[AR] Re: Freeman Dyson, RIP

  • From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
  • To: Arocket List <arocket@xxxxxxxxxxxxx>
  • Date: Thu, 5 Mar 2020 15:07:10 -0500 (EST)

On Wed, 4 Mar 2020, David Summers wrote:

...The solar sail would just be a large reflecting mirror, about 1 um thick (which implies in-space film processing - perhaps take a heavy Mylar and then chemically remove the polymer).

No, it needn't require in-space processing: Dupont has made 0.9um Mylar in quantity with one side aluminized and the other side black, for solar sail use. (This was for the abortive Team Encounter mission.) It was a custom job, not an off-the-shelf product, and it turned out to be a hassle so they would charge a lot to do it again, but <1um can be done on Earth if you want it badly enough.

If you want <100nm (skip the Mylar and keep the aluminum), now *that* probably needs an in-space factory.

The solar cells would be hanging below the sail on the sunward side. This would allow you to concentrate the sunlight for higher efficiency, and be more picky about the direction you emit waste heat. As long as the waste heat does not go up, it will (on average) push outward...

But then, again, we're talking about sending most of the light, and the radiated heat, *inward*. Which is fine for individual craft, and totally unacceptable for a Dyson sphere because of what it would eventually do to the Sun (hard to say just what that would be, but the possibilities all look unappealing). For a viable Dyson sphere, the heat flow *outward* (measured in watts) must equal the Sun's output, so there is no net buildup of energy inside.

Henry

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