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In recent photos of DSCOVR I've noticed it appears to be missing eight solar cells in its array.

DSCOVR undergoing testing

Photo of the DSCOVR spacraft at NASA/GSFC prior to shipment to Cape Canaveral, FL (image credit: NASA)

In photos from the original "unboxing" from storage it appears to have all its cells.

DSCOVR in Feb 2009.  Credit: Stephen Clark/Spaceflight Now

Why is this?

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    $\begingroup$ Just a guess, but DSCOVR was mothballed and refurbished twice since being built in the late '90s, so possibly at some point they replaced the old cells with newer, more efficient ones, and left 8 of them off for weight and cost savings. An 11% increase in efficiency would allow that. Alternately, changing power requirements during the original design and development phase could have justified removing the panels without redesigning the frames. $\endgroup$ – Russell Borogove Feb 9 '15 at 1:16
  • $\begingroup$ The fact that the gaps are symmetrical - mirrored about the central axis - seems to support @RussellBorogove's theory. It seems deliberate, as opposed to incidental damage. $\endgroup$ – HDE 226868 Feb 9 '15 at 1:28
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    $\begingroup$ @RussellBorogove Well that was just a conjecture anyway, but the frame has clips top and bottom spaced symmetrically between the two halves, so that could explain why both halves use same frame shape (clamshell deployment). Dunno, Triana was built around an old SMEX-Lite (SMall EXplorer) bus and much of it was fairly standard, including 24% power envelope margin, so if cells were upgraded then your theory is more likely. I just can't find if they were, and some old photos seem to show same design. $\endgroup$ – TildalWave Feb 9 '15 at 4:37
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    $\begingroup$ @Schwern, re your update and the "unboxing" photo, it appears those panels are merely folded the other way and the innermost panel covering the outermost one. Some old diagrams I found (e.g. figure 1 here still depicting "Triana observatory" in Shuttle config) also show top frame row empty when folded "inside out" like on the second photo you now attached to your question. Same if you zoom in on the image in the SMEX-Lite PDF ;) $\endgroup$ – TildalWave Feb 9 '15 at 5:56
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    $\begingroup$ In the "unboxing" photo, it looks to me like only the inner frame is present, the outer not attached. $\endgroup$ – Russell Borogove Feb 9 '15 at 18:06
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@TildalWave mentioned DSCOVR was originally Triana built around the SMEX-Lite bus. I think the key is that Triana originally had all its cells, but DSCOVR does not. I believe, as @RussellBorogove commented, the original solar array has been replaced with an upgrade. The cells have been left off to save weight, cost and heat dissipation.

SMEX-Lite, when proposed in 1996, was said to be able to deliver 150W to the instruments via its solar arrays. Schematics for DSCOVR say 600W. SMEX-Lite was specifically designed with modular solar arrays. I can only assume with the advances in solar power in the last 10 years they have taken advantage of this and upgraded them.

SMEX-Lite also talks about the problem of excess power...

Excess power is dissipated in shunts mounted on the backside of the solar arrays. TRACE and WIRE are working towards the total elimination of the shunts.

I agree with @RussellBorogove that modern solar cells are so much better than the originals that a full array would produce excess power, so they simply eliminated them. This eliminates the need for shunts and reduces the weight of the array.

Sources:

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