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So, returning (once more) to the prospect of the terraformation of Venus, I have finally come up with a method of spinning this rock to something approaching 24 hours which (cross fingers) might work for a sub-kardashev civilisation.

A huge mass driver is built along venus’ equator. At base of this driver are engines that extract carbon from the atmosphere (forget about where the oxygen goes for now). The carbon is compacted and shot from mass driver into space. Each launch has a “knock-back” effect which slightly increases venus’ rotation. All this is being powered by solar farms placed between venus and mercury.

Some fact figures:

  • Each payload of carbon weighs about a million grams
  • One payload is launched every second
  • The mass driver is 6,000 miles long
  • The payloads are launched at about 23,200 mph, or 10.37 km/s

How long would it take for this method to speed up venus’ rotation to 24 hours?

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  • $\begingroup$ Are you sure than slingshotting a one-tonne projectile at cosmic speed inside the atmosphere of Venus will result in something other than a big explosion? $\endgroup$
    – AlexP
    Feb 7 at 12:29
  • $\begingroup$ @AlexP if the mass driver is made like a proposed "space tram" device, it can well be above the atmosphere. $\endgroup$
    – Vesper
    Feb 7 at 12:36
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    $\begingroup$ Why do you want to spin the place up? Place a big starshade at L1, use it to freeze out the atmosphere, export, pave over or process the ice. The starshade can double as a solar power plant beaming power home. After the surface is ready, set up L4 and L5 mirrors as well as orbital ones. This lets you set the daylength. Move an asteroid into orbit and create a plasma ring from it, then run a current through it. Out of the box magnetosphere. $\endgroup$
    – TheDyingOfLight
    Feb 7 at 13:18
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    $\begingroup$ I think you overestimate the mass of the atmosphere relative to Venus. Also, if your goal is terraforming, you kind of need that carbon for biomass... $\endgroup$
    – Christopher James Huff
    Feb 7 at 15:18
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    $\begingroup$ @TheDyingOfLight It won't, unless the starshade is charged in some way. Anyway the worst it can do is to very slowly remove the atmosphere by heating up the outermost parts of it. Solar particles are far too low in energy to reach the ground through an atmosphere of 1 bar. $\endgroup$
    – Karl
    Feb 8 at 20:51

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You need to launch faster

10.37 km/s is just about the escape velocity from Venus, meaning that the object thrown away from Venus at this speed at surface would lose most if not all of its speed while leaving the planet's gravity well. Thus the answer for this speed is about infinity, or rather in the basspark of 10^21 seconds as you would then tear off the significant part of Venus' mass thus lowering the escape velocity, making more impulse count. Yet, if you would launch mass faster, the times would be able to get calculated.

Let's say your Lofstrom loop is actually encompassing the entirety of Venus' equator, being connected to the surface in at least two locations opposite each other, and the launch point of that loop is located above Venus' atmosphere. This configuration could potentially launch projectiles at higher speeds, limited by tensile strength of the loop's belt. Here on Earth the loop is believed to be able to operate at sub-escape velocity only, yet let's say your civilization has made discoveries allowing the loop to operate at no less than 12.0 km/s, sustaining launches of one-ton masses per second. This device will allow those masses to escape Venus at speeds of sqrt(12.0^2-10.36^2) ~= 6.055 km/s, essentially applying an integral force of 10^3*6055*10 = 6.055e7 N at level length of 6051.8 km plus the loop's height (arbitrary, but can't be too high in order to allow the loop to ever start - let's say 100 km), or 6.152e6 m. This amasses to 7.725e14 N*m, at least a factor of 100 better than what the proposed solar reflectors could do (that answer had a 0.5x mistake in radius, read 6051 km as diameter instead), thus the expected time to accelerate Venus would be correspondingly smaller - 1.435e18 seconds, or 4.55e10 years. Still too slow. Note that during all these years Venus would lose 1.435e21 kg of mass, or 0.0295% its total mass, making escape velocity slightly lower over the process and speeding up Venus' rotation slightly more effective.

Note that increasing final speed would only lessen the gravity-related momentum losses, without any cardinal increases in the acceleration speed (at most effective change of momentum would be equal to mass multiplied by starting speed), thus increasing linear speed of projectiles would only give linear effect on time required to speed Venus up, at most to double (12/6.055) effectiveness, so having that mass driver to launch stuff at 100 km/s would shorten the required time by a factor of 2*100/12~=16.66, thus even while this idea looks quite more attractive, it would still not allow that civ to terraform Venus before it'll get consumed by the Sun.

(Perhaps next time calculate the idea of having ion engines at space towers? These don't require as much mass down the drain as the exhaust speed is quite high, and you can make quite a lot of those around Venus' equator.)

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