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So, returning again to the subject of terraforming Venus, what if these terraformers collide an asteroid with the planet? This meteor hits Venus at an angle, and thus speeds up its rotation to 24 hours, and this rotation in turn generates a stronger magnetic field.

How massive would an asteroid have to be to achieve this, and at what speed must it collide? To help, Venus has a current rotation period of 243 days, and I’m trying to reduce that to 24 hours.

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  • $\begingroup$ I seem to recall a very similar question on here about a moon of Jupiter. You just need to account for the heating. This will heat the surface of the resulting planet to lava temperatures. It is going to be 100's of thousands of years (millions?) before that cools to a solid surface. $\endgroup$
    – BillOnne
    Commented Dec 4, 2022 at 14:29
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    $\begingroup$ I understand that after many meetings the committee has decided on increasing Venus' rotation; I am all for it! But just for my own education, how will spinning get a magnetic field, and how will a magnetic field help with terraforming Venus? $\endgroup$
    – Willk
    Commented Dec 4, 2022 at 15:21
  • $\begingroup$ A magnetic field is generated by a planet’s rotation, which creates an internal dynamo in the molten metal core. A magnetic field is necessary to protect Venus from solar wind, which as seen on Mars, whose core has solidified, can strip away the atmosphere and water. Venus has a molten core but it’s magnetic field is not generated from within, which I rationalise use because of its slow rotation. $\endgroup$
    – user98816
    Commented Dec 5, 2022 at 10:16
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    $\begingroup$ Venus's lack of magnetic field may not necessarily be related to its slow rotation. See web.archive.org/web/20100714051425/http://www-spc.igpp.ucla.edu/…: "It is important to note that, contrary to popular belief, dynamo theory does not credit the smallness of the magnetic moment to the slow rotation of Venus... It is also notable that Venus would not have maintained any remanent crustal magnetic fields from its proposed early period of dynamo activity because the temperatures in the crust are expected to be above the Curie point..." $\endgroup$ Commented Dec 5, 2022 at 11:28
  • $\begingroup$ Notably, Mercury has a magnetic field despite its very slow rotation. And the atmosphere of Venus obviously hasn't been stripped, despite it having about 4x the solar radiation intensity as Mars and no magnetic field. $\endgroup$ Commented Dec 5, 2022 at 14:01

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Too much!

A back-of-the-envelope calculation: being so slow, Venus's rotation energy is a rounding error compared to Earth's, and its mass is very similar to Earth's. So the problem is comparable to imparting Earth's entire rotational energy, which is 2.14 × 10^29 Joules.

One calculator returns approximately this value for an asteroid diameter of ~750 km (assuming density = 2 g cm^-3, velocity = 30 km s^-1). 4 Vesta (diameter 525 km) is the Solar System's largest asteroid, with a mass of 2.5 × 10^20 kg, unless you count the dwarf planet Ceres.

And this assumes that the entire impact energy goes into spinning up Venus, which of course it wouldn't (much would be lost as heat, or shifting Venus's orbit), so the actual required diameter and mass would be larger still.

Basically, we're not talking about an asteroid collision here. You'd need to fire a large moon at Venus to make this happen.

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    $\begingroup$ What would happen is that Venus would be instantly converted into a liquid molten blob... which may or may not spin faster after is cools down. $\endgroup$
    – AlexP
    Commented Dec 4, 2022 at 14:37

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