International Space Station Major Constituent Analyzer (MCA) On-orbit Performance

The Major Constituent Analyzer (MCA) is an integral part of the International Space Station (ISS) Environmental Control and Life Support System (ECLSS). It is a mass spectrometer-based instrument designed to provide critical monitoring of six major atmospheric constituents: nitrogen, oxygen, hydrogen, carbon dioxide, methane and water vapor. These gases are sampled continuously and automatically in all United States On-Orbit Segment (USOS) modules via the Sample Distribution System (SDS). The MCA is the primary tool for management of atmosphere constituents and is therefore critical for ensuring a habitable ISS environment during both nominal ISS operations and isolated airlock operations including extravehicular activity (EVA) preparation.  This paper discusses the performance of the MCA on orbit over the past year, with particular attention paid to lessons learned regarding the operational life of critical components. This year a second MCA was activated on orbit, and the operation of two MCA systems simultaneously provides the opportunity to evaluate the consistency of their performance. Recent data have also helped validate design upgrades to the filaments and ion pumps for a new set of orbit-replaceable units (ORUs) – upgrades that were implemented in the last several years for both production and depot overhaul units.