Terraforming Venus

Venus is a hellish, uninhabitable planet that has undergone a runaway greenhouse effect.

The good news is that Venus' surface gravity is 92% as strong as Earth's, so a surface-dweller could escape the harmful effects of micro-gravity (unlike Mars which is about one-third the strength of Earth's surface gravity). However, the rest is bad news. On the surface of Venus, temperatures can be up to 500° C (hotter than the boiling point of lead) with an atmospheric pressure over 90 times greater than that of Earth's—this would be the equivalent of standing (and getting crushed) underneath 1km of ocean water on Earth. The lower portion of Venus' atmosphere is comprised of hurricanes and perpetual rainstorms of sulphuric acid.

Due to these hellish surface conditions, the first humans to live on Venus would likely live in enormous blimps about 50−55 km above Venus' surface, where the temperature and atmospheric pressure is Earth-like (lower down, Venus' atmosphere is unprotected by the Sun's radiation). These sky cities could also incorporate ultra-long cables dangling dozens of kilometers from their bases, and be used to collect resources from Venus's atmosphere. This first generation would be confined to living in the sky, and would begin the long and arduous process of terraforming Venus, taking hundreds or even thousands of years to complete.

Sequestering C02 from Venus' atmosphere
According to Freeman Dyson, the first step in terraforming Venus should be to cool the planet since this step would take the least amount of time (hundreds of years). This could be accomplished by constructing an enormous soletta of orbital mirrors, reaching at least twice the circumference of the planet. Venus receives an enormous amount of solar insolation (solar radiation received by a planet) and its thick C02 atmosphere traps the Sun's heat energy. The soletta could eliminate both of these problems. It would reflect sunlight away from Venus, causing the planet to cool. Eventually it will rain C02 creating seas of liquid C02 which will eventually freeze into vast ice sheets, turning Venus into a kind of Snowball Earth. A serious side-effect would be the subsequent melting C02 due to Venus' geological activity, so a solution to reduce this would have to be devised.

Liquid water and life on Venus
The next stage of terraforming could be to use the soletta to heat up Venus' surface to Earth-like temperatures and importing enormous amounts of water to create seas of liquid water as well as a functioning hydrosphere. One way could be to lift Saturn's moon Enceladus towards Venus and allow it to collide with the planet, or split it lower down in the atmosphere. Enormous quantities of water would then be imported onto the planet, creating seas of liquid water with a depth of 140 meters. We could also lift comets made of ice from the Oort Cloud to release water.

The final step in terraforming Venus would be to create an Earth-like atmosphere. We could lift asteroids rich in nitrogen from the Kuiper belt towards Venus, making its atmosphere rich in nitrogen and oxygen. Photosynthetic microbes could use C02 to generate enormous amounts of oxygen, while nanobots could be dispersed to absorb carbon dioxide and other toxic gases to replace them with breathable air. Gradually, after microbes create arable soil, plants could be introduced into the Venusian environment which would continue oxygenating this world. And, eventually, Venus' atmosphere would have Earth-like proportions of carbon dioxide, oxygen and nitrogen; at this point, animals and humans could live on Venus' surface.

Planetlifting
Finally, the entire orbit of Venus could be planetlifted further away from the Sun to bring it closer to the Goldilocks Zone where biological life is able to exist at stable temperatures.