A Solar Powered Station at a Lunar Pole

Arnold G. Reinhold

[Confirmation of the presence of water ice crystals at the Lunar polar regions by NASA's Lunar Prospector spacecraft has stimulated interest in colonizing the Lunar poles. While there is some controversy over the feasibility of extracting water from the low concentration of ice observed so far, there are other advantages to the poles as an initial location for a Lunar colony. Here are some ideas I had on that topic in 1990. In a separate submission, I also proposed to deliver key light elements, thought then to be absent from lunar soil, at a much lower cost by building supply craft as much as possible out of plastic or wood.

In December 1989, the Bush administration directed NASA to set up an Outreach Program to solicit ideas for future space exploration. The papers received were synthesized by a group led by former astronaut Gen. Thomas P. Stafford into a report titled America at the Threshold, Report of the Synthesis Group on America's Space Exploration Initiative. The following papers were submitted to NASA on July 17, 1990 as part of that initiative, and is acknowledged, in fine print, on page A-51 of that report. The papers have been formatted for HTML and figures added, but otherwise they are presented as submitted.]


Establishing a continuous source of power is a major problem faced by designers of any lunar station. Solar power would be ideal but for the month long rotation period of the Moon. The long lunar night forces storage of enough power for two weeks of station operation. Equipment to store that much energy would be too heavy to transport from Earth. This proposal shows how a station located at the North or South pole of the Moon could enjoy continuous solar power month and year round.

An observer standing at a lunar pole during the six month period when that pole tilts toward the Sun (local summer) would see the Sun circle the horizon once a month. This is precisely the same as the "midnight sun" effect on Earth. However because of the low angle of inclination of the Moon's equatorial plane with respect to the ecliptic, the altitude of the sun would never be higher than 1.533 degrees (three solar diameters). At the beginning and end of this six month period the sun would circle with its center on the nominal horizon. During the "winter" the Sun would drop no more that 1.533 degrees below the horizon. A solar collector at a lunar pole and 622 meters (2042 feet) higher than the terrain out to the horizon (only 45 km away) would always see at least half of the solar disk. Brief periods of blockage by a mountain or crater rim near the horizon might be acceptable, with the station running off of stored power.

The feasibility of this proposal depends on the actual topography at the lunar poles. Unfortunately, photo coverage of the poles is very oblique and incomplete. Better polar imagery could come from the upcoming Galileo lunar flybys in Dec. 1990 and Dec. 1992. It appears that the Moon's South pole is located in the middle of a 45 km diameter crater and the North pole is on a hill on the rim of the crater Peary. These facts are encouraging.

Payoff and value

Continuous solar power would greatly simplify a lunar station and costs would be significantly reduced. Other advantages and disadvantages of a polar site are discussed in the back up paper.

Performance characteristics

A realistic specification might be:

Relation to mission objectives:

This proposal could significantly reduce mission cost and complexity. In addition, since solar collectors and mirrors could eventually be made from lunar materials, an obstacle to a self sufficiency would be removed.

Complete "Backup" Paper

[NASA requested a summary and optional, separate backup paper on each topic. This is the backup I submitted.]

There is currently a great deal of interest in establishing a permanent, manned station on the Moon. Reasons for building such a station include unique opportunities scientific research, serving as a way point for exploring the rest of the solar system and as an alternate venue for human civilization. To succeed in the long run, such a station should have a goal of being as self sustainable as possible, that is the station should be able to function and expand using materials available on the Moon, with minimal resupply from Earth.

Description of idea

A major problem faced by designers of any lunar station is the difficulty of establishing a continuous source of power on the Moon. Solar power has been used for decades on Earth satellites and missions to the inner planets. Solar would be the obvious choice but for the month long rotation period of the Moon which keeps a typical lunar station site in darkness 14.75 (Earth) days out of every 29.5 days in the lunar day. This two week lunar night means that any solar power equipment would be non-productive for half the time. The lunar station would have to be store enough energy to sustain itself during the two week lunar night. Power storage equipment is very heavy and expensive to import from Earth. It would represent one more manufacturing technology that would have set up on the Moon for a sustainable station. Nuclear power is also bulky and expensive and would be very difficult to fabricate locally.

This paper suggests locating a solar powered lunar station at a high point near the north or south pole of the Moon. If high enough, the solar collector will always be in sunlight and thus a source of continuous power. Depending on the actual topography this might require putting the collector on top of a tower or other structure. *

At first glance this might seem impractical. Indeed such at tower on the Earth would be prohibitively expensive, if not impossible given known materials. This is because the Earth's axis is tilted 23.45° to the ecliptic, the plane of the Earth's orbit around the sun. By simple trigonometry, such a tower would have to be 355 miles high!

The Moon's rotation axes however is almost perpendicular to the plane of the ecliptic. In fact it is only tilted 1.533° [HBK, p. F-165]. Along with the smaller radius of the Moon this means that the height of a lunar polar power tower need only be 622 meters. (See fig 2.) By comparison, the KTHI-TV tower in Fargo, ND is 628 meters high. The Sears Tower in Chicago is 443 meters high, not counting its TV tower. (See Table 1.)

One may question the feasibility of erecting a power tower taller than the Sears building on the Moon. However the tower height calculation above is based on a perfectly spherical Moon. In fact the Moon has significant topographic relief. This relief would be used to reduce the required height of the tower. The Moon's North pole is apparently located on a hill on the rim of crater Peary [MAP]. Depending on the height of this hill and surrounding features it might be sufficient to just erect a collector on top of this hill. The South pole is in the center of a 22 km diameter crater. A high point on the rim might suffice. Or series of collectors could be built around the crater and connected by a electric grid. Better mapping would is clearly called for.

It may turn out that some sort of tower is needed to assure adequate winter power coverage. There are several reasons for optimism about the feasibility of tower construction on the Moon. The 1/6th gravity and the absence of weather and tectonic activity will dramatically reduce structure weight. Locally available material might be used in construction. For example lunar soil might be compressed and sintered using solar heat to make ceramic structural elements.

The actual power conversion equipment could be on or close to the ground with only thin membrane mirrors mounted on the tower. Since they are only redirecting and concentrating sunlight, the mirrors do not need to be very accurate or rigid. The assembly would only have to rotate at a one revolution per month rate to track the Sun. It might not be necessary to track in elevation since the Sun's altitude will only vary 3.1 degrees.

The six month polar summer would allow construction to proceed with the station powered with solar collectors temporarily located at high ground level. If construction progress were slower than expected the would be plenty of time to return personnel to Earth for another try next summer. Or a nuclear generator might power the facility during construction and the first winter.

Relay mirrors, constructed from locally refined or imported metal, could be set up on the ground or on near by hills to increase the power density at the collector during the summer and to fill gaps in coverage during the winter.

The solar constant in vacuum is 1.353 Kw/sq meter [HBK], so a collector of 50 x 100 meters could produce 1.7 MW of electricity, assuming a 25% conversion efficiency. NASA estimates electrical power requirements of 25 to 200 kW for a lunar outpost, 500 kW for an interim base and 2 MW for a permanent base [EET]. Some of the concentrated sunlight could be simply collected and transmitted by relay mirrors into station buildings and facilities where it could be used directly for lighting, heating, photosynthesis, and as process heat for chemical synthesis, metallurgy, etc. Other mirrors could supply outside flood lighting. Power used this way could be transmitted with efficiencies of 80% or more. Indeed concentrated sunlight could be transmitted significant distances by relay mirrors, reducing the need for an electric grid.

The 662 meter figure is based on the setting of the center of the sun. While only the area of the tower above 858 meters would be under constant full illumination, the region from 425 meters and up would be under at least partial illumination, since the sun would never set completely. At the 521 meter level 20% of the area of the sun's disk would be above nominal horizon at nadir (Table 1). Areas below the 425 meter level would by out of sunlight for part of the year. The station would experience a power availability cycle but would always have enough power to sustain life and agricultural processes.

As the lunar settlement developed, a chain of manned stations could be established, linked linked together by an electric grid and/or network of relay mirrors. It is not inconceivable that moderate temperature superconductors could be fabricated on the Moon and and buried below the lunar arctic soil at suitable temperature level and used for power transmission.

Potential disadvantages:

There are several apparent and real disadvantages to a polar location for a lunar station, but I believe they can all be addressed.

First the extreme cold at the poles. Estimates place the surface temperature at the lunar poles close to absolute zero. But there should be no difficulty maintaining adequate temperatures inside the living quarters despite the extreme cold since high grade insulators are easy to build in the near vacuum of the lunar atmosphere. Indeed most space station designs face the problem of dissipating waste heat. Having a constant external temperature could simplify the design of the station's environmental control system. By contrast Surveyor 1, which landed near the equator recorded temperature variations of from 235°F at noon to -250°F an hour after sundown [SP350 p. 85].

The availability of extreme cold might be useful for a number of process applications. And there is at least reason to hope that over the 4 billion years of lunar history the cold lunar polar surface soil may have trapped gasses from comet and carbonaceous meteor impacts on the Moon and from the solar wind much like a cold finger in a lab vacuum system. These gases might be recoverable in economic quantities, simplifying the supply chain for a sustainable lunar station. Also gasses that leak from partially buried station buildings might freeze and be trapped in the soil from which the could be periodically recovered.

Another problem is that the lunar poles are in the area of the Moon subject to the phenomenon of libration. This means that for half the month the Earth will be below the local horizon, disrupting line of sight communications. The lunar latitude below which the entire Earth is always visible is 82.3° or only 232 Km (145 Mi.) from the pole. A small unmanned relay station could be established there that would provide constant communications. (The Apollo Lunar Rover had battery power sufficient for 55 miles [SP350 p. 270], so an excursion to such a site is not unreasonable.) Taking advantage of lunar relief could reduce the distance somewhat, as would locating the Earth based communication stations high in the opposite hemisphere. The south rim of crater Gioja for a North polar station and a rim/hill shown at 0° E, 82° S for the South are likely possibilities [MAP].

The relay station might be connected by a fiber optic cable or by a chain of microwave or laser links. It might be powered by wire or by solar+storage battery or by a nuclear thermoelectric generator. Power requirements would be quite modest. Other communications solutions might include use of long wavelengths to maintain over-the-horizon communications by refraction, or using a constellation of satellites in high polar orbit around the Earth (5488 km or 3430 Mi. high) to maintain line of sight. The high elliptical orbits used for the Soviet Moilyna communication satellites would probably work. Lunar polar orbits could work but they are not stable. Communications relays at the L4 and L5 Lagrangian points of the Earth-Moon system would reduce the blackout interval by 2/3 (1/3 each).

Another objection is that a station located at a pole might have reduced scientific value since it could only observe half the sky. Balancing this however is the possibility of maintaining continuous surveillance of selected astronomical objects or areas of interest. Also a station on the backside of the Moon could be built with close logistical links to the polar station. Such a station could survey the celestial hemisphere over the entire spectrum, free of electromagnetic pollution from the Earth. Since the galactic center is of particular scientific interest and it is located in the southern celestial hemisphere, the South pole is a logical site for a first station. Ideally bases would eventually be established at both poles, which would solve the coverage problem.

There may also be morale issues caused by the Earth being out of sight for long times. A TV camera at the communications relay station might help.

A location far from the Earth's equator is undesirable for launching and recovering spacecraft. Unlike on Earth however, the orbital delta V penalty for being at the lunar pole rather than at the equator is quite low --- only 4.6 meters/sec (15 ft/sec or 10.3 Mi./hr.) -- due to the Moon's low rotation rate and smaller radius.

An advantage of a polar lunar station is the ease of providing shielding from solar storms. Methods might include burying part of the station, locating it in a crater that is always shaded, building a wall to shade the station or a shelter area, and building a barrier that could be erected on short notice.

Likelihood of Success:

A major risk factor is the possible lack of a suitable site at either pole. Another is the need to complete the power collector within the six month period when the sun is above the horizon. Soil mechanics is another issue. The heat escaping from the station may cause local thawing which could result in shifting. Solar collector technology itself seems mature enough but might need adaptation.

Key demonstrations:

The key short term need is accurate mapping of the north and south poles. These seem to have been poorly covered in the Lunar Orbiter series. The Galileo mission to Jupiter will make two passes by the Moon in December 1990 and December 1992. Targeting the polar regions for imaging should be a high priority. In the interim it may be possible to extract more data from the Lunar Orbiter photos. New ground and Earth orbit telescope imagery might be useful, taking advantage of libration.

Once adequate imagery is available, a digital terrain map should be produced. It should extend out to about 80 km from the poles (the nominal horizon is 47 km from the top of a 622 m tower). A computer program could then be written that would systematically evaluate each high point for year round power availability at various collector heights. Detailed design concepts could then be developed for the most promising sites.

Estimated costs:

Initial evaluation of this proposal could be done inexpensively. The U.S. Geodetic Survey could be asked to prepare topographic maps of the poles on a "best efforts" basis using available data. Universities might be invited to select and evaluate sites as part of student courses, e.g. [MIT].

The next major expense would be to get imagery scheduled in the Galileo flybys and to prepare accurate maps based on them. Once this is done another round of evaluations by universities and/or industry would be initiated.

If the concept still deemed promising compared with other station approaches, more detailed surveys including a lunar polar orbiter mission and, possibly, an unmanned probe might be in order.


Applications beyond space exploration

Setting maximum self sufficiency as a major goal of a lunar station design should result in many more beneficial spinoffs. A wide array of new manufacturing, construction, agricultural and resource management technologies will have to be developed.

Other factors: Educational

Since the early phases of evaluating this proposal require very little beyond topographic data and since there will be many candidate sites and architectures, there is a real opportunities to involve students in the work and introduce them to the various disciplines involved.

Other Factors: National Security

To the extent that a sustainable station on the Moon has strategic value and to the extent that this proposal facilitates such a station, it is worth underlining the fact that there are only two poles and perhaps only one that is suitable. In real estate, location is everything.

A Consumable Lunar Supply Craft

An obstacle to establishing a long term lunar station is the is the near absence of low atomic weight elements on the Moon's surface. One way to reduce the cost of importing these materials from Earth is to build supply craft out of plastics and composites with a low atomic weight composition. Then the entire structure of the supply craft would be usable at the Moon, ameliorating the normal rocket logistic equations with their low net-payload efficiencies. Also since many supplies would not be shock sensitive, a landing deceleration system could be made of crushable honeycomb, again made out of plastic (the Ranger missions attempted this with balsa wood).

One way to use the plastic material might be to mix it with lunar soil and heat it, yielding CO2, H2O, NOx, metals, etc. Further economy could be achieved by using such supply modules to transport waste material (which would be valuable on the Moon) from Earth orbiting space stations. Indeed economies might dictate that equipment on a space station that otherwise might be made out of metal should be made out of light elements to maximize their scrap value on the moon. Conversely, on the Moon, ceramics and metals might be used in place of materials that would be made of light elements on the Earth. Electrical wiring might be insulated with ceramics for example. Ceramics might also be used for station furnishings. The result might be a colony that could eventually achieve self sufficiency based on trade with Earth, say by shipping lunar materials to low Earth orbit.

Electronic modules in the supply craft could be designed for reuse on the moon. For example, the communications system might be adaptable as a space suit walkie-talkie, the guidance computer could be a single board CPU adaptable both as a personal computer or a process logic controller. Wiring harnesses and connectors could be designed for easy disassembly. Power converters, motors and actuators could also be designed with reuse in mind. The selection of integrated circuits and other components could be limited to a "lunar station parts list" so that other circuit boards could be cannibalized for spare parts and locally designed products. Screw and nut fasteners might be in place of rivets, etc.

Performance Characteristics:

One kg of polyethylene from Earth reacted with FeO3 from lunar soil would yield 4 kg Iron, 3.1 kg CO2 and 1.3 kg H2O, a total of 8.4 kg of useful material. Elimination of landing rocket and metal spacecraft structure would yield additional efficiencies.

Other Enabling Technologies:

An Earth orbit to lunar orbit tug could reduce operating costs. Fine terminal guidance would allow landing in a bed of loosened lunar soil, providing further passive deceleration.


[MAP] Map Showing Relief and Surface Markings of the Lunar Polar Regions, 1:5,000,000, MAP I-1326-B, U.S. Geological Survey, 1981

[EET] Jim Van Nostrand, Electronic Engineering Times, Sept. 25, 1989, p.24

[HBK] Robert C. West, Ed., Handbook of Chemistry and Physics, 54th Edition, CRC Press, Cleveland, Ohio, 1973

[SP350] Edgar M. Cortright, "Apollo Expeditions to the Moon," NASA SP-350, National Aeronautics and Space Administration, Washington, DC 1975

[MIT] John F. McCarthy, Oscar Orringer, et al., "A Systems Design for a Prototype Space Colony," Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA, 1976

agr HTML version March 13, 1998