Outgassing issues and measurement protocols for spacecraft coatings

Spacecraft surface coatings will outgas various molecular components when the spacecraft is subjected to the vacuum of space. Some of these outgassing molecules have the potential to contaminate sensitive, critical optical surfaces on the spacecraft. Some of this outgassing can be mitigated through a thermal vacuum bakeout of the coated structures. The residual outgassing, the return flux, and the deposition of outgassing molecules for the as-flown spacecraft environment needs to be modeled to assess the vulnerability of the spacecraft to this contamination. Laboratory outgassing measurements provide the data that can be used to set the protocols for thermal vacuum bakeout procedures and to develop reasonable models of the in-flight contamination process. Several questions arise: How is the outgassing information affected by the outgassing measurement procedure? After a thermal vacuum bakeout, what happens to the outgassing properties of a sample, especially after long term interim storage? Is the rate of outgassing affected by temperature cycling? How does exposure to ultraviolet radiation affect the rate of outgassing of molecules from coatings and the rate and nature of deposition onto receptor surfaces? What is the influence of primers when used as a conditioner of the metal surface to be coated? How can laboratory outgassing information be related to the to-be-flown temperature of spacecraft outgassing? The purpose of this paper is to describe these issues and to discuss some of them with examples taken from outgassing measurements performed on a black optical paint (Chemglaze Z306).