PoSSUM, an acronym for Polar Suborbital Science in the Upper Mesosphere, is a 501(c)(3) non-profit international research program that uses manned commercial spacecraft and high-altitude manned balloons to study rare "space clouds" called noctilucent clouds. These elusive clouds can help scientists address critical questions about the Earth's climate, but they can only be seen from polar latitudes during a small window of time in the summer. The project evolved from the Noctilucent Cloud Imagery and Tomography experiment, selected by NASA's Flight Opportunities Program in March 2012 as experiment 46-S. Written by former NASA astronaut instructors and some of the leading upper-atmospheric and noctilucent cloud scientists, the Project PoSSUM Scientist-Astronaut Manual is the official text of the Project PoSSUM Scientist-Astronaut Qualification Program. This program, hosted by the Embry-Riddle Aeronautical University in Daytona Beach, Fla., provides its candidates with the skills required to effectively conduct research as part of Project PoSSUM. The PoSSUM Scientist-Astronaut Manual is intended for the student or professional interested in manned spaceflight and how it applies to upper-atmospheric research. The manual contains all you need to know to be prepared for the Project PoSSUM Scientist-Astronaut Qualification Program, including an overview of Project PoSSUM, an introduction to the upper atmosphere and noctilucent clouds, suborbital cinematography and PoSSUMCam operations, spaceflight physiology and hypoxia awareness, life support systems and spacesuit operations, and PoSSUM mission planning and mission simulation techniques. A series of questions are provided to assess the student's retention of the subject material.