Two deployed radiators are visible behind Expedition 20 Flight Engineer Nicole Stott during the STS-128 mission's first session of extravehicular activity. Credit: NASAMost 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.