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The amount of energy required to change the rotational velocity of Venus in any meaningful way is immense. Requiring far more energy than humanity can muster at the present date. But I wonder if we can use the most powerful thing in our solar system, the sun, to affect the Venutian rotation? Can we turn Venus into a radiometer?

Would the rotational velocity of Venus change if we shaded half of the planet? Shade the half rotating toward the sun and only allow sunlight to strike the half rotating away from the sun? Would the radiation pressure of the sun on the lighted half help to increase the rotational velocity of the planet? And if so, how long would it take for the rotation to match that of Earth?

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  • $\begingroup$ Apparently this would work for changing orbital velocity. I'm not sure how you would change rotational velocity, though. $\endgroup$
    – Cadence
    Jul 19, 2021 at 23:46
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    $\begingroup$ No time for a full answer now, but how is the shading happening? Putting a sunshade in orbit would require stationkeeping drives which would use at least as much energy as would be imparted to the unshaded side of Venus, by necessity (since it would be encountering the same amount of radiation pressure as the unshaded side of the planet). Thermodynamics being the cruel mistress it is, that would seem like something of a waste of energy. $\endgroup$
    – jdunlop
    Jul 19, 2021 at 23:47
  • $\begingroup$ It's essentially a huge solar sail, I'd imagine you could reflect the light in such a way (multiple directions, not simply a flat plane) as to maintain proper positioning. $\endgroup$
    – loweryjk
    Jul 20, 2021 at 15:00
  • $\begingroup$ @jdunlop I can't remember where, but I believe I've seen a strategy for using multiple mirrors to help counteract that radiation pressure... I'll look around and see if I can find it again. $\endgroup$ Jul 20, 2021 at 15:48
  • $\begingroup$ If you're blocking the light from impacting half the planet, though, the vector sum would still push the sail towards the planet, I'd think. $\endgroup$
    – jdunlop
    Jul 20, 2021 at 17:54

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This would be difficult to do, but it's easy to make an order-of-magnitude estimate of how difficult it is. We can estimate the angular acceleration this would produce on Venus by calculating the radiation pressure at its orbit, in an ideal scenario: $$P_{\text{rad}}=\frac{2G_{\text{SC}}}{c}\left(\frac{r}{\text{AU}}\right)^2\approx1.74\times10^{-5}\text{ Pascals}$$ with $G_{\text{SC}}$ the solar constant and $r$ the orbital radius of Venus. The force is then $P_{\text{rad}}$ multiplied by half of the cross-sectional area of Venus: $$F=P_{\text{rad}}\cdot\frac{\pi}{2}R^2\approx10^9\text{ Newtons}$$ We can estimate the torque applied to Venus by $\tau\approx F\cdot(R/2)$ and calculate the angular acceleration by dividing this by the moment of inertia, giving $$\alpha\approx\frac{\frac{1}{2}FR}{\frac{2}{5}MR^2}\approx4.24\times10^{-23}\text{ radians s}^{-2}$$ Venus currently has a rotation speed of $\omega=3\times10^{-7}\text{ radians s}^{-1}$, so you'd need timescales of $$\Delta t=\frac{\omega}{\alpha}\approx300\text{ million years}$$ to make any significant change.

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  • $\begingroup$ Unless we somehow lens the radiation instead of blocking half of it... then it's only 150 million years :P $\endgroup$
    – Corey
    Jul 20, 2021 at 0:01
  • $\begingroup$ I would add earth rotation velocity as well, 0.000072 rad/sec, hm around 50 billion years, hm surprisingly fast. Also this has effect on comets asteroids, it has some wiki article but forgot the name of the effect, it more about rotating ones for which one side is emitting more IR than the another one. $\endgroup$
    – MolbOrg
    Jul 20, 2021 at 8:16
  • $\begingroup$ Only six orders of magnitude to go in order to get that time scale down to 300yrs! We could of course gather more sunlight with reflection to increase the effectiveness and lower the time scale. $\endgroup$
    – loweryjk
    Jul 20, 2021 at 14:50
  • $\begingroup$ @loweryjk Yeah, reflection or lensing, like Corey suggested, would be interesting. With the time required inversely related to the flux or area the light is incident upon, it looks like (running the numbers) you'd need to absorb about 0.1% of the total light of the Sun and funnel it onto half of Venus in order to get the timescale down to ~300 years. On the other hand . . . the planet would certainly heat up. Although given how hellish it is already, maybe that wouldn't be a huge loss. . . $\endgroup$
    – HDE 226868
    Jul 20, 2021 at 15:14
  • $\begingroup$ @HDE226868 - at six orders of magnitude more energy, I rather suspect "heat up" is an understatement. $\endgroup$
    – jdunlop
    Jul 20, 2021 at 20:18

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