…could be up to five kilometers wide.
Mycroft Holmes, call your office.
[Update a few minutes later]
Speaking of the moon, Paul Spudis has some ideas about how to make space great again. I actually agree with most of it, except for this:
The Orion spacecraft and its SLS launch vehicle are currently in final stages of development, with initial test flights planned for 2018. We can use these existing systems to return to the Moon; indeed, as the remnants of the cancelled Constellation program, they are already optimized for cislunar missions. The only missing piece is a lander to put people on to the lunar surface. NASA’s Altair lander program was cancelled in 2011, but fortunately, a lander may be ready very soon. The United Launch Alliance has outlined a plan for a human-rated lander based around the venerable Centaur stage, using modified RL-10 engines. This vehicle is almost perfectly configured to return people to the Moon, as it is intended to be reusable and utilizes the LOX-hydrogen propellant that we will produce on the lunar surface.
The surest way to ensure that this doesn’t happen is to plan it around SLS/Orion, which will fly so rarely that we will make very little progress. He’s postulating the existence of a ULA lander, while ignoring the firmer plans for Vulcan ACES, which would be the natural way to carry out these mission (Orion might be usable in that scenario, but not SLS, and Dragon would probably be more cost effective). And as usual over there, the comments, particularly from “Bilgamesh,” are idiotic. And even more particularly the fantasy about flying SLS a dozen times a year.
38,000,000 sq. km. or 3,800,000,000 hectare.
Access to lava tube condos should raise the minimum bid considerably. Raise enough money so the moon development corp has an annual budget of perhaps $50 billion?
It’s doubtful those are actual lava tubes. It’s far more likely that they were made by the Tok’ra with their tunneling crystals as part of a secret base to support their war on the Go’uld system lords.
In any event, their size and state of disrepair means we’ll face the difficulty of finding a tube that’s accessible from the surface and then sealing it so it can hold pressure. It’s not particularly easy to glue something like Bigelow fabric to rock walls, especially when the ceiling is perhaps 300 feet up.
Then there’s the problem of shipping enough atmosphere to the moon to pressurize such a large tube. We can liberate oxygen from lunar rocks (but that’s still energy intensive without a good source of carbon) but we need another gas to avoid a pure oxygen environment. The obvious choice is nitrogen, but the moon is very poor in that, so we’ll probably have to ship it up there.
Combined with the size of the tubes, that creates a severe problem. A tunnel 320 meters wide and 100 meters tall has a cross sectional area of 25,000 square meters. If the tunnel is 2 km long the volume is 50 million cubic meters, and for atmospheric pressure we’d need about 0.85 kg of nitrogen per cubic meter. So it would take about 40,000 tonnes of nitrogen, which would take about a thousand super heavy launches to deliver.
So until development is much further along, we’ll be restricted to something like a Bigelow tent in a tunnel, and it would be a long time before anyone brags about the view inside the dark empty cave.
George said:
“In any event, their size and state of disrepair means we’ll face the difficulty of finding a tube that’s accessible from the surface….”
There are more than 200 “pits” already found in LRO pics that look very familiar to cavers, as skylight entrances to lava tubes. Since the first thing to do utilize this particular In Situ Resource is to orbit a Ground-Penetrating Radar with a resolution <10 meters, we will then be able to see the bright glow in radar wavelengths from the bottom of remaining pits/skylight entrances.
" ….and then sealing it so it can hold pressure."
This can also be done, by using In Situ Resources. The lunar regolith fines contain the re-condensed native Nickel/Iron material from metallic meteorites that vaporized on impact. Use magnetic separation to pick these out of the fines. Then, run them through the Mond Process, to create nickel Tetracarbonyl and Iron Pentacarbonyl. Distill them to separate them, and make Nickel and Iron powders out of them, just as has been done here for 100 years+.
Reducing the basaltic Iron content can be done by several methods, then run the particles through the Mond Process to extract the Iron, leaving behind waste enriched in Silicon and Aluminum Oxides. Use the Iron to seal the lava tube cracks. (*All* basalt cracks as it cools) To do this use an electrostatic version of the Cold-Spray technology, that would spray the particles at the cave walls at about 3-5 kilometers/second. Each soft Iron particle will self-forge into a thin layer covering whatever is underneath it, building up a structurally sound seal over the whole cave surface. Then, cover the Iron with a similar, but far thinner layer, of Nickel, sealing the Iron away from the watervapor/Oxygen combination of the atmosphere.
Finally, use the high silica/high alumina waste material in the same way, to coat the Nickel layer, separating it from the percentage of humans that are allergic to Nickel. Since Alumina and Silica crystals are white, the final white coating on the cave walls can be used as a projection screen for lasers to project scenes from the lunar surface, or from Earth, or from the outer planets if we want. Not boring at all.
"It’s not particularly easy to glue something like Bigelow fabric to rock walls, especially when the ceiling is perhaps 300 feet up."
Easier than traveling over a lava tube floor littered with breakdown talus boulders the size of a house! Far easier if we use the Gecko-tech pads now being developed (Yes, geckos' pads stick to rock quite well in vacuum) that can be laid in place by robots with so many thin cables stringing up each inflatable they'll look like a caterpillar's cocoon. Redundancy to the max is the idea.
Another good reason for stringing the initial base area to the ceiling is that it is close to the surface, without being part of it! The bottom of a 300 meter diameter tube is an Empire State building length away from the surface. Since you are still getting major supplies and equipment from Earth at this point, don't make that last transport distance harder on yourself, and more expensive. The bottom of the roof may be only 50 meters from the surface, by comparison. Also, those highly abrasive lunar regolith fines will be more easily excluded from a suspended base than from one on the floor of a lava tube, because they can fall away.
"So it would take about 40,000 tonnes of nitrogen, which would take about a thousand super heavy launches to deliver."
Only if you get it from Earth. If you get it from CC asteroidal resources, the shipping costs to the lunar surface drop substantially. That is why any *single* node in the industrial network will likely be uneconomical, while the productivity of market networks of industrial society will be what makes settling the Solar System possible.
You’ve laid out some of the difficulties. It’s doable, but it’s going to take a whole lot of man hours and an even larger number of robot hours, plus the development of more than a few in situ resource projects.
Dropping in from the ceiling raises it’s own initial problems, such as stabilizing the top of the collapsed entrance area so rocks and dust don’t rain down on people and equipment below. That alone is a pretty major project, far more involved than anything we did with Apollo. And then you have to clear out or stabilize the mountain of collapsed debris that fell from the tube’s ceiling, which of course made the entrance. Breakdown piles tend to be quite unstable, and lunar ones have never been subject to wind and water and the rumblings of shifting continents, so they’ll probably be doubly so.
Then you’re probably going to want to build a structure over the entire entrance pit, since you’ll eventually want to hold pressure and you’re going to have to raise and lower large quantities of equipment and materials, implying a large elevator of some sort because having astronauts rapel and ascend on a rope in a vacuum would give everyone at NASA a heart attack.
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And you still have to build a more conventional surface habitat to house the people who are doing the initial work on developing the lava tube. That habitat will have to be the main base until the lava tube entrance is completed. To get the most rapid return on investment, you’d want to develop the smallest practical tube, preferably one with a horizontal entrance instead of a vertical one.
Perhaps the idea is a small tube that connects to a much larger one way in the back.
The project is easier than building an O’Neill colony, but it’s still a major, major project. The potential does, however, provide ample justification for some really good lunar exploration projects because the cost savings of developing the best tube compared to a bad tube would more than outweigh the cost of the initial probe.
“Mycroft Holmes, call your office!”
More: “The Menace from Earth”! I think that trying to strap on wings and fly in those (by then air filled) lunar caverns will (as Heinlein foretold) be a great sport!
Somewhat related, Mars ice homes at Space.com