The problem isn’t Venus’ hot, dense atmosphere per se, it’s the difficulties involved in:
[li]what makes it hot[/li][li]what makes it dense[/li][li]Hi, Opal![/li][/list=1]
However, let’s look at these, with a bit of handwaving thrown in as necessary.
One of reasons for Venus’ being hot is, of course, the fact that it gets about twice as much energy from the Sun as does the Earth. Therefore, we need a mirror. A really big mirror. A thin, mylarized silicone sheet orbiting Venus in the ecliptic is probably the best that we can do. Putting the mirror in a cytherocentric orbit, or even at the Sun-Venus L1 point, would have it just too far from the planet to cast much of a shadow unless it were really, really big, and, whilst I am willing to wave my hands, I’m not willing to wave them so vigorously that they fly off of my wrists. Therefore, a (fairly) closely orbiting mirror. We may wish to put several of these in orbit, to reflect the maximum amount of light (although in that case we can have problems with radiation from Venus) and arrange them so as get a decent day-night cycle (ISTR that the sideral period of Venus (essentially, the time between the sun being overhead at the equator, and the sun being overhead again at the equator, is 117 days). Manufacturing the mirrors is an exercise left to the student.
The atmosphere of Venus is mostly CO[sub]2[/sub]; contrary to popular belief, however, that’s an effect of the temperature, not a cause of it. The Urey reaction is:
CO[sub]2[/sub] + *X*SiO[sub]3[/sub] <-> *X*CO[sub]3[/sub] + SiO[sub]2[/sub]
or, in layman’s terms, carbon dioxide and silicates will be in equilibrium with carbonates and silica (essentially, sand). The Urey reaction runs to the right at lower temperatures, to the left at higher temperatures, and faster (but not fast enough to do our terraforming project any good, alas) in the presence of liquid water. Earth has about the same amount of CO[sub]2[/sub] that Venus has; however, the Urey reaction on Earth has gone almost completely to the right, Venus, almost completely to the left. Sulfuric acid is one of the real culprits in the Cytherean “runaway greenhouse”.
Now, Venus has very little water at present; it probably outgassed the same amount as the Earth, but the water has long since been photolysized by solar UV, the hydrogen escaping and the oxygen combining with the regolith. We will want to bring in both water and an alkali (to neutraliz the sulfuric acid); fortunately, we can get both in the outer system in the form of impure water ice laced with ammonia ice and/or ammonium hydroxide. The task of delivering it is again left as an exercise for the student.
Even cooled down, Venus’ atmosphere will remain a problem. Assuming that we convert the carbon dioxide to oxygen by handwaving, we still have about 30 atmospheres of oxygen – a wee bit too much for Terrestrial life.
More should be said on this, but I have to devote my attention to other, more mundane things (like earning a living) just now.