Home, Home on the Moon
Last December, President Trump signed the new Space Policy Directive that instructs NASA to prepare for the return of humans to the Moon for long-term exploration and utilization. Unlike the quick land—explore—take-off timeline of the Apollo missions, this plan envisions much longer stays on the Moon’s surface by many more astronauts.
This all sounds really exciting: Who wouldn’t love to bound gaily over the lunar landscape in the freedom of 1/6 of the Earth’s gravity and hit golf balls beyond the horizon? However, the Moon’s surface is a tough place to survive, much less enjoy. By comparison, the open air of an Antarctic winter is a joyride. Temperatures on the Moon’s surface range from 298 °F at night to +224 at high noon—and each lunar day lasts a month.
There is no air, no free water and deadly radiation pours down from the sky, especially when the active sun ejects one of its frequent flares . On the Moon, the meteors that make lovely streaks across the Earth’s night sky are deadly missiles that travel 20 times faster than a rifle bullet, and when a big one strikes, it creates secondary shrapnel that, upon falling back, churns up the soil miles away from the impact itself.
Finally, the gritty gray lunar dust clings to everything, fouls the seals of doors and space suits and brings exposed moving parts to a grinding halt.
Does this sound unpleasant? How, in fact, are astronauts even going to survive under conditions like this? The internet is filled with artist’s images of lunar habitats, comfortable-looking domed buildings that look like the real-life Biosphere II in Arizona. However, the reality is that cosmic rays will zip through such domes like tissue paper, expansion and contraction of materials under the huge temperature swings will alternately buckle the walls during the day and open wide cracks at night, and meteoroid impacts will punch holes in those lovely windows over the course of a few years.
To be safe from radiation and temperature swings, lunar habitats will have to be buried under 15 to 20 feet of lunar soil, and windows are out of the question. Astronauts will only be able to venture out onto the lunar surface for short periods, and will have to be constantly wary of solar flares and meteor showers.
With threats like these, the idea that astronauts will land on the Moon and quickly pop up shelters that will protect them is pure fantasy. Long term lunar survival will depend on heavy “earth”-moving machinery and a lot of clever engineering to create safe, comfortable habitats that will endure the Moon’s challenges for years.
With the high cost of lifting material from the Earth’s surface, a high premium must be placed on the use of available materials such as lunar soil and rock that, at first sight, does not seem particularly well suited for a habitat’s needs. Oxygen and food must be somehow extracted from the local materials and water, a scarce resource on the Moon, must be found.
None of this is impossible: Many studies by NASA and other organizations have already pointed to solutions. However, continued development and investment by both government and private organizations will be needed before lunar shelters are ready and we can begin planning our dream vacations in a low gravity playground.
One attractive solution might mimic humanities first shelters: Caves. Just as our distant ancestors found shelter from the harsh conditions of the untamed Earth, natural caves on the Moon might provide our modern space-going cavemen a secure haven from most of the hazards, such as temperature swings, radiation and meteoroid strikes. And we now know that caves do exist there.
A Japanese spacecraft, Kaguya, was the first to discover “skylights” on the Moon, deep pits that seemed to connect to caverns beneath the surface. These caves originated as “lava tubes,” underground channels that once carried hot lava from volcanic vents out to the Moon’s vast lava plains, but then drained and remained open caverns after the lava cooled.
The full size and extent of these cavern systems was not known until NASA’s GRAIL mission mapped the Moon’s gravity in exquisite detail, revealing long meandering traces of slightly lower gravity acceleration beneath the mare surfaces. The gravity data indicate that these caves could be huge—a mile wide, half a mile high and dozens of miles long—room enough to accommodate the city of greater Chicago. GRAIL’s discoveries have been at least partially confirmed by direct ground-penetrating radar studies from Kaguya, although further dedicated studies are necessary to fully map this lunar resource.
Lava tubes may serve as ready-made shelters from the worst of the Moon’s harsh conditions. They can be accessed, at first, from the few known open skylights, but later access may be created by tunneling down from the surface. Light can be brought from the surface by the kinds of light tubes already used in many homes.
Power may come from solar cells or nuclear reactors safely located on the surface. Depending on the number of fractures in the lava surrounding the lava tubes, they might be sealed airtight from the inside, creating large, airy, open spaces that might someday be the sites of agricultural fields and home to millions of lunarians who could not imagine living anywhere else. We do not yet know what resources might be found sealed away inside the Moon’s lava tubes, but they may be rich in volatiles that were originally dissolved in the ancient lavas.
This may all seem like a science fiction fantasy. Indeed, lunar caverns were centerpieces of lunar tales by authors such as Jules Verne and H. G. Wells, but modern technology has now revealed that they actually exist. Given the harsh reality of lunar surface conditions, such caverns, plus a lot of engineering knowhow and human inventiveness, may enable our first step in really expanding humanity off the Earth and into the wider universe.
Purdue University, now celebrating its 150th anniversary, has been at the forefront of these efforts. A Purdue team of graduate students and faculty from several departments first analyzed the GRAIL data and discovered the great size and extent of the lunar lava tubes. Purdue is home to the Resilient Extraterrestrial Habitat (RETH) program, a broadly-based effort to delineate the hazards encountered on the surfaces of the Moon and Mars, or open space, and to explore the concept of resilience, of how to keep a complex system such as a habitat functioning through unforeseen accidents and challenges. Purdue, through its outstanding faculty, students and alumnae has long been at the forefront of space exploration and research and, going into the future, is well positioned to remain there.
H.J. Melosh, Purdue University