Characterization of supersonic flowing discharges (SFD) is important to many applications in aerospace, like plasma-assisted supersonic combustion. In conditions close to the real combustion environment, a cylindrical microwave cavity was used to study the effects of H2 and air admixtures to plasma parameters in an Ar SFD. Ar and H2 were chosen since their atomic and molecular structure are well known. However, the presence of H2, N2, and O2 leads to complex branching inter- radical chemistry, which may result in a decrease in the degree of ionization and loss of combustion enhancing radicals. A qualitative description of the ionization loss was the main goal of this work. The second goal was to develop a microwave SFD to validate Martian atmospheric entry models and explore the prospect of harvesting Martian entry plasma.An atmospheric and kinetic model was developed for Martian atmospheric entry plasma based on the existing Mars data. We found that variations in the entry plasma composition were considerable and must be included in future harvesting schemes.