Managing the temperatures of the various components of a spacecraft, so they remain within the temperature range in which they have been tested on Earth, is the task of the spacecraft’s thermal control system. While we generally think of space as being very cold, spacecraft are exposed to extremes of temperature, from extremely hot on the sunward side to extremely cold on the shadow side or in the outer solar system. In addition, heat generated by electronic components inside the spacecraft can warm components unacceptably unless any excess heat is transported to a cooler part of the spacecraft or to a radiator that will reject it to space.

Space Technology 5 is another New Millennium Program mission that pushes the technology of spacecraft miniaturization. Designing a low-mass, low-power thermal control system for these three tiny spacecraft has been a challenge for thermal engineers.

The usual approach to thermal control is to connect the components that generate heat to a radiator that conducts the waste heat to space, while warming colder components with electric heaters. More mass is often added to various components solely to enhance their properties of heat conduction. The thermal control system is usually 4 to 6% of the mass of the spacecraft. These approaches are not suitable for smaller, more efficient spacecraft because they do not make efficient use of power resources and they do put constraints on the locations of individual components within the spacecraft.

The need for mass savings becomes ever more critical as spacecraft shrink to accommodate smaller and more efficient payloads. There is a critical need for advanced thermal control technology that would both accommodate the low mass, low power, and compactness necessary for future spacecraft and help achieve these goals by improving the efficiency with which the spacecraft manages its own heat. The new technology provided by the ST8 Thermal Loop will not only save mass, but it will also allow greater design flexibility in deciding where components are placed. It will also reduce—if not eliminate—the need for supplemental electrical heaters.

The thermal control system to be validated on ST8 is called a "Thermal Loop." It is a technology advance that will provide the improved efficiency in thermal control that will help spacecraft be lighter and use less power for managing spacecraft temperatures.