Venus has a high surface temperature and pressure. This is due to the high concentration of Carbon dioxide (CO2) gas. When all CO2 gas is removed, Venus will be colder than Earth by 35 K. The highly reflective SO2 in the upper atmosphere will precipitate within a few years, warming the planet by a few Kelvin. CO2 is dissociated, the Carbon stored and the Oxygen released. The atmosphere will then be mostly O2 with a small portion of N2. Venus is then relocated to an orbit between Earth and Mars. H2 is then imported from the gas giants and released directly into the atmosphere from space. H2 will react with atmospheric O2 producing water that will rain down across the planet. Water is a greenhouse gas and Venus will warm again until it is slightly cooler than Earth. After a hydrosphere is established the excess atmosphere is then seperated into its constituents. Each constituent is liquified and stored under pressure for later use or incremental release. The planet is now ready for colonisation!
It's interesting to note: Reduced atmospheric pressure will cause an uplifting of the planets surface, forced by internal pressure. High-pressure CO2, trapped below the surface, will explosively release during the early stages of caeliforming.
Construct a moon orbitting Venus. Orient the moon at 0° inclination to the solar plane. Position the moon to maximise solar shading. Use low-density mass. Tidally lock the moon to maximise stability. The new moon is made from the orbital transfer of a moon from the outer system, or constructed from the mining waste of the asteroid belt. The moon will provide regular shade, allowing the planet to cool slightly.
Advantage: Start-stop process. No ongoing-maintenance.
Disadvantage: High initial investment. High energy requirement. Requires high-precision moving of mass. Low-fatality for orbital transfer, medium-fatality if mining waste is used.
Deploy carbon dissociators across the surface of Venus. Carbon dissociators seperate CO2 at the atomic level, into carbon powder and O2 gas. The carbon powder is then stored in airtight flexible thermoplastic surface domes or, solid wall containers.
Advantage: Surface dissociation removes the need for direct human involvement and avoids the high energy costs associated with an orbital distillery.
Disadvantage: Numerous units will be required as components will corrode quickly, or become clogged with dust. Simple mechanical faults can't be fix and parts can't be replaced. Reliant on the integrity of the airtight storage of carbon.
Near-identical to CO2 dissociators except they produce diamonds instead of carbon powder.
Advantage: Diamonds are inert and don't react with the high oxygen atmosphere. There's no need for airtight storage as diamonds can be left scattered across the surface in the open air. Smaller unit, requiring few resources. Start-stop process. Diamond can be later collected and used as currency. Non-human process, zero fatality.
Disadvantage: Converting CO2 to diamond, instead of carbon powder, requires highly tuned equipment to operate with smaller volumes and at a lower rate of conversion. The diamonds need to be large as diamond dust is a health hazard for animals (including humans) who will colonise later.
Construct an orbital distillery in low orbit of Venus. Cloud skimmers (spacecraft) are used to collect atmospheric gas from the upper atmosphere and return it to the orbital distillery. The distillery then seperates the different atmospheric gases by vacuum distillation. The extracted heat will supplement the distillery, fusion reactor and/or black (solar) panels. Carbon dioxide and Carbon monoxide are disassociated to carbon powder and O2 gas. Carbon is then transported to the new moon for storage, it'll be needed later. The additional mass will help build the moon. Nitrogen oxides are dissociated to N2 and O2 gas. Water is a greenhouse gas. At this stage it needs to be removed from the atmosphere. Water is distilled and stored on the distillery, or transported to the moon. SO2 is a highly reflective gas, it cools the planet. It's immediately returned to the upper atmosphere. Other return gases include N2, O2, Neon, Argon, Krypton, Xenon. These gases can be used as thruster gases to maintain the orbit of the distillery. The remaining return gases are thrown back to the planet in a supercooled state, to help the planet cool faster.
Advantage: Removal of atmospheric gases producing immediate results. Start-stop process.
Disadvantage: High initial investment. High on-going maintenance. Cloud skimming is a high-fatality occupation.
Drop tanks are gas cyclinders dropped into the upper atmosphere by rockets launched from Earth. Their outer surface is highly reflective. They start with a vacuum inside and so have some bouyancy in the thick atmosphere. They have an atomic sieve that allows the smaller Carbon dioxide to enter the tank, while larger atmospheric gases remain outside. As the tank fills it looses it bouyancy and falls to the surface. The tanks fill with Carbon dioxide at the same pressure as the surrounding atmosphere. The seperation of CO2 from other atmospheric gases has negligable effect on atmospheric pressure. What it does do is decrease the optical depth of the atmosphere. As the optical depth decreases, the ability to loose heat increases, and the planet cools down.
Advantage: Can be performed without us leaving Earth, without the need for orbital distilleries or a moon base. Non-human process, zero-fatality.
Disadvantage: Most of the atmosphere will need to be sieved to have the desired result. Sieving half the atmosphere will only reduce the surface temperature to 621 K. Sieves will clog with dust particles. Very high investment of resources with low possibility of success.
Additional mass should be added to Venus, Mercury is the best source. Mining Mercury will produce an abundance of precious metals for construction and trade. It'll also produce an abundance of waste mass, primarily high-density Iron and Nickel, which can be dumped on Venus. The capture and mining of asteroids is an alternative.
Advantage: Abundant precious metals. Stop-start process.
Disadvantage: High energy cost. High-fatality mining. Meterorite impact of mass is not possible after colonisation.
All of the water on Venus is currently in the form of gas. Reducing the atmospheric temperature and pressure will allow water to precipitate to the surface. Water stored on the orbital distillery can be thrown back into the atmosphere in incremental stages. The high O2 concentration in the atmosphere, from the dissociated CO2, is only temporary. Hydrogen-1 (not deuterium) is imported from the outer system. Hydrogen trapped from solarwind will supplement imports, but should not be relied on.
Advantage: Water regulates planetary temperature and buffers against extreme temperature fluctuations. Start-stop process.
Disadvantage: Mining Hydrogen from the gas giants is energy expensive, and a high-fatality process.
Venus has a negligable magnetosphere. A magnetosphere protects the planet surface from low-energy cosmic radiation and coronal mass ejections (CME). The high atmospheric pressure on Venus has compressed and cooled the planetary core. The high atmospheric temperature has slowed the convection of heat from the core. These inhibitors have stopped the planetary dynamo from generating a magnetic field. Reducing atmospheric pressure and temperature will allow the core to expand, reliquify and for convection currents to start again. This is not an automatic process. The core will need time to recover and until we get a closer look we won't know if the dynamo will start again and at what strength. The planetary dynamo may need technological help. Superconductive magnets fixed in the mantle could be used to accelerate the dynamos recovery and possibly increase the rotation of the core. The rotation of the planets core will create friction with the mantle, increasing volcanic activity at the surface and heating the core further.
The high surface temperature has bleached all volatiles from the planet surface. Metal carbonates have all decomposed into metal oxides and Carbon dioxide. By reducing temperatures, metal carbonates will slowly reform. This process will be accelerated by the presence of water.
A majority of the planets Carbon was removed from circulation. The carbon was dissociated and stored. When the hydrosphere stabilises carbon is reintroduced, incrementally and in small amounts to allow life to start growing.