During the first U.S. orbital flight of Mercury 6, astronaut John Glenn observed particles outside the spacecraft which he called "fireflies." They were later identified as condensed ice crystals from the small hydrogen peroxide rockets used for altitude control.
Since this early observation, it has been established that a spacecraft creates a small cloud of debris or contamination around itself. The cloud is formed from gas released by external material surfaces, minor vehicle leaks, control rocket firings, and vented waste materials. It can cause degraded "vision" for any viewing instruments that must observe through it and can produce a deterioration of external surfaces by the deposition of contaminants.
Figure 7-1 shows a darkening of Skylab's external white-painted surfaces, which resulted from such contamination. The contaminants are characteristic of gases released from nonmetallic materials.
Skylab, the first true space laboratory with an extended flight time (several months), provided an opportunity to study the contamination cloud and the effect of its surface deposition over a long period. The space station contained monitors of external contamination, experiments to evaluate the effects of the space environment on materials, and experiments to investigate contaminant deposition on typical external materials in the space station.
Contamination information was also obtained, as secondary data, from instruments designed for other purposes. Furthermore, the astronauts visually observed contamination both inside and outside Skylab.
Skylab had several sensitive optical instruments that were susceptible to contamination. Design criteria were generated before the mission to select materials that would produce minimum contaminants in space. Mathematical models were developed and laboratory tests were made to simulate the orbital environment of the space station. On this basis, optimum locations were chosen for experiments and vents, covers were installed over susceptible apertures and windows, and operational procedures were generated to minimize contamination of critical surfaces. During the mission, these procedures permitted controlled venting activities and the performance of experiments with minimum possibilities for contamination.
The deposition of contaminants was measured with a device called a quartz-crystal microbalance. Two quartz crystals vibrate mechanically, driven by electronic circuits. The frequency of vibration depends on the effective mass of the crystal. The addition of foreign matter to the surface of a crystal increases its effective mass and decreases its frequency of vibration. The crystals are as nearly identical as they can be made and vibrate at  almost the same frequency. An electronic circuit compares the frequencies of the two crystals and generates a signal that depends on the difference. One crystal is exposed to the environment being studied, the other is at the same location (same temperature, etc.) but shielded from contaminants. Under these circumstances, the signal produced depends entirely on the mass of material deposited on the exposed crystal.
The device is extremely sensitive. An idea of its sensitivity is conveyed by the fact that a monolayer (a layer of contaminants one atom thick) will produce a signal (frequency difference) of 1 Hz, which is quite substantial with modern techniques. Figure 7-2 shows the device.
In all, there were six quartz-crystal microbalances placed in critical locations, including two mounted on the solar observatory sunshield looking directly at the Sun and four located beneath the docking adapter, near the Earth resources instruments (fig. 7-3). Of these, two were pointed away from the vehicle and the other two were pointed along the vehicle, one toward the command and service module and the other toward the orbital workshop.
Data from these devices were telemetered to the ground and were monitored throughout the mission. If contamination rates rose to a level that would interfere with a particular experiment, that experiment was postponed until a more suitable time. In addition, during the time that the externally mounted solar observatory or airlock instruments were operating, controlled vents were closed. Conversely, the optical surfaces were.....
....covered by doors except during periods when they were in use.
The time history of mass accumulation on the quartz-crystal microbalances is shown in figure 7-4. Crystals facing along the Skylab's longitudinal axis registered the highest contamination rates. It is believed that structures in the line of sight of these crystals released gases and caused the deposition. Crystals that faced away from the vehicle collected deposits presumably consisting of contaminants which originated from the space station and whose molecules were backscattered by the atmosphere around the space station. Gases released from wire bundles in the field of view of these crystals were also a source of contamination. Early in the mission, the crystal facing the command module was contaminated by the steering-rocket exhaust. The two crystals mounted on the solar telescope were much warmer than the others and did not show any deposition, because the contaminants did not adhere well to warm surfaces.
For temperature control, large areas of the external surfaces of Skylab were painted with a special white thermal-control paint. These surfaces were photographed by the first Skylab crew from the command and service module, before they boarded Skylab. At that time all the surfaces were still pristine white (fig. 7-5). Later, it was reported during a spacewalk that some brownish discoloration was visible on the underside of the solar observatory's solar panels. Photographs taken by the following two crews showed that this surface darkening increased with time (fig. 7-6). The brownish color appeared darkest near the hatch through which the astronauts went in and out for spacewalks (fig. 7-7). The airlock, in which the spacesuited astronaut went through decompression immediately before his walk in space, normally maintained cabin pressure. During a spacewalk the materials in the airlock were subjected to space vacuum and these apparently emitted gases that....
.....condensed on surfaces near the hatch, causing the discoloration.
Discoloration was particularly heavy along seams (fig. 7-8) in the airlock's meteoroid curtains, as though material had leaked from inside them. During the mission, a leak developed in the vehicle's coolant system. The molecules had an affinity for surfaces covered with silicone-based paints. In the presence of solar radiation, and water vapor in the external Skylab environment, a nonvolatile material resembling silica gel formed on the surfaces. The ultraviolet radiation from the Sun caused a color change from clear to golden brown in this material.
Some darkened spots appeared on painted surfaces of the sunshield, the sides of the solar observatory, and the command module. These spots were probably localized contaminants introduced during touch-up work or other ground handling and became visible as they underwent chemical change in the space environment. Most organic or silicone materials will polymerize in the presence of ultraviolet or energetic-particle radiation, and such polymerization affects the optical properties, generally resulting in a brownish appearance. Surfaces that are partially shadowed from sunlight show distinct.....
.....differences in coloration between the exposed and the shadowed areas. Either the contaminants on the surface in the shadow remain unchanged or they migrate along the surface until they are exposed, whereupon they become immobilized.
Despite the visible contamination on the sunshield paint, neither the quartz-crystal microbalance on the surface of the solar observatory nor the optics of its instruments detected the presence of any contaminant. The solar observatory's optics were covered when not in use (fig. 7-9) and had an acceptance angle for contamination 100 times less than that of the quartz-crystal microbalances. Apparently, the contaminants that deposited did not readily adhere to warm bare-metal surfaces but had a much greater affinity for cold painted surfaces, whose molecular structure resembled that of the contaminants.
The fact that Skylab was of long duration and its crew could return samples made it easy to experiment with materials and to study the effect of the external environment on these materials.
A thermal control and polymeric film experiment (D024) was conducted for William L. Lehn of the Wright-Patterson Air Force Base, Ohio (fig. 7-10). Samples of thermal control materials and of polymeric films were exposed to the environment outside the space station. The thermal control materials are used on space equipment to control its temperature by controlled absorption and re-radiation. The polymeric films studied were of the type used for thermal insulation of spacecraft. On each of the three manned Skylab missions, samples of thermal-control materials and polymeric films were deployed externally and returned by the crew for laboratory evaluation.
The experiment (fig. 7-11) used four sample panels: two duplicate sample trays each containing 36 individual sample buttons coated with 27 different thermal control coating materials, and two duplicate sample trays each holding eight different polymeric film specimens. The four specimen trays and two hermetically sealable return containers were mounted on the exterior (fig. 7-12) of Skylab and were retrieved during a scheduled spacewalk by the crew.
Figure 7-13 shows four sets of thermal control coatings: an unexposed set not flown in Skylab (left) and one set returned from each of the three manned Skylab missions. The specimens returned with the first crew (second from left) were exposed for approximately 550 hr (35 days) of solar radiation. The specimens returned with the second crew (third from left) were retrieved after approximately 2040 hr (131 days) and returned to Earth for analysis. Severe contamination on the first two manned missions tended to obscure the desired
.....measurements. The experiment was therefore repeated on the last flight. These specimens (extreme right) show the effects of approximately 1150 hr (74 days) of solar exposure, but no exposure to fly-around by the command and service module.
Since this experiment was located near the EVA hatch, the sources of the contaminants are most likely those discussed previously: outgassing from the materials in the airlock and vapor from leaks in the cooling system. A postmission analysis of the contamination indicated the presence of organic silicon compounds and traces of phosphorus and other elements. The vast variety of silicone materials used on the spacecraft, together with the fact that the space environment causes chemical changes in the contaminant deposits, makes it impossible to identify the specific source. A significant finding in the analysis was the total lack of nitrogen. This rules out the hypergolic thrusters as sources because they operated on monomethylhydrazine nitrate.
Clearly defined "shadow" patterns (fig. 7-14B) on the returned polymeric film trays gave a clear indication of the contamination and the degree of solar orientation that was maintained by Skylab. These "shadow" patterns were also observed on the containers and the.....
.....thermal control coatings returned to Earth. The apparent absence of contamination in the shadowed area of the returned samples indicated that the contaminants either did not polymerize, were able to deposit and reevaporate, or could migrate along the surface until irradiated and fixed by sunlight.
Curves of reflectance versus wavelength (fig. 7-15) were based on optical measurements made before and after the flight. The samples of thermal control coatings generally showed gross changes in solar absorbency explicable only on the basis of contamination and solar degradation.
Many instruments expose optical surfaces to the orbital environment. An experiment was conducted in which samples of various optical surfaces on various materials were deliberately exposed to the orbital environment. Although the purpose of this experiment, designed by Joseph A. Muscari, of the Martin Marietta Aerospace Corporation, Denver, Colorado, was to study the behavior of optical surfaces in the orbital environment, it furnished valuable data on the deposition of contaminants. Figure 7-16 shows the exposed samples. Twenty-five different kinds of optical surface, such as transmissive windows, mirrors, and diffraction gratings, were used on a total of 248 optical samples. These were mounted on an extendable boom and exposed to the space environment through the antisolar airlock of the orbital workshop during the first manned Skylab mission. The optical surfaces had varied characteristics and exposure durations. The entire experiment lasted 46.5 hr. From the extremely small amount of contaminant deposition observed on the samples, it was concluded that the antisolar side of the vehicle was very clean.
Preflight and postflight laboratory measurements of optical transmission from 0.28 to 15 000 nm and reflection from 0.834 to 21000 nm were made to determine the effects of surface contaminants. Computer programs were developed to determine the variations in deposition of contaminants due to surface material, solar radiation, period of exposure, and direction of exposure. Spectral and polarization and spectral measurements were used to determine the thickness and composition of the contaminants. Only trace amounts of surface contamination were found, near the sensitivity limit of the laboratory instrumentation. The maximum thickness was less than I nm. Some indication of silicone was present in the spectroscopic data, but the data obtained were inconclusive.
The results of this experiment were unfortunately compromised by the initial launch problem. The experiment had originally been planned for operation through the solar airlock of the orbital workshop within the first 4 days after the unmanned Skylab launch. The first manned launch was delayed approximately 10 days, which caused the experiment to be performed later than desired, so that no observations were obtained during the early phase of outgassing. The parasol was deployed through the solar airlock, preventing the airlock's use for experiments. The experiment was therefore deployed on the other, "cleaner," side of the space station.
The surfaces of equipment for other experiments mounted outside the spacecraft were examined for con-.....
.....-tamination and yielded valuable supplementary data. An experiment by Don Lind to monitor magnetospheric particle composition is described in Chapter 5. The various foils were visibly contaminated with a film of various colors, indicating different contaminant thickness (fig. 7-17). As a consequence of the visible contamination, eleven of these small foil strips were measured for total reflectance at wavelengths from 0.25 to 2.5 µm and for directional reflection with hemispherical incident radiation at wavelengths from 1.2 to 20.5 µm. The reflectance (fig. 7-18) was significantly lower than that of control foils that had remained on the ground.
The reflectance measurements were used to calculate the absorption of solar radiation by the foils. This value was used to determine the effectiveness of surfaces as thermal shields or radiators. Increases in absorption of up to 5 percent per day were measured, which would greatly decrease thermal shield effectiveness in a short time.
 Lind prepared cross sections of contaminated foils in order to measure contaminant thickness with an electron microscope. Measurements of suitable cross sections on one specimen showed thicknesses of 30 to 150 nm and on another thicknesses from 70 to 800 nm. The latter specimen was exposed to the space environment for 200 days, a period significantly longer than the exposure of the former.
Richard Tousey of the Naval Research Laboratory, Washington, D.C., was the principal investigator for a camera (S020) with grazing-incidence mirrors and used to photograph the Sun in the extreme ultraviolet and soft X-ray regions. The camera was mounted externally during a spacewalk on the third mission. It showed a decrease in transmission in its indium and beryllium thin-film filters and lost some of its data at far-ultraviolet and X-ray wavelengths. The data loss was probably caused by contamination deposited on the filters. Laboratory tests, which attempted to identify the contaminant composition, were unable to determine the species, although its characteristics were similar to those of silicone-type materials.
Another experiment (T027) designed by Joseph A. Muscari used a photometer system to determine the spatial distribution and temporal variation of the cloud of particles around Skylab (fig. 7-19). This instrument, the same photometer that was used to measure zodiacal light and Gegenschein, also measured the brightness and polarization of sunlight scattered from the contaminant cloud around the space station.
Various areas around Skylab were scanned in daylight and in darkness to separate the natural sky background from the contaminant-scattered light. A significant amount of contaminant scattering of a transitory nature was found on one observation. This may have resulted from a single bright particle moving through the field of view.
Jerry L. Weinberg, the investigator for the zodiacal light experiment, was also concerned about the effects of contaminant material from around the space station. To determine amount of optical contamination early in the mission, measurements of sky brightness in a fixed direction (95° from the Sun) were taken on June 12, 1973, using the instrument described in Chapter 3. Observations were made of 10 wavelengths and were repeated every 2 min, starting in the Earth's shadow and ending in daylight. The plot of relative intensities shows that there were levels of sky brightness in daylight only 5 percent above those at night (fig. 7-20).
Photographs of the Gegenschein were taken with a 35-mm camera, through the airlock of the orbital workshop on the third mission. Photometric analysis of the film was made by K. S. Clifton of the Marshall Space Flight Center. Considerable amounts of scattered light were observed at the edge of the frame. The phenomenon is believed to be the result of light reflected from a Skylab discone antenna into the airlock. However, at the center of the photograph the difference in light intensity between sunlight and shadow conditions was only 5 percent above the Gegenschein brightness. The results indicate that the amount of sunlight reflected by contaminant material was small. Observations of faint....
....astronomical phenomena could therefore be made from Skylab in daylight.
Figures 7-21, 7-22, and 7-23 are photographs taken through the white-light coronagraph (S052) of Skylab's solar observatory. They show dramatic effects caused
by inadvertent venting from Skylab. The photograph in figure 7-21, taken at 1027 G M T on January 21, 1974, shows a typical image of the solar corona, except that the coronal transient in the lower right side of the field is unusual. The picture is free of particulate contamination.
The next photograph, figure 7-22, taken at 0440 GMT on June 9, 1973, shows the track of a particle illuminated by sunlight as it passed across the field of the coronagraph. The particle apparently was spinning, and the brighter portions described perfect spirals on the photograph. Such uniform motion was possible because the vacuum outside Skylab offered no appreciable atmospheric drag. Instrumental vignetting caused the track's brightness to be reduced near the center of the picture. The solar coronal background is typical, with a single feature in the northeast and several streamers along the western limb.
The photograph in figure 7-23 was taken at 1408 GMT on August 2, 1973. A few minutes before this exposure, inadvertent venting of the water-condensate system occurred, producing a cloud of particles around Skylab. The photograph shows random motion of many condensed ice particles that passed through the coronagraph's 3.2° field of view. It demonstrates why contamination is of such concern to sensitive instruments. Venting was usually scheduled during space-station night, when the solar experiments were not operating and instrument covers were closed to protect the lenses. This scheduling allowed the contaminant cloud to disperse before observations were resumed. Fortunately, the lens did not suffer contamination from the inadvertent ventings.
An experiment that recorded micrometeoroid impacts is described in Chapter 3. Among other specimens, this experiment, designed by C. L. Hemenway, exposed silver samples in the space environment. Figure 7-24 shows three exposed silver slides and an unexposed silver slide used as a control. A thick black corrosion layer developed on the silver slides during exposure in the solar direction; a thinner layer, resembling tarnish, developed during exposure in the antisolar direction. Analysis by X-ray diffraction showed the black material to be silver oxide. The silver oxide layer on the solar facing sample was approximately 2 µm thick. The corrosion was so thick that it was impossible to measure micrometeoroid impact craters in the silver samples. Copper slides showed a smaller, but noticeable, amount of oxidation. The oxidation is thought to be the result of a chemical reaction with atomic oxygen in the extremely thin atmosphere at Skylab's orbital altitude. The thermal control coatings experiment, discussed earlier, exposed a thin silver coating, which also tarnished. The oxidation of bare silver and copper in the orbital environment was unexpected, but it provided valuable information on the behavior of materials.
Contamination also occurred on window surfaces and the mirror that was used for photography through the antisolar airlock of the orbital workshop.
Windows are important for both photography and visual observations. Windows for photography must be of extremely high quality; lower quality windows are adequate for visual observation. A single-pane window on Skylab was designed solely for photographing the Earth. It had a heater that maintained the surface temperature above the cabin dewpoint temperature, and no condensation formed on it. The window was kept covered when not needed for experiments. It remained clean throughout the entire mission. In contrast, the double-paned wardroom window of the orbital workshop was designed primarily for astronaut viewing. It had a heater, which was not on all the time. When the heater was off, a crazed ice condensed on the inner surface of the outer pane (fig. 7-25). It had no external cover, and critical inspection by the astronauts revealed a streaky and oil-like film on its outer surface.
A window in the command module of the last Skylab mission (fig. 7-26) turned brown and was found to have acquired a coating. After return to Earth, material was....
....scraped off for analysis. The coating, approximately 1.7 µm thick, proved to be a methyl silicone. Infrared spectroscopy revealed chemical differences from the methyl silicones used on the vehicle, such as the presence of hydroxyl and carbonyl groups, which probably resulted from photochemical processes during the 80-day exposure. The effect was noticed by the third Skylab crew after 38 days in orbit but was not observed on the earlier manned missions.
The inside atmosphere of Skylab was monitored for aerosol particles with an instrument (fig. 7-27) supplied by W. Z. Leavitt of the U.S. Department of Transportation, Cambridge, Massachusetts. A thorough survey was conducted on each mission. Readings were obtained....
....at many locations to assess sources of airborne particles and to evaluate the capability of the space station's environmental control system to maintain a clean atmosphere. Particle sizes from 1 to 100 µm and counts from 0 to 20000 particles per liter were measurable. Filters were returned to Earth for analysis.
In general, the number of larger particles remained constant throughout, but the number of smaller particles decreased with time. For the three manned missions, the particle loading in the air averaged 187 particles per liter, which indicates that the environmental control system operated efficiently. Such an atmosphere was judged to be extremely clean and comparable to that of a hospital operating room. Particles such as skin flakes, food, and fragmented and abraded materials from the space station were collected by two aerosol analyzers.
Figure 7-28 shows the filter of the ventilation mixing chamber being vacuum cleaned. Objects that were misplaced or "dropped" drifted to these inlets along with debris. The astronauts vacuumed the inlet screens as part of their regular housekeeping tasks every other day.
Figure 7-29 shows an inlet filter from the environmental control system's ventilation fan, before vacuum cleaning. Fans operated continuously and circulated the....
.....cabin air to provide a habitable environment for the astronauts.
Since the discovery of the Van Allen belt, much has been learned about orbital radiation environments from a number of Earth satellites. The long-duration Skylab provided a unique opportunity to study radiation and its effects on man and photographic films.
Before Skylab was launched, an elaborate mathematical model was constructed to describe the radiation environment inside the spacecraft. This model was needed to design the space station for astronaut safety and to determine the required film-vault wall thickness for protection against film fogging (fig. 7-30). Radiation doses from trapped protons, trapped electrons, and galactic cosmic rays were calculated. These curves show the total radiation dose rates and their contributors as a function of shielding. They are based on an idealized mathematical model of the radiation devised before the Skylab mission.
The measured flux of protons with energies higher than 50 MeV was approximately 80 percent of the predicted values, and its spectrum was more energetic than predicted; that is, there were more high-energy protons than expected.
Studies predicted the amount of fogging on all types of film, both in the film vault and in operational locations. Operational procedures, such as returning film to.....
.....the vault when not in use, were established to minimize exposure to radiation within the space station.
The film vault (fig. 7-31), located in the orbital workshop, had various thicknesses of aluminum shielding to protect various types of film. Films most susceptible to radiation damage were stored in the bottom drawers, which had the thickest walls. Postflight analysis showed that the actual radiation levels were within approximately 20 percent of the predicted levels, a value considered extremely accurate for a vehicle with such a complex geometry and radiation environment as Skylab.
Van Allen belt dosimeters (fig. 7-32) were used to monitor radiation levels inside the orbital workshop. The device consisted of two ion chambers. The first was covered with an aluminum shield to simulate a layer of skin and tissue 2 in. thick; the second, unshielded, chamber provided information on the amount of radiation impinging on skin. In addition to this dosimeter, the astronauts had personal radiation dosimeters, which they carried whenever they were wearing spacesuits. A radiation survey meter was also on board for measurements throughout the space station, and passive radiation dosimeters were placed in specific locations, such as....
....the film vault. These passive devices were returned to Earth to aid in updating the localized radiation-level models to be used in the future design of large space stations. The Van Allen belt dosimeter and the radiation survey meter read approximately three-fourths of the conservatively predicted radiation dose rates, and the readings were in extremely good agreement with each other.
The many experiments of Skylab recorded data on several miles of photographic film, covering spectral ranges from the X-ray to the infrared. Film degradation occurred from two distinct causes, high temperatures and radiation.
The high temperatures occurred early in the Skylab mission, before the installation of the parasol. The effect.....
.....of increased temperature (although dependent on film type) is to cause a loss of contrast. Radiation effects were anticipated, and vaults were designed to minimize film fogging. For most films, the ratio of actual to predicted fog ranged from 0.6 to 1.0 in spite of radiation levels sometimes being higher than anticipated.
Figure 7-33 is a plot of density versus log exposure for a typical film, showing control sample, heating effects, and radiation effects.
An experiment that measured radiation in the command module on the first manned mission used active and passive radiation dosimeters. The dosimeters (fig. 7-34) were designed to give a time history of radiation exposure to the astronauts and to define radiation doses behind various wall thicknesses. In figure 7-34, the spherical object is the active dosimeter, and the red and white striped cylinder below it is the passive instrument. The investigators for this experiment were Andrew D. Grimm, Joseph F. Janni, and Glenn C. Ainsworth of the Kirtland Air Force Base, New Mexico.
Comparison of the results of this ionization chamber with those recorded by the orbital workshop's Van Allen belt dosimeter yielded predictable agreement. Any differences in the measured levels can be explained by differences in metallic shielding for each instrument; the command module provided more shielding than did the thin-walled orbital workshop.
Five passive-dosimeter units were located at different shielding locations (fig. 7-35); their readings over the first 28-day mission ranged from 0.96 to 1.67 reds. Such doses were routinely experienced on earlier manned....
....spaceflights and did not produce any measurable biological effects in the astronauts.
Dose-rate curves are shown in figure 7-36 for two typical orbital revolutions passing through the South Atlantic anomaly. Figure 7-37 shows the ground trace of these orbits on a map that also shows the regions in which Skylab encountered the Van Allen belts. Radiation dose rates encountered by Gemini 4, in 1965, were lower than the Skylab measurements, as expected. The former were taken at a lower altitude (325 km) and a 30° inclination and were 0.02 red/day. The Skylab measurements were made at 435 km and a 50° inclination and were 0.05 red/day.
On the third day of the first manned Skylab mission, the fire alarm sounded. The astronauts quickly investigated the entire space station to ascertain the cause and found nothing. Analysis showed that this was the first occasion that the vehicle had passed through the highest radiation region of the South Atlantic anomaly. The increased radiation level had triggered the detector. The alarm was disabled before later passages through the region.
As a result of the knowledge gained from Skylab in both the contamination and radiation disciplines, future manned space vehicles such as the Space Shuttle will be better designed to protect their crews, supplies, and instruments. The first orbital flight test of the Shuttle will carry a package of instruments, the Induced Environment and Contamination Monitor, which is a direct outgrowth of Skylab instrumentation.
In a similar manner, of course, the experience and knowledge generated by Skylab and portrayed in part in the previous chapters will be a broad foundation for the coming Shuttle era. The first Shuttle mission carrying a Spacelab produced by the European Space Agency is scheduled, in fact, to emphasize atmospheric measurements. The second Spacelab mission will emphasize astrophysical observations, for which the Skylab experience surveyed in this volume again provides an informative predecessor. Skylab has thus earned a place in the history of astronomy and space sciences.