Two deployed radiators are visible behind Expedition 20 Flight Engineer Nicole Stott during the STS-128 mission's first session of extravehicular activity. Credit: NASA Most of the space station's systems produce heat, which needs to be transferred from the station to space to achieve thermal control and maintain acceptable temperatures. The complex has two different Thermal Control Systems, a Passive Thermal Control System (PTCS) and an Active Thermal Control System (ATCS). The passive system includes things such as insulation, special surface coatings, heaters and heat pipes that help keep the station’s systems at a regular temperature. The active system takes over when the passive system is not enough.
The space station’s radiator system, which is a critical component of the active system, consists of seven panels (each about 6 by 12 feet) designed to deploy in orbit from a 2-foot-high stowed position to a 50-foot-long extended position. There is a series of tubes that are routed throughout the radiators, and ammonia is circulated through the tubes. The ammonia collects heat from the space station's electronic equipment and module cooling components and transfers it to the radiator panels to be dissipated into space. Ammonia was selected because it was found to be the best heat transport fluid that meets all of NASA's thermal performance and safety requirements (toxicity, flammability, freeze temperature, stability, cost and successful commercial and industrial use).