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tgb comments on Server Sky: lots of very thin computer satellites - Less Wrong
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This struck me as an interesting estimate so here's my attempt at checking it:
Wikipedia quotes 300W/kg solar cells. Medium Earth Orbit ranges from over 2,000km to 35,000km above sea level - let's pick a fairly low estimate of 6000km. Gravitational potential is 3.32 J for elevating a 1kg mass to 6000km from sea level. So the solar panel must operate for 1.28 days to recoup the energy cost of elevating the object. This is, of course, a lower bound (assuming perfect launch mechanism, no kinetic energy of orbit, etc. etc.), but it seems unreasonable to assume that launching solar panels has no benefit given this tiny lower bound. Furthermore, the fact that solar panels are routinely launched into orbit suggests that they do have a net energy production.
What do existing computers-in-space do? Shielding of some sort?
The object- what about the rocket? (I also should have included the energy cost of making the solar panel in the first place, which tends to seriously reduce their attractiveness.)
Well, solar is cheap to get to space. (I know our recent Mars rover is using nuclear energy (powered by decay, not fission or fusion) rather than solar panels to reduce the impact of Mars dust, and that deep space probes used similar technology because solar radiance decreases the further away you get.) Batteries in particular are pretty heavy- and so solar panels probably represent the most joules per kilogram in Earth's orbit.
But the comparison isn't "solar in space" vs. "chemical in space", it's "solar in space" vs. "anything on earth". The idea of "let's put computers out in space, where the variable cost of running them is zero" misses that the fixed cost of putting them in space is massively high, probably to the point where it eats any benefit from zero variable cost.
That is, this technology looks cool but I don't yet see the comparative advantage.
Check out the wiki page on radiation hardening. I believe that the primary thing to do with cosmic rays is just noticing when they happen and fixing the flip. I think it's a mostly solved problem, but that the hardware / software is slightly more expensive because of that. (Buying RAM with ECC appears to be difficult for general consumers, but I imagine it's standard in the satellite industry.)
Isn't decay a subset of fission? (Excluding things like lone protons that don't technically have a nucleus or whatever.)
Yeah, that was sloppy of me. I meant to specify that it was spontaneous fission rather than chain reaction fission.
The term 'fission' is generally reserved for daughter species of vaguely similar mass. Decays are generally alpha (He-4) or beta (electron and neutrinos), maybe with some others mixed in.
ECC RAM is standard for servers, so it's not especially hard to get. Fixing bit errors outside the memory (e.g. in CPU) is harder; I imagine something like http://en.wikipedia.org/wiki/Tandem_Computers, essentially running two computers in parallel and checking them against one another, would work. But all of this drives the cost up, which, as you note, is already a problem.
There are other clever things you can do, like including redundant hardware and error-checking within the CPU, but they all drive up the die area used. Some of this stuff might be able to actually drive down cost by increasing the manufacturing yield, but in general, it will probably be more expensive.
You seemed to have missed my sentence between the two that you quoted:
My point was that even if the launch is only 0.1% efficient at moving solar cells into space, you're looking at more than recouping the cost of the launch in the putting the solar panel up. If you think the launch is much less than 0.1% efficient, I'd be interested in hearing why you think that. They might actually be that inefficient, but I would be hesitant to assume such without having a reason to do so.
Now that lsparrish has posted a link to a better discussion of the subject, my post is more or less obsolete.
I agree and was not trying to say that this plan was practical - I do not believe it is. I was just pointing out that something you stated as true doesn't appear to be so from a very quick look at the numbers.
Sure. That's why they would have to be very lightweight for this to work.