NASA confronts a forward contamination problem with the launch of spacecraft and probes into extraterrestrial environments with potential for detection of life. Earth-origin microbes risk threatening planetary systems, compromising the data obtained, and require implementing microbial control for planetary protection from cross contamination. While generally effective, currently approved decontamination methods are costly, involve high heat and chemical treatments, and risk damaging thermally sensitive spacecraft materials and components. Alternative methods, including atmospheric pressure plasma jets (APPJs), show promise as effective technologies for microbial control. Researchers at Edmonds Community College partnered with Eagle Harbor Technologies (EHT) of Seattle, WA to test the capabilities of an APPJ developed by EHT, performing characterization of killing efficacy. Bacillus atrophaeus endospores were spread on tryptic soy agar plates to quantify killing efficacy of over 900 combinations of APPJ parameters including pulse width, frequency, voltage, distance, time, electrode placement, jet configuration, gas composition, and flow rate. Unprecedented independent control of APPJ pulse width, frequency, and voltage allowed researchers to identify combinations of these parameters resulting in greater than 4-log reduction of endospores. A decimal reduction time (D-value) was additionally determined at an average of 160 seconds on aluminum. High humidity and low concentrations of O2 were identified as conditions that enhanced killing of endospores. Further characterization of higher voltage, frequency, and humidity, as well as variable pulse width and low O2 should lead to improved APPJ killing efficacy with decreased exposure time and allow for development of an APPJ optimized for microbial reduction for large-area spacecraft.