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Comment by londons_explore | Hacker News Reader

On earth, the weatherproofing needed for solar cells by far outweighs the weight of the silicon itself.

In space, we generally don't allow use of anything that can't be tested on earth - that means the silicon has to be able to hold its own weight under gravity. Therefore, it is the mechanical supports that are the weight limiting factor.

One day, when solar cells are built, tested, and deployed all in space, we can probably get rid of these limitations and see 10x or more improvements in power to weight ratios.

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Most panels shipped to space can't support themselves: Google "solar array deploy" and you'll see videos from lots of space missions where there are complex deployment jigs to hold the panels from collapsing under their weight during testing. This isn't the entire story though - launch loads still meaningful contribute some to the weight. A certain amount of mechanical stiffness will still be required even for in-space builds for stability and controllability against slew, solar wind, drag, etc, putting a lower bound on mass. I'd like to see technologies that can avoid coverglass requirements (for atomic oxygen, solar wind), but that's a fundamental physics problem that looks tough to me.

And silicon is not the best semiconductor for cell mass. It's an indirect bandgap semiconductor, and requires ~100 microns of thickness to absorb photons (depending on texturing which allows photon trapping.)

CdTe cells, on the other hand, can be just 1 micron thick.

Is this actually true for present time satellites? I find it hard to believe that teams designing satellites today would not go for such an 'easy' weight reduction, however limitted it might be.

well it isn't easy... If your foldable solar panels can't be unfolded in gravity because they'd immediately break, then you can't test that they work.

>you can't test that they work.

Sure you can: one needn't test the full exact as-will-be deployed assembly to have confidence a system will work. Systems can be tested in parts or with surrogates.

Will the exact system in the exact conditions have been tried? No - but that's not really the goal; the goal of testing is only ever achieving an adequate[0] confidence of success in actual use.

[0] A situationally defined term.

Test them submerged in a fluid with (approximately) the same density and the material they're made of and they are "weightless" for all intents and purposes. The fluid will add some other dynamics which are not present in a vacuum so the test will not be entirely accurate but it will show whether the mechanism does what it is supposed to, mostly.

Plenty of amazing ground systems aren't flown because they can't be tested. Solar panel tests are usually done with a careful support system that takes the load from gravity and allows the panel to be unfolded in an approximately equivalent environment to microgravity.

What?

A. just set up a rig that can support the panel in earth gravity

B. Send up a test panel as part of some other satellite. You don't exactly have to spend much of your weight budget.

C. Use your theoretical and practical knowledge of space to determine whether this will work or not.

How do you think they tested the Apollo buggies? Do you think they built it strong enough to support 2 men in earth gravity?

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