What is the function of this array of what looks like mirrors on TESS?

Question

The Mashable.com article TESS is about to become your new favorite NASA mission shows a beautiful photo of the TESS spacecraft in a state that looks like it is nearly ready to launch.

There is a panel with array of what looks like mirrors on one side. What are they for? How do they work? Are they related to or the same as the large panels shown in the question What are these very large, square panels on Inmarsat 5's? or something completely different?

below: TESS on Earth. Cropped from Mashable.com. Credit: NASA

below: "A look at TESS inside the PHSF at NASA's Kennedy Space Center in Florida." Cropped from Space.com's NASA's TESS Exoplanet-Hunting Mission in Pictures. Credit: Kim Shiflett/NASA

below left: TESS on Earth. Mashable.com. Credit: NASA. right: Screenshot of YouTube video NASA’s New Planet Hunter: TESS Click for full size.

From the video of tess, it looks like place for antenna... Wasnt able to spot this is the video — zephyr0110, Apr 10 '18 at 04:51
@Prakhar the dish antenna is the large circular thing facing the camera, but it is covered and so you may have missed it. See The appearance of TESS' dish seems to be evolving, what will be the final configuration? for more about that. The panel I am asking about is to the right of the dish, as seen in the image. — uhoh, Apr 10 '18 at 07:00
@Prakhar check out the extremely low noise of the CCD sensors, and how this is achieved. It's just a wild guess, I could be way off... — uhoh, Apr 10 '18 at 11:14
I found a very in depth discussion of the thermal characteristics of TESS, but they do not mention this shielding on the body. This might mean it isn't for thermal reasons, or might mean it's a lower level design feature (ie cover all blank areas with a shield, versus specifically needing to shield this area for something) https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160007929.pdf — Saiboogu, Apr 10 '18 at 14:14
@uhoh I agree, but thermal seemed a suggestion that I felt was at least partly ruled out by the PDF I found. I'll review your other links more thoroughly for more clues. — Saiboogu, Apr 10 '18 at 14:39
@Saiboogu a PDF of a Master's Thesis in 2016 can't rule out things in a final configuration two years later. — uhoh, Apr 10 '18 at 14:40
I'm going to hazard a guess that it is a phased array antenna, where each of those squares is an array element. This would allow steering a high-gain beam without requiring changes to the vehicle attitude in a way that a dish antenna wouldn't. I don't have strong support for this guess, but I am having trouble thinking of what else it could be -- maybe a radiator, but it's facing the wrong way to really be useful for that purpose (same direction as the solar arrays) — Tristan, Apr 10 '18 at 15:12
@Tristan see (at)Puffin's comment. 2nd surface mirrors reflect visible light while the glass is still a good blackbody radiator at longer wavelengths. I'm hoping to get a good description of this effect and design in an answer here since all I can do for now is keep linking to that comment! — uhoh, Apr 10 '18 at 15:17
I guess that is not a very sexy part of the spacecraft bus. Every paper I can find just ignores it. I saw a reference in one to "heatpipes and radiators" but not tied back to a diagram. My throwdown answer is radiator but I can't confirm it yet. — Organic Marble, Apr 10 '18 at 21:26
@OrganicMarble I find the use of glass' unique optical properties extremely sexy, but that's just me. — uhoh, Apr 11 '18 at 01:58
Here is one super wild guess. What if it used for optical ranging ? Like they did in apollo mission by placing mirror in surface of moon ! I love guessing when there is not a definite answer available .. any more interesting guesses ? — zephyr0110, Apr 11 '18 at 03:56
@Prakhar the pod bay might be a better place to chat about guesses. — uhoh, Apr 11 '18 at 04:13
@uhoh I think you may be on the right track with the thermal radiator hunch. Do you have any diagrams of the interior of the satellite? It would help if there were clues as to whether there might be a transmitter amplfier underneath (large heatsource) and whether the panel is part of the structure, bolted on, or even deployed as three panels. — Puffin, Apr 13 '18 at 18:33
Just another thought, and I don't know the TESS mission at all, is there something about the payload that requires exceptional thermal stability given the nature of the instrument and the orbit/yearly evolution in solar aspect? — Puffin, Apr 13 '18 at 21:32
@Puffin it looks like the entire spacecraft is wrapped in a white blanket to reflect sunlight, so I suspect it can not radiate effectively. The CCDs are cooled for low noise, but (at least at one point in the design) the hot side of their thermoelectric coolers radiate to space via the black light shields for each camera, quite a "cool" and interesting design! — uhoh, Apr 14 '18 at 02:40

score 9 · Accepted Answer · edited Jun 16 '18 at 02:20

The silvery/mirror like surface on the spacecraft are Optical Solar Reflectors. They are very effective radiators to dissipate the internal heat from the spacecraft. The mirror effect reduces absorption (i.e. reflects visible light where the Sun's intensity is strongest) while the quartz material itself radiates very effectively in the infrared (i.e. radiates heat).

Your guess that these are effectively the same as those on Inmarsat 5 is correct. However Inmarsat, being a geocomm satellite, has significant higher heat dissipation, and therefore needs much larger radiator area - hence the deployable radiator.

Here's an image of a recent GOES satellite with an engineer "admiring themselves in the mirror" to illustrate the widespread use of these devices, be they deployable or not.

Thanks for your answer and for finding the Wikipedia article! With such an ambiguous name, no wonder nobody found it until now. — uhoh, Apr 30 '18 at 02:51

What is the function of this array of what looks like mirrors on TESS?

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