I like to believe we could circumvent some of these problems technologically. After all, there is no particular reason to feel bad about macroscopic terraforming on a lifeless planet. Yes, it would be more work than Mars. But it's also bigger than Mars and further from the asteroid belt. No planet but Earth is going to be ideal.
The amount of energy you'd have to expend to fix Venus would be far more costly than trying to keep Mars warm by using baseboard heaters on every square metre.
One of the largest issues facing Venus is that it is effectively a dead planet. Mars is a dead planet too but it's far easier to make it warmer than to cool down Venus so we can live on it. On top of that, Mars has water frozen at its surface and perhaps in liquid form beneath; Venus only has water vapour that accounts for less than 0.01% of the total atmospheric make up.
And if it is size that you want to take into account, then think about this: it's far easier to fix a smaller planet that is geologically dead than it is a planet of our size.
Venus is never going to be colonised. Mars maybe not, but at least that is less of an impossibility.
I appreciate your taking the time to reply, but I think it's a little soon to be saying never. For one thing, we have spent a lot more time and energy trying to figure out what's going on with Mars than Venus.
Though of course Mars is a better prospect today. Frankly, I think the important thing is to have a backup, as it were, and to that end whatever serves will do. But if one backup is nice, two is even better, and the cost of making a good backup is highly justified.
In the case of Venus, the Soviets had sent several probes (Venera) from the 60s into the mid-80s. When the first probes were sent to land on the surface, it was discovered that the atmospheric pressure induced (about 100x the pressure here on Earth at sea level) was crushing the probes before they even touched down.
It took until 1970 for the Soviets to succeed and the landing device lasted a grand total of a half hour once it had landed on the surface. In fact, it barely survived due to the fact that the parachute broke and it ended up hitting the surface at 60 KM/h. It registered CO2 levels of 97% during its descent.
Venera 9 was similar in concept to the Viking landers and landed five years later, but it managed to last for almost an hour before failing due to the immense heat. The longest any Venusian surface programme by the Soviets was just shy of two hours.
Let's compare this to the Americans' probes to Mars: all of them have managed to out-last their stated mission. We have a rover that was intended to do its job within a 90-day period which instead has defied its masters and instead continued on to this very day.
The reason why I completely discount Venus as a place we'll ever visit or colonise for that matter is because it comes down the old principle that I like to follow: it's easier to bundle up than to bundle down. I can put on layers to keep my body heat in, but I cannot do much without expending energy to cool myself down.
I can appreciate that it isn't viable today. But looking back on the history of our species, things that seemed useless or even harmful frequently turned out later to be indispensable resources. Sometimes they did turn out to be simply unhelpful and harmful and I can appreciate that this is the current state of knowledge with Venus, and that it isn't especially economically meaningful to hammer on it further at this time. I just don't think it's safe to consider the matter settled for all time.
