Space 20: Dyson Spheres

[wrog]

(you'll want to read about solar power satellites first, where we cover all of the sensible stuff)

On Dyson Spheres and the other ridiculous things truly advanced civilizations will be doing with all of their free time

I should note that the scenario that I fleshed out in the previous post, i.e., having a cloud of independently orbiting power satellites that would start blocking out the sun if we really were able to populate huge numbers of orbits, is a lot more what Freeman Dyson was talking about in his original paper. The fixed sphere that people tend to imagine when they hear about this stuff really makes no sense at all if you think about it (and Dyson knew this at the time and said so). I mean sure, let's waste a whole lot of effort and materiel fighting solar gravity directly; that should be oodles of fun.

Making only slightly less sense is the flotilla of stationary habitats supported by solar sails. We're doing this why? Because we have this pathological hatred of orbital mechanics and don't want to make use of it? So much better to be depending on a sail that can get a hole in it and then we go plummeting straight into the sun. That sounds so much like a place I would want to live; really. Sign me up, please.

At best, I could see it as a kind of Bite Me, Universe gesture by a civilization that really has Done Everything, is now completely bored, and just wants to build some kind of insane, completely pointless artifact, just for the hell of it, because they can. Sure. Why not?

But I can't see it as something we're going to do while we're still on our way to the stars.

And I still wonder:

Can we really build all that?

Forget the full Dyson sphere, let's consider just one of the solar power satellite orbits. The chart in my previous post is calling for anywhere between 600,000 and 10 million square kilometers of satellite area.

Now, at this point, I don't even know what we're going to be making them from or what the preferred tech is going to be for solar power generation. Photovoltaic? Big-ass mirror driving some kind of heat engine? Or maybe we'll be supplying light to a mini-farm that's going to grow megatons of gerbil food, and then, in the next module over, we have billions of gerbils running their little wheels at top speed (at which point our movie instantly loses its "No Animals Were Harmed..." designation)?

Yeah. No idea.

But we can always make some kind of half-assed estimate. Let's just build aluminum sheeting; whatever the solar collector is, we'll need someplace to mount it. How much sheeting can we make out of the asteroid belt? We can vaguely do this:

• total mass of the asteroid belt (kg) is 2.3910×10²¹ kg.
  
  
• people estimating relative abundances in the universe say aluminum is 58 ppm of everything. Fine, so asteroid belt might have 1.38678×10¹⁷ kg of aluminum.
  
  
• let's be pessimistic and imagine that only 1% of it is mineable, or we lose 99% in the smelting process for whatever reason. Now we're at 1.38678×10¹⁵ kg.
  
  
• the thinnest sheeting you can buy online is 1/32"=0.79375mm thick. Meaning we need 793.75 m³ of aluminum to make a 1 km² sheet.
  
  
• Density of aluminum is 2.7g/cm³=2700kg/m³, so that's 2.143 kg to make a 1 m² sheet or 2.143 million kg to make a km² sheet.
  
  
• So we get 650 million 1 km² aluminum sheets if we harvest the entire asteroid belt. Which is somewhere between 60 and 1000 times the number of km² we need to populate one of our orbits, i.e., if it were the case that collecting solar energy only needs the km² aluminum mirror and everything else on the satellite is really cheap and easily available.
  
  
• So by this completely stupid measure, depending on which orbit we choose, anywhere between 60 and 1000 orbits are doable using the h= 36.635km⁻¹ separation and the Mercury L2 point.
  
  
• Note that these orbits were designed to suck off 86400 kg/day which is 1/4.26×10⁹ of the sun's output. Meaning blotting out the sun will entail filling 4.26 billion orbits, assuming we get the geometry exactly right.
  
  

Which seems to suggest that we'll get our 86400 kg/day power production and maybe even be able to go up to 1000 times that, but as far as blocking out the sun goes, just forget about it.

But then we have this interesting fact that I only learned about recently:

The Asteroid Belt is way smaller than you think

On the off-chance that anybody's still trying to convince you that the asteroid belt is the remains of a planet that got destroyed, here's something that really makes that not work:

The total mass of the asteroid belt is about 3% of the mass of the moon.

To be sure, I always knew the asteroid scenes you see in The Empire Strikes Back were bogus (space is big), but I'm still surprised that there's not even remotely enough there for any kind of respectable planet. (Sorry, James Hogan and whoever else wrote SF stories that had a planet breaking up a million years ago)

Which calls into question some of the premises of asteroid mining.

I will grant that there's stuff that won't be available on the moon. But for what is available, which probably includes all sorts of building materials, why not just mine the moon?

We can obtain 3% of the moon by strip-mining the top 12 km of its surface, and we might even vaguely be able to do that with present-day tech. And it's right here; no needing to travel hundreds of millions of km to get to Ceres or wherever else. Probably get huge economies of scale, too.

Granted, this won't really help with the Dyson Sphere. Even consuming the entire moon is only giving us a factor of 30, which is a long way from the 4.26 billion we need. Even eating all of Jupiter only gets us a factor of 800,000.

I think I can safely say that there is not enough aluminum in the solar system to cover the sun. Switching to a more abundant metal like iron gains us another order of magnitude or two, but I suspect we're still hosed.

Though, again, I'm obliged to point out how dubious this particular estimate is. It's not an impossibility proof by any means, and if the materials science folks do manage to come up with some carbon-nanotube/ceramic bullshit unobtainium that's insanely lightweight and can be built out of anything — much like I'm expecting them to do for the laser ships and light-sails we need for the transit tube — then all bets are off.

But I still think I'm pretty safe in putting the Dyson Sphere on the Not Gonna Happen list. If not because it's impossible, then because I'm not convinced we're going to need it.

Granted it is a bit weird finding myself in the position of wanting to say that 86,400 kg of matter+antimatter per day really ought to be enough for anyone — sounds a little too like that apocryphal Bill Gates quote — and having to stop myself.