On June 20, 1996, Thirsk flew aboard space shuttle mission STS-78 (the life and microgravity Spacelab mission) as a payload specialist. During this 17-day flight aboard the Space Shuttle "Columbia", he and his six crew mates performed 43 experiments devoted to the study of life and materials science. Most of these experiments were conducted within the pressurized Spacelab laboratory module situated in the orbiter's payload bay. The life science experiments investigated changes in plants, animals and humans under spaceflight conditions. The materials science experiments examined protein crystallization, fluid dynamics, and high-temperature solidification of multi-phase materials in microgravity.