5

Physical Sciences Research on the Lunar Surface

Like Session 3, Session 4 had the goal of illustrating the types of research that could benefit from being performed on the lunar surface. However, Session 4 focused on physical sciences rather than biological sciences. In addition, the session offered some details on what is involved in conducting such research. The session was moderated by planning committee members Steven Collicott and Douglas Matson, and had four presenters.

Paul Ferkul, the project scientist for the Solid Fuel Ignition and Extinction investigation at the Universities Space Research Association, described an upcoming experiment to be carried to the Moon by the Commercial Lunar Payload Services (CLPS) program to study the behavior of flames on the lunar surface. Karen Daniels, a professor of physics at North Carolina State University, spoke about research on granular materials carried out in reduced gravity. Jack Burns, a professor in the Department of Astrophysical and Planetary Sciences and in the Department of Physics, both at the University of Colorado, Boulder, spoke about radio astronomy on the Moon that is being enabled by CLPS missions. Last, Clive Neal, a professor in the Department of Civil and Environmental Engineering and Earth Sciences at Notre Dame University, spoke about lunar research opportunities linking the physical and biological sciences.

FIRE ON THE MOON

Fire safety in space has been a major NASA concern for some time, Ferkul said. For example, he noted, the 2011 National Research Council report Recapturing a Future for Space Exploration: Life and Physical Sciences Research for a New Era¹ called for fire safety research aimed at improving methods for screening materials for flammability and fire suppression in space environments. Specifically, NASA is interested in understanding combustion in low gravity because of its implications for fire safety aboard space vehicles and in other space-related environments, such as the surface of the Moon. The experiments he described were aimed at advancing this understanding.

Currently, Ferkul said, NASA relies on a 1 g test for screening material flammability for flight, and if a material passes this test carried out in normal gravity, it is considered safe for spaceflight. However, he continued, his group’s research has shown that some materials that will not burn in 1 g will burn in zero gravity or in partial gravity, such as the gravity found on the Moon. Furthermore, the direction of burning can be very different in low or zero gravity than on Earth’s surface, he said. He offered an example of a particular material that will not burn downward on Earth, but will burn in any direction in microgravity, where there is no up or down as far as the flame is concerned. Thus, it is important to test the flammability of materials in much lower gravities than 1 g.

To a certain extent it is possible to get an idea of how a flame will behave in different gravities. This can be done by working in the microgravity of the space station and applying an air flow to mimic the buoyancy of heated air under a certain amount of gravitational pull, such as the gravity found on the

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¹ National Research Council, 2011, Recapturing a Future for Space Exploration: Life and Physical Sciences Research for a New Era, The National Academies Press, Washington, D.C.

Moon. However, Ferkul said, by actually carrying out experiments on the Moon, it will be possible to observe directly how flames burn in this environment.

Next, he described the experiment that he and his team have proposed to carry out on the Moon to test the flammability of various materials under lunar gravity. The hypothesis they will be testing is that some materials burning in lunar gravity are more flammable than on Earth. If that hypothesis is confirmed, it will have important implications for the current 1 g screening method used by NASA.

The experiment will be carried out in a small chamber. Four fuel samples will be burned, one at a time, with cameras and other sensors recording the characteristics of the flame. Oxygen levels will be varied to determine the lowest oxygen level at which each fuel burns for upward and downward spread. Those oxygen limits will be compared with the limits measured on Earth. Furthermore, the measured flame characteristics in both 1 g and lunar gravity will be compared with detailed model predictions. The researchers will use these comparisons to refine the pressure-gravity scaling relations not just at these two gravity levels, but at other levels as well.

Displaying a diagram of the experimental combustion chamber (Figure 5.1), Ferkul described how the experiment will be done. Everything is carried out within the cylindrical chamber. It will be a small chamber—25 cm in diameter by 20 cm tall, for a volume of about 10 liters. It will contain two gas bottles, two fabric samples and two plastic rods that be burned in the tests, igniters, a heater to control the temperature in the chamber, a fan to keep the air in the chamber well mixed, two cameras, and several sensors.

In one mode, the experiment will be carried out with the oxygen level kept approximately constant for flammability tests. In another, no additional oxygen will be added after ignition, and the plastic rods will be allowed to burn until the flame goes out to determine the lowest oxygen level at which the flame can be maintained.

The chamber will be self-contained and will hold everything needed to run the experiments except for power, which will be supplied by the lander through a power line into the chamber. The other two connections between the chamber and the lander will be a chamber vent valve and a communications line to export the experimental data.

To carry out on experiment, the vent valve will be closed, and the chamber will be filled with gas and, if necessary, heated to 20°C. Then the cameras and instruments will be initiated, the fuel will be ignited, and data recorded. Once the burn is complete, the valve will be opened to vent the chamber. Then the sequence will be repeated for the next experiment.

At present, Ferkul concluded, his team has finished the design of an engineering prototype that is now being manufactured. Cameras and other electronics are now being procured, and once everything is built and assembled, the team will test various sample configurations and operations scenarios in 1 g. At the same time, the team will be going through the CLPS process to arrange for the transport of the experiment to the Moon and its execution there.

In response to a question in the discussion period about modifying the parameters for a test to address previous run results, Ferkul said that the current plan is to make everything automatic because communications between Earth and the Moon are likely to be limited and will also have a time lag. However, he added, if it becomes possible to use feedback to modify the experiment between runs, that could be valuable.

GRANULAR MATERIALS RESEARCH IN LUNAR GRAVITY

Daniels began her presentation by asking the question, why would one want to do research on granular materials in lunar gravity? To begin with, granular materials—things like sand, coal, or cereal—are common, and it is useful to have a good understanding of their behavior. Doing experiments on them in different environments, such as the one-sixth g environment of the Moon, opens up opportunities to discover new phenomena and improve models of granular dynamics. That, in turn, will help improve the current understanding of planetary geomorphology and of the formation of the solar system. Ultimately, an improved understanding of the granular materials on the surface of the Moon and Mars should help in the design of vehicles to explore these places.

Humans have been working with granular materials for millennia, Daniels said, but it has, of course, always been on Earth. Even on Earth, these remain difficult materials to work with, she said, referring to a RAND study from the 1980s that found that the factories working with granular materials ran at only two-thirds of their capacity because the machinery often did not work. Thus, it will be important to gain a better understanding of the behavior of granular materials in other environments, such as the Moon, if these materials are going to be worked with in those other environments.

The study of granular materials is also interesting fundamental science, she said. There is a huge community of physicists and engineers around the world working on the question of how granular-scale properties (size, shape, roughness, friction coefficient, density, etc.) are associated with bulk properties (elastic modulus, tensile strength, flowability, and so on). That question is, in turn, one of a whole collection of open physics questions related to collective effects, she said.

Daniels described two examples of the sort of fundamental physics questions concerning granular material that physicists are interested in. One is how gravity affects mechanical rigidity and the speed of sound in granular materials. Lower gravity causes the grains to pack less densely, making the material less rigid and slowing the speed at which sound moves through it. Physicists have used simulations to examine this phenomenon, but have not done the actual studies.

A second example is the question of how a granular material’s flow properties change under different gravities. Simulations have shown some interesting patterns, but at this point it is all theoretical.

As an example of what can be learned by doing experiments with granular materials under different levels of gravity, Daniels showed videos created by her own group. Experiments were flown on a plane that can simulate low or zero gravity for a short time by dropping rapidly through the atmosphere. The

experiment involved a needle being pushed into a box of granular material. At zero gravity, the needle slid right in with no resistance, while at Martian gravity—about two-fifths that of Earth—the material had various points where it resisted the needle strongly. Eventually, the needle was pushed off its path. “We don’t have a general theory of this,” she said.

Researchers are even further from understanding the bulk properties of the regolith, the granular material that makes up the lunar surface, Daniels said. One reason is that the individual grains of the regolith have complex, irregular shapes. By contrast, the experiments that Daniels’s group has been doing are with spherical shapes. The shape makes it easier to model and understand the results of the experiments, but it means that the work does not represent what happens in the regolith.

There are also other factors influencing granular materials that are very different away from Earth, she said. For instance, electric forces play a much larger role. Under Earth’s gravity, granular materials settle quickly, and van der Waals forces and electric charges on particles are typically significant only for very fine powders, where the grains are less than 10 microns in diameter. On the Moon, by contrast, regolith dust tends to stick to everything (because of the greater role played by van der Waals forces), and electric-levitated dust is significant. The effects on an asteroid would be even more dramatic, she said, and van der Waals forces are significant enough there that they are what stabilize rotating objects and prevent them from flying apart.

“This is not stuff that we have intuition for,” Daniels said, “so unless we’re doing experiments in low-gravity environments, we’re not going to understand what’s actually going on.”

There are a number of other open questions, she said, including some that have implications for humans on the Moon. For instance, how is the stability of soils affected by gravity? That will be important to know if structures are going to be built on the Moon. How does gravity affect the drag on something moving through granular material? Such drag affects, for example, how easy it is to anchor something in the material.

In closing, Daniels said, “The successful completion of a lot of lunar activities is going to depend on us being able to understand granular dynamics in this context.” That, in turn, will require doing the experiments on the Moon, under lunar gravity, with the Moon’s radiation and atmosphere, and, eventually, with real lunar regolith.

Later, during the discussion period, Daniels responded to a question about the largest challenge to this sort of work by pointing to the need to build a self-contained apparatus for carrying out the experiment. “Figuring out how to develop an experiment that required one button push—that was a leap for me,” she said. She has learned that carrying out her experiments with a robotic apparatus is “a completely doable paradigm.” Her team puts together automated motors and cameras into an assembly, and then sees if the assembly will run for a week without human contact. If it does, it should work on the Moon as well.

RADIO SCIENCE FROM THE MOON ENABLED BY NASA COMMERCIAL LANDERS

Next, Burns spoke about radio science studies from the Moon. He described three missions, two of which are under way and one that is a mission concept that is fairly well developed at this point.

Because the far side of the Moon is “radio quiet,” that is, it is relatively little affected by electromagnetic radiation in frequencies below 100 megahertz, there is some “compelling astrophysics” that can only be done on that side, Burns said. He mentioned two examples. One is radio-frequency observations of the early universe, and the other is studies of magnetospheres and space weather. Both areas were identified as top priority in the 2010 decadal survey, he said.

Expanding on the value of the far side of the Moon, he explained that it is radio quiet because the body of the Moon blocks radio-frequency electromagnetic radiation from Earth. The far side also lacks a significant ionosphere, at least at the radio frequencies of interest, and it offers a dry stable environment. This environment allows for sensitive radio-frequency observations.

In November, a CLPS mission operated by Intuitive Machines will carry a payload, Radio-Wave Observations at the Lunar Surface of the Photoelectron Sheath (ROLSES), to the lunar surface. Here, it

will carry out the first radio-frequency observations from the lunar surface. The landing will be on the near side, Burns said, so it will not be possible to take advantage of the radio quiet of the far side. However, ROLSES will still be able to make a number of important observations. It will examine the mini-ionosphere created by the solar wind hitting the regolith on the Moon’s surface. In addition, it will observe the sun, and, most important, it will examine aspects of the lunar environment vital for designing future radio-frequency observations from the Moon.

The next payload, which is already scheduled for a CLPS mission in the second quarter of 2024, will be placed on the far side of the Moon. Using antennae that can detect radio-frequency radiation, that payload “will allow us to do the first cosmology from the surface of the Moon,” Burns said, “which is very exciting.”

In particular, he explained, the purpose of that study, the Dark Ages Polarimeter Pathfinder (DAPPER), will be to begin to map the evolution of the early universe, within the first few hundred years after the Big Bang. In that early era, the universe was filled with neutral hydrogen, which emits radio-frequency waves DAPPER will be able to detect. This is a capability that the other observers of the universe, such as the Hubble Telescope or the James Webb Space Telescope, do not have, he said. “We’re opening up an entirely new part of the universe that’s never been observed before.” In particular, he explained, data from the study can be used to distinguish among the predictions of various theoretical models of the development of the early universe and to look for details in the radio waves that differ from what is predicted by theory. “Any deviation . . . means there is additional physics that is operating here.”

In closing, Burns described a concept for a mission that would greatly expand what can be seen from the Moon via radio-frequency observations. In this case, the Moon lander would carry a number of rovers that would head out in four directions from the lander, laying long tethers along the ground in each direction. The tethers will hold antennas along with power and communications lines. The result would be a giant X-shaped antenna 10 kilometers across that would have a high enough resolution to make out many individual details from the early universe. The system would also be used to look for exoplanets—that is, planets circling stars other than the sun.

In answer to a question during the session’s discussion period, Burns made the point that building telescopes on the Moon is going to become more like how they are built on Earth than the way they have been done in space up to now. That is, telescopes in space have been one-off projects, where a telescope is built and launched and never touched again. However, with the CLPS program and regular trips to the Moon, he said, telescopes will be projects that evolve and develop over time while doing science at the same time.

In response to another question, Burns agreed with a comment that Sean Mahoney had made in Session 2 that “risk is your friend.” Because CLPS makes it possible to do things quickly, it is less important to be 100 percent certain of success. This represents breaking with a philosophy that has dominated NASA’s thinking for the past 50–60 years, he said. If risk-taking proves successful with the CLPS program, he said, “it may open up that paradigm, more broadly, for NASA as well.”

PHYSICAL AND BIOLOGICAL SCIENCE RESEARCH POSSIBILITIES ON THE LUNAR SURFACE

Neal, the final speaker of the day, wrapped up the presentations with a talk that included discussion of research in both the physical and biological sciences.

One of his emphases was on the importance of the lunar regolith for the long-term presence of humans on the Moon. One likely use of the regolith is as the basis of soil in which to grow plants on the lunar surface. Early experiments on the regolith done during the Apollo era showed, he said, that the regolith is not toxic to Earth plants. However, the tests were done by bringing the plants into contact with the regolith, not by attempting to grow them in a regolith-based soil. Referring to the study described in the previous session by Anna-Lisa Paul that demonstrated that Earth seeds could sprout and grow on the Moon, Neal said that it will be important to repeat such experiments in the regolith.

More generally, he said, the regolith is likely to be important not just for biology, but for the production of various materials on the Moon because “all of the resources that we know and think are present on the Moon are in the regolith.” It is the regolith that will allow astronauts to extract water, mine metals, get oxygen and hydrogen for breathing and fuel, and perhaps even export materials back to Earth. “Understanding how to move the regolith in the lunar environment is one critical thing that we desperately need in order to allow humans to survive and thrive up there in any Artemis base camp,” he said.

He also spoke of the importance of laser retroreflectors that have been placed on the Moon during previous visits; both by the Apollo program and by the Soviet Union’s Luna program. Those reflectors, which are used to reflect back laser light sent from Earth, are used by the astrophysics community to test a number of important theories, he said. There are plans to add other laser reflectors to the network sometime in the future.

In the final portion of his presentation, Neal spoke about the biological and physical sciences research that will be necessary to help humans establish a long-term presence on the Moon. In particular, he displayed a long list of NASA research objectives; researchers interested in participating in the CLPS program could design experiments that offered insight into any of these objectives. The 21 objectives he listed included such things as investigate and characterize the fundamental interactions of combustion and buoyant convection in lunar gravity, study the behavior of granular media in the lunar environment, investigate the production of oxygen from lunar regolith in lunar gravity, study and assess the effects on materials of long-duration exposure to the lunar environment, assess the effect on plants of long-duration exposure to the lunar environment, study the fundamental biological and physical effects of the integrated lunar environment on human health and the fundamental biological processes and subsystems upon which health depends, and study the effects of lunar radiation on biological model systems.

Then, in the discussion period, Neal added that another important object will be developing a supply of power on the Moon. Kilowatts of power will not be enough, he said; eventually it will be necessary to provide megawatts of power on the lunar surface. “Having a reliable source of power is something that’s going to be critical for all of these endeavors.”

“Again,” he said in closing, “this is research that’s going to enable humans to survive and thrive not only on the Moon, but also forward on to Mars.”