It might be possible to terraform Venus some day, when our technology gets good enough. The challenges for Venus are totally different than for Mars. How will we need to fix Venus?
This planet has been the center of corny nerd fantasies and awful behavioral gender oversimplifications from days of yore.
There are many real reasons to admire Venus from afar, but my favorite, much like Mars, is the potential to turn it into a vacation spot and haven for mad science planetary engineering. Venus is a virtual twin of Earth. It has a solid surface and very similar gravity to Earth.
You're already sold, right? You can already picture yourself there in a sun hat on a beach towel. There's are just few complications. There's insane atmospheric pressure, clocking in at a hull crushing 91 atmospheres, and what's worse for all this atmospheric trouble, there's no oxygen. It's a just a huge tank of suffocating CO2. It also rains sulfuric acid.
So, in order to terraform Venus, we need to decrease the temperature, reduce the thickness of the atmosphere, and there's the simple matter of making breathable air. Here's the best part. They're all connected, and not a John Gray book in sight.
Venus is so darned hot because of this thick CO2 atmosphere, the first thing we need to is cool the planet down. If you set up a huge space-based shade and block all sunlight from hitting the atmosphere, the temperature would drop, and I mean *drop*. It would cool hundreds of degrees until it was so cold the CO2 would freeze out of the atmosphere, and pile up in drifts on the ground. Then you could scoop up the carbon, bury it or shoot it off into space.
Another, equally mad idea would be to build floating cities high up in the atmosphere of Venus. They would need to contain factories which sucked carbon dioxide from the atmosphere and split it into carbon and oxygen. The carbon would be made into graphene structures, and the oxygen would become the lifting gas to keep the cities afloat. With more cities, it would block the sunlight and help cool the planet down.
Unfortunately, the slow rotation of the planet is still a big problem. A solar day on Venus is 116 Earth days in length. You could speed its rotation by close asteroid flybys, or use that crazy space shade contraption to create an artificial day/night cycle.
This is but the tip of the CO2 iceberg. Here are the really crazy ideas. Start by smashing thousands of asteroids into the planet and splash the atmosphere into space. This plan has some flaws beyond the obvious engineering requirements and "Hail Mary" qualities. There's a pretty good chance Venus would just scoop its atmosphere back up on the next orbit around.
You could also dump massive amounts of calcium or magnesium into the atmosphere to sequester the carbon away. Unfortunately, you'd need more mass than one of the largest asteroids we currently have available. There's still that problem of no magnetic field, but I'm sure by the time we've worked out the first parts, that would seem like a walk in the park.
Terraforming Venus is insanely harder than terraforming Mars. But it's still possible with enough planning, technology and patience, and that's pretty amazing.
Explore further: ESA: Unveiling Venus