Added to my list of interesting things that will likely never happen is engineer Ken Roy’s plan for a type of terraforming without actually changing a planet’s atmosphere. He wants to use “shells” to create enclosed cities–biospheres of a sort in outer space. From Miriam Kramer at Space.com:

“Roy’s terraforming vision hinges upon what he calls ‘shell worlds.’ Upon arrival at an ideal planet, humans would literally encase the alien world inside of a protective shell made from Kevlar, dirt and steel.

‘We have a central world. We put an atmosphere on it,’ Roy said. We can have the ‘composition, temperature, pressure of our choosing. Let’s assume we want ‘Earth-normal,’ and we put a shell around the central world to contain this atmosphere. The atmosphere then exists between the shell and the central world. The outer part of the shell is essentially a vacuum.’

While the planet’s gravity would remain unchanged, the rest of the world could be made very similar to Earth after importing vital materials,”