In 1998, Thirsk was assigned by the Canadian Space Agency to NASA's Johnson Space Center in Houston to pursue mission specialist training. This training program involves advanced instruction on both shuttle and space station systems, EVA (spacewalking), robotic operations, and Russian language. Within the NASA Astronaut Office, Dr. Thirsk served as a CapCom (capsule communicator) for the International Space Station (ISS) program. CapComs participate in actual and simulated space missions as a communication link between the ground team at Mission Control and the astronauts in orbit. CapComs speak directly with the space station crew, and assist with technical planning for the mission and last-minute troubleshooting.