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So, given that my earlier question, amount of mass needed to speed up Venus rotation, is a no-no, I am resorting to one last method of changing the planet’s rotation before giving up on Venus entirely.

Huge pipes are constructed on the surface, and I do mean huge. These immense tubes, built of woven carbon nanotubes, extend from the surface of the planet to outside the atmosphere. Openings at the bottom of each pipe thus allow gases through, and due to the differences in pressure the atmosphere is “sucked” through these tubes and vented into space.

These tubes therefore function as huge jets; by being set at an angle, the pipes not only vent the atmosphere of Venus and dissipate heat into space, but slowly speed up the planet’s rotation. All the pipes open in the opposite direction of venus’ rotation, so they gradually turn the planet around and around faster and faster.

Eventually the rotation has sped up to 24 hours, the atmosphere is much less thick, and the faster rotation has created a magnetic field. The planet can now be seeded with water and the atmosphere replaced with one of nitrogen and oxygen, much like earth’s.

Would this system work?

user98816
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    "Due to the differences in pressure the atmosphere is “sucked” through these tubes and vented into space": No it won't. (Why would it? The atmospheric pressure is the weight of the column of atmosphere over the unit area. The presence of the pipe is irrelevant.) – AlexP Dec 04 '22 at 15:59
  • Even if this worked (as @AlexP points out, it won't), you've discarded vast quantities of carbon, oxygen, and nitrogen that you now have to replace to terraform Venus. For every ton of H2 you would have had to import previously, to be combined with the oxygen from CO2 to form water, you now need to import 9 tons of water. For every ton of H2 that would have ended up combined with the carbon, it's now around 7 tons of hydrocarbons that you have to import. And then there's the entire atmosphere worth of nitrogen that you have to import. You need what's in that atmosphere. – Christopher James Huff Dec 04 '22 at 18:28
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    @user98816 - Why would you give up on terraformng Venus if you were unable to speed up its rotation? I guess you want the days and nights on Venus to be short enough that it doesn't get too hot in the day or too cold at night. But you can change the length of day and night without changing the physcial rotational period of Venus. Just use orbital sunshades to cool off the sunfaceing side and orbital mirrors to heat up the side away from the Sun. That would take billions and trillions of times less energy and effort than changing the physical rotation period of Venus. – M. A. Golding Dec 04 '22 at 21:19
  • Building on @AlexP's comment: you're assuming if you build pipes from just above the ground to space that gas would naturally flow through them. That's the natural state of the atmosphere. There's nothing stopping gas from venting as you suggest - except that it doesn't. That's where everyone's explanations are coming in. Off the top of my head, baring exotic solutions like bending space time, the only way I can think of to speed any planet's rotation is to be hit by a meteor of sufficient mass at exactly the right angle. Such a hit would destroy the atmosphere and might result in a moon. – JBH Dec 04 '22 at 23:58
  • I'd also like to build on @M.A.Golding's comment. You can't reduce the temperature of a planet by speeding its rotation. The same energy-per-second is impacting on the surface. That's a function of the distance to the sun, not the speed of rotation. 50% of the planet is exposed to the sun at all times. Cooling the planet requires more distance or less exposure by somehow blocking sunlight. – JBH Dec 05 '22 at 00:01
  • @AlexP "Due to the difference in pressure" is it at all possible do you think that this post is from someone with an occasional Goons-esk sense of humour rather than your every day brain fart – Pelinore Dec 05 '22 at 06:32
  • @JBH there is one other way that might conceivably make use of the planets atmosphere .. two 'offcentre' rockets pushing in opposite directions (much as the OP may have imagined this, but without the magic of a localised breakdown in the laws of reality) such that the sphere (planet), spins, positioned to accelerate the existing spin, the mechanics of something like that boggle the mind somewhat but at least it doesn't rely on a sudden and inexplicable breakdown of the laws of gravity in it's normal everyday application – Pelinore Dec 05 '22 at 06:58
  • @JBH Oh! Hang on! I have another workaround we can enclose the atmosphere in a giant skin then constrict this balloons skin to put the atmosphere under extreme pressure and then insert the nozzles to vent it, effectively you'll be creating two giant atmosphere / gas volcanoes, enough pressure and some of the atmosphere spewing out of the two nozzles can achieve escape velocity and in conjunction with each other impart some spin were going to need some next level rubber for this, and a lot of it – Pelinore Dec 05 '22 at 07:10

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This method won't work at all. The pressure of Venus' atmosphere is simply the weight of atmosphere above the location where you measure the pressure. Venus' gravity holds onto the atmosphere.

Running a pipe up to orbital altitude doesn't do anything: the weight of atmosphere in the pipe means that the pressure in the pipe is just the same as the pressure outside the pipe at the same altitude. There won't be any flow up the pipe. Nothing happens.

If you're willing to have anti-gravity mechanisms of some kind, you can get rid of the atmosphere, but the best way to do that will depend on the limitations of the anti-gravity. You're also discarding conservation of energy, which will make further reasoning about the setting challenging.

John Dallman
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    It's quite a bit like asking why a drink doesn't continuously squirt up out of a straw. Close off the top of a straw, dip it into a glass of water, and then release the seal at the top, and yes, the ambient pressure around the bottom of the straw will force water into it...until the water in the straw reaches an equal level with the surrounding water. – Christopher James Huff Dec 05 '22 at 14:09