At about 100 km above the Earth's surface the magnetosphere gradually begins. It contains ions and electrons whose density is very low and whose behavior is strongly influenced by the Earth's magnetic field. The upper boundary of the magnetosphere is not spherical. On the dayside it is about 60 000 km above the Earth; on the nightside it is on the order of a million kilometers.
The particles within the magnetosphere have various origins and some have complex histories. Among those observed by instruments on Skylab were galactic cosmic rays, trapped particles probably originating in the solar wind, and neutrons formed by the reaction of high-energy particles with the atmosphere. In addition, there were secondary particles produced by high-energy particles striking the structure of Skylab itself.
Skylab's orbit was entirely within the magnetosphere (fig 5-1).
A magnetospheric particle experiment (S230) was designed by Don Lind of the Johnson Space Center and Johannes Geiss of the University of Bern, Switzerland, to determine the numbers and kinds of atomic particles impinging on Skylab and, from their isotopic abundance, to gain insight about their origin.
Figure 5-2 shows a cuff-shaped collector used to measure the fraction of various atomic species within the total flux of magnetospheric particles. It carried strips of thin aluminum and platinum foils mounted on fabric. Some strips consisted of a single metal; others were multilayered composites of both metals. The foils trapped the energetic atoms that struck them and were returned to Earth for analysis.
The collectors were mounted on an external truss of the Skylab structure (fig. 5-3). The foils were able to trap particles approaching from almost any direction.
Collector assemblies were stacked so that the outer one collected while protecting the inner one. At intervals....
....the astronauts removed outer collectors, exposing the inner ones. In this way a time series of collections was obtained. The mounting spools (fig. 5-4) were designed to protect the delicate foils. The exhaust plumes of the Apollo spacecraft attitude-control thrusters could have damaged the foils during maneuvers if the flanges of the spools had not shielded the collectors. In addition, shields clipped on the edge of the spools covered portions of the foils at specified times. These shields were used as "shutters" to limit exposure to certain areas of the foils.
After the metallic foils had collected particles for the planned period, they were brought back to Earth for analysis in an extremely sensitive mass spectrometer at the University of Bern (fig. 5-5). Sections of the foils were removed and heated to drive off the trapped particles. All extraneous atoms were discounted, including those from a thin layer of contamination, seen as a colored film on the collector. (The contamination environment of Skylab is described further in Chapter 7.) Specifically, the trapped noble gases-helium, neon, and argon-were carefully measured because the relative abundances of their various isotopes are sensitive indicators of the origin of the particles.
Results of the experiment indicated, for example, that the majority of helium atoms impinging on Skylab with energies greater than 3 keV are transported to the Earth by the solar wind, and are accelerated in the magnetosphere. This conclusion is supported by the recognition that the 3He/4He ratio in the solar wind is greater by a factor of 300 than that in atmospheric helium and by the finding that the collected helium has nearly the solar wind ratio.
Also from measurements on neon, Lind and Geiss detected, for the first time, isotopic fractionation in the upper atmosphere. This process occurs because a lighter isotope of neon (20Ne) tends to diffuse higher in the atmosphere than does a heavier isotope (22Ne).
The two square test patches shown on the collectors are sections of foil bombarded with a known number of particles before flight. Any loss of these particles as a result of solar heating would have been detected, and the trapped particle data would have been corrected accordingly. However, no such losses were found.
Neutrons are somewhat more difficult to detect than energetic protons, nuclei, or electrons, largely because they have no electrical charge. This absence of charge limits the interaction of neutrons with matter. They lose no energy through ionization or atomic excitation as they pass through matter and are thus able to penetrate matter much more easily than can charged particles with the same energy.
 A free neutron is not a stable particle; it decays with a half-life of 11 min into a proton and an electron. Therefore, the vast majority of the neutrons detected in Skylab must have been generated either within the spacecraft or within the Earth's atmosphere. These neutrons are products of nuclear reactions initiated by primary radiation striking the spacecraft or the atmosphere.
When an energetic neutron collides with a proton (i.e., a hydrogen nucleus), there is a high probability that the neutron will exchange substantial momentum with the proton. Since human beings are largely composed of hydrogen-rich compounds such as proteins, fat, and especially water, neutrons passing through the human body have a substantial probability of reacting with it. Therefore, a knowledge of the ambient neutron fluxes within a manned space station is essential in order to assess the total radiation dosage encountered by the astronauts.
Neutron-flux measurements are important for other reasons. Interactions by high-energy neutrons can damage film and sensitive experimental equipment. Furthermore, neutrons are a complicating factor in sensitive X- and gamma-ray astronomy observations since their potential interactions with nuclei can result in secondary emissions.
The measurement of neutron fluxes inside Skylab was an experiment proposed by Terry Quist, a participant in Skylab's high school student program. The objective was to measure the neutron fluxes present in Skylab, to determine the energies of the neutrons and to establish the origins of the particles observed.
The experimenter chose to detect neutrons with a "solid-state track recorder." In this technique, a target material and a recording material are used side by side. The target materials are foils of metals that have a relatively large neutron cross section; that is, they present a good target for the particles. When a neutron strikes the nucleus of an atom in such a foil detector, the target nucleus is fissioned into fragments. The recording medium, in contact with the detector foil, is typically a dielectric material such as natural muscovite mica or a manufactured polycarbonate material (e.g., Lexan or cellulose triacetate). The fission fragments from the detector foils pass through the recording sheets, disrupting the polymer chains in the plastic or disturbing the crystal lattice in the muscovite. In either case, etching with appropriate chemicals makes the fission tracks visible for counting under a microscope.
Figure 5-6 shows tracks from fission in a uranium foil recorded in muscovite and developed by etching with hydrofluoric acid. The energy range of the neutrons was differentiated into thermal, intermediate, and fast by means of a multiple-foil dielectric assembly shown in figure 5-7. The target foils were made of boron, bismuth, thorium, and uranium. The recording media were cellulose triacetate and muscovite mica. Each detector was activated by withdrawing an aluminum shield, shown on the right, which was interposed between the target foils and the recorders.
Ten of these detectors were deployed by the first Skylab crew at various locations throughout the space station (see, for example, fig. 5-8). Four of the detectors were returned to Earth by the first Skylab crew. Analysis by Terry Quist, under the supervision of Donald Burnett of the California Institute of Technology, indicated that....
....the neutron fluy was much higher than had been expected. As a result, one of the four returned detectors was refurbished for the third mission. Ultimately, all detectors were returned, four having been exposed to the Skylab environment for 24 days, six for 251 days, and one for 81 days.
The high neutron fluxes observed were attributed to secondary neutrons produced by bombardment of Skylab by charged particles (mostly protons) from the Van Allen belts. Although not nearly as energetic as cosmic rays, these protons are still energetic and numerous enough to generate the secondary neutrons observed in the space station by nuclear reactions with its structural materials.
A different technique for measuring the neutron flux aboard Skylab was used by Gerald Fishman of the Mar-....
 ...-shall Space Flight Center. Fishman's method used specimens of stable elements whose nuclei readily react with neutrons. In each case a radioactive nucleus is formed which decays with a known half-life by the emission of a gamma ray of known energy. Measurements of the gamma rays can thus be used to calculate the flux of neutrons to which the samples of stable elements were exposed.
Four "activation packets," containing samples of tantalum, nickel, titanium, hafnium, and cadmium-covered tantalum, were launched with the third Skylab crew. Figure 5-9 shows the samples contained in cloth bags. They were deployed in the Skylab orbital workshop in a film-vault drawer, on the outside of a water storage tank, on the dome of the forward compartment of the workshop, and on the outside wall of the sleep compartment at the location shown in figure 5-10. The packets remained in place for 76 days and were then returned to Earth. In addition to these specially designed activation packets, analyses were made of a large sample of stainless steel from an experiment container and samples of tantalum and indium antimonide from other Skylab experiments.
Some of Fishman's activation packets were designed to enter into nuclear reactions with the neutrons, while others were expected to react with the protons that penetrated Skylab's wall. The amount of activity traceable to each of these components was further studied by the differences in behavior at various locations within the space station. The production of radioisotopes by proton reactions should have been greatest in locations with least shielding from the outside, since the protons came from the exterior. Radioisotopes produced by neutron reactions would logically show the reverse behavior, since the samples would be more heavily irradiated by neutrons in the densely shielded locations, assuming such neutrons were generated almost entirely by interactions in the space station material itself. The plot of activity against shielding, figure 5-11, shows the expected pattern. The proton-induced reactions fell off with increasing shielding, while the neutron-induced radioactivity increased in locations of high-shield density.
A gamma-ray spectrometer system was used to measure the gamma-ray decay rates of all the samples over an extended period of time after their return to Earth. All induced radioactivities were very weak, as expected. Nevertheless, the neutron fluxes derived from these measurements agreed well with those obtained from Quist's experiment. In essence, Fishman's results confirmed that the neutron environment of a space station the size of Skylab, and in its orbit, is dominated by high-energy (fast) neutrons and that their flux is higher than had been predicted.
Together, the two neutron experiments have led to important conclusions for the design of future space stations. First, the flux of neutrons observed in Skylab was much too high to be attributed to solar neutrons, Earth albedo neutrons, or even neutrons induced by cosmic rays in space-station materials. The other conclusion is that the higher neutron flux must come primarily from bombardment of space-station material by trapped protons in the Van Allen belt. However, the neutrons did not pose a biological hazard to the crew, nor did they produce significant film fogging.
Cosmic rays are atomic nuclei moving at velocities approaching the speed of light. The commonest are the nuclei of hydrogen and helium. The abundance decreases generally with mass except for elements close to iron in....
 ....the periodic table, which are present in excess. The heaviest are among the most interesting. The effects of cosmic rays are due primarily not to the number of particles but rather to the prodigious individual particle energies, in some instances up to 1020 electron volts. This is an amount of energy per particle that, if converted to mechanical work, could lift this book a meter or so. In fact, the total energy contained in cosmic rays in the Milky Way may be comparable to the energy stored in the galaxy's magnetic fields or to the turbulent kinetic energy of its gaseous clouds. Astrophysicists are presented with an intriguing puzzle in trying to determine the natural processes that can concentrate so much energy in such a small amount of matter.
One example of a prodigious energy source on an astronomical scale was noted as early as 1054 AD by Chinese astronomers who witnessed the cataclysmic death of a star, a supernova explosion. The remnant of this explosion is still conspicuous in the sky today as a huge, irregular shape some 3500 light-years away, the Crab Nebula. It is widely believed that each supernova event produces cosmic rays that speed out into the galaxy, where they may wander for millions of years, deflected first one way and then another by magnetic fields.
Skylab performed two important cosmic-ray experiments; the first used nuclear emulsion film stacks to track heavy atomic nuclei close to iron in atomic mass. The other used large plastic sheets to record the rarely encountered, ultra-heavy particles close to uranium in atomic mass.
An interesting feature resulting presumably from the later stages of the formation of elements (known as nucleosynthesis) is a large relative abundance of iron nuclei. This is referred to as the iron peak in the universal elemental composition of matter. An iron peak is prominent in the galactic cosmic ray population. Furthermore, iron is often observed among the energetic nuclei emitted from solar flares.
The abundance of iron ejected in solar flares can only partly be explained by the hypothesis that the Sun is a second-generation star (i.e., one formed from the debris of heavy elements of a previous supernova explosion). Preferential acceleration of heavier nuclei such as iron may also be at work in some of the flares.
The galactic cosmic rays approaching the Earth still possess an iron peak, but they also have a far larger than universal abundance of elements just below iron in atomic number. The breakup of cosmic ray iron by collisions with interstellar matter during millions of years of traveling through the galaxy could contribute importantly to the buildup of elements lighter than iron. Analysis of Earth-arriving cosmic rays for elemental constituents in this part of the periodic table was a prime objective of the Skylab nuclear emulsion experiment (S009). The experiment was prepared by Maurice M. Shapiro and colleagues at the U.S. Naval Research Laboratory, Washington, D.C., and used a refined version of the nuclear emulsion detection method first used in high-altitude balloons and later employed on a Gemini space mission.
The nuclear emulsion detector flown on Skylab consisted of stacks of special photographic film. The emulsion layers differ from ordinary photographic film principally in being thicker and having a higher density of silver halide grains. On passing through the stack, a cosmic ray particle activates the tiny silver crystals in its path and makes them developable. The silver ions become specks of metallic silver after development. The tracks are typically several micrometers or less in width;
...hence magnifications greater than 1000x are commonly used to examine their details. Figure 5-12 shows some typical microscopic tracks formed by various cosmic ray nuclei moving with relativistic velocities through such emulsions. The densest track was left behind by a calcium nucleus, the next one by a nitrogen nucleus, and the thinnest track by a hydrogen nucleus.
The Skylab nuclear emulsion package (S009), shown in figure 5-13, was activated by the crew of the third mission. It remained inside Skylab for 38 days at a location chosen to give it an unobstructed view of deep space througha section of the skin of the docking adapter purposely milled down to minimize the amount of matter between the incoming cosmic rays and the package.
 Two operational constraints had to be met during the experiment. First, the exposure faces of the emulsion package had always to point away from the Earth to avoid collecting albedo particles from its atmosphere. This was accomplished by a daily readjustment of the mechanism by the crew to compensate for the slowly changing tilt of Skylab's orbit relative to the Earth-Sun line.
Second, the package was built in two parts and hinged like a book (fig. 5-14). This permitted a timer to open and close the package. Data were collected when Skylab was in the equatorial zone between 30°N and 25°S, where only the highest velocity cosmic ray nuclei, of interest to the experiment, could penetrate the Earth's magnetic field to Skylab's altitude and leave tracks in the detector package. At higher latitudes (both north and south), where the magnetic field bends closer to the Earth, heavy-but slower-nuclei also penetrated to the space station's altitude and left tracks in the package. Since these nuclei were of lesser interest, the package was closed in those regions. Sufficiently penetrating particles recorded at those latitudes were recognizable because they crossed the two faces of the closed package and could be subtracted out during analysis.
Package closure south of 25° latitude also provided partial shielding from the Van Allen belt radiation, which penetrated Skylab's skin as Skylab passed through the South Atlantic anomaly.
At the end of the mission, the nuclear emulsion package was returned to Earth for development and analysis of the recorded tracks. The large volume of data obtained (estimated to be more than 4000 tracks) added a great deal of knowledge about the composition of cosmic rays and provided insight into the cosmological significance.
Although the iron peak is a conspicuous feature of the cosmic rays that impinge on Earth, most cosmic rays....
.....are protons. Helium is the next most abundant element. Heavier nuclei become progressively rarer with increasing mass. Skylab was the first vehicle that was capable of deploying a large enough detector, for long enough to record a statistically significant sample of ultra-heavy nuclei and then return the experiment to Earth for analysis.
The second Skylab cosmic ray experiment, searching for ultra-heavy cosmic rays, was designed by P. Buford Price and E. K. Shirk of the University of California at Berkeley. They have had considerable experience in the detection of heavily ionizing cosmic ray particles in meteorite fragments and have pioneered in the use of plastic detectors, such as Lexan, for experiments flown first in balloons and later in space. After exposure to energetic radiation, the Lexan is treated with a chemical that preferentially etches tracks of radiation damage left behind by heavy nuclei.
Figure 5-15 is a photomicrograph of etched pits showing the paths of ultra-heavy cosmic rays such as those detected aboard Skylab. It shows three different penetration depths, corresponding to cosmic rays with atomic numbers of approximately 75, 8O, and 96.
The Lexan detectors for the transuranic cosmic ray experiment were arranged in 36 stacks with a total detector area of 1.3 m2. A harness, with pockets containing the detectors, was attached to the wall of the workshop, as shown in figure 5-16.
One module was returned to Earth on September 25, 1973, after a 119-day exposure. At the end of the final mission, 34 of the modules were brought back to Earth, having had a 253-day exposure. One was left behind in the space station's docking module, conveniently located for retrieval by any astronauts who might someday visit Skylab.
The sheets of Lexan in the returned modules were chemically etched and then scanned under a low-power microscope for tracks of cosmic rays with atomic number Z>65. Approximately 150 separate events (etched pits) were located. Three events were found for which the best estimates of charge correspond to those of transuranic elements (Z [greater or equal to] 94). This is 4 percent of the 77 events from nuclei in the range 74 [smaller than or equal to] Z [smaller than or equal to] 87. No nuclei with atomic number greater than 110 were detected.
The transuranic cosmic ray experiment also permitted the determination of other abundance ratios of astrophysical significance without the need to correct for atmospheric interactions. The ratios thus obtained shed...
....light on theories of the birth of elements in the universe. For example, the elements close to platinum (Z= 78) in the periodic table are generally formed in processes involving rapid neutron capture, such as in high-neutron-density reactions in supernovae. Those close to lead (Z= 82) are associated with slow neutron capture reactions, more typical of helium-burning phases (e.g., in red-giant stars). The thorium-uranium ratio is an indication of origin since it should vary from less than I in newborn stars to about 3 in older systems such as the Earth.
The observation of visual phenomena termed light flashes during manned space flight was first reported by Edwin Aldrin during the Apollo 11 mission. Similar observations were made on subsequent Apollo missions. No flashes were reported during earlier Mercury or Gemini flights, previous Earth-orbital Apollo missions, or the circumlunar flight of Apollo 8. Why no flashes were observed before Apollo 11 has not been explained. The most likely reason appears to be that the eye must be adapted to darkness and the observer must be reasonably relaxed and free from distracting activities. These conditions, by and large, did not obtain in flights pre
 ....ceding Apollo 11. A special effort was made to observe them on Skylab.
Although the cause of the flashes is not certain, the interaction of heavily ionizing particles with the visual apparatus, most probably the retina itself, appears to be the most likely explanation, as illustrated in figure 5-17.
If cosmic rays were the cause, there would be a latitude effect on the light-flash rate for an observer orbiting Earth because of the geomagnetic cutoff of charged particles and the steep energy spectrum of primary cosmic ray fluxes. Near the Equator, only cosmic particles with very high energy penetrate Earth's magnetic field to Skylab's orbital altitude; nearer the magnetic poles, particles of much lower energies reach comparable altitudes because the magnetic lines bend toward the Earth.
The primary objective of a study designed by R. A. Hoffman of the Johnson Space Center was to investigate the frequency and character of visual light flashes in near-Earth orbit as Skylab's path passed from northern to southern latitudes. Because the path periodically passed through the South Atlantic anomaly, a well-known region where the Van Allen belt bends closer to the Earth, another objective was to determine whether light flashes would occur as a consequence of the many energetic particles trapped there.
During the last Skylab mission, two separate lightflash observation sessions were conducted by astronaut William Pogue. Two particular orbits were selected to provide data on the effects of both latitude and the South Atlantic anomaly. The first session provided the best latitude conditions, but the space station passed only through the edge of the anomaly. During the second session, it passed through the center of the anomaly but did not achieve optimal geomagnetic latitudes. During each session, the astronaut, while in his "bed," donned a blindfold, allowed 10 min for adaptation to darkness, and then began to voice record his observations of each flash experienced before falling to sleep.
Figure 5-18 depicts the light flashes observed as the Skylab passed from the northern to the southern latitudes in its orbit. Each point represents the occurrence of a flash. Because of the high frequency of flashes detected during one of the two passes through the South Atlantic anomaly, the numbers shown for that case represent the number of flashes observed per minute.
Figure 5-19 represents the flashes observed before and after equatorial passage on session I of figure 5-18, a plot that roughly indicates latitude position. For example, Skylab was at the highest magnetic latitude approximately 20 min before and 30 min after equatorial passage. The calculated cosmic ray fluxes at the latitudes corresponding to the times on the abscissa are also shown.
Two conclusions are apparent. The occurrence of light flashes correlates with the flux of cosmic particles. However, the greatly increased number of flashes in the South Atlantic anomaly is not fully understood; they must result from either trapped protons in this region or possibly from trapped heavier nuclei.
The first United States satellite, Explorer 1, carried an experiment prepared by James Van Allen to measure cosmic radiation. However, the primary result of the experiment was the discovery of the belts of trapped particle radiation around the Earth. These radiation belts subsequently became the subject of intense research. Features of the belts now have become so well known that they have acquired a name, e.g., the South Atlantic anomaly. Still, many aspects of the energetic particle flux above the atmosphere remain to be understood. The fact that samples could easily be returned to Earth for analysis after long exposures in space allowed Skylab experimenters to make significant advances in this important, continuing topic of investigation.