When we think about humans developing into a multiplanet species, the common vision is a future with humans living on both Earth and Mars. But Venus might be a better choice.
It wasn't all that long ago that people thought life might exist on Venus. In the late 19th and early 20th centuries, some astronomers theorized that life existed on every planet in the solar system. Astronomers thought Venus, which is close to the Sun and shrouded in a thick layer of clouds, could be a tropical paradise, a world where the global climate would be similar to the Earth's equatorial region—temperate and lush all the time.
But that dream was shattered in the 1920s: When astronomers began studying the composition of Venus, they found the planet's clouds we made of toxic carbon dioxide, not water vapor.
And as astronomers soon found, Venus isn't tropical, it's blazing hot. Radio observations in the 1950s revealed the planet was far hotter than any Earthly climate, and the first spacecraft to fly by Venus in the early 1960s, Mariner 2, determined the average surface temperature to be around 900 degrees Fahrenheit.
When the Soviet Union landed a handful of Venera probes on the surface in the 1970s and 1980s, they lasted just hours in the intense heat and pressure; the atmospheric pressure on Venus' surface is 92 bar, or 92 times what we feel on the Earth's surface.
More modern observations have added to our knowledge of our planetary neighbour. There are hotspots on the surface that get up to 1,295 degrees Fahrenheit, and the atmospheric pressure is closer to 96 bar, which is equivalent to the pressure you would feel roughly 0.6 miles under water. The thick cloud cover means little to no sunlight is available at the surface for solar power. And the atmosphere is poisonous: In addition to carbon dioxide it contains nitrogen and clouds of sulfuric acid droplets.
Venus is, in short, a hellish world.
Yet it's exactly that thick, heavy, toxic atmosphere that makes Venus an appealing world for humans to live on. Nothing says that to live on another planet means actually living on that planet's surface. We could, in theory, use Venus' thick atmosphere to our advantage, and build floating cities.
At the very least, it's an argument Geoffrey Landis, a scientist at the NASA's Glenn Research Center and science fiction author, made in a presentation at the Conference on Human Space Exploration, Space Technology & Applications International Forum in New Mexico in 2003. More than a decade later, it's still a compelling idea.
Landis points out that at an altitude of about 31 miles above the surface, Venus becomes less of a hell world and more of a paradise. At that altitude, the environment is about as Earth-like as it gets without being on our own planet. There's plenty of sunlight for solar energy, the temperature is similar to Earth's in that it allows for liquid water to exist, and the gravity is about 90 percent of what we feel on Earth.
And most importantly, the gases we need to support life are readily available. Carbon dioxide for plant growth and nitrogen both exist in the Venusian atmosphere in abundance. These gases along with hydrogen gathered from the sulfuric acid droplets could be used to generate the basic elements humans need for survival.
As noted in a great blog post by Robert Walker that's recently been making the rounds, doing all this while floating isn't impossible. As Landis argued, "on Venus, breathable air is a lifting gas"; it's light enough to rise above the atmosphere on its own.
Habitats wouldn't need a separate system like a helium balloon to lift then to that ideal 31 mile altitude. The gas inside the lightweight habitat that its inhabitants would breathe could theoretically be enough to keep it aloft, as suggested by Soviet research conducted in the 70s that Walker references.
From these floating habitats, the first humans on Venus could explore the surface telerobotically. Guiding rovers from high atmospheric perches, we could learn about the planet, gather samples, and bring them back to a habitat for analysis in a floating laboratory. Eventually, as more in learned about how to survive 31 miles above Venus' surface, these habitats could expand to become fully functional floating cities.
Also, because Venus is relatively close to the asteroid belt, it could serve as a base for the burgeoning asteroid mining industry. Landis says that this could provide a natural economic model for Venus colonization, making it perhaps more attractive than Mars.
Colonizing Venus like this might seem insane, but it's not, strictly speaking, impossible. It's not something we're close to achieving for the moment, but it might be time to turn our planetary attention to Venus in the ongoing quest to find a way to live on another world.