Service mission for the Hubble Space Telescope. The films that are clearly torn to the left of the astronauts are the outer layers of the thermal blankets. Many such areas had to be covered with new material. (NASA)

Ground Testing of Spacecraft Materials

Wayne Stuckey and Michael J. Meshishnek

Spacecraft paints, films, and coatings are more than cosmetic—they contribute to the vehicle's thermal design. Ground-based testing can help determine how well and how long these materials will survive the harsh space environment.

Despite its apparent scarcity of matter, the near vacuum of space presents a hostile environment for external spacecraft surfaces. A spacecraft receives the full spectrum of solar radiation, and these electromagnetic waves, charged particles, atoms, and plasmas can cause surface materials to grow dark or brittle, or even erode away. Such changes can lead to increases in spacecraft temperatures or degradation of optical and power-system components. Aerospace has developed environmental models, simulations, and ground-based testing methodologies to identify the most stable materials and provide data that can be used to design spacecraft that can tolerate on-orbit material degradation.

Ramifications of Material Change

Thermal control plays a central role in spacecraft operations. The lack of atmospheric convection in space limits a satellite's ability to dissipate heat. The thermal design must therefore consider how much solar radiation will be reflected or absorbed by external surfaces. In addition, onboard electronics usually generate waste heat that must be dissipated. Reflective paints and thermal-control films can influence the reflection and absorption of solar radiation and the dissipation of heat by emission of infrared radiation. Ideally, these paints and films would not change over time, but both flight experience and ground experiments have shown that they do. Thus, to produce a suitable thermal design, the spacecraft developer needs to know what the thermo-optical properties of these materials will be both at the beginning and end of the intended mission.

Various paints are used for spacecraft applications, including polyurethanes, silicones, and silicates, some of which are formulated specifically for space use. The choice of a paint might depend on several factors, including cost and durability. Polyurethanes, for example, tend to be cheaper, but suffer greater degradation on orbit. The silicates are more stable, but are also more expensive, more brittle, and harder to apply. Knowing how different paints will hold up in a particular orbit can help designers choose the best one for meeting cost and performance requirements.

Samples of commonly used white paints were exposed to simulated radiation environments. The paints started out pure white, as in the photo on the left. After a simulated 10-year exposure in geosynchronous orbit, the paints turned brown, as shown in the photo on the right.

A similar situation exists for thermal-control films, such as Kapton and Teflon. These polymeric films may be applied in single layers to a spacecraft surface or, more often, fastened together as part of a thermal blanket. These films and blankets work the same as thermal-control paints: they insulate and shield components from solar radiation and allow heat generated onboard to be rejected. Thermooptical properties of the outer layer of these blankets, exposed to the space environment, must be known at the design stage to ensure proper thermal performance for the duration of the mission.

The harsh space environment can also degrade the solar array—a critical component of the onboard power system. Optical coatings, applied to solar-cell cover glasses, are typically used to increase the efficiency of solar cells, and these can grow darker after a long exposure to the space environment. These surfaces almost always face the sun, which means they can also attract and hold outgassed contaminants produced when a satellite settles into orbit. This deposition process involves a photochemical reaction between the surface and the contaminant molecules, causing them to stick irreversibly. Solar-array degradation is of course predicted for the mission lifetime; but such contamination can cause the solar array to degrade much faster than anticipated. In some cases, the solar-cell interconnects can also be eroded, eliminating their ability to convey electrical power. Degradation of solar-cell cover glasses from solar radiation and contamination is suspected as the cause of the anomalous Global Positioning System (GPS) solar-array power degradation.

Long-term flight data for films, paints, and optical coatings are not always available, so the spacecraft designer is challenged to select materials that will perform as intended for the duration of the mission. Compounding matters, manufacturers sometimes change their paint formulas, often with unforeseen consequences for space durability. In such cases, ground-based testing is required, but such testing first requires an adequate model of the space radiation environment—and the details of this complex environment are still being explored.

The Space Materials Laboratory space simulation chamber can accommodate large samples for testing—even a part of a solar array, as shown in this photograph.

Ground Test Design

The solar spectrum that propagates through space is not the same as the atmospherically filtered spectrum that reaches Earth's surface. For example, the shorter-wavelength, higher-energy vacuum ultraviolet rays do not penetrate Earth's atmosphere, but these can be the most damaging to spacecraft materials. Including this radiation is only one of the challenges in a ground test of space environment effects.

The electron and proton particle populations are also difficult to simulate. These particles range in energy from a few electron volts to millions of electron volts, with densities that vary widely depending on the orbit. The energy level of a particle will dictate how it reacts with a material, determining whether it will effectively "bounce off" the surface, become buried in the surface layers, or deposit its energy deeper into the material. Recent improvements in space-environment modeling with a more complete consideration of low-energy contributions to the total radiation environment, and the inclusion of these models with particle energy transport codes, have led to better approaches to ground simulation.

The low-energy or plasma portion of the spectrum is especially significant for surface materials such as thermal-control paints and coatings, exposed optics and their coatings, multilayer insulation, solar-cell cover glasses, uncoated thin structures, and some inflatable structures. These materials encounter the full force of the space environment, while most electronics or other materials are, in effect, shielded by the spacecraft itself. A ground-based simulation test of surface materials in high-radiation environments must include this part of the spectrum as well as the higher-energy part of the spectrum.

The Van Allen radiation belts present different hazards for different orbits. The lower belt is dominated by protons, while the upper belt is predominantly electrons. A low Earth orbit generally stays below the proton belt but still passes through it at some point in its orbit. A geosynchronous satellite is primarily affected by the outer radiation belt. The highly elliptical and medium Earth orbits spend a considerable amount of time in both belts. In addition, a satellite in low Earth orbit can encounter high levels of atomic oxygen, formed by sunlight splitting oxygen molecules into constituent atoms. Atomic oxygen is highly reactive and can steal atoms of carbon, hydrogen, nitrogen, and other elements from material surfaces, eroding them layer by layer. Clearly, then, the first consideration in the design of a test is the definition of the orbital parameters, and hence the environment that the spacecraft will encounter.

Plots from AE8 MAX of the electron fluence as a function of energy for various orbits. AE8 MAX represents the predictions at a period of maximum solar activity. The significant difference in levels between low and high orbits is illustrated. As shown, the number of electrons increases sharply at lower energies. The low-energy part of the spectrum is very important for surface materials (view larger image).

Predictive Models

Radiation levels change by orders of magnitude depending on the particular orbit. For example, the integrated fluence of trapped protons is four orders of magnitude higher in a geostationary orbit than in a low Earth orbit. The fluence for a medium Earth orbit is even higher. In general, the low-energy plasma environments are not as well known as the trapped radiation environments.

The most commonly used models for estimating particle fluxes are known as AE8 and AP8. Based on flight data, these statistical models were developed by Aerospace (the "A" stands for "Aerospace") and are used extensively by the space community. AE8 MAX and AE8 MIN model electron flux conditions at solar maximum and solar minimum, respectively. Similarly, AP8 MAX and AP8 MIN model proton fluxes as solar maximum and minimum.

These electron radiation dose profiles for Kapton in geosynchronous orbit were calculated using both AE8 MAX and ATS-6 models. At 40 keV, the peak in the dose-depth curve occurs at about 0.3 mil and does not penetrate significantly beyond 1 mil. At low energies, 10 keV, for instance, the dose might be close but only at about 0.1 mil. A combination of energies is the only way to reproduce the complete dose-depth curve (view larger image).

The AE8 paradigm allows spacecraft designers to calculate the total radiation dose deposited in a material in a specified orbit. Different materials will absorb radiation in different ways, depending on their density and chemical composition. Thus, the composition and density of the material is used, together with the predicted electron spectrum and fluence, to generate a so-called dose-depth curve for that material. The result is a prediction of the absorbed radiation dose, expressed as rads, versus material thickness or depth that is expected for a specified time on orbit. This prediction can then be used to design a simulation test.

The AE8 MAX dose-depth curve for Kapton, for example, shows a wide range in the absorbed radiation dose depending on the orbit, but even at low Earth orbit, the total deposited surface dose is greater than 1 megarad. For comparison, a dose of 400 to 450 rads is fatal to humans. Any materials exposed on the surface must be able to tolerate radiation levels many orders of magnitude higher than any electronic device (which would be shielded). Typical damage thresholds for most polymeric materials are in the 0.1–100 megarad range. Most polymers should be able to survive a low Earth orbit but may be susceptible to damage at higher orbits.

One-year electron depth-dose profiles for Kapton in geosynchronous orbit with the low-energy predictions included. The surface dose is two orders of magnitude higher than that predicted by AE8 alone. The combined depth-dose curve calculated using both the AE8 MAX, and the low-energy ATS plasma electron contribution, are used in Aerospace laboratories for a proper ground simulation (view larger image).

The AE8 algorithm indicates that at any orbit, low-energy electrons (which are most important for surface effects) will be far more prevalent than high-energy electrons. Nonetheless, AE8 does not adequately model this environment and was never designed to do so. In fact, generally applicable plasma models are not available, especially for low Earth and highly elliptical orbits, though data from the SCATHA (Spacecraft Charging At High Altitudes) ATS-5 and ATS-6 experimental satellites have been used to model the geosynchronous environment. Significantly, a 1-year dose-depth curve for Kapton in geosynchronous orbit using both AE8 and ATS-6 data shows the surface dose to be two orders of magnitude higher than the curve plotted using AE8 alone. Thus, Aerospace now uses combined AE8 and ATS-6 dose-depth curves to generate ground simulations.

From Models to Test

The problem of simulating the space environment in the laboratory is one of attempting to reproduce the myriad of particle energies and fluxes along with the wide spectrum of radiation emitted from the sun. As for solar radiation, the spectrum of interest—the vacuum ultraviolet, ultraviolet, and visible wavelengths—can be simulated appropriately using xenon and deuterium arc lamps together. The charged particle spectrum is another matter, because the energy of the particles varies so significantly. Simulating this environment is done by calculating the effect of all of the fluxes and energies (through a dose-depth curve) and then mimicking this energy deposition with several selected energies and fluences.

Ten-year electron dose-depth profiles for Kapton in various orbits. The graph clearly shows the wide range in the deposited radiation dose depending on the orbit. The surface dose is greater than 1 megarad even at low Earth orbit (view larger image).

Thus, to simulate a space environment dose-depth curve in the laboratory, a combination of electron energies must be selected that will reproduce the on-orbit curve as closely as possible. For example, the dose-depth curve for Kapton in geosynchronous orbit shows that the peak penetration depth for a 40-keV electron is approximately 0.3 mil and that these particles do not penetrate significantly beyond 1 mil. A simulation using a 40-keV electron beam would come close to matching the total on-orbit dose at 0.3 mil, but would not adequately reproduce the dose absorbed at any other depth. Similarly, a low-energy simulation—10 keV, for instance—might approximate the dose absorbed at about 0.1 mil, but not deeper. A combination of energies is the only way to reproduce the complete depth-dose curve. The 100-keV electrons penetrate to a depth of about 4 mils, more than adequate for surface phenomena. Higher-energy electron irradiation—up to 1 MeV—can be used for bulk damage at depths beyond 5 mils where the dose-depth curve is nearly flat. Irradiation using 1-MeV electrons only would never be capable of an acceptable simulation for surface materials because of the mismatch to the surface areas of the curve. The dose from a single energy can be matched at one point on the curve but can never match the complete dose-depth profile.

Flight Data Comparison

The true measure of any ground test methodology is how closely results agree with flight data. Flight data are not available for as many different materials and for as many different orbital exposures as designers might like, but there are some cases where ground and flight data can be compared. The space shuttle, for example, has returned numerous samples to Earth from the Solar Max satellite, the Hubble Space Telescope, various shuttle-based experiments, and the Long Duration Exposure Facility (LDEF). All of these samples came from low Earth orbits, where the electron and proton populations are low, but where ultraviolet and atomic-oxygen levels are high. On-orbit exposure time for these samples varied, with LDEF providing the longest exposure of 69 months (see LDEF sidebar).

Data from the Long Duration Exposure Facility (LDEF) mission confirmed the usefulness of Aerospace environmental testing methods. In this graph, the solar absorptance of the white polyurethane paint is plotted as a function of years on orbit. Simulated data corresponds closely with observed data (view larger image).

For other orbits, data are sometimes transmitted back to Earth that provide insight into materials degradation. For example, the geosynchronous SCATHA satellite has provided data on a few commonly used spacecraft materials. SCATHA identified radiation effects such as surface and bulk charging, attraction of outgassed contaminants by charged surfaces, radiation-induced conductivity of dielectric materials, and deterioration of thermal-control materials and coatings. Aerospace experiments on SCATHA included thermal-control materials like silver Teflon. Other experiments flown on the Defense Support Program and GPS spacecraft have also provided data on paint degradation in high-radiation environments.

Overall, Aerospace simulations match flight-test data fairly well. For example, in ground tests, white paints turned brown after a simulated 10-year exposure to the solar ultraviolet, electron, and proton environment encountered in a geosynchronous orbit. The color change represents an increase in the solar absorptance—which can lead to unacceptable increases in spacecraft temperatures, if not anticipated in the thermal design. Similarly, white polyurethane paints on LDEF had already turned brown when the satellite was retrieved. The change in solar absorptance, as measured by a spectrophotometer, was consistent with Aerospace predictions from ground testing.

Another ground study exposed Tedlar (a white film made from a fluoropolymer) to two different simulated orbital environments. The samples had an ultraviolet rejection coating to block the most damaging part of the ultraviolet spectrum. The samples exposed to a low Earth environment, where the radiation levels are not high, remained relatively stable; however, the samples exposed to geosynchronous conditions degraded severely, becoming shredded and cracked. Researchers attributed this effect to the penetration of high fluxes of low-energy electrons through the thin coating, causing degradation of the Tedlar polymer.

Samples of Tedlar, a white fluoropolymer film, were exposed to simulated radiation environments for various satellite orbits. Supplied with an ultraviolet rejection coating to block the most damaging part of the ultraviolet spectrum, the samples started out bright white, as in the left photo. When exposed to a simulated low Earth orbit, the material remained relatively stable, exhibiting only small changes. However, after a 1-year simulated exposure to a geosynchronous orbit, the samples turned a purple-brown color, as in the middle photo. Such a color change increases the solar absorptance, which can lead to unacceptable increases in spacecraft temperatures. After a simulated 3-year exposure in geosynchronous orbit, the Tedlar samples became severely degraded and would have been useless for any space application, as shown in the right photo.

Conclusion

The agreement between Aerospace tests for the low Earth orbit environment and data from the LDEF experiment is good. Similarly, data from SCATHA support predictions based on laboratory models of the geosynchronous realm. This agreement with flight data gives confidence that ground tests are providing reliable data for the performance of materials in space. More important, such ground simulations enable Aerospace to make straightforward recommendations to satellite designers to ensure that all materials used will be suitable for a given mission.

To Summer 2003 Table of Contents