The primary objective of the STS-104 was to connect the Joint Airlock Module to the starboard side of the Unity Node using the station's Canadarm2 robotic arm. The airlock is a critical space station element because of design differences between American and Russian spacesuits. American suits do not fit through Russian-designed airlocks. During a series of integration tests, the Russian suits were connected to the airlock to assure that they worked together. The airlock is specially designed to accommodate both suits, providing a chamber where astronauts from every nation can suit up for space walks to conduct science experiments and perform maintenance outside the station.
The airlock serves two key purposes: to keep air from escaping when the hatch to space is opened and to regulate the air pressure before an astronaut enters or leaves the ISS. Over 90% of the gases, previously lost when airlocks were vented to the vacuum of space, will now be recovered. The airlock has two compartments: the crew lock, from which astronauts enter and leave the station, and the equipment lock, where the space walkers can change into and out of their suits and stow all necessary gear.
During the second and third space walks, the crew used the Canadarm2 to install the High-Pressure Gas Assembly, which included four high-pressure gas tanks (two oxygen and two nitrogen). A meteor debris shield and a multi-layer insulation blanket cover each tank to protect them from the harsh elements of space. Installation of these tanks was necessary before space walks could be performed from the airlock without a Shuttle present.
While docked, the crewmembers transferred space walk and airlock outfitting equipment, science payloads, water and other supplies to the Expedition Two crew. Additional activities included IMAX filming and installation of equipment and a work site on the airlock for use by space walkers on subsequent missions.
Science payloads on board Atlantis included several Detailed Supplementary Objectives (DSOs). A DSO is a NASA-sponsored investigation performed by Space Shuttle crewmembers, who serve as the test subjects. These studies are designed to require minimal crew time, power, and stowage. Biomedical DSOs focus on operational concerns, including space motion sickness, cardiovascular deconditioning, muscle loss, changes in coordination and balance strategies, radiation exposure, pharmacokinetics and changes in the body's biochemistry. The DSOs flown on STS-104 were Individual Susceptibility to Post-Spaceflight Orthostatic Intolerance (DSO 496), Monitoring Latent Virus Reactivation and Shedding in Astronauts (DSO 493), Sleep-Wake Actigraphy and Light Exposure during Spaceflight (DSO 634), Spaceflight and Immune Function (DSO 498), and Spatial Reorientation Following Spaceflight (DSO 635).
Six Development Test Objectives (DTOs) were also performed during this mission (Crosswind Landing Performance; ISS On-Orbit Loads Validation; ISS Waste Collector Subsystem Refurbishment; Micro-Wireless Instrumentation System; On-Orbit Bicycle Ergometer Loads Measurement; and Single-String Global Positioning System). A DTO is a NASA-sponsored investigation that is performed by Space Shuttle crewmembers to evaluate new hardware and procedures involving the orbiter, its subsystems, and its support equipment.
The Shuttle Atlantis undocked from the space station on July 21,2001, and landed at KSC on July 24, 2001, concluding a successful mission.