STS-128 was a NASA Space Shuttle mission to the International Space Station (ISS) that launched on August 28, 2009. Space Shuttle Discovery carried the Multi-Purpose Logistics Module Leonardo as its primary payload. It was Discovery's 37th flight. Leonardo contained a collection of experiments for studying the physics and chemistry of microgravity. Three spacewalks were carried out during the mission, which removed and replaced a materials processing experiment outside ESA's Columbus module, and returned an empty ammonia tank assembly.