To explore higher, farther, and faster, scientists and engineers have developed advanced materials for manned spacecraft and satellites for a range of sophisticated applications in transportation, global positioning, exploration, and communication. Materials used in space are exposed to vacuum, intense ultraviolet radiation from the sun, and ionizing radiation that results in material damage as well as charging (electrostatic discharge effects), micrometeoroids and debris impacts, and thermal cycling (typically from -175 to 160°C). In terms of materials degradation in space, the low Earth orbit (LEO), where LEO is defined as 200—1000 km above the Earth’s surface, is a particularly challenging synergistic environment, since atomic oxygen (AO) is present along with all other environmental elements. Hence, this special issue focuses primarily on the materials issues experienced in LEO by space environmental exposure, such as on the exterior of the International Space Station and the Hubble Space Telescope, and the challenges and opportunities of ground-based laboratory sources to mimic LEO. The combination and comparison of both in-flight and groundbased experiments are needed for the development of predictive understanding of the materials degradation and AO passivation mechanisms in LEO. Such insights are essential for the development of advanced materials and coatings to ensure the longterm durability and performance of vehicles employed in space.

When we think of space, we typically think of a vacuum containing very little matter that lies between the Earth and other planetary and stellar bodies. However, the space above Earth's breathable atmosphere and beyond contains many things that make designing durable spacecraft a challenge. Depending on where the spacecraft is flying, it may encounter atomic oxygen, ultraviolet and other forms of radiation, charged particles, micrometeoroids and debris, and temperature extremes. These environments on their own and in combination can cause degradation and failure of polymers, composites, paints and other materials used on the exterior of spacecraft for thermal control, structure, and power generation. This article briefly discusses and gives examples of some of the degradation experienced on spacecraft and flight experiments as a result of the space environment and the use of ground and space data to predict durability.

Space environmental effects on materials are very severe and complex because of the synergistic interaction of orbital environments such as high-energy radiation particles, atomic oxygen, micrometeoroids, orbital debris, and ultraviolet irradiation interacting synergistically, along with thermal exposure. In addition, surface degradation associated with contamination can negatively impact optics performance. Materials flight experiments are critical to understanding the engineering performance of materials exposed to specific space environments. Likewise, the spacecraft designer must have an understanding of the specific environment in which a spacecraft will operate, enabling appropriate selection of materials to maximize engineering performance, increase mission lifetimes, and reduce risk. This article will present a methodology for assessing the engineering performance of materials baselined for a specific spacecraft or mission. In addition, an overview of the space environment, from low Earth orbit to interplanetary space, will be provided along with an overview on the effects of the space environment on materials performance. The majority of this article is devoted to materials flight experiments from the European Space Agency (ESA), the Japan Aerospace Exploration Agency (JAXA), and from the National Aeronautics and Space Administration (NASA). Some of the experiments reviewed include ESA's Materials Exposure and Degradation Experiment on the International Space Station (ISS), JAXA's Micro-Particles Capturer and Space Environment Exposure Device experiments on the ISS Service Module and on the ISS Japanese Experiment Module Exposed Facility, and NASA's Long Duration Exposure Facility satellite and the Materials International Space Station Experiment series flown on the exterior of ISS.

This article focuses on mechanistic aspects of hyperthermal atomic oxygen reactions with polymers, which are the major contributor to material degradation in low Earth orbit. Due to the importance of well-controlled experiments in the understanding of the reaction mechanisms, ground-based experimental results obtained by a hyperthermal atomic oxygen beam generated by laser detonation facilities are mainly surveyed. Combined effects of atomic oxygen and vacuum ultraviolet (VUV) light on fluorinated polymers are also described. Such combined effects of hyperthermal atomic oxygen and VUV light are important not only from a fundamental point of view but also for engineering purposes (i.e., methodology for ground-based space environmental simulation). The VUV-sensitive polymers, poly(methyl methacrylate), and Teflon fluorinated ethylene-propylene do not show significant synergistic effects. Instead, the effect of combining atomic oxygen and VUV light produces erosion of the polymer that is the sum of the erosion caused by atomic oxygen and UV light acting individually. The experimental results suggest that material erosion in a complicated space environment may be quantitatively predicted if the erosion yields caused by the individual action of atomic oxygen and VUV light are known.

In the last 40 years, the increased space activity created a new form of space environment of hypervelocity objects—space debris—that have no functional use. The space debris, together with naturally occurring ultrahigh velocity meteoroids, presents a significant hazard to spacecraft. Collision with space debris or meteoroids might result in disfunction of external units such as solar cells, affecting materials properties, contaminating optical devices, or destroying satellites. The collision normally results in the formation of additional debris, increasing the hazard for future missions. The hypervelocity debris effect is studied by retrieving materials from space or by using ground simulation facilities. Simulation facilities, which include the light gas gun and Laser Driven Flyer methods, are used for studying the materials degradation due to debris impact. The impact effect could be accelerated when occurring simultaneously with other space environment components, such as atomic oxygen, ultraviolet, or x-ray radiation. Understanding the degradation mechanism might help in developing materials that will withstand the increasing hazard from the space debris, allowing for longer space missions. The large increase in space debris population and the associated risk to space activity requires significant measures to mitigate this hazard. Most current efforts are being devoted to prevention of collisions by keeping track of the larger debris and avoiding formation of new debris.

Virtually all spacecraft employ photovoltaic energy conversion for continuous power generation. Compared to their counterpart on Earth, photovoltaic modules in the space environment face a unique set of performance requirements. Among the most demanding ones are the need to have the highest possible specific power output with regard to mass and surface area while showing as little degradation as possible under intense particle and ultraviolet radiation during lifetimes of up to 15 years. In addition, the thermomechanical stresses induced by temperature fluctuations up to 200°C are not to result in additional electrical degradation. This article briefly outlines the state-of-the-art design solution to meet these requirements before it focuses on current materials issues in two core areas: On the solar cell itself, which requires new materials systems and cell concepts to surpass the efficiency of the lattice-matched triple junction solar cell technology, and on materials issues concerning the encapsulation of solar cells for space use. Closely linked to these materials challenges are testing-related issues that arise in verifying the expected material behavior during extended periods in the space environment. These are discussed in conjunction with the materials challenges.

Many polymers, paints, and organic-based materials exposed to the space environment undergo dramatic changes and irreversible degradation of physical and functional characteristics. While many protective approaches, including protective coatings and mechanical metal foil wrapping or cladding—especially for synthesized bulk materials, are used to reduce the effects of the space environment, the protection of such materials in space remains a major challenge, especially for future long-duration exploration missions or permanent space stations. In addition to the traditional approaches, surface modification processes are used increasingly to protect or to impart new properties to materials used in the space environment. This article presents a brief overview of the present situation in the field of surface modification of space materials. A number of surface modification solutions that differ from the traditional protective coating approaches are discussed that change the surface properties of treated materials, thus protecting them from the hazards of low Earth orbit and geostationary orbit environments or imparting new functional properties. Examples of their testing, characterization, and applications are provided.

Surface plasmon polaritons are responsible for various optical phenomena, including negative refraction, enhanced optical transmission, and nanoscale focusing. Although many materials support plasmons, the choice of metal for most applications has been based on traditional plasmonic materials, such as Ag and Au, because there have been no side-by-side comparisons of different materials on well-defined, nanostructured surfaces. This article will describe how a multiscale patterning approach based on soft interference lithography can be used to create plasmonic crystals with different unit cell shapes—circular holes or square pyramids—which can be used as a platform to screen for new materials. The dispersion diagrams of plasmonic crystals made from unconventional metals will be presented, and the implications of discovering new optical coupling mechanisms and protein-sensing substrates based on Pd will be described. Finally, the opportunities enabled by this plasmonic library to dial into specific resonances for any angle or material will be discussed.