With interest growing in a return of humans to the moon, says Jonathan Spencer Jones, attention is turning to the possibility of establishing a long-term presence there and the form that such a settlement might take.
One such study has recently emerged in Europe with a concept for a ‘Moon Village’, which was prepared by the European Space Agency (ESA) with input from the US architectural firm Skidmore, Owings & Merrill and the Massachusetts Institute of Technology.
The word ‘village’ perhaps sounds a bit grand as the base is modelled for a crew of just four people for a period of up to 300 days at a time – the maximum duration recommended based on radiation exposure assessment. But with the various systems envisaged for living and working on the moon and transfer to and from the Earth, over time it would come to expand into a village and perhaps eventually even into a town or city.
A key consideration in the development of a base on the moon is that all the components have to be delivered by a spacecraft, with the inherent size and weight restrictions this presents.
Another is the environmental conditions for living and working there. With this in mind the location selected for the village is on the elevated rim of a crater, the Shackleton crater, at the lunar south pole. There, there is a high level of illumination with the highest grounds in near permanent sunlight and the longest periods of darkness typically 3-5 days. Elsewhere the periods of darkness would be longer and the thermal control requirements for the habitable systems more demanding due to the extreme changes in temperature – from 127oC in daytime to -173oC when the sun is shaded.
The location also has other advantages. One is an ongoing view of Earth but more importantly is the potential of access to water ice deposits in the permanently-shadowed areas of nearby craters which could provide a ready source of water.
The basic living structure proposed, named the Habitat, is comprised of a vertical rigid central frame with an inflatable multilayer shell. When deployed it would provide a four-storey structure, roughly ellipsoidal in shape, about 15.5m in height and 10.5m in diameter with a pressurised volume of almost 700m3.
Within the structure, the space would be modularised to provide for private quarters, cooking and dining, work, exercise, hygiene and other activities.
Core to the running of the Habitat is the power availability. Its external structure could provide a limited space for solar PV, but with a maximum output unlikely to exceed around 1kW this would be inadequate for living, although it could be utilised during the transfer flight and as back-up. Thus an external power plant would be essential.
Various studies have provided a range of estimates for the power requirements from a base of 10kW/person up to 60kW continuous consumption for a crewed habitat at full power. A subsystem breakdown for the proposed Habitat indicates an average power requirement (including 20% margin) of 57kW during the day and 60kW during the night.
Two proposals are made for a lunar power plant, using either solar PV or nuclear fusion.
Solar power plant
A solar power plant at the south pole would have to maintain an array of solar panels facing horizontally, with a rotational capability in order to track the sun through 360° over the course of the month. However, such a power station would have to overcome the problem of mutual shadowing, in which some panels would always be shadowed from the horizontal sun by some neighbouring panels, or indeed by any other surface constructions such as the Habitat itself.
Furthermore, the darkness periods demand the inclusion of large energy storage capability.
Based on a 59kW power requirement for the Habitat, a solar system with a battery would require approximately 282m2 of solar panels, while a solar system with regenerative fuel cell storage would require 329m2 of solar panels. The latter, however, is much lighter at a total mass of 14t, whereas the solar battery power station comes in at 68t due to the much greater weight of the lithium-ion batteries.
Nuclear fission plant
The second option of a nuclear fission reactor is a technology that is already under development for space travel and lunar surface applications, particularly within NASA.
As far back as 2018 NASA and the US Department of Energy demonstrated the Kilopower reactor named Krusty (Kilopower Reactor Using Stirling Technology) capable of providing up to 10kW of electrical power for at least 10 years.
The prototype system uses a solid cast uranium-235 reactor core, about the size of a paper towel roll, and passive sodium heat pipes transfer reactor heat to high efficiency Stirling engines, which convert the heat to electricity.
As many Krustys as are required to meet the power requirements could be implemented together.
A nuclear fission reactor would be more compact, for a given power capability, than a solar power farm. However, extensive cooling radiators are required to reject the waste heat at a temperature low enough to suit the power conversion principle involved.
Protection of crew and systems from the ionising radiation emissions of an operating reactor also is a requirement and would be achieved by a combination of distance and shielding by regolith (unconsolidated residual or transported material that overlies the solid rock on the earth, moon, or a planet [Merriam-Webster Dictionary]), most likely by burial.
The mass of a space fission reactor system depends on various design parameters and assumptions, but a broad estimate indicates a total power station mass of about 5.6t to deliver the Habitat’s 59kW requirement.
This is at least an order of magnitude lighter than a solar power plant. A nuclear fission reactor approach also has the advantage that its development would be applicable to non-polar lunar applications, providing wider mission flexibility.
NASA says the Kilopower project team is developing mission concepts and performing additional risk reduction activities to prepare for a possible future flight demonstration.
ESA’s moon village study is still very much a concept. But Skidmore, Owings & Merrill’s study leader Daniel Inocente, while describing it as extremely costly to build and technically challenging, says it is conceivable given the rate of improvement in technology and engineering.
Advenit Makaya, study leader at ESA, adds that the study is clearly looking into the future, beyond the horizon of currently planned lunar exploration activities.
“It has been a very interesting exercise for the various ESA experts, to collaborate with architecture experts, to identify and address the drivers and ways in which this innovative design could be deployed on the Moon,” he says.
On the power front the study does not indicate a preference for either the solar PV or nuclear fission reactor option. Potentially some hybrid combination is the more likely, if only to provide a level of technology redundancy.
Either way as NASA’s progress on the latter as its preferred solution indicates, much further work is required to get either to a state of operational readiness for the moon. That is without mentioning the similar demands for the Habitat and other systems that are integral to the construction and operation of a moon village.
But the technological developments are occurring increasingly fast and some of us may yet see the implementation of the first lunar power station. One of us may even be lucky enough to become its operator.
NASA and lunar surface innovation
The ‘Moon Village’ concept is a first of its kind study in terms of its scope, but European space scientists are not alone in working on such concepts. On the other side of the Atlantic scientists at NASA, the US space agency, also are investigating a sustained lunar development plan and particularly for using it as a springboard to establishing a human presence on Mars.
NASA’s Artemis programme also targets the Shackleton crater area for the lunar base, with the base camp serving both for the development of commercial activities on the moon and for the testing of systems for the first mission to Mars.
NASA envisages the US to return to the moon in 2024, a base to be established circa 2028 and the first human mission to Mars to take place in the 2030s.
NASA’s Krusty nuclear fission development as its primary power system is already under way. The organisation also plans to develop an advanced solar collection system and proposals for a 10kW vertical solar array technology are being sought. The concept calls for the system to be relocatable to different sites by a rover.
Through the ‘Watts on the Moon’ challenge, NASA seeks proposals to develop and demonstrate solutions for the distribution, management and/or storage of energy. The intermittency and resiliency of solar with the extended dark periods are among the issues that must be addressed.
Three power delivery activities are up for solutions, which are expected by September 2023.
1. Deliver power from a power plant on the rim of a crater to a mobility platform operating in the crater that collects and delivers icy regolith2 to a water extraction plant.
2. Deliver power from the power plant to the water extraction and purification plant operating inside the crater.
3. Deliver power from the power plant to an oxygen-producing pilot plant operating outside the crater, which extracts oxygen from the delivered material.
Other areas that NASA is pursuing in its lunar surface technology research drive include wireless energy transmission technologies for power distribution to difficult to reach areas such as those in permanent shadow and for mobile applications; batteries for sustained low-temperature operation and advanced power control technologies for interconnected systems.
In many, if not all these cases, the developments also could help to advance similar technologies for use and commercialisation here on Earth.
US DOE gets behind NASA
Energy for space applications covers a multitude of use cases besides power generation for lunar and Martian bases. Rocket propulsion, powering robotic vehicles on the lunar surface and orbiting space stations and delivering solar power to the Earth are among others.
The growing and likely sustained presence of humans in space has spurred the US Department of Energy to develop a 10-year strategy for energy for space, formalising its input to the country’s space programme since its earliest days and setting out clear intent to contribute to advancing space exploration.
Under the strategy the DOE commits to developing space-capable energy technologies, both nuclear and nonnuclear, exploring energy management systems for their potential application to space missions and advancing innovation in energy generation, storage, distribution and other technologies for space systems.
Thus far, the DOE’s space power systems work has focused primarily on space nuclear power and propulsion technologies, ranging from small radioisotope power systems to fission power reactors. The department expects to continue to play a key role in the development of such systems, which it indicates also could be exploited in efforts to revitalise the country’s commercial nuclear industry.
The DOE considers that its advanced R&D of photovoltaics and concentrating solar thermal power could be of great value for a sustained crewed presence on the Moon. Its expertise in batteries and in particular hydrogen and fuel cell technologies could be applied for exploiting lunar water for the synthesis of hydrogen fuel, as well as for power storage on the surface of the moon and for powering other applications in a lunar base camp.
Thermal control is another key area in which the DOE offers its skills, drawing on its energy management expertise.
Other areas the DOE commits to in the strategy include support for astronomers and other space scientists in areas such as AI, data collection and advanced computing, the development of capabilities to support the peaceful use of space and the driving of innovation in technologies for future space missions.
“Under the vision, DOE’s scientific and engineering capabilities will be applied to overcome the challenges of vast distances, extreme conditions, complex operations and unfamiliar environments to propel and power exploration, security and commerce in space,” says Dan Brouillette, Secretary of Energy, of the strategy.