A group at Purdue University has released an interesting study of lava tubes on the Moon in which they examined the possibility that there are ones big enough and stable enough to hold cities:

• Theoretical study suggests huge lava tubes could exist on moon Purdue News
• Tunnel Vision: Underground Cities on the Moon – Leonard David

This diagram illustrates a lava tube protecting a city the size
of Philadelphia. Purdue University/courtesy of David Blair

Lava tubes are

tunnels formed from the lava flow of volcanic eruptions. The edges of the lava cool as it flows to form a pipe-like crust around the flowing river of lava. When the eruption ends and the lava flow stops, the pipe drains leave behind a hollow tunnel, said Jay Melosh, a Purdue University distinguished professor of earth, atmospheric and planetary sciences who is involved in the research.

There is indirect evidence that lava tubes exist on the Moon. A number of large holes, for example, have been seen on the Moon and it is speculated that these could be where the roofs of lava tubes have collapsed. See Lunar and Martian Lava Tube: Exploration as Part of an Overall Scientific Survey, Daga et al. (pdf).

The Purdue study considered the case of really large tubes:

David Blair, a graduate student in Purdue’s Department of Earth, Atmospheric and Planetary Sciences, led the study that examined whether empty lava tubes more than 1 kilometer wide could remain structurally stable on the moon.

“We found that if lunar lava tubes existed with a strong arched shape like those on Earth, they would be stable at sizes up to 5,000 meters, or several miles wide, on the moon,” Blair said. “This wouldn’t be possible on Earth, but gravity is much lower on the moon and lunar rock doesn’t have to withstand the same weathering and erosion. In theory, huge lava tubes – big enough to easily house a city – could be structurally sound on the moon.”

Blair worked with Antonio Bobet, a Purdue professor of civil engineering, and applied known information about lunar rock and the moon’s environment to civil engineering technology used to design tunnels on Earth.

The team found that a lava tube’s stability depended on the width, roof thickness and the stress state of the cooled lava, and the team modeled a range of these variables. The researchers also modeled lava tubes with walls created by lava placed in one thick layer and with lava placed in many thin layers, Blair said.

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Personally, rather than on the Moon I would prefer to live in a large, island-sized, in-space habitat like that promoted by the late Princeton physics professor Gerard K. O’Neill . Rotation could provide a full 1 g of spin-gravity, no lunar dust to hassle with and breath, sunlight all around, and there would be a greater sense of freedom of movement. That is, why work so hard to get out of earth’s gravity well just to jump down another one?

Every so often, someone on the web discovers the great space colony artwork from the workshops that studied such structures: How we’ll live in space, according to people in the 1970s – mashable

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Interior including human powered flight. Art work: Rick Guidice.
Image via NASA Ames Research Center.

At the time, such ambitions were believed justified by the big drop in space transportation costs that would come when the Space Shuttle began to fly. The Moon would still be useful – as a source of the materials to build such huge structures. It’s low gravity would allow building materials literally to be thrown into space by mass drivers, i.e. electromagnetic catapults.

Unfortunately, the hyper-complex and fragile Shuttles failed to lower space transport costs at all. The original fully reusable design for the Shuttles was down-graded by budget cuts and the Shuttles became only partially refurbish-able, requiring a standing army of thousands to work for months to return one to flight.

Today we finally are seeing new approaches to reusable space vehicles that intend to achieve full reusability and fast turnaround. We could see the cost of getting to space drop by factors of 100 in the coming decade.

Low cost spaceflight combined with new technologies like 3D printing and advanced robotics will make it affordable to build large scale structures in space.  Today’s space pessimism towards concepts like in-space colonies will give way to taking them granted just as we take granted the giant structures on earth that once would have been considered fantasies.