SP-404 Skylab's Astronomy and Space Sciences

 

3. Interplanetary Dust

 


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Figure 3-1. Micrometeoroid collection instrument (S149) deployed by scientist pilot Owen K. Garriott during the second manned mission. The collection cassettes are mounted on the rim of the solar observatory (in front of Garriott's head) and remained facing the Sun for 46 days before being retrieved.

Figure 3-1. Micrometeoroid collection instrument (S149) deployed by scientist pilot Owen K. Garriott during the second manned mission. The collection cassettes are mounted on the rim of the solar observatory (in front of Garriott's head) and remained facing the Sun for 46 days before being retrieved.


 

[27] Meteors, or "shooting stars," were the first evidence of small solid objects in interplanetary space. They are caused by the passage through the atmosphere of chunks of solid matter that heat the air to incandescence by friction. The large remnants that reach the Earth's surface are called meteorites. Not all of these solid objects are big enough to cause shooting stars or to be conspicuous on the surface of the Earth after falling. Those smaller than about 0.1 mm in diameter are called micrometeorites when they are recovered from the surface of the Earth. Their surface-to-mass ratio is large and radiative cooling is so efficient that the object does not become hot enough to leave a trail of incandescent air. The terms "meteoroid" and "micrometeoroid" are used for individual solid objects in interplanetary space. The micrometeoroids, particularly the very small particles, are often called interplanetary dust. The mass of solid extraterrestrial matter reaching the Earth's surface has been estimated to be 10000 metric tons per day, most of it as micrometeorites.

Meteorites recovered from the Earth's surface are of two types, stony and iron. About 61 percent are stony and about 35 percent are iron, with 4 percent being stony-iron. Over 90 percent of the stony meteorites are called chondrites because they contain small spherical inclusions called chondrules, which suggests that they formed from a rapidly cooling melt. Some 3 to 4 percent of recovered meteorites are black chondrites with a carbon content of about 3 percent and are called carbonaceous chondrites. Carbonaceous chondrites are sometimes very fragile; hence, many of them must be destroyed in the atmosphere before they reach the ground.

Meteors are classified as shower meteors or sporadic meteors. Sporadic meteors can occur at any time and can come from any direction. They are more common than shower meteors. Shower meteors are seen at about the same time each year from a particular direction in space.

Meteor showers are the trail of debris left by a comet along its orbit. The shower occurs when the Earth in its orbit around the Sun passes through the comet's trail. If the material in the trail is fairly evenly spread out, the Earth will pass through it and the shower will be seen each year. If the debris occurs in clusters along the trail, the Earth will sometimes pass through a gap between clusters and there will be no shower that year. An example of an annual meteor shower is the Orionids, so named because the meteors appear to originate from a point in the sky in the direction of the constellation Orion. The Orionids are in the orbit of Halley's comet and are observed each year around October 21.

The existence of micrometeoroids in space is indicated from studies of the zodiacal light. The zodiacal light is sunlight reflected from micrometeoroids orbiting the Sun near the ecliptic plane. Studies of it suggest that the number of micrometeoroids increases rapidly with decreasing size. Micrometeoroids presumably travel at the same speeds as meteoroids, about 12 to 72 km/sec.

 


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Figure 3-2. Sample cassettes being loaded into the micrometeoroid collection instrument.

Figure 3-2. Sample cassettes being loaded into the micrometeoroid collection instrument.

 

In the early days of space flight, therefore, it was thought that they might present a hazard to spacecraft even though they were very small. Zodiacal light is the best source of information about the average properties of in-place interplanetary dust as a whole, and particle impacts are the best source of data on the properties of individual micrometeoroids. Both the zodiacal light and micrometeoroid impacts were studied by instruments aboard Skylab.

 

The S149 Skylab Micrometeoroid Collection Experiment

 

Micrometeoroids in the near-Earth vicinity were expected to strike Skylab. To take advantage of the fact, C. L. Hemenway, director of the Dudley Observatory in Albany, New York, devised an experiment in which thin foils and polished metal plates were exposed in space to record penetrations by such particles. This experiment is shown deployed in figure 3-1. The exposed materials were returned to Earth and studied with optical microscopes and scanning electron microscopes.

Sample cassettes for the Skylab micrometeoroid collection experiment are shown being loaded in figure 3-2. A total area of 1200 cm2 was exposed when the instrument covers were opened, deploying their sample surfaces and uncovering other samples in the pans around the cassette body (fig. 3-3). The surfaces included slides of stainless steel, copper, and silver. There also were stacked, aligned layers of thin carbon-coated nitrocellulose films on electron-microscope grids; nitrocellulose films over glass; and two layers of thin gold foil over stainless steel. The cassettes were opened, exposed, and sealed in the space environment before being retrieved and returned to Earth for analysis.

The micrometeoroid collection cassettes were deployed in two different locations on the Skylab vehicle, as shown in figure 3-4. For 34 days between the first and second manned missions, they faced away from the Sun on the end of a boom. This boom was also used to deploy and point a photometer that measured starlight and zodiacal light in another experiment. The cassettes were opened and closed by ground command after the Apollo spacecraft had left Skylab.

During the second and third manned missions, new cassettes were installed to face the Sun for 46 and 34 days, respectively. These cassettes were manually attached to the rim of the solar observatory and opened during spacewalks. A fourth set of cassettes was left on the rim of the solar observatory for later retrieval should there be a return visit to the Skylab. The letters A, B, C, and D indicate the orientation of the cassettes.

 

Table 3-1. Distribution and Location of Craters.

Number of craters

.

A

B

C

D

Total

.

Antisolar airlock - between first and second mission

Covers

2(-Z)

1(-Z)

1(-Z)

1(-Z)

6

Pans

6(-Y)

5(-X)

2(-Y)

2(-X)

17

.

Observatory rim - second mission

Covers

0(+Z)

0(+Z)

0(+Z)

1(+Z)

1

Pans

7(+Y)

3(+X)

3(-Y)

3(-X)

16

.

Observatory rim - third mission

Covers

0(+Z)

0(+Z)

2(+Z)

1(+Z)

3

Pans

1(+Y)

1(+X)

13(-Y)

3(-X)

18

 

[29] The distribution of craters in the various pans and covers of the micrometeoroid collection experiment is shown in table 3-1. The X, Y, Z directions are the directions perpendicular to the exposed surface with respect to the space station as shown in figure 3-4. The data are a summary of the results of scanning all metal surfaces once at magnifications of 200X and some of them at 500X. Because of Skylab's orientation, with the solar panels pointing at the Sun 99 percent of the time, the pans facing in the direction of Earth's motion around the Sun collected more particles per unit time than did the covers. The covers facing away from the Sun had.....

 


Figure 3-3. Micrometeroid collection experiment deployed. Link to a larger picture.

Figure 3-3. Micrometeroid collection experiment deploy.


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Figure 3-4. Location of micrometeoroid collection cassettes on the Skylab vehicle. For 34 days between the first and second manned missions, the cassettes faced away from the Sun on the end of a boom (lower right). During the second and third manned missions, they were attached to the rim of the solar observatory and faced the Sun for 46 and 34 days, respectively.

Figure 3-4. Location of micrometeoroid collection cassettes on the Skylab vehicle. For 34 days between the first and second manned missions, the cassettes faced away from the Sun on the end of a boom (lower right). During the second and third manned missions, they were attached to the rim of the solar observatory and faced the Sun for 46 and 34 days, respectively.

.

Figure 3-5. High-velocity micrometeoroid impact crater on a copper slide.

Figure 3-6. Closeup of figure 3-5, showing the frozen droplet structure within the high-velocity impact crater.

Figure 3-5. High-velocity micrometeoroid impact crater on a copper slide.

Figure 3-6. Closeup of figure 3-5, showing the frozen droplet structure within the high-velocity impact crater.


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Figure 3-7. Small, asymmetric impact crater in stainless steel, probably produced by a particle striking at an angle to the surface.

Figure 3-8. Extremely small impact crater, produced by a micrometeoroid between 0.1 and 0.2 µm in diameter.

Figure 3-7. Small, asymmetric impact crater in stainless steel, probably produced by a particle striking at an angle to the surface.

Figure 3-8. Extremely small impact crater, produced by a micrometeoroid between 0.1 and 0.2 µm in diameter.

 

.....more craters than those facing the Sun, which is consistent with the concept that the dust particles are spiraling slowly inward toward the Sun.

 

Impact Craters

 

Typical high-velocity impact craters observed in stainless steel and copper slides with a scanning electron microscope are shown in figures 3-5 through 3-8. Figures 3-5 and 3-6 show the same crater in a copper slide. This was one of the largest craters observed in the micrometeoroid collection experiment; figure 3-6 is a closeup that shows the frozen droplet structure within the crater. The small, asymmetric crater in stainless steel, shown in figure 3-7, was probably made by a particle striking at an angle to the surface. Figure 3-8 shows one of the smallest craters observed, made by a particle believed to have been between 0.1 and 0.2 µm in diameter. The structure of the crater appears similar to that of larger craters.

Analysis of the craters produces information on the numbers, sizes, and velocities of the micrometeoroids and some information about their chemical composition. Considerable amounts of micrometeoroid residue were found in the bottom of rough-textured craters or on the lip of smooth craters. Elements found in such residue were aluminum, silicon, sulfur, chlorine, potassium, calcium, iron, and zinc.

Figures 3-9 and 3-10 are from a pair of stacked gold foils over a stainless steel substrate. A micrometeoroid struck the first gold foil and shattered into fragments, which in turn penetrated the second gold foil. The micrometeoroid must have been quite fragile, since it fragmented upon striking a foil much thinner than its dimension. In one case, two small craters were found in the stainless steel substrate after a particle penetrated two layers of gold foil. These pictures, incidentally, illustrate the principle of the micrometeoroid shield. Fragmentation of micrometeorites striking the shield [32] would greatly reduce the possibility of damage to the spacecraft wall. Although Skylab's 0.6-mm-thick micrometeoroid shield was lost, the orbital workshop's wall, 3.18 mm thick, was not penetrated, indicating that there is little meteoroid hazard to spacecraft in Earth orbit with such wall thickness.

 

Multiple Craters

 

Figure 3-11 shows two small craters close together, suggesting a clustering of micrometeoroids. Several such multiple events were observed. Figure 3-12 shows several penetration holes close together. As this picture indicates, debris from the impacting particle sometimes remained near the penetration hole. One of the best examples of the clustering of impacts was observed in a cover facing away from the Sun, where approximately 1000 penetration holes in the thin gold foil were observed in an area of only 8 mm2.

 

"Evil Eyes"

 

Micrometeoroids penetrating the thin nitrocellulose films produced a cylindrical hole with a halo, giving it....

 


Figure 3-9. Penetration hole in gold foil.

Figure 3-9. Penetration hole in gold foil.


Figure 3-10. Fine penetration holes in the second gold foil positioned beneath the gold foil shown in figure 3-9.

Figure 3-10. Fine penetration holes in the second gold foil positioned beneath the gold foil shown in figure 3-9.


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Figure 3-11.-Two small impact craters close together.

Figure 3-11.-Two small impact craters close together.

 

....an "evil eye" appearance when viewed in an electron microscope (fig. 3-13). The halo around the "evil eye" is probably residue from the particle. The "evil eyes" were first seen in an exposure of a Gemini micrometeoroid collector but were unexplained. The particles causing them may consist of a hard core surrounded by soft material. The hard core produces the hole, and the soft material causes the halo.

 

Meteoroid Residue

 

An unrelated experiment (S228) by P. Buford Price on transuranic cosmic rays provided an unexpected additional observation of a micrometeoroid. A large micrometeoroid crater was found in the pure aluminum foil covering a detector stack that was deployed on the outside of the space station during the third manned Skylab mission and subsequently returned to the Earth. The crater is approximately 110 µm in diameter and 75 µm deep, as shown in figure 3-14. Donald Brownlee, of the University of Washington, found that the crater wall contained enough micrometeoroid residue to allow determination of the relative elemental composition by electron-microprobe techniques. It is thought that

 


Figure 3-12. Multiple penetration holes close together.

Figure 3-13. <<Evil eye>> hole produced in thin nitrocellulose film. the halo is probably residue from impacting micrometeoroid.

Figure 3-12. Multiple penetration holes close together.

Figure 3-13. "Evil eye" hole produced in thin nitrocellulose film. The halo is probably residue from the impacting micrometeoroid.


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Figure 3.14. Large impact crater (~110 µm in diameter, 75 µm deep) in pure aluminum foil.

Figure 3.14. Large impact crater (~110 µm in diameter, 75 µm deep) in pure aluminum foil.

 

...micrometeoroids of this size (~30 µm in diameter) probably originated from comets, since meteor showers containing particles of this size or larger occur when the Earth crosses a comet's orbit. If so, this crater analysis provides a laboratory measurement of actual cometary material.

The results of two electron-probe analyses are shown in figure 3-15, the relative abundance being normalized to the amount of silicon found. Elements identified were iron, silicon, magnesium, calcium, nickel, chromium, and manganese. Upper limits were also obtained for titanium and cobalt. For comparison, the relative elemental abundances for two types of carbonaceous chondrite meteorites (C1 and C3) are also given. There is a....

 


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Figure 3-15. Element composition (normalized to silicon) of micrometeroid residue found in the crater shown in figure 3-14. The open squares and circles represent different electron-beam probe runs. The elemental compositions of two types of carbonaceous chondrite meteorites (C1 and C3), represented by solid circles and by crosses, respectively, are shown for comparison. Link to a larger picture.

Figure 3-15. Element composition (normalized to silicon) of micrometeroid residue found in the crater shown in figure 3-14. The open squares and circles represent different electron-beam probe runs. The elemental compositions of two types of carbonaceous chondrite meteorites (C1 and C3), represented by solid circles and by crosses, respectively, are shown for comparison.

 

.....marked similarity, but this should not be construed as evidence that both objects have a common source. The similarities are possibly only a consequence of their both being primitive, well-preserved samples of early solar system materials. A sulfur analysis at a later date indicated that sulfur is also present in the crater with an abundance similar to the abundances of iron, magnesium, and silicon and also comparable to the abundances for carbonaceous chondrites.

Apparently the pure aluminum was a good material to trap the residue of the particle that caused the crater, since similar sensitive analyses of craters in lunar rocks failed to detect any micrometeoroid residue. Based on the analysis, it was estimated that approximately 10 percent of the particle survived as residue.

The successful detection of residue in the Skylab micrometeoroid craters suggests that, if other large craters can be analyzed, it may be possible to make a statistical estimate of elemental abundances in comet debris. Primitive meteorites found on Earth contain large (> 30 µm) inclusions of materials such as the minerals olivine and magnetite, for which the relative elemental abundances are very different from the accepted cosmic abundances. Residue analysis of a large number of craters would show whether or not these types of materials are also in comets. The results of such a search would have implications regarding the interrelationships between comets and meteorites.

 

The Zodiacal Light

 

The zodiacal light is sunlight reflected from microscopic dust particles in orbit about the Sun and concentrated near the ecliptic plane. Figure 3-16 is a picture of the zodiacal light taken by P. Hutchinson from atop 3048-m Mt. Haleakala, Hawaii, in January 1967, just before sunrise. From the Earth, the zodiacal light is best seen when the ecliptic plane is approximately perpendicular to the horizon. In the Northern Hemisphere, the best times to view the zodiacal light are on clear, moonless nights after twilight in February and March and before dawn in September and October. In equatorial regions it is visible throughout the year. Figure 3-16 shows the rapid dimming of the zodiacal light at increasing angular distances from the Sun. At 30° from the Sun in the ecliptic, it is approximately three times brighter than the brightest part of the Milky Way.

 

The Gegenschein

 

The faint patch of light in the night sky directly opposite the Sun is called the Gegenschein, which is German for counterglow; it is an intensification of the zodiacal light. The phenomenon is difficult to observe or photograph from the ground because of interference from atmospheric airglow.

Figure 3-17 shows the Gegenschein photographed during the second manned Skylab mission. The photograph required a long exposure (6 min) using a very fast but grainy film (Kodak 2485) in a 35-mm camera. The camera was provided with occulting disks to blot out the Sun in an "artificial eclipse" when operated in a Sun-pointed mode (Experiment T025). The occulting disks and the supporting rod were rotated out of the way and are seen as the out-of-focus dark areas on the right. If the Gegenschein were due to a reflection from dust near the Earth, it might have been possible to see the Earth's shadow in the center of the Gegenschein in this photograph. Such a shadow is not seen, although a bright star (3.8 magnitude) near the center of the Gegenschein may have masked any existing shadowing to some degree. This result is in agreement with earlier ground-based observations.

 

[36] The S073 Gegenschein-Zodiacal Light Experiment

 

Detailed study of the characteristics of the zodiacal light can in principle provide information on the dust particles that gave rise to it: their numbers and sizes, distribution in space, refractive index, and shape. Ground-based observations have given discordant results, and it has not been possible to determine from these observations the nature or origin of the zodiacal dust.

The Skylab study of zodiacal light designed by Jerry Weinberg, of the State University of New York in Albany, used a photomultiplier photometer provided....

 


Figure 3-16. Zodiacal light observed just before sunrise on top of 3048-m Mt. Haleakala in Hawaii.

Figure 3-16. Zodiacal light observed just before sunrise on top of 3048-m Mt. Haleakala in Hawaii.

 

[37] ....with filters and polarizers, The data are obtained as a plot of brightness and polarization at a number of wavelengths across the spectrum. The photometer was attached to the end of a boom that extended from the airlock in the wall of the orbital workshop. A 16-mm camera was mounted alongside the photometer in order to verify the direction in which it was pointed.

The two instruments were positioned at the end of the boom on a two-axis mounting that permitted a fixed-position or a scanning observation of selected areas of.....

 


Figure 3-17. Gegenschein photographed during the second manned Skylab mission.

Figure 3-17. Gegenschein photographed during the second manned Skylab mission.


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Figure 3-18. Photometer deployed preflight as for the Gegenschein/zodiacal light experiment (S073).

Figure 3-18. Photometer deployed preflight as for the Gegenschein/zodiacal light experiment (S073).

 

.....the sky. The experiment measured the properties of the zodiacal light and starlight in the hemisphere of the sky centered in the antisolar direction. Before the mission, it had been hoped to scan the sunward area of the sky to within 15° to 20° of the Sun, where the zodiacal light blends into the outer corona of the Sun. The blockage of the Sun-facing airlock by the parasol deployed by the first crew, however, prevented such scanning.

The instrument is shown in figures 3-18 and 3-19. Figure 3-18 depicts it with the boom fully extended, which placed the photometer head some 6 m beyond the space-station wall. Figure 3-19 shows the photometer mounted in the airlock during the first Skylab mission. The long canister and panels contained power supplies, automatic and manual control equipment, and the extension mechanism. An astronaut's spacesuit is shown drying out alongside the photometer. Astronauts programmed the photometer scan pattern and scan limits so that it operated automatically, telemetering data on the observed region of the sky for up to 10 consecutive orbits while the astronauts slept or performed their duties.

When the meteoroid shield was torn off the spacecraft, the backup unit of the photometer canister was modified, launched with the first manned mission, and used at the solar airlock to deploy the parasol that shaded the vehicle.

 

The Poynting-Robertson Effect

 

Interplanetary dust particles, including those which give rise to the zodiacal light, are subject to gravitational and radiation forces. Gravitational forces cause them to orbit the Sun, while radiation pressure tends to push them away from the Sun. An additional force on the small micrometeoroids responsible for the zodiacal light is the Poynting-Robertson effect. The particles absorb sunlight from only one direction, but reradiate this energy in all directions. Calculations for small orbiting particles show that the result of this action is a drag force that, under most conditions, is greater than the radiation pressure and causes the particles to spiral inward toward the Sun. The Poynting-Robertson effect is greatest for particles of low density and small size. Consequently, after millions of years, first the smaller particles and then the larger ones will fall in toward the Sun, causing a sorting out process to occur, with the smaller particles being nearer the Sun. The particles will.....

 


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Figure 3-19. Photometer mounted in the airlock during the first manned Skylab mission.

Figure 3-19. Photometer mounted in the airlock during the first manned Skylab mission.

 

.....not actually fall into the Sun but will be partially or totally vaporized. The vaporized and the residual material are subsequently blown out of the solar system by radiation pressure. The fact that the dust is present, as is known from the zodiacal light, and is being depleted by the Poynting-Robertson effect means that there must be a source from which it is being replenished. Among the possible sources of fresh material are comets, with their long tails of ejected material, and fragments from collisions in the asteroid belt. Data on these effects were obtained from Skylab's sky-mapping observations, which were coordinated with observations from the zodiacal light experiments on the Pioneer 10 and 11 Jupiter probes, which passed through the asteroid belt. Weinberg's Pioneer data indicate that the interplanetary dust responsible for the zodiacal light does not [40] extend much beyond the asteroid belt. The rate at which the brightness decreases from the Sun out to the asteroid belt, combined with Pioneer and Skylab data on color and polarization, may make it possible to determine whether zodiacal dust has its origin in comets, in the asteroid belt, or in both.

 

Brightness of the Skyglow

 

Figure 3-20 is a color-coded brightness map of half a hemisphere of sky opposite the Sun. The colors show the variations in the level of brightness measured by the photometer through a red filter at a wavelength of 710 nm. The scan started in the center and worked outward in ever larger rings. Each block represents the average brightness in the area swept across by the photometer in 10 sec. The white rings are missing data due to automatic photometer shutdown caused by bright objects (e.g., Jupiter or the illuminated Earth). The lighter colored vertical band corresponds to the Milky Way, which intersects the ecliptic near the antisolar point, located at the center of the figure. The Gegenschein was in the Milky Way, in the region near the center colored dark red. This photometer scan was performed during the second manned mission. Similar maps were made at other wavelengths and for other periods of time.

 

Polarization of the Skyglow

 

Figure 3-21 is a color-coded polarization map for the scan program shown in figure 3-20. It shows the percentage of the total light that is polarized. Regions near the Gegenschein and in the Milky Way show the smallest amount of polarization.

Weinberg and associates report that the polarized brightness of the zodiacal light at intermediate elongations (i.e., when the apparent angle from the Sun is near 90°) has the same color as the Sun. Since the total brightness also has the same color as the Sun, the degree of polarization is independent of wavelength between 4000 and 8200 Å. This suggests that the zodiacal particles near the Earth's orbit are substantially larger than these wavelengths, in agreement with the results of discrete particle detectors and of lunar cratering studies, which find the size of the particles to be primarily in the range of tens to hundreds of micrometers in radius.

 


Figure 3-20. [Left] Color-coded brightness map of half a hemisphere of sky opposite the Sun (red filter, 710-nm wavelength). The positions of the galactic equator and the ecliptic are indicated by G and E, respectively, and the lighter colored vertical band corresponds to the Milky Way. Figure 3-21. [Right] Color-coded map showing the percentage of total light that is polarized (same scan program as in fig. 3-20). Link to a larger picture.

Figure 3-20. [Left] Color-coded brightness map of half a hemisphere of sky opposite the Sun (red filter, 710-nm wavelength). The positions of the galactic equator and the ecliptic are indicated by G and E, respectively, and the lighter colored vertical band corresponds to the Milky Way. Figure 3-21. [Right] Color-coded map showing the percentage of total light that is polarized (same scan program as in fig. 3-20).


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Figure 3-22. Left: lines of constant gravitational potential for a small mass near two orbiting celestial bodies, one of which is comparatively large. The points L1, L2, etc., are the libration regions. Right: the Sun-Jupiter-Trojan asteroids system.

Figure 3-22. Left: lines of constant gravitational potential for a small mass near two orbiting celestial bodies, one of which is comparatively large. The points L1, L2, etc., are the libration regions. Right: the Sun-Jupiter-Trojan asteroids system.


 

Lunar Libration Regions

 

Figure 3-22, left, shows lines of constant gravitational potential for a small mass near two orbiting celestial bodies, one of which is comparatively large. The points labeled L1, L2, L3, L4, and L5 are called libration regions.

In the absence of perturbations by other celestial bodies, a small mass finding itself at rest near one of these equilibrium positions would tend to remain there. An example is the Sun-Jupiter-Trojan asteroids system. The Trojan asteroids are located near libration regions L4 and L5, as shown on the right in figure 3-22.

A number of ground-based observers have studied the L4 and L5 regions associated with the system Earth-Moon-particle. If small particles had accumulated in these regions, their presence might be detected by "excess" brightness from reflected sunlight. Since the light of the night sky is very faint and nonuniform, it is very difficult to identify the origin of localized patches of "excess" brightness. Both positive and negative results have been obtained in studies from the ground, where the L4 and L5 regions must be observed at large scattering angles.

At smaller scattering angles, the brightness from any material in these regions would be greater than that seen at the larger angle. Observation at these angles (near the Sun) is possible only from above the Earth's atmosphere. The brightness of the L4 and L5 regions at small Sun-region-Skylab angles was measured on photographs taken with the Skylab coronagraph (S052).

The photographs were examined by Robert MacQueen, of the High Altitude Observatory in Boulder, Colorado, the principal investigator of the coronagraph experiment, and by Alison Hopfield, of the Princeton Day School, Princeton, New Jersey. Miss Hopfield's suggestions that the lunar libration regions be studied was one of 25 experiments that were proposed by high school students and selected for the Skylab student experiment program. No excess brightness and hence no dust clouds could be distinguished against the solar coronal background in the photographs. An upper limit to the libration cloud radiance of 2.5 x10-11 of the mean radiance of the solar disk was determined. When this upper limit was combined with past measurements of the back-scattered radiance of the libration region, certain candidates for the nature of the interplanetary dust could be eliminated. It was calculated that the radiance contrast of a possible libration cloud, composed of remaining candidate materials, would be maximum at about 30° from the Sun. However, that location was not within the field of view of the coronagraph. Thus the observations serve as a guide for future ground-based or spaceborne experiments.


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