In any plan to establish a presence on the Moon, the South Pole is key. There, in the deep permanent shadows of the region’s craters, there are vast amounts of water ice. And water ice means water, oxygen and even rocket fuel.
But the region is shrouded in shadow.
Science has advanced greatly in the decades since the Apollo era, the last time astronauts walked on the moon. Soon, NASA’s Artemis mission will bring another generation of explorers to the lunar surface. But while the Apollo astronauts faced many unknowns during their missions, including the fear that their landers might sink into the dust, we now have a much clearer understanding of the lunar environment.
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This photo of Apollo 11’s Eagle lunar module shows the three sensor probes on three of the four landing capsules. They told the crew when the Eagle was safely on the moon and when the descent engine could be turned off. They were 67.2 inches (1.71 m) long because no one was sure if the Eagle would sink into a thick layer of fine lunar dust. (There were originally four probes, but they had a tendency to bend, so the one next to the ladder was removed due to the risk of puncturing a spacesuit.) Image credit: NASA.
We have mapped the surface of the Moon in detail and know how much water ice is there and where it is. A 2018 study based on lunar orbit data showed that the Moon contains vast amounts of water in its shadowy polar craters. When this study was published, NASA Administrator Jim Bridenstine said, “We know that on the surface of the Moon there are
There are hundreds of billions of tons of water ice at the lunar poles, with most of it at the south pole. Image credit: NASA/Shuai Li.
We also know where the minerals are found and that some regions of the Moon are 10 times richer in titanium than Earth’s rocks. We know that there is an abundance of iron and rich sources of aluminum, silicon and magnesium. We also mapped the lunar terrain KREEP, which contains rare earth elements that are important in electronics manufacturing.
This image of the Moon shows the near side (left) and the far side (right). Shows thorium concentrations that indicate where the KREEP terrain is. Image Credit: From NASA – Public Domain,
We have topographic maps that allow us to plot paths that the rovers can follow. We know where the safe places to land are and where the most intriguing places to explore are. Sunlit areas of the moon are open to exploration by solar-powered rovers, although they will have to contend with two-week periods of darkness.
But the poles are another matter. Unlike the Earth, with its axial tilt of 23.4 degrees and accompanying seasons, the Moon is seasonless. With an axial tilt of only about 1.5 degrees, the Moon basically experiences one continuous season. Unlike Earth’s polar regions, the Moon’s polar regions do not have a well-lit winter to support solar rover exploration. But although the variations at the equator are slight, they are more pronounced at the poles.
Due to the angle of the Sun at the south pole, the lower altitude in the craters is almost continuously dark, while the high areas are almost continuously illuminated. The problem is that high altitude areas are not smooth terrain easily navigated by a rover. But a new study based on detailed lunar maps shows how a clever series of routes and traverses could open up the lunar south pole for rover exploration.
The study is “Sunlit Paths Between South Pole Objects of Interest to Lunar Exploration.” The article will be published in the journal Acta Astronautica. The authors are from NASA Goddard Space Flight Center, NASA Johnson Space Center, NASA Headquarters, and Jacobs Technology in Houston, Texas. The lead author is Erwan Mazarico of Goddard.
The Moon’s axial tilt is only about 1.5 degrees, so it has no seasons. The polar regions see very little sunlight in their deep craters. Image credit: NASA/Richard Papa, Jeff Rose, Dave Paddock and Roger Lepsch.
NASA’s upcoming Artemis Moon missions are designed not only to explore the Moon, but also to understand how it can be used as a base for further exploration of the Solar System. Launching everything needed to explore the Solar System from Earth is cumbersome and expensive. If we can use the resources of the Moon, then we can more efficiently explore the rest of the Solar System.
Another goal of Artemis is to use the Moon as a testing ground to develop technologies that will contribute to NASA’s exploration goals. Developing expertise goes hand in hand with this as well as making cutting edge discoveries along the way.
A critical element in this endeavor is water. From the moon’s water ice we get oxygen for breathing, hydrogen for fuel and, of course, water itself. Since most of the moon’s water ice is at the south pole, it is critical that NASA opens up the region for exploration. But the deep shadows that are responsible for all this coveted water are also the obstacles to its exploitation.
Almost everything in Artemis requires mobility. Sample acquisition, sample preparation, instrument deployment, and all other activities require mobile rovers. Mars rovers like Curiosity and Perseverance don’t need solar power. They have MMRTGs — Multipurpose Radioisotope Thermoelectric Generators. But MMRTGs are expensive, costing over $100 million to build. Their fuel – plutonium 238 – is also complex and expensive to produce. And it’s hard to store because it decomposes.
The Moon has enough solar energy because it is so close to the Sun. Scientists have identified elevated locations at the South Pole that are free of shadows and can provide energy. Their favorable location with respect to solar energy makes them strategic positions for exploration. The question is, how affordable are they? “However, it has not previously been established whether travel between these locations is possible given the complexity of lighting conditions and the dynamic shadows cast by distant topography passing through the area,” the authors of the new paper write.
This image from the Lunar and Planetary Institute shows the permanently shadowed regions in the moon’s south pole craters. Image credit: Stopar J. and Meyer H. (2019) Topography and Permanent Shadow Regions (PSR) of the Moon’s South Pole (80°S to the Pole), Lunar and Planetary Institute Regional Planetary Imaging Center, LPI Contribution 2170
This is where this new article steps in. This study points out how finely planned routes across the Moon’s south pole can provide enough sunlight to traverse the region. “The goal of our study is to demonstrate, with minimal assumptions about rover capabilities, that travel exists between selected distant well-lit locations,” they explain.
The researchers identified possible “journeys” the rover could follow that minimized the time spent in shadow, cut off from the sun’s energy. They point out that more challenging journeys may be possible depending on the rover’s design. For example, higher speeds or the ability to survive in the dark or even drive in the dark can extend these journeys.
The team based their trips on data from the Lunar Orbiter Laser Altimeter (LOLA) instrument aboard the Lunar Reconnaissance Orbiter (LRO) spacecraft. They used software to find the routes, and the software “…calculates the cheapest path from source to destination, taking into account the length of the route, the slope of the terrain, and the average sunlight of the terrain,” they explain.
The research builds on previous research that identified four brightly lit regions at the moon’s south pole. In this paper, the researchers created four different paths between the regions.
LOLA topographic maps of the South Pole region shown in polar stereographic projection; The South Pole appears at coordinates (0,0). The color indicates the altitude relative to the reference sphere of 1737.4 km. Paths are shown in black. Image credit: Mazarico et al. 2023
The trails are very detailed and sometimes require waiting periods in sunny areas. These periods can be used to recharge batteries or do local research while waiting for the next ride to emerge from the shadows. “Sometimes it can be beneficial to make a ‘stop’ to recharge or wait for (current or upcoming) shadow en route,” the authors explain. “The duration of such segments can be quite long (days), and exploration of the local area may be possible during that time.” Typically, some of the areas around the path are sunlit and open for exploration while the rover awaits the next leg of its journey .
This animation shows Path One connecting the Ridge to de Gerlaches. The horizontal sections in the left panel represent “stops” in the path.
The authors point out that these may not be the most optimal transitions between points at the South Pole. Instead, they demonstrate the existence of predominantly solar paths. The roads account for the “…extreme time-varying illumination conditions at the Moon’s south pole.”
They also point out that the exact routes will be a combination of optimal sunlight and rover design, with specific science points of interest rounding them out. In this work, they avoid or at least minimize travel on shaded terrain. This means that some of the trips are of long duration, around 30 days. Depending on the rover and mission design, there may be more flexibility in the actual paths.
This animation shows an example of traveling from Connecting Ridge to the Slater Ridge along predefined path 2.
This work does not outline any specific dangers in these journeys…
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