Conceptual illustration of a lunar base in Mare Tranquilitatis Hole, believed to be an entrance to a lava tube about 100 meters below the lunar surface. Credits: Dipl.-Ing. Werner Grandl
In a new paper in Acta Astronautica Raymond P. Martin, a propulsion test engineer at Blue Origin and Haym Benaroya, a professor of mechanical and aerospace engineering at Rutgers describe the former’s research he carried out as a graduate student under the latter analyzing the structural integrity of lunar lava tubes after pressurization with breathable air. As reported previously on SSP, subterranean lava tubes on the Moon and Mars hold much promise as naturally occurring enclosures that are believed to be structurally sound, thermally stable and would provide natural protection from micrometeoroids as well as radiation. If they could be sealed off for habitation and filled with breathable air, life could be simplified for colonists as they would not have to don space suits for routine activities.
“This paper makes the argument that … lunar lava tubes present the most readily available route to long-term human habitation of the Moon”
Two views of a lunar skylight revealing a potential subsurface lava tube in Mare Ingenii. Credit: NASA/Goddard Space Flight Center/Arizona State University
Martin opens the paper with a history of the discovery and physical characteristics of lunar lava tubes tapping geological data dating back to the Apollo program. The existence of a lava tube is sometimes revealed by the presence of a “skylight”, a location where the roof of the tube has collapsed, leaving a hole that can be observed from space. Using an engineering simulation software called ANSYS, he developed a computer model to assess the structural integrity of these formations when subjected to internal atmospheric pressure.
Martin creates a model for his simulation based on the morphology of a relatively small lava tube known to exist from imagery taken by the Chandrayaan-1 spacecraft, the first lunar probe launched by the Indian Space Research Organisation . This structure averages 120 meters in diameter and was chosen because it has a rille-type opening level to the surface and could be sealed off at two locations. This approach makes sense as a starting point because the cavern would be easy to access and less energy would be be required to pressurize a smaller enclosure. Thus, the amount of infrastructure needed to establish early settlements would be minimized.
The goal of the simulation was to assess the integrity of the enclosed space under varying roof thicknesses and pressurization levels. Failure conditions were defined using commonly employed methods of assessing stability of tunnels in civil engineering and based on lunar basaltic rock general material properties known from testing of samples brought back from the Moon in the Apollo program and lunar meteorites. Finally, a formula was derived for safety factors associated with the failure conditions to ensure robustness of the design.
When running the simulation over various roof thicknesses and internal pressures, an optimum solution was found indicating that it is possible to pressurize a lava tube with a roof thickness of 10 meters with breathable air at nearly a fully atmosphere while maintaining its structural integrity. This would would feel like sea level conditions to people living there.
Being able to pressurize a lava tube for habitation could significantly simplify operations on the Moon as the infrastructure needed to make surface dwellings safe from radiation, micrometeorite bombardment and thermal extremes would be extensive adding costs to the settlement.
“A habitat within a pressurized tube would offer large reductions in weight, complexity, and shielding, as compared to surface habitats.”
Once a permanent settlement has been established and engineering knowledge advances to enable expansion into larger lava tubes, we can imagine how cities could be built within these spacious caverns, and what it would be like to live and work there. SSP explored just this scenario with Brian P. Dunn, who painted a scientifically accurate picture of such a future in Tube Town – Frontier, a hard science fiction book visualizing life beneath the surface of the Moon. Dunn envisions a thriving cislunar economy with factories producing spacecraft for Mars exploration.
Conceptual illustration of a spacecraft manufacturer inside a lava tube. Credit: Riley Dunn
Martin and Benaroya dedicated their paper to the memory of Brad Blair, a mining engineer who was a widely recognized authority on space resources.
SSP featured a post in 2020 on the promise of lava tubes as ideal natural structures on the Moon or Mars in which space settlements could be established. Some are quite voluminous and could contain very large cities. Lava tubes provide excellent protection from radiation, micrometeorite bombardment and temperature extremes while being very ancient and geologically stable.
How would a city be established inside a lava tube? What would it be like to live and work there? Brian P. Dunn paints a scientifically accurate picture of such a future in Tube Town – Frontier, a hard science fiction book visualizing life beneath the surface of the Moon. Dunn recently appeared on The Space Show where he provided tantalizing details on his book scheduled to be published later this year. You can also get a taste of the story through excerpts available on his website.
I’ve had the opportunity to get an advanced copy of his book and will be providing feedback to Dunn prior to publication. He agreed to an interview via email, summarized below, answering some of my initial questions:
SSP: Your first chapter of the book takes place in 2028 and starts out with teleoperated “SciBots” networked together in swarms to explore and prospect for resources at the Moon’s south pole. They are battery powered and need to periodically recharge at stations at the base of solar power towers at the Peaks of Eternal Light, similar to what Trans Astronautical Corp. is planning with their Sunflower system. This time frame seems overly optimistic given that NASA’s Artemis program won’t return astronauts to the Moon until the mid 2020s and Jeff Foust reported recently that a second landing won’t take place until 2 years later. Would it be more realistic to move out the timeline 5-10 years?
BPD: As Kathy Lueders at NASA has said, our strategy with both Moon and Mars is ‘Bots then Boots’. There is much scientific and ISRU work that can be done before the humans arrive. (See the article on my blog “The Mother of All CLPS Missions.”) With the Moon’s close proximity and communications satellites, we can teleoperate rovers much easier than on Mars. Regarding the SLS/Artemis timeline, I don’t believe it will ever reach full fruition. The Artemis/Gateway architecture is too expensive and too slow. There is a paradigm shift happening now as the concept of large, re-usable, re-fuel able, high payload, quick launch cadence rockets is being proven out with SpaceX’s Starship.
SSP: After discovery of the lava tube in which Tube Town is eventually established, the public “was clamoring for more” and the “excitement of the discovery of the tube breathed new life into lunar and space exploration”. I know that I would be excited, and most space cadets would be as well, but why would the general public be so supportive of space exploration because of the discovery of a lava tube on the Moon? A recent poll found that a majority of people think that sending astronauts to the moon or Mars should be either low or not a priority.
BPD: Now that we’re starting to get the rockets, the American public will soon see landers and rovers return to the Moon. This time it will be in HD TV. At some point Americans will return to the Moon. This will be must-see TV. Taikonauts will eventually land on the Moon. This will definitely light a fire under the Americans. Interest in the Moon and lunar exploration will go up. The problem will be sustaining interest (We have an incredibly short attention span). After the world record TV event, interest will wane. We will only be able to put a few astronauts in small habitats on the surface for short periods of time. Upon discovery of an intact lava tube people will know that we could actually build a town on the Moon. Even better than that guy described in that book… what was it called?
SSP: Tube Town is operated by an umbrella organization of national space programs led by NASA called the International Space Program. How do you envision this cost sharing structure getting started?
BPD: Although much cheaper than a comparable sized surface base, outfitting a lava tube for human habitation will not be cheap. Much of the materials can be made in situ, such as aluminum sheeting for the floors and airlocks, waterless concrete, steel for pressure vessels to hold volatile gasses, but much will need to come from Earth such as Factory machines, computers, electronics, medical equipment, etc.
In Tube Town, this cost is spread among the space programs of 27 countries of the International Space Program (NASA, ESA plus 9 countries that signed the Artemis Accords).
US, Canada, Australia, New Zealand, Japan, South Korea, India, Brazil, Israel, United Arab Emirates, and the 17 member countries of the European Space Agency (Austria, Belgium, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, the Netherlands, Norway, Portugal, Spain Sweden, Switzerland and the United Kingdom). Notable holdouts were China (CNSA) and Russia (Roscosmos).
The ISP is a cost and opportunity sharing umbrella organization for building and maintaining a large Moon base and robotic creation of a Mars base and the first crewed mission to Mars.
NASA would be the lead partner of the ISP, but project decisions were approved and administered by the ISP Board of Directors consisting of the member countries of the organization with weighted voting rights proportionate to their contribution. Many countries wanted to get in on the ground floor of a new space economy but couldn’t afford to duplicate the resources and infrastructure that already existed at NASA. With their combined buying power, the ISP could source rockets, landers, robotics, space suits, etc. from the most efficient and innovative private suppliers. In return, ISP countries received habitation services (shelter, atmosphere, food, water) and discounted rates for:
leasing habitation space in the Tube for scientific or commercial enterprise,
buying propellant and other in situ resources, and
payload return to Earth
ISP construction costs of the Tube are initially off-set by lunar tourism and bespoke mining. Tourism licenses are issued by the ISP to private companies. The contracts include revenue sharing, ISP Code of Conduct compliance and Space Heritage sites preservation requirements. In exchange, the licensees get transportation, medical emergency and habitation services on the Moon.
In Tube Town, the first ISP tourism licensee is with Lunar Experience, LLC. LE licensed 50 seats for a seven Earth day stay. They ran two tours per Earth month to take advantage of the Nearside lunar day (in early days, most of the popular attractions were on the Nearside). LE agreed to give away 25% of the seats to people who could not afford the price. So, of the 50 seats per trip, 12 were free and 38 were paying customers. Assuming a ticket price of $5m for a trip to the Moon for a week, a flight made $190m. The revenue sharing agreement with the ISP was 60/40 (LE 60%, ISP 40%) so for that $190m flight, LE earned $114m and ISP $76m. If only two trips were completed per month, the yearly income would be LE $1.3B and ISP $912m. The ticket price would double to watch the uncrewed launches to Mars and the price would triple to be a part of history to witness the crewed launch to Mars.
In addition, the ISP or commercial customers could take advantage of very reasonable freight rates to backhaul refined payload on the returning tourist rockets to Earth. When would the price become affordable for regular people? Probably after the third tube is discovered. I could see an ISP member like UAE opening a large lava tube exclusively as a vacation resort.
SSP: The main product produced by Tube Town’s factory is spacecraft for Mars exploration and the eventual establishment of an outpost on the Red Planet. Presumably, at least at first, not all electronic components can be made on the Moon so will have to be imported from Earth via a space-based supply chain. Elon Musk is designing Starship to go directly to Mars from Earth. Why does building spacecraft on the Moon for a Mars mission make economic sense when compared to “going direct” like Starship, and why isn’t Starship mentioned in the book?
BPD: The book is a work of fiction so I try not to use real names or products. Although I think Starship is the first of its class of big, reusable rockets, I also think the concept will be replicated (like airliners) and hopefully there will be several options in the Earth to Moon supply chain. If you can make a big re-usable rocket on a beach in Texas, you can make one inside a nice lava tube on the Moon. We will also need to get lots of bots and machinery to Mars before the humans. This can also be manufactured on the Moon. When you launch, you don’t have to fight the giant gravity well of Earth (12.6 km/s vs 2.6 km/s) and you may not even have to re-fuel to head for Mars. Huge payloads will be much more economical from the Moon.
Artist conception of a spacecraft manufacturer inside a lava tube. Credit: Riley Dunn
SSP: Tube Town has a Farm devoted to food production, waste re-cycling, and ice processing. However, without insects or wind pollination it is not possible to grow desirable fruits and vegetables like apples, squash, melons and many more. You devised an innovative way to pollinate the plants. Tell us about that!
BPD: Nearly all of the technology described in the book is based on existing technology, whether in the lab or in production. Harvey’s pollinating space bees are based on a combination of miniature drone-delivered soap bubble pollination and AI image recognition software.
SSP: In your book, the Apollo 11 landing site becomes a tourist destination. What steps are taken to preserve this fragile heritage site?
BPD: I think the Apollo 11 site is the must-see tourist attraction on the Moon. Part of that attraction is that you can still see the boot prints of the astronauts in the regolith. On the moon, boot prints are forever- unless another human destroys them. It only takes one knucklehead.
In my book, a regolith wall is built around the site to protect from plume drift from vehicles. The entrance is a good distance away from the site. Access into the site is in a plexiglass pod that is suspended above the surface. A cable system mounted on tall towers maneuvers pods of tourists through the site from above, giving them a close-up encounter, yet not disturbing the artifacts nor the regolith.
There should be multiple Space Heritage Sites on the Moon consisting of artificial artifacts from multiple countries and natural wonders like Schroter’s Valley. They should be identified and preserved by the tourist licensees that will profit from them.
Vallis Schröteri (Schroter’s Valley), believed to be volcanic in origin, is the largest sinuous rille on the Moon seen here as imaged by Apollo 15. Credits: NASA via Wikipedia
SSP: Tube Town has a centrifuge in the Rec Section to provide artificial gravity for residents to maintain their physical health, but very little detail is provided. How often do residents use this facility, on average, and is it’s radius optimized to minimize Coriolis forces? You might consider this well thought out design for a centrifuge.
BPD: I love this design for a lunar lava tube environment! The Rec section of Tube Town is over 400m wide so this is the perfect place for a floor mounted Dorais Gravity Train. In my book, this would be used for scientific study of the effects of artificial gravity treatment in a low gravity environment. They would do studies on both animals, plants and humans. I see crewmembers and tourists using the gravity train as a health spa and treatment against ‘gravity sickness’.
SSP: There are a couple of resident dogs in Tube Town and one them actually becomes pregnant. This has huge implications for biomedical research on mammalian reproduction in lunar gravity and in particular, determination of the gravity prescription for healthy human gestation. In my opinion, determination of the gravity prescription is one of the most significant questions to be answered for long term space settlement. Tell us about how this research is carried out in Tube Town in an ethical manner?
BPD: The studies would start with mice. Only when and if the studies show that mammalian reproduction in low gravity is safe, would the crew move up to higher level mammals. If safe, the female dog would be taken off the canine birth control medication she is on. BTW, all the ISP crewmembers and commercial residents must agree to be on birth control medication while living on the Moon. Many may choose to freeze eggs or sperm on Earth before a long deployment in space.
SSP: Where on the Moon should we look for lava tubes?
BPD: Nearly all of the volcanic activity of the Moon was on the Nearside, not the Farside. So we should definitely concentrate on the Nearside. We can see lots of collapsed lava tubes on the surface of the Moon, the intact ones are probably in the same regions.
My suggestion is to look for them where we would like to find them, in other words, lets look in strategic lunar base locations where there is water and power and easy access to other useful minerals (like metals).
I’m sure NASA knows better than me, but my target priorities would be:
North Pole – because its near water and solar power and metals (the Northern Oceanus Procellarum and the highlands between the maria).
South Pole – because its near water and solar power. The South Pole-Aitken basin is a large impact crater but apparently there was some later volcanic activity so it is possible to find tubes in the South Pole area but they may be smaller in size and length than the ones in the Maria.
Marius Hills (southwest of Schroter’s Valley in Oceanus Procellarum) – because there is lots of volcanic activity and collapsed tubes and it is near minerals and metals.
SSP: Thanks Brian for your exciting vision of our future on the Moon and for the opportunity to get a sneak peek. I’m enjoying the story of Tube Town and wish you much success with the release of the book.
