In January 1993, Fossum was employed by NASA as a systems engineer. His primary responsibilities were to evaluate the Russian Soyuz spacecraft for use as an emergency escape vehicle for the new space station. Later in 1993, Fossum was selected to represent the Flight Crew Operations Directorate in an extensive redesign of the International Space Station. After this, he continued work for the crew office and Mission Operations Directorate in the area of assembly operations. In 1996, Fossum supported the Astronaut Office as a Technical Assistant for Space Shuttle, supporting design and management reviews. In 1997, he served as a Flight Test Engineer on the X-38, a prototype crew escape vehicle for the new Space Station, which is under development in-house by the Engineering Directorate at NASA-JSC and being flight tested at NASA-Dryden.
Selected by NASA in June 1998, he reported for training in August 1998. Astronaut Candidate Training includes orientation briefings and tours, numerous scientific and technical briefings, intensive instruction in Shuttle and International Space Station systems, physiological training and ground school to prepare for T-38 flight training, as well as learning water and wilderness survival techniques. Following a period of training and evaluation, Fossum will receive technical assignments within the Astronaut Office before being assigned to a space flight.