Coatings Used in Space
Requirements and Solutions
Optical coatings have been used in discovery, exploratory and monitoring missions since the beginning of space-borne missions. The first application was in the 1958 US launch of the Vanguard satellite in reaction to the USSR Sputnik a year earlier.
Coatings that perform critical optical functions have been used in space instrument applications for the National Aeronautics & Space Administration (NASA), the National Oceanic & Atmospheric Administration (NOAA), and the Dept. of Defense (DoD) for 30+ years. The performance of the first coatings launched into space had been observed to change with time. Investigations seeking the cause of the instability were initiated. Pre-flight testing on the earth’s surface in simulated space environments revealed changes in spectral and efficiency performance that are comparable to those changes observed in space. It has been learned that coating layers tend to absorb water in the atmosphere, and when inserted in the vacuum of space, the volatile water leaves. Another effect discovered early was the loss of transmission caused by energetic irradiation from electrons and protons trapped in the radiation belts with equatorial orientation, known as earth’s Van Allen belts.
The Space Environment
It is normally considered that the environment of space imposes benign influences on optical instruments and their coatings compared to that of the earth’s surface where weather and other environmental stresses are factors. Thin-film coatings employed in the extraterrestrial environment for numerous mission-specific functions have compositions such as:
- Anti-reflective coatings
- UV to IR band-pass filters
- Thermal control films - Environmentally-protective coatings
For critical optical coatings, the foundation of environmental performance criteria are the coating test standards in MIL-C 48497: specifically temperature cycling, humid-arid, salt fog, blowing sand, and abrasive wear exposure. Coatings intended for space applications must tolerate and survive a set of environmental requirements that include most of these earth-bound specifications as well as an additional set peculiar to the space environment. Coated optics operate in environments that range from Low-Earth Orbit (LEO) (where the International Space Station (ISS) flies), to planetary and deep space probes. In addition to extended vacuum exposure and damage from micro-meteorite impact, the space environment poses additional radiation and thermal exposure which varies according to the orbital properties of the space mission. Specific to each orbital environment, the ionizing (charged particle) radiation exposure is characterized by the species energy and its density (particles / cm^2 sec = fluence).
In Low-Earth Orbit at 400 km altitude, the space environment contains the full spectrum of solar energy, trapped protons, occasional high-energy protons from solar flares and atomic oxygen. High-energy protons can cause optical damage in the form of transmission loss (darkening) through ionization to coatings and optical materials located internally; the other forms are non-ionizing and affect exposed surfaces and materials such as those on solar-cell power generating panels. Observe that the fluence of electrons is relatively low.