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Physics of the Moon NASA TECHNICAL NOTE -cNASA TN D-2944 e. / PHYSICS OF THE MOON SELECTED TOPICS CONCERNING LUNAR EXPLORATION Edited by George C. ‘Bucker and Henry E. Siern George C. Marsball Space Flight Center Hzmtsville, A Za: NATIONAL AERONAUTICSAND SPACEADMINISTRATION - WASHINGTON;D. C. TECH LIBRARY KAFB. NM llL5475b NASA TN D-2944 PHYSICS OF THE MOON SELECTED TOPICS CONCERNING LUNAR EXPLORATION Edited by George C. Bucher and Henry E. Stern George C. Marshall Space Flight Center Huntsville, Ala. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION For sole by the Clearinghouse for Federal Scientific and Technical information Springfield, Virginia 22151 - Price 56.00 I - TABLE OF CONTENTS Page SUMMA=. 1 INTRODUCTION. i SECTION I. CHARACTERISTICS OF THE MOON. i . 3 Chapter 1. The Moon’s .History, by Ernst Stuhlinger. 5 Chapter 2. Physical Characteristics of the Lunar Surface, by John Bensko . 39 Chapter 3. The’ Lunar Atmosphere, by Spencer G. Frary . , 55 Chapter 4. Energetic Radiation Environment of the Moon, by Martin 0. Burrell . 65 Chapter 5. The Lunar Thermal Environment , . 9i The Thermal Model of the Moon, by Gerhard B. Heller . 91 p Thermal Properties of the Moon as a Conductor of Heat,byBillyP. Jones.. 121 Infrared Methods of Measuring the Moon’s Temperature, by Charles D. Cochran. 135 SECTION II. EXPLORATION OF THE MOON . .I59 Chapter I. A Lunar Scientific Mission, by Daniel Payne Hale. 161 Chapter 2. Some Suggested Landing Sites for Exploration of the Moon, by Daniel Payne Hale. 177 Chapter 3. Environmental Control for Early Lunar Missions, by ‘Herman P. Gierow and James A. Downey, III . , . .2i I Chapter 4. Advanced Life Support Techniques, by Jerry L. Johnson . .235 Chapter 5. Thermal Control Analysis of Objects on the Lunar Surface, by James M. Zwiener . .265 . 111 PHYSICS OF THE MOON SUMMARY This volume consists of thirteen papers dealing with lunar physics, and is based upon a series of symposia conducted at Marshall Space Flight Center in the fall of 1963. The writers are members of the Research Projects Labora- tory of the Center. Topics include The Moon’s History, Physical Characteristics of the Lunar Surface, The Lunar Atmosphere, Energetic Radiation Environment of the Moon, The Lunar Thermal Environment, A Lunar Scientific Mission, Some Suggested Landing Sites for Exploration of the Moon, Environmental Control ‘for Early Lunar Missions, and Advanced Life Support Techniques. INTRODUCTION In the fall of 1963, the Research Projects Laboratory of the George C. Marshall Space Flight Center conducted a series of symposia in which members of the laboratory presented lectures on selected topics related to lunar physics. The symposia were held at MSFC, primarily for the purpose of better acquainting Center personnel with known information about the moon and the planned ex- ploration of the moon. This volume consists of a number of papers on most of the subjects covered in the symposia. The volume is divided into two sections: Section I de- scribes some of the characteristics of the moon and its environment; Section II describes some aspects of a scientific lunar exploration program. Man’s knowledge of the moon is quite fragmentary, as the contents of this volume will readily show. Unmanned lunar payloads, such as the phenomenally successful Rangers VII, VIII, and M, will provide much additional valuable knowledge. However, only when man finally sets foot on the moon and conducts a comprehensive scientific exploration program will his knowledge become fairly complete. SECTIONI CHARACTERISTICSOFTHEMOON 3 - * Chapter 1 THEMOON’S H I STORY BY Ernst Stuhlinger ” The early history of the moon is as dark as the early history of the earth. Roth came into existence about 4. 5 billion years ago. Unlike the earth, the moon completed the decisive phases of its evolution during the first few hundred million years of its life. Since then, it must have remained almost unchanged. When the first living beings that were capable of seeing appeared on earth one 01’ two billion years ago, they probably saw the moon much the same as we see it today. A number of theories were offered on the origin of the moon. Sir George Darwin [ I] , in 1898, developed the idea that the moon was pulled out of the earth by solar gravitational forces at a time .when the earth was still in a molten stage. However, even long before Darwin’s theory, in 1850, Roche [ 21 had shoivn that ‘the moon could never have been in the very close vicinity of the earth ivithout breaking up. In fact, when a celestial body moves around another one, a differen- tial force between the gravitational and the centrifugal forces develops within the body which tends to break it up. As long as this difference force is small, the breakup tendency is counteracted by internal gravity forces. Roche showed that for each two-body system, a critical distance exists below which breakup occurs. This critical distance, the Roche limit, is equal to 18,500 km in the earth-moon system (Fig. I). In this derivation of the Roche limit, cohesive forces \\ere neglected, and only internal gravitational forces were considered. Another school of thought assumes that the moon, and also the planets, originated in clouds of hot gas. Condensation of the gas at discrete places led i&J the formation of planets and moons. It seems, though, that severe difficulties arise when condensation of a tenuous gas is considered as the initial step in the formation of a planet or a moon. A very interesting modification of the gas cloud theory was developed by Alfv& in 1954 [ 3,4] . It starts with a central body, such as a newly formed star or planet, possessing a magnetic field and surrounded by a plasma. Neutral gas atoms fall from great distances towards this central body under the action of gravity forces. Alfv& assumed that the atoms become ionized by interaction with the plasma in the vicinity of the central body as soon as their kinetic energy has become equal to their ionization energy, as expressed by equation I : ::: Director, Research Projects Laboratory 3 EARTH- MOON R,d=l8,500 IJRI RROCHE 3. q 2.44 pf q 2.89 RE Jr-P M RR=ROCHE LIMIT RE=RADIUS OF CARTH PE q DENSlTYOF EARTH PM q DENSlTYOf MOON FIGURE 1. DIAGRAM OF THE EARTH-MOON SYSTEM, ILLUSTRATING THE ROCHE LIMIT I - rnvi = e V, 2 i where vc = critical veloci@;V=ionization potential; m = atomic mass; and e = ion charge. As soon as the atom is ionized, it is forced by the magnetic field into t i a trajectory which, in essence, follows a helical path around the field lines. At t ; the same time, centrifugal forces become noticeable and keep the ions from i falling into the central body, provided that they were not formed too close to one 1 of the poles. This magnetic stopping of the falling particles occurs at a critical i, distance R, where the potential energy equals the critical kinetic energy: (2) , or, with equation 1, R ==m (3) C V e where y = gravitational constant, and M = mass of central body. Hence, we should expect that matter will be accumulated as a plasma around a central body at about the distance R,. Electromagnetic forces will transfer angular momentum from the central body into the plasma. Planets or moons will later develop out of the plasma at about the distance Rc. Figure 2 shows that the critical velocities of a number of elements are indeed of the correct order to account for planets and moons of our solar system. It need not be expected that each of the newly ’ formed planets and moons consists only of that element whose ionization energy i corresponds to its distance from the central body; actually, a mixing of different i elements will occur over wide regions around the central body because of dif- ;I ferences in condensation temperature, gas density, ionization cross section, and %other parameters influencing the ionization and capture of falling atoms. A more ! serious criticism of Alfv&‘s theory arises from the fact that the ionization cross sections of atoms, if they are to be ionized by collisions with ions, are almost t a vanishingly small at the velocities in question. Transfer of the kinetic energy ’ of the falling atoms to electrons, which are more efficient ionizing particles, \ would be required . I It appears that the most widely accepted theory for the formation of planets k and moons is the accretion theory. Fragments of matter, orbiting in loose clouds I d, or rings around a central body, accumulated slowly under the action of gravita- ,I tional forces. Scooping up more and more solid particles, the newly formed 7 cn SOLAR JUPITER SATURN URANUS M( MOONS 1o18 _ PLANETS MOONS NS r OBERON 0 PLUTO t NEPTUNE TITAN ARIEL 0 URANUS CALLISTO 5 Hx 0 SATURN GANYMEDE l 0 JUPITER EUROPA 20 . MIMAS c IO = 0 ti 0 MARS 3o Ho > E t EARTH 40 I 0 VENUS 50 N MERCURY 60 5 80 f FIGURE 2. DIAGRAM OF SOLAR PLANETS, AND OF SOME PLANETARY MOONS Left ordinate: potential of central force fields; right ordinate: free fall velocity in central force fields. Elements are shown at places where free fall kinetic energies of atoms are equal to their ionization energies (Alfvb, 3). bodies gradually swept all orbiting fragments from the regions around the central body. This theory, which is supported by Kuiper, [ 51 , Urey [ 6, 71 , and others, seems to present fewer difficulties than the gas cloud theory. A body formed by accretion would not be hot during its formation. It has been speculated that it may heat up, and even melt, at later times under the action of radioactive heating; however, Urey [ 81 , and later MacDonald [ 91 , have shown that our moon could not reach melting temperatures from radioactive heating alone.
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