[ben_w]

> You can't "boil off" the atmosphere. You need to accelerate the gas molecules past the planetary escape velocity, otherwise they'll just cool down and drop back onto the surface.

Velocity in a gas is a distribution; raise temperature and increasing fraction exceeds escape velocity.

This is why Earth has ~ no hydrogen or helium in the air.

> There's no realistic way to evacuate that much gas (the surface pressure on Venus is almost 100 atmospheres!).

"Realistic" for values including "let's build a mirror the size of a planet, in space, and keep it together for centuries".

It's currently scifi to send more than a mere few tons total mass that way, and mirrors wouldn't even survive decades, so "realistic" is a bad criticism.

[cyberax]

> Velocity in a gas is a distribution; raise temperature and increasing fraction exceeds escape velocity.

It's not going to be a significant fraction to matter, except of truly geological timeframes.

> "Realistic" for values including "let's build a mirror the size of a planet, in space, and keep it together for centuries".

Probably thousands of years. But not hundreds of thousands.