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The image below, found in this answer shows four "Solar Pressure Vanes" on Mariner 4.

I've never heard of such a thing.

I can imagine that it might have been thought to help favor the spacecraft's direction more towards the Sun than away from it which might at least keeping the low gain antenna from being completely behind the spacecraft relative to Earth in the event of loss of attitude control.

But I wonder, without damping would they just induce crazy though extremely slow oscillations?

Question: What is the principle behind Mariner 4's "Solar Pressure Vanes"? In what case(s) would they be effective?

Mariner's Solar Pressure Vane

The Mariner 4 spacecraft

above x2: Cropped from image in COSMOS Magazine. "The Mariner 4 spacecraft. CREDIT: NASA / JPL"

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    $\begingroup$ @uhoh, they were a passive system, even though they were adjusted. It is, as you pointed out, unlikely the damping would be in a practical timeframe; and without trim adjustment it is unlikely it would work in the real solar system. $\endgroup$ – JCRM May 7 '18 at 2:13
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    $\begingroup$ Per the comment by @WayneConrad , in the 1990's I came up with a method for using light pressure to keep a spin-stabilized spacecraft in a heliocentric orbit roughly sun-pointed. NASA gave me a little money to analyze it (I got out my coin purse for the award money! ;-) and I found it would work for small satellites, stuff roughly the size of a 12U CubeSat or smaller. But NASA decided they weren't really interested, published a summary of my report in Tech Briefs, and that was the end of it. $\endgroup$ – Tom Spilker May 7 '18 at 2:38
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    $\begingroup$ @uhoh , both sites refer to the same work. I gave them a full report but I don't think they ever published it. $\endgroup$ – Tom Spilker May 7 '18 at 3:04
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    $\begingroup$ I wonder if they could be made into active system while remaining solid-state. Using materials like e-paper to change reflectivity of the vanes, providing active stabilization. $\endgroup$ – SF. May 7 '18 at 6:58
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    $\begingroup$ FYI, James Webb will have some too ! $\endgroup$ – Antzi Apr 9 at 5:25
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The vanes move to stabilse the spacecraft - although that may only be a first order stabilisation "Compensation for an unbalance in solar radiation pressure is provided by moveable paddles located on the tips of the solar panels"

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