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Is it possible to do anything to a coin in space, so it begins to spin, using sunlight as the energy source?

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    $\begingroup$ Does a solar panel connected to an electric motor count? Also connected to whatever stabilization system you need to prevent the whole thing from spinning $\endgroup$ – user253751 Jun 19 at 21:13
  • $\begingroup$ It will probably spin anyway. Any lack of symmetry on the coin's surface will differentially be affected by radiation pressure. $\endgroup$ – chasly - reinstate Monica Jun 20 at 0:38
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Yes.

Paint half (as in a filled semicircle) a face of the coin black, while leaving the other half shiny. Place the painted face parallel to the Sun's surface.

The two sides of that face will now experience different radiation pressures from the solar radiation: in particular, the still-shiny half will experience greater radiation pressure because it reflects the radiation (requiring twice the counter-momentum to conserve momentum), while the black half absorbs it (as it simply absorbs the momentum of the light), and hence a torque will be created that favors the shiny half.

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    $\begingroup$ Does the coin just keep spinning faster and faster until it rips itself apart? I can't think of anything that would cause the torque to change over time. $\endgroup$ – Nuclear Wang Jun 19 at 13:14
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    $\begingroup$ See also the weird example of the light mill which paradoxically does not work according to this principle. (But would, if it weren't for friction and residual pressure) $\endgroup$ – mlk Jun 19 at 13:23
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    $\begingroup$ It will not only rotate, but also accelerate away from the sun. $\endgroup$ – bogl Jun 19 at 14:36
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    $\begingroup$ A nontrivial part of this is that the shiny part needs to be shiny in the (near) visible spectrum, but black in the lower IR spectrum. Otherwise, the coin will reradiate the absorbed heat chiefly from the black region (cancelling out the momentum initially absorbed) $\endgroup$ – poncho Jun 19 at 16:40
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    $\begingroup$ @JohnDvorak - Assuming the coin is in orbit, and pretending that there were nothing else to interact with, and that the sun would shine steady state far longer than it it actually will, the radiation pressure would slowly increase the eccentricity of the orbit until it eventually becomes hyperbolic and the coin escapes. $\endgroup$ – Paul Sinclair Jun 19 at 17:58

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