How to keep a giant shell around venus rotating normally

Question

So, let’s face it, terraforming Venus is a bust. There’s simply no good way to do it without speeding up its rotation, which demands huge quantiles of energy. Any civilisation that could produce this much energy is advanced enough to find and terraform some more equable planet in a different Star system.

But, what if we’ve got it wrong by trying to terraform the surface? What if, (hear me out) we seeded nanotech into the atmosphere, where it converted the carbon dioxide into carbon nanotubes and oxygen. The nanotubes are woven to form a huge layer surrounding the entire planet, which is buoyed up by the pressure of the atmosphere below and by tiny pockets of oxygen within it.

This strong yet flexible layer is then terraformed; rocks and soil is dumped on the layer, water is dumped on the rock, top with a biosphere and serve fresh to hungry colonists! The best part? The shell rotates independently of the planet below, emulating a day/night cycle!

The only problem I can foresee is in keeping the shell spinning around a central axis, the problem being that there isn’t one. How can I keep this giant shell spinning at a set rate on venus?

Answer 1

From a practical perspective, no can do...

Let's assume a shell with no stabilizing rockets and no attachment to the ground. It's been reduced to force the planetary atmosphere to a fairly high pressure. Could that be stable?

Nope.

Almost anything would move it into contact with the ground. My first guess would be the solar wind. My second would be storms (shifting low/high pressure regions of the atmosphere). So, from a practical perspective, this can't work.

It would certainly work with magically strong connections to the surface

Imagine connecting the shell with six herculean cables. One at each pole and four equally distanced around the equator. In reality you'd want thousands or millions. Give each one a turnbuckle so you can tighten them until the shell is forcibly stable over the planet. Boom. Works like a charm.

But it won't rotate independently. In fact, it won't move (for the intent of this question) at all. So this doesn't solve your problem.

What about rockets? Lots and lots of rockets?

OK, given a computing infrastructure that could control rockets with excellent precision, we could theoretically create a stable shell that rotates independently and doesn't collide with the planet. It doesn't matter if it's a lot of rockets or a few... what maters is that the same amount of energy in either case would be expended (you know, F=mA) to keep the shell in place.

But those rockets would be very energy hungry. What would be the point of using them if they cost more than just building a bunch of orbiting megastructures somewhere more practical? In the long run, it might cost less to move Mars into a more favorable orbit. But, believably, it would work.

And there are some issues with a spinning shell...

An independently spinning shell suspended over an atmosphere, no matter how smooth and regular the inner surface of the shell, will cause friction with the atmosphere. That can cause storms and internal shifts in pressure that start putting those rockets to the test. The heat that friction causes might be beneficial to your inhabitants... but it's more likely to eventually cook them because there's pretty much nothing to cool the planet now that you've blocked off its ability to radiate into space on the night side.

And as the atmosphere heats up, it expands. Even carbon nanotubes aren't infinitely strong. Perhaps it's unlikely, but I can believe your shell eventually popping like a balloon. If the shell isn't compressing the atmosphere enough to make this a possibility, it'll either be expending a lot of energy to keep everything stable or it must be built so far away from the planet that the atmosphere becomes irrelevant. But that would decrease your available gravity. Consequences...

Answer 2

A frame challenge:

Why do you assume that the rotation rate of Venus needs to be changed to make it habitable?

The aim of terraforming a planet is to turn it into a "habitable planet for Man". In Habitable Planets for Man (1964) Stephen H. Dole discussed the properties of a planet habitable for humans.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

On page 58 Dole wrote that:

From the standpoint of human habituation there are two limits related to rotation rate. for slow rotation rates a limit would be reached when daytime temperatures became excessively high in the low latitudes below a critical latitude and when nighttime temperatures became excessively low poleward from that same latitude, or when the light-darkness cycle becomes too slow for plants to live through the long hot days and long cold nights.

On page 60 Dole guess that the longest possible day for a human habitable planet might be 96 hours or 4 Earth days long.

Dole then goes on to discuss the special case of a tidally locked planet with a rotation rate equal to its orbital period, so one side always faces the star and one side always faces away from the star in eternal night. Dole decided that all the water and maybe atmosphere would probably freeze out on the dark side, leaving the planet waterless and maybe airless.

Astronomers for many years ruled out red dwarfs as potential abodes for life. Their small size (from 0.08 to 0.45 solar masses) means that their nuclear reactions proceed exceptionally slowly, and they emit very little light (from 3% of that produced by the Sun to as little as 0.01%). Any planet in orbit around a red dwarf would have to huddle very close to its parent star to attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of Mercury) for a star like Lacaille 8760, to as little as 0.032 AU for a star like Proxima Centauri[84] (such a world would have a year lasting just 6.3 days). At those distances, the star's gravity would cause tidal locking. One side of the planet would eternally face the star, while the other would always face away from it. The only ways in which potential life could avoid either an inferno or a deep freeze would be if the planet had an atmosphere thick enough to transfer the star's heat from the day side to the night side, or if there was a gas giant in the habitable zone, with a habitable moon, which would be locked to the planet instead of the star, allowing a more even distribution of radiation over the planet. It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, preventing photosynthesis.

This pessimism has been tempered by research. Studies by Robert Haberle and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it included greenhouse gases CO2 and H2O) need only be 100 millibars (0.10 atm), for the star's heat to be effectively carried to the night side.[85] This is well within the levels required for photosynthesis, though water would still remain frozen on the dark side in some of their models. Martin Heath of Greenwich Community College, has shown that seawater, too, could be effectively circulated without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Further research—including a consideration of the amount of photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.[86]

https://en.wikipedia.org/wiki/Planetary_habitability#Suitable_star_systems

So if a tidally locked planet with eternal day on one side and eternal night on the other side might possibly have suitable temperatures for life if it has sufficient atmospheric and water circulation to equalize the temperatures between hemispheres, a planet with very long days and nights but less than eternal ones might also be able to have suitable temperatures for life.

If humans terraformed Venus they might possibly have to help the heat diffusion around the planet by building giant pipes and pumps to pump water between the hot and the cold sides of the planet. That would be a vast project, but almost infinitely smaller than trying to change the rotation rate of Venus.

And the might have to build their cities with super powerful heating and air conditioning systems to star warm in the coldest period of the night and cool in the hottest period of the day. But that would be almost infinitely less vast a project than trying to change the rotation rate of Venus.

The solar of day of Venus is 116.75 Earth days.

An annual plant is a plant that completes its life cycle, from germination to the production of seeds, within one growing season, and then dies. The length of growing seasons and period in which they take place vary according to geographical location, and may not correspond to the four traditional seasonal divisions of the year.

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

The growing season for annual plants on Venus might be one quarter to one half of the solar day of Venus, or about 29.1875 to 58.375 Earth days.

One seed-to-seed life cycle for an annual plant can occur in as little as a month in some species, though most last several months.

So some Earth annual plants might flourish in the long days on Venus, though many others might have to be genetically engineered to do so.

A perennial plant or simply perennial is a plant that lives more than two years.1 The term (per- + -ennial, "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also widely used to distinguish plants with little or no woody growth (secondary growth in girth) from trees and shrubs, which are also technically perennials.2

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

In temperate climate zones, many perennial plants shed their leaves and go domant during the winter months. And maybe they could react to the 58.375 day long nights on Venus like they were short winter season.

So I don't see any reason why the rotation rate of Venus would need to be changed to terraform it.

Answer 3

Yes you can build a shell, but having it spin independently from the surface could not work. However, we are talking super tech here, so we can figure out an equivalent set up.

First, build an Orbital Ring around Venus. Then build additional Orbital Rings at slightly different angles and elevations, until you have solidly enclosed the planet.

This is your foundation. Now you can build your "surface" upon which you will construct your living space.

As this is a supertech megastructure, you could just plan on having bands of this surface move on tracks on your foundation around the planet.

However, it will be much, much easier (and allows you to skip the 'shell' entirely) to just put a solar shade construction at the L1 Lagrange point and just have panels that slowly spin between edge-on and surface-facing to the Sun to match your desired day length. Do the same thing with mirrors at the L2 Lagrange point. You now have a completely customizable day-night cycle for your planet.

Answer 4

Neigh-impossible

This is a terrible idea, as the 'surface' will be squishy, prone to tears and leaks, and the rotation will be transferred as friction into the Venusian atmosphere below, slowly heating up into a fiery inferno that will eventually destroy the surface.

There is basically no way to get around it, but why even bother with that, if theres a simple, elegant solution to your day-night problems?

Why not move the Sun?

By using mirrors creatively. Let's say, for the sake of argument, you have a mirror in place to reflect the sun away from Venus so that it cools down and the CO2 freezes.

Why do you need to make the planet spin? If you want a day-night cycle, just put a mirror in orbit that acts as an artificial sun. It will catch some of the sunlight and focus it back on the planet, but instead of being at the mercy of orbital mechanics, you can change the speed, size and angle of your sun-mirror (or mirrors) to control how the sun works on your planet.

Give the mirror a little bit of an inclination relative to the sun and planet to give it seasons like on earth, although you might make them less extreme, cooler summers and warmer winters.

And you can even use it to power your Venusian cities as a free bonus.