Couldn't we just build a MegaMaid and suck the atmosphere out? If we're going to go sci-fi, it seem easier to hoover it out than capture an asteroid and nick the atmosphere just right.

Then, move the MegaMaid into an orbit around Mars and go from suck to blow. Venus has too much atmosphere, Mars has too little. Win Win.

It wouldn't do Mars any good. Lacking a magnetosphere, any atmosphere you add will get stripped away by the Sun.

The atmosphere will be stripped away over a time period of millions of years though.

I’ve read that a large asteroid could be positioned at a particular Mars L point and it would protect the planets atmosphere from being stripped by the sun.

I've read something similar, but I believe it has to be an asteroid with a magnetic field, whether permanent or electromagnet.

The estimate I heard is it would take 100,000 years for the atmosphere to be stripped off. That’s a long long time.

Something like a solar powered space elevator that just blows atmosphere into the sun

That would require a lot of energy to ensure the gasses escape Venus' gravitational pull, which would in turn effectively be a rocket. So then we'd be adjusting to ensure we don't mess with Venus' orbit too much.

We can avoid orbital drift through the magic of building two of them.

Venus outweighs its atmosphere by about 10,000 times. This is actually less than I thought -- for comparison Earth outweighs its atmosphere by over 1,000,000 times, which is still far less than I would have guessed.

Venus's escape velocity is less than 1/3rd of its orbital velocity. According to google, Venus's orbit, despite being very circular, causes its velocity to vary by a KM/s from aphelion to perihelion.

So I believe you could send all of Venus's atmosphere off permanently into space at the cost of about 1/30,000th of Venus's orbital velocity, meaning you could very slightly circularize its orbit further.

Spinning it up would be handy.

I recall one of the plans being to use the sun shield to entirely freeze out the atmosphere, then use a mass driver to chuck most of the CO2 into space. I don't recall exactly where that was supposed to be on the feasible to scifi spectrum.

I'd rather try to keep the carbon around for organic molecules. Are we sure we can't get in enough H2O and N to balance it out and build a nice thick biosphere?

You'd have to shoot it out of the solar system, otherwise much of it would reaccrete onto Venus... or onto Earth!

I guess I'll count that as another reason to prefer managing it in situ. But it might be fine, actually? Slow re-accretion can probably be managed by whatever terraforming process we've kickstarted while most of it was gone.

Oh wait, I remember the plan, ship it to Mars so they can have some decent atmospheric pressure.

Mars could use only a small fraction of it. If the entire atmosphere of Venus were moved to Mars, the surface pressure there would be 120 bars (more at the lowest point).

There’s a book where they freeze the atmosphere then cover the ice with a thick layer of dirt. But I cannot recall which.

"Venus is dry"

I thought the Venus theory was runaway greenhouse driven initially by water vapor. Going off memory, H2O is roughly 10 times as effective a greenhouse gas as CO2, with Venus being closer to the sun A larger percentage of water ended up in vapor form, leading to a feed back loop where the increased heat pushes more water to vapor leading to more heat, eventually liberating the co2 from the rock, making everything worse, ending up with the current situation where venus has way too much atmosphere.

Which is the long way to say, I think there is a lot of water on venus.

https://en.wikipedia.org/wiki/Atmosphere_of_Venus

From your link:

> Lighter gases, including water vapour, are continuously blown away by the solar wind through the induced magnetotail.

There used to be a lot of water on Venus.

Moreover, I think Venus has lost most of its hydrogen to space, so you can't even make water anymore. Hydrogen escapes the atmosphere relatively easily for Earth- and Venus-sized planets[1], and the vaporization of all the water and subsequent disassociation[2] of H2O allowed the hydrogen to escape into space.

[1] See https://en.wikipedia.org/wiki/Atmosphere#/media/File:Solar_s...

[2] See https://en.wikipedia.org/wiki/Atmospheric_escape

Earth's oxygen rich atmosphere is really important here-- any free hydrogen can readily combine with the free hydrogen, decreasing the chance of any being loss.

According to that wiki article, the hydrogen was mostly lost in the form of water vapor. The hydrogen and oxygen in the magnetotail are in almost a perfect 2:1 ratio. At least,

> Currently the main ion types being lost are O+, H+ and He+. The ratio of hydrogen to oxygen losses is around 2 (i.e. almost stoichiometric for water) indicating the ongoing loss of water.

Also Earth didn't have an oxygenated atmosphere until relatively late, a couple billion years in, so I didn't know if that could be the thing that saved it.

I don't have primary sources, unfortunately -- I'm recalling something I read in _Oxygen_, which I found reassuring. It said that, one of the benefits of our oxygen-rich atmosphere was that we would lose an insignificant amount of hydrogen/water over the next billion years. It's one of those things that doesn't actually matter to me in my daily life, but I still find comforting, so it's stuck with me.