Answer: Venus

These days, it is Mars that gets all the terraforming hype. “We’re going to the red planet in my lifetime!”, billionaire visionaries say. “This is how we could turn the red planet green!”, the futurists and sci-fi authors shout. “We should totally get Matt Damon stranded on Mars again and make a boatload of money at the box office!”, Ripley Scott says (probably).

But long before everybody was starry eyed for stories about Mars and planning to turn it into Earth II, famous astronomer and astrophysicist Carl Sagan had high hopes for Venus. Sagan was the first person to seriously propose terraforming Venus. We now know that Venus is a very hot and very inhospitable planet, so what prompted Sagan to focus on it?

Prior to his proposal, the atmosphere of Venus was believed to have an Earth-like temperature, but when scientists learned that Venus had a thick carbon dioxide atmosphere (with a corresponding greenhouse effect), Sagan proposed injecting photosynthetic bacteria into Venus’ atmosphere. The purpose of the bacteria would be to convert the carbon dioxide into reduced carbon in organic form in order to greatly reduce the amount of carbon dioxide in the atmosphere (while also dealing with the severe greenhouse effect).

Three decades later in his 1991 book Pale Blue Dot, Sagan acknowledged how advances in science had shown his plan to be both premature and exceptionally difficult to pull off due to how much denser the atmosphere on Venus was discovered to be:

Here’s the fatal flaw: In 1961, I thought the atmospheric pressure at the surface of Venus was a few bars … We now know it to be 90 bars, so if the scheme worked, the result would be a surface buried in hundreds of meters of fine graphite, and an atmosphere made of 65 bars of almost pure molecular oxygen. Whether we would first implode under the atmospheric pressure or spontaneously burst into flames in all that oxygen is open to question. However, long before so much oxygen could build up, the graphite would spontaneously burn back into CO2, short-circuiting the process.

It’s possible that we might terraform Venus one day, but Carl Sagan was right, it won’t be by seeding photosynthetic bacteria alone. The current theories on how we could terraform Venus rely on advanced technology like solar shades, capturing hydrogen from gas giants to inject into the atmosphere, and using giant space mirrors to shade the dayside while providing sunlight to the nightside. In other words, if we get around to it at all, we won’t be getting around to it in our lifetimes—alas, no Venusian vacations for any of us.