Surviving the Temperamental Moon

Temperature map of the Moon’s south pole. The bottoms of some of the craters in this region are never exposed to sunlight, thus have extremely cold temperatures. Alternatively, some peaks and crater rims are exposed to sunlight over 80% of the time. Data was captured from the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment. Credit: UCLA/NASA/JPL/Goddard.

As indicated by the title of Robert Heinlein's 1966 novel The Moon is a Harsh Mistress, the Moon is a harsh environment to operate in. There is a scant atmosphere, abrasive dust, and extreme temperature ranges to deal with. Observed temperatures range from a blistering 127 C (260 F) in equatorial sunlight down to a frosty -238 C (-397 F) in the permanently shadowed regions of the poles. Designing missions to survive these conditions pushes current technology to its limits.

With respect to the stars, the Moon makes a complete rotation about its axis once every 27.3 days (sidereal month). However, the lunar day/night cycle is based on the 29.5 days it takes the Moon to return to the same phase (synodic period). This means that a given location on the Moon experiences about 14 days of sunlight and then 14 days of darkness, although this isn't always the case.

At the lunar poles, some craters experience complete and continuous darkness, while a few peaks and ridges receive sunlight over 80% of the time. This is caused by the Moon's axial tilt of only 1.54 degrees away from the ecliptic (compared to the 23.44 degrees of Earth’s axial tilt).

The Moon's rotation and orbit play a large part in understanding its surface temperature, especially its day and night cycle. The lack of any substantial atmosphere prevents heat retention from air. The surface regolith is the only store of heat at the surface, and it is highly insulative (low thermal conductivity), meaning heat has difficulty penetrating much more than a few centimeters through it.

These factors contribute to the Moon's extreme temperature range. Equatorial daytime temperatures have been observed to reach around 127 C (260 F), while equatorial nighttime temperatures fall under -173 C (-280 F).

Animated map showing the average brightness temperatures across the Moon. Values collected from the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment over a 5.5 year period. Equatorial temperatures swing from around 127 C during the lunar day to around -179 C during the lunar night. Credit: Williams et al 2017

The China National Space Administration (CNSA) recently announced that their Chang'e 4 lander recorded a low temperature of -190 C (-310 F) during its first lunar night on the far side of the Moon. This exceeded the low temperature expected, along with the low temperatures observed at similar latitudes on the near side of the Moon. Possible reasons include different surface material, lack of Earth shine, or something else.

Model calculations of the lunar surface temperature across the lunar time of day based on latitude. Equatorial latitudes (closer to 0 degrees) show the most dramatic temperature swings between lunar day and lunar night. The lunar poles (around 90 degrees) have a more mild temperature swing, but are generally colder. Credit: Paige et al. 2010, Figure 1.

The permanently shadowed regions (PSRs) at the lunar poles represent the coldest areas yet observed in the Solar System. The Diviner instrument on the Lunar Reconnaissance Orbiter (LRO) has recorded temperatures as low as -238 C (-397 F) within the south pole PSRs. This is even colder than the observed surface temperature of Pluto. For reference, -273 C (-460 F) or absolute zero, is the lowest limit of the thermodynamic temperature scale.

These extreme temperatures present unique opportunities and massive challenges for lunar surface missions. The extreme cold temperatures in PSRs allow the trapping of water ice and other volatiles. Yet the extreme cold requires systems to withstand temperatures rarely encountered before.

Most known materials become very brittle when they approach temperatures experienced within PSRs. Electronics cease to function at these temperatures unless large heaters are used to keep them warm. Additionally, there are no facilities available for testing complete mission hardware at temperatures this cold.

Missions sent to areas outside PSRs don't have to survive temperatures as cold, but they need to survive a wider range of temperatures along with being in alternating 14 days of darkness and direct sunlight. The direct sunlight also causes visibility issues around lunar mid-day. In addition to producing an extremely bright environment that can over saturate cameras, direct overhead light obscures surface details including craters and boulders. The difficulty seeing around lunar mid-day has forced prior missions to take a mid-day nap until seeing conditions improved to allow safe navigation.

Illustration of the TransFormers concept of beaming sunlight down into permanently shadowed regions (PSRs). Large heliostats on the crater rim track the sun, and beam the power down to rovers within the PSR. Systems like this allow non-nuclear systems to survive the extreme cold within PSRs. Credit: Stoica et al. 2017, Figure 1.6.

The simplest approach to surviving the extreme lunar temperatures is not to try to survive. Many lunar missions are designed to only operate while the lunar day is ongoing, utilize solar cells for generating power. Once the lunar night starts, these systems run out of power and become frozen. If the mission can be completed within a lunar day, this approach minimizes overall complexity and costs.

Nuclear based systems provide a good setup for large and long duration missions, however, launching a nuclear system can be politically challenging. Nuclear based systems include the use of Radioisotope Thermoelectric Generators (RTGs) and Radioisotope Heater Units (RHUs), both of which use a form of plutonium for fuel. RTGs have been used on many NASA missions to provide electrical and thermal power to large, multiyear missions. RHUs only provide thermal power, but are compact and compliment solar/battery systems. By using several RHUs and putting its systems to sleep, the Chang’e 3 lander has survived over 60 lunar nights so far.

Regolith based materials for insulation around lunar bases can provide a large temperature protection because lunar regolith is a good insulator with low thermal conductivity. Temperatures under a meter of regolith hover around -53 C, which is much warmer and more consistent than the surface temperature. This concept shows the SinterHab being constructed from a large 3D printer mounted on an ATHLETE rover. Credit: Howe et al. 2013, Figure 26.

Another interesting approach uses regolith to insulate the system. Lunar regolith is a good insulator, where temperatures under a meter of regolith hover around -53 C (-64 F). This approach allows the system to use less energy heating the system. Both human bases and rover shelters could utilize this approach.

Within PSRs, power beaming is a viable approach for transferring power from sunlit regions to shadowed areas. A system like this could use either heliostats, microwaves, or lasers for transferring energy down to a receiver within the shadowed PSR. This approach has many challenges, including building a power transmission system, generating enough power, and placing the transmission system on a crater rim and the receiving system in a pitch black PSR.

Surviving the extreme temperatures on the Moon is a massive challenge. With temperatures ranging from -238 C (-397 F) to 127 C (260 F), the lunar thermal environment is unforgiving. The LRO and surface missions are building our understanding of this rough environment, yet there are still many unknowns. Building missions to survive long durations on the Moon is required for establishing a permanent lunar presence. Current work shows promise of achieving these goals over the medium term.


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