I used to belong to the L5 Society in NYC back in 1985, which was a group of space enthusiasts. We would meet at the Intrepid Sea, Air and Space museum in New York City. Scientists, poets, sci-fi geeks, artists, musicians, and even NASA employees and astronauts. Ideas like this for using the other Lagrange equilibrium points were always being discussed.
The Lagrange points allow for insertion of objects that while at those equilibrium points, require minimal energy to maintain that position or orbit. AFAIK, there are 5. L5 was a choice for a space colony hence the L5 Society. The L5 Society became absorbed along with the National Space Instititute into the National Space Society. I believe Carl Sagan was President.
The biggest problem I see with this is that we will lose the incentive to dial down extraction of fossil energy. The non-warming side-effects of oil and other deposits such as carbonic acid in the oceans would become just as severe if not moreso.
Although this would be a major engineering feat, it could be done. It would stop global warming almost immediately. What it won't do is reduce the CO2 in the atmosphere, so ocean acidification and the resulting dieoff of aquatic life would still happen. Perhaps even worse, it would enable the continued "business as usual" burning of fossil fuels and likely cause CO2 levels to rise even higher than they would have otherwise.
This also makes for a somewhat spooky dystopian science fiction. Society engineers a gigantic sunshade. Something goes wrong and they lose contact with the shade unable to adjust it. Leading to crop failures, ecosystem collapse, and a global ice age. Frantically, the world governments race to launch a mission to repair or destroy the shade before life on earth is plunged into a permanent ice age from which a collapsed technological civilization could never hope to avert.
That is, this thing should have a dead man's switch on it, and if it isn't renewed every few years, it will self destruct, alter course, stop station keeping, etc Of course, this is probably an implausible scenario but it would make for a great TV movie.
Radiation pressure will push it out of the L1 point pretty quickly (months?) without active stationkeeping. Tidal forces will want to rotate it edge-on to the Earth, and the tug of the Moon's gravity will want to pull it out of alignment between the Sun and the Earth.
"Causing a ice age" is pretty rich, too. It can only block 1.7% of the light the Earth gets. If it gets any closer to the Earth, then it ends up in a non-geosynchronous orbit, (Well, not strictly geosynchronous. You know what I mean) and stops blocking the Sun. All the failure modes of a L1 sunshade are pretty benign.
Can you account for radiation pressure by modifying the orbit? It's a constant radial force. Searching for Lagrange points is searching for points where (in the rotating reference frame fixing the Earth) the combined gravitational forces of Earth and Sun are zero. Just add in the radiation pressure and you should get a Lagrange point slightly closer to the sun.
Buut I haven't done the math so this is speculation.
I believe gp was talking about a failure mode in which station-keeping goes on automatically when communication fails, as we learn that we massively overdone it because climate science is hard.
Right, I was talking about a sci-fi failure mode where the onboard program goes haywire and it ends up actively station keeping and maintaining the block for hundreds of years. Is 1.7% not something to worry about? IIRC, Solar variance of 0.1% has correlated with a 0.32F temperature variance. If that relation were linear, that would amount to almost a 5.5F variance.
I'm not proposing this as a 'hard scifi' model for a story, but you could tweak the scenario to get suitably scary failures (multiple shades)
Quoting further, "the mass and scale of the sunshades will also be similar to current terrestrial civil engineering projects such as the Chinese Three Gorges Dam [20], and require a mass production of coated thin material equivalent to the current world decadal production of aluminium foil."
They continue with "Nevertheless, scholarly work has yet to identify a scientific showstopper for its implementation". To me, the idea of designing a practical support framework for a disc of aluminium foil having a diameter of one thousand four hundred kilometers -- a framework that has to be shipped from earth one rocket-load at a time and assembled in space -- would be at least a show slower-downer.
There's a substantial amount of aluminium on the Moon, as well as in some asteroids. Extracting and turning it into sunshade won't be easy, obviously, but when budgets in the tens or hundreds of trillions are being talked about, the options widen a bit.
Wow you're right! Its apparently a huge thing on the moon:
"Aluminum composes 10% of the atoms and 13% of the mass of lunar highland regolith, being the third most abundant element. In the mare basins, aluminum makes up only 4.5% of the atoms and 5% of the weight, strongly suggesting the use of highland feedstocks for aluminum extraction."
Pretty much a little solar energy and a crucible and you'd turn out aluminum like mad. Absolutely the right choice for a lunar civilization, instead of lifting structures from Earth's gravity well.
Aluminum is similarly abundant in the Earth's crust. Most common element after oxygen and silicon, twice as abundant as iron. Correctly tempered and hardened aluminum is lighter and much stronger than iron.
The sole reason aluminum isn't the "default metal" in the way iron is, is that it's so much harder and more expensive to smelt:
Electric power represents about 20% to 40% of the cost of
producing aluminium, depending on the location of the
smelter. Aluminium production consumes roughly 5% of
electricity generated in the U.S.[26]
I agree that pretty much everything manufactured on the Moon is going to be made of aluminum, for precisely those reasons, but needing a "little" solar power is underselling it a little.
But the two things you have on the moon - unrestricted view of the sun, aluminum dust lying around in piles - means its certainly the easiest of the alternatives. That's the point.
It's aluminum-containing dust, not bulk metallic aluminum. All of the aluminum-containing minerals on the Moon have the aluminum atoms bound to oxygen, silicon, etc. Smelting aluminum is expensive because oxygen bonds to aluminum very tightly, and requires a lot of electricity to pull it off.
Right! So all we have to do is establish a base on the moon, build a mining and smelting facility, build a fabrication plant, build a rail-gun launcher, operate all three long enough to fling at least 10e7 tonnes [1] of finished aluminum parts toward L1, where we catch them and assemble them.
The rail-gun is not likely to be the pinch-point in the process, but suppose that the mining, smelting, fabrication, packaging and transport process can deliver a shipping unit to the rail head every minute, around the clock [2]. That's 10e7 minutes, pretty close to 20 years of continuous operation.
This stream of 1-tonne bundles of finished rods and other parts has to cross space to L1. I have no idea what the orbital feasibility of Luna-to-L1 is; I'm just assuming that a rail-gun can impart enough energy to get it there. But, one, if a bundle requires any guidance or course corrections en route, that would mean every bundle would have to be equipped with at least a computer, gyros and some type of propulsion units; and two if it approaches L1 with any significant relative velocity, it needs all that and a serious rocket engine to slow down [3]. So we are not merely flinging passive bales of aluminum parts from the rail-gun, we are launching little one-tonne spacecraft. One per minute. For 20 years. [4]
So supposing all that is in place, then you need to have sufficient personnel (plus a swarm of quite intelligent robots) at L1 to receive them, unbundle them, and distribute the parts across a disc that even in the earliest stages is hundreds of fucking kilometers in diameter, and assemble them. For 20 years.
There is nothing in this scenario that is inherently impossible. On the other hand, given it currently takes us a decade to design and transport a 1/2-tonne robot to Mars, how many decades would it take to design and build the proposed Lunar infrastructure? On time alone, never mind the costs, wouldn't it be simpler, quicker and cheaper to just stop burning petroleum?
[1] the article says 10e7 - 10e8, so we're giving them the low side of a power of 10 here.
[2] something like a factory that builds a new economy car every 2 minutes. On the moon, in a vacuum.
[3] or it has an ion engine like the Dawn craft and does continuous thrust, yadda yadda, same basic problem.
[4] you might say we could recycle the computer and ion engine from each bale, but then you need to figure out how to fire about 10e6 bundles of used engines from L1 back to the moon and soft-land them there next to the factory.
Fernly, great enumeration of some of the challenges!
I'm completely with you on the difficulty, and agree that this won't happen soon. Even so, 200 years ago the steam engine was just being developed; since then we've had the rest of the scientific, industrial, and information revolutions. So the question is what will happen 200 or 2000 years from now? A sunshade seems plausible on that time-frame, and the paper is one way to begin the conversation about such a project.
Here's some help: regolith is broken rock and dust on the ground. So no mining necessary - just earth movers (luna movers?) and a conveyor belt.
Probably the first thing you'd build would be - more mining equipment. Let the thing bootstrap itself, including the fabrication plant, the rail-gun and more mining equipment.
In 20 years you'd be ready to begin doing the rest of the project, but it would be much easier with the orders-of-magnitude larger infrastructure you'd bootstrapped.
Incidentally, NASA commissioned a study back in 1980 about self-replicating factories harvesting lunar regolith. It's quite good, and goes into excellent detail:
If we use the super cheap Proton rocket costing $16,620 per kg to lift into space. (from wikip) A tonne is 1000kg. Then this will come in at a price of 1.7^14-1.7^15 which I think is 170-1700 trillion dollars.
What about the impact of reduced insolation on photosynthesis? Wouldn't crop yields be lower? And what about reduced carbon sequestration via buried biomass? Not to mention ecosystem-wide impacts of reduced photosynthesis.
But as long as it's under control, I suppose that shading could be adjusted as appropriate.
One possibly overlooked problem with this is that alien races / robots may be able to see this sun shield from a vast distance. They will know that we are primitive people who have not yet figured out quantum dot solar and who still die from cancer.
No wait... if we put it up then we'll still have 50 or so years before this information reaches such alien planet. So we should be OK if we can figure those things out soon.
Given our current telescope technology, and assuming that others in the galaxy have the same thing, they could probably easily figure out that we're here - in the same amount of time. (Limit of light speed, and all that.)
And unless they've got FTL, it'll take awhile for them to do anything about it, anyway.
