STS-128 (ISS assembly flight 17A) 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.

The mission's first launch attempt was delayed due to weather concerns, including multiple weather violations in NASA's launch rules, beginning over two hours before the scheduled launch. The second launch attempt, scheduled for August 26, 2009, at 01:10:22 EDT, was called off the previous evening due to an anomaly in one of the orbiter's fuel valves. The launch finally took place on August 28, 2009, at 23:59 EDT. Discovery landed on September 11, 2009, at Edwards Air Force Base, which was the last landing of a shuttle to occur at the California site.

The primary payload of STS-128 was the Multi-Purpose Logistics Module Leonardo to assist with establishing a six-man crew capacity by bringing extra supplies and equipment to the station. The Multi-Purpose Logistics Module contained three racks for life support, a crew quarter to be installed in Kibo, a new treadmill (COLBERT) that will temporarily be placed in Node 2 and later in Node 3, and an Air Revitalization System (ARS) that will temporarily be placed in Kibo and later in Node 3.

The shuttle carried a Lightweight Multi-Purpose Experiment Support Structure Carrier (LMC) with Ammonia Tank Assembly (ATA). The new ammonia tank replaced an empty tank during an EVA.

The shuttle flew the first test flight of the TriDAR, a 3D dual-sensing laser camera, intended for potential use as an autonomous rendezvous and docking sensor. The TriDAR successfully tracked the ISS position and orientation from the shuttle during docking operations.

It also contained three racks dedicated to science, FIR (Fluids Integrated Rack) and the first Materials Science Research Rack (MSRR-1) to be placed in Destiny and MELFI-2 (Minus Eighty Laboratory Freezer for ISS) to be placed in Kibo. The FIR enabled detailed study of how liquids behave in microgravity, a crucial detail for many chemical reactions. One experiment, for instance, examined how mixtures known as colloids behave without being stirred by sedimentation and convection. Another using the Light Microscopy Module (LMM) will examine how an ideal heat pipe works without the distortions of gravity.

The STS-128 mission (as did STS-125 and STS-127) took part in crew seat vibration tests designed to help engineers on the ground understand how astronauts experience launch. They will use the information to help design the crew seats that will be used in future NASA spacecraft.

STS-128 repeated the Boundary Layer Transition (BLT) Detailed Test Objective (DTO) experiment that was done by the same shuttle during STS-119. In this experiment, one of the thermal protection systems was raised to create a boundary layer transition in which the air flow becomes turbulent beyond a certain speed. During STS-119 the tile was raised 0.25 inches (6.4 mm) above the others, tripping the flow at Mach 15 during reentry. In the modification being done, the tile has been raised 0.35 inches (8.9 mm) to trip at Mach 18 producing more heat.