The start of the space era was ignited by the development of rockets, materials, and electronics. This odyssey was propelled by the human thirst for knowledge and adventure. Space endeavor relies heavily on materials with outstanding properties – they must survive in an environment that combines ionizing radiation, extreme temperatures, and micrometeorites. Certain missions add extra threats: low earth and geostationary orbits inflict ferocious ozone-induced degradation, while deep-space missions involve high levels of ionizing radiation and, eventually, extremely low temperatures.
The main requirements for space materials are: light weight (to reduce mission costs); resistance to ionizing radiation (accelerated electrons, protons, and ions); multifunctional capabilities; smart features; self-healing capabilities; and outstanding thermal stability. Space materials research is concentrated on composites obtained by dispersing nanofillers with designed functionalities within different polymeric matrices. The polymeric matrix gives low weight. An appropriate choice may also add structural and thermal stability. Polymers also act as a radiation shield because of their high hydrogen content, reduced radioactive activation, and light weight.
Much research is focused on multifunctional materials combining radiation shielding, structural capabilities, and electrical conductivity, such as polymer-carbon nanotube composites. Nanotubes can enhance the mechanical strength of polymers and add high electrical and thermal conductivity. Minute amounts give polymers antistatic features, while concentrations as low as 1 wt.% trigger electrical conductivity. The intimate relationship between the electrical and mechanical properties of these composites adds smart capabilities. The correlation between the position of Raman peaks of nanotubes embedded within polymeric matrices and the stress acting on the composite opens up an alternative route for predicting their mechanical failure.
Self-healing capabilities will protect polymers and composites from the effect of ionizing radiation, temperature, and micrometeorites. Further advances are required to extend the temperature range over which the polymeric matrix is protected and to decrease the size of the microbubbles containing the healing agent. Importantly, polymers are not ideal matrices – most are sensitive to ionizing radiation, temperature, and atomic oxygen. Synergistic effects caused by competition of these degradation processes have been reported.
The search for space materials includes other nanomaterials for extreme temperatures, conversion of light into electricity, and optical and magnetic applications. Nanomaterials are extremely appealing, as they promise reduced volume, weight, and energy consumption. However, their survival in the space environment has yet to be assessed. The details of the interaction between ionizing radiation and nanometer-sized features are not yet fully understood and a new theoretical description – nanodosimetry – is under development.
New projects that would make our presence in space easier and cheaper are maturing. The solar sail will offer an alternative method of space travel. The discovery of carbon nanotubes revived the space-elevator project. But, while the mechanical properties of nanotubes are promising (with a Young's modulus of about 1 TPa), they degrade when the size is increased toward the micron scale. To fulfill the technical requirements for the elevator cable, it is mandatory to project the outstanding properties of nanotubes to larger scales, and to understand and control adhesion between nanotubes and polymers.
Both space shuttle disasters were caused by material failures; the cooling of a polymeric O ring below the glass transition temperature caused the Challenger disaster and a piece of insulating foam peeling from the external tank and striking a reinforced carbon-carbon panel triggered the Columbia tragedy. So, it is mandatory to understand the behavior of materials in space and to improve Earth-bound simulation by focusing on synergistic effects triggered by the combined action of ionizing radiation and temperature.
US president George W. Bush's new vision revives space exploration. Most debate concerns the emphasis on manned missions. Opponents focus on robot-based exploration, citing the reduced price and risk. The reallocation of NASA's limited resources could endanger previously approved projects – already, some missions have been delayed or abandoned. But manned missions should accelerate materials development. Enhanced safety requirements will fuel a search for materials with self-healing and smart capabilities, and open up new research, such as in-space repair and in situ fabrication of materials. But, for safe missions, NASA must contribute and support materials science research, improve Earth-bound simulation, and improve estimation of the lifetime of materials in space.
[1] Mircea Chipara is at the Indiana University Cyclotron Facility in Bloomington, Indiana.