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« No More Nanotechnology? | Main | Big Day for CRN in Venezuela »

November 10, 2006


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Tom Craver

20M tons, at 1000000 shades/ton - so he's planning on 20 trillion of these devices, each with an area of about 0.4m^2, or about 8 million sqkm - i.e. about 10% of the earth's solar intercept cross-section.

Since he's aiming to cut light by 2%, his refractive approach is about 20% as efficient as using reflectors.

He apparently has the sunshades dynamically use sunlight pressure to maintain position between the sun and Earth - but he deliberately made the shades very non-reflective so that photonic pressure wouldn't quickly blow them away.

Making the shades refractive won't prevent interaction with the solar wind - does anyone have access to the full PNAS paper, to check if he took that into consideration?

And as long as he's putting intelligence on board, why not have them balance gravity, sunlight, solar wind, position etc? Then they could be reflective and near 100% efficient, reducing the mass to 4 million tons. They might be thinner as well, further shaving the mass.

Tom Craver

One way to amplify the effects of space sunshades, would be to cool the sea near Earth's poles - increasing ice cover, and countering a strong global warming feedback loop. That should give on the order of a 8x gain (albedo of snow = 0.8 vs seawater's 0.1). But it might not work with his refractive rather than reflective shades.

It'd still be cheaper, even with $20/kg launch costs, to use reflectors floating high in earth's atmosphere, with just enough smarts on board to stay at a specified altitude and maybe the a fixed latitude (to use the ice leveraging scheme).


I hope that "big solutions [that] will be desparately needed" do not only include absurdly oversized mega-engineering (running out of superlatives here), but also political, inconvenient, almost boring solutions like international agreement on lowering harmful emission which are actually enforced. Otherwise, hypothetical international regulation of MM, which is the only possible kind of MM regulation, wouldn't stand a chance.


Physicist Gregory Benford proposed injecting particles into the stratosphere over the polar regions to preferentially cool those areas (save the polar bears!). It is much cheaper to focus on smaller areas like that, not to mention this technology is basically feasible today. Wind currents allow the aerosols to remain concentrated over the poles, and they would rain out every year so if things went wrong the experiment could be stopped.


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