[AR] Re: Freeman Dyson, RIP

  • From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
  • To: arocket <arocket@xxxxxxxxxxxxx>
  • Date: Tue, 3 Mar 2020 16:54:27 -0500 (EST)

On Tue, 3 Mar 2020, David Summers wrote:

...So it is fairly obvious that you could make a bunch of "fixed" solar sail like structures in a sphere around the sun...

If you can get total mass down to about 1.5g/m^2, which is what sunlight will support at 1AU. That's challenging -- the Team Encounter sail design, circa 20 years ago, was aiming for 3.63g/m^2 (including structure and a minimal payload), and that was a very aggressive design, much lighter than anything actually built so far. The fundamental limits are much lower, but approaching them is not easy, especially if you want a substantial payload.

Now for the fun part:
All those mirrors around the sun would be internally reflecting most of the energy - which would greatly increase the internal energy of the system. So you'd get much more power out of your solar cells. But then there is the downside: it probably eventually makes the sun go nova, by runaway heating. Unless you really can extract most of the energy in solar cells! ;-}

Well, the Sun wouldn't literally go nova -- novas are now known to involve a double-star system with one star a white dwarf, they don't happen to isolated main-sequence stars -- but having most of its output reflected back into it definitely wouldn't be good. :-)

At the cost of having only half the thrust, your sails can be black instead of reflective. You're still radiating thermal infrared back at the Sun, but only half as much because you're also radiating from the starward side. Good solar cells are actually pretty close to black, although it's not clear that you can make them thin enough for this.

Actually, what you want on the sunward side is something that absorbs most of the visible but doesn't emit much thermal IR (with an IR-emitter surface on the starward side). Again there's a question of whether you can make a suitable material thin enough, but it might be possible.

Doesn't sound like a good way to support substantial payloads, though.

Henry

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