would just fall down.

https://en.wikipedia.org/wiki/Statite

Interesting; I'd seen this before but forgotten about it.

Don't think it works here though - if the pressure of reflected sunlight is counteracting gravity, then I don't see how the statite can be acting as a sunshade. If it were in position for that, the sunlight would be pushing it toward the Earth, no?

If anything this sounds more like the reflectors often proposed for Martian terraforming, which are designed to heat the planet rather than cool it.

A very different mode of Statite system operation is possible that would allow the Statite to be placed at any point around the Earth, at all times of the year, even over the sunlit side, at the expense of slightly greater operating distance. Instead of the Statite being balanced by sunlight in the gravitational field of the Earth, the Statite would be placed in orbit around the Sun, at such a distance from the earth that the gravitational field of the Earth plus the moon is only a perturbation on the gravitational field of the Sun. [1]

But in practice, I think you're right. Forward wrote a short story about such a "pole sitter" ("Race to the Pole"), and had one of the characters say this:

The control problem of keeping the [statite] balanced over the pole is very tricky, especially during the summer season of that hemisphere when the polar axis is over on the sunlit side of the Earth. That’s why ‘pole-sitters’ have to be placed so far away from the Earth. If they get any closer than 250 Earth radii, they become unstable during the summer. [2]

[1] https://patents.google.com/patent/US5183225A/en

[2] https://www.centauri-dreams.org/2010/07/29/statites-hovering...

I think geo-engineering will open Pandora's box. On one hand, we need to invest in geo-engineering as a last resort, on the other hand, if people see this as the last resort the problem of CO2 will seem less urgent. Very dangerous stuff.

People already don’t care about carbon emissions. Those on the right think it’s a hoax or humans aren’t causing climate change, those on the left largely pay lip service, but still do very little.

Wouldn't sun shades blocking any amount of sunlight have a negative effect on agriculture and endanger the food supply?

What impact would sun-shades have on agriculture?

...Mr. Burns?

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