Cities in Flight

July 25, 2008

In the 1970s the acclaimed space artist Robert McCall created a number of arresting high-tech fantasy paintings of cities floating over the southwestern desert. These probably helped inspire George Lucas to create the opulent “Cloud City” on the planet Bespin in Star Wars Episode IV, The Empire Strikes Back.

Geoffrey Landis of NASA's Glenn Research Center in Cleveland, Ohio has taken this idea a step further by proposing to someday build a floating city on perpetually cloudy Venus.

At 800 degrees Fahrenheit, Venus’ surface is hot enough to melt lead. The sulfuric acid rain would eat the chrome off you car, and then dissolve the entire vehicle. Rivers of molten lava snake across the surface and relentless lighting activity explodes across the sky.

But Landis points out that things are comparatively peaceful and balmy 30 miles above the Dantesque landscape. Temperatures are the same as in your living room and the air pressure is equal to the pressure at Earth’s surface. This would be the place to put a science outpost or perhaps an entire city, he says.

But don’t expect to go walking around on outdoor terraces to get a panoramic view. The city would have to be enclose in some sort of ceramic material to prevent it from being eaten away by battery-acid rain. So, a cloud city on Venus might more closely resemble the Star Wars Death Star battle station. You might not even have windows to look out across the billowing Venusian cloud tops or volcanic ranges.

The city would easily float in the Venusian atmosphere. All you have to do is fill it with the equivalent mix of Earth’s atmosphere: nitrogen and oxygen. These gasses are lighter than Venus’ dense carbon dioxide atmosphere, and so the city would be buoyant like a helium balloon.

The idea of lighter-than-air travel on planets isn't new. It’s where 19th century flight meets 21st century exploration. NASA is studying a solar-powered hot air balloon for Mars. A hot air balloon could drift in the chilly nitrogen atmosphere on the frigid Saturnian moon Titan. Located one billion miles from the sun, the balloon would be heated by radioactive material. 

This was best dramatized in Arthur C. Clarke’s 1971 story, Meeting with Medusa. A “bionic-astronaut" gently descends into Jupiter’s atmosphere using a huge hot-hydrogen balloon. A mile-wide floating jellyfish-looking creature sees the balloon and gets amorous.

One challenge for a floating base on Venus is that there aren’t any natural resources to support the inhabitants. There’s no water on Venus’ surface and the hot basalt certainly could not be mined. So everything the colonists need to survive would have to be imported from elsewhere. Imagine the cost of a bottle of Perrier.

If you want to live off Earth, simply build a city in space and keep it there. There’s plenty of energy available from the sun and there's a hard vacuum for manufacturing applications. Resources, including water, could be harvested from the moon and asteroids. Spin the structure like a giant carousel and you have artificial gravity.

This idea was widely popularized by Princeton physicist Gerard O’Neill in the late 1970s in a book called The High Frontier. His self-sufficient drum-shaped “O’Neill Colonies” were free-flying city-sized space stations in the Earth-moon system. Colonists would live along the wall of the spinning drum and mirrors would reflect sunlight in an artificial day-night cycle.

These space colonies could be built from regolith electromagnetically launched off the moon. O’Neil proposed that the colonists would earn a living from building solar power satellites, with square mile solar cell arrays, and selling the electricity to Earth by beaming it down as microwaves. 

There’s no practical limit to the number of space city-states that could be built and parked in any of the Lagrange points in the Earth-moon and Earth-sun systems.

A fanciful precursor to this idea is in James Blish’s series Cities in Flight. In the story humans develop an antigravity device known as the "spindizzy.” The antigravity drive is so powerful it propels entire cities off the Earth and their inhabitants roam the stars. 

Arthur C. Clarke’s and Genrty Lee’s Rendezvous with Rama series gave exquisite details of the engineering of such a space colony, which even has a “cylindrical sea.” In their story the colony is really a mysterious space ark from another planetary system that passes by the sun on an interstellar journey. A survey team from Earth becomes interplanetary tomb raiders when they enter the creepy, apparently hibernating dark city.

Over the long term I believe free-flying space colonies will be integral to our colonization of the solar system. These city-states may require much less effort to build and maintain than trying to survive on a hellish planet like Venus, or even trying to warm up frigid Mars through terraforming.

Photos, top to bottom: NASA; Analog