# What are these air-vent-like structures on this satellite?

I struck out with my previous question so I'll try again.

What are these large circular 4-vane structures on the sides of NOAA-19 - the satellite that "fell down"?

They remind me of adjustable air vents but I have a feeling that's not quite right.

You can see them from another angle here.

Spacecraft rotated for better view:

• Spacecraft rotated for better view :-) – gerrit May 17 '16 at 13:37
• The report from NASA on the mishap listed "get-home-itis" as one of the proposed reasons for this thing falling over. – Samuel May 17 '16 at 22:19
• Those guys in the first photo look pretty casual. – Organic Marble May 17 '16 at 23:19
• How thoughtful of them to rotate the spacecraft for you! – Anton Hengst May 20 at 0:07

They are thermal control pinwheels. Reference: figure on page 3 of this document .

Note: odd how the diagram shows 4 devices, but the note says 3. In the photos there are 3 on one side, and 4 on another.

"Elements of Space Technology for Aerospace Engineers", page 302, describes thermal control pinwheels as follows:

Pinwheel louvers are similar, but in place of rectangular blades incorporate vanes (typically four) in the form of sectors of a circle that together cover one-half of the area under the pinwheel. They are also rotated by bimetal actuators. Depending on the position of the sectors, they will cover (uncover) areas that have high ratios of solar absorptivity to emissivity and uncover (cover) areas that have a low such ratio.

And, there is a cross-section:

• They are some kind of radiator. I am not sure if the quad blades rotate or not. Looking for reference. – Organic Marble May 17 '16 at 12:07
• @uhoh Looks like it's what I thought, and they do rotate. Edited answer. – Organic Marble May 17 '16 at 12:40
• Bravo - thank you for that!! Extra credit for a speedy conclusion. I am puzzled, usually absorptivity and emissivity are similar - something back absorbs light and (thermally) radiates light well, something white or metallic is usually poor at both. I'm not sure what's going on here. Maybe the key is wavelength - solar radiation (~5000K) is mostly near IR and visible, while "ambient temperature emissivity" (~300K) would be more like 10 to 20 microns. So whatever they use is dark in the visible, but shiny (low $\epsilon$) in IR. – uhoh May 17 '16 at 14:22
• There may be more info in the linked document, I just skimmed. Key to the google success was "pinwheel", I never would have come up with that. – Organic Marble May 17 '16 at 14:27
• @Mark The top of the linked page in the book Elements of Space Technology by R. X. Meyer it says "If this surface is provided with a high ratio of solar radiation absorptivity to ambient temperature emissivity, control over a wide range of temperatures can be obtained." I think I'm interpreting it correctly - let me know if I'm missing something. – uhoh May 18 '16 at 4:38