The Integrated Science Instrument Module (ISIM) forms the scientific heart of the James Webb Space Telescope (JWST) and is being developed at the Goddard Space Flight Center. It is one of the three major elements that comprise the Webb Observatory flight system. The others are the Optical Telescope Element (OTE) and the Spacecraft Element (Spacecraft Bus and Sunshield). It consists of four science instruments, guider, and nine key subsystems that are shared by each of the science instruments.

The ISIM has a distributed architecture consisting of cold and warm components. The cold portion of the ISIM is integrated with the Optical Telescope Element (OTE). This passively cooled cryogenic (30 K) section houses the instruments and fine-guidance sensor. A mechanical cryo-cooler provides additional cooling for the mid-infrared detectors.

A warm section (250 K) of the ISIM is located in the spacecraft. This more benign environment allows for relaxed requirements on major portions of the electronics and avoids unnecessary heat loads in the cold section.

The ISIM development philosophy will follow an Engineering Test Unit (ETU)/Flight Program model. The ISIM team will first perform a detailed design and analysis, determine methods of construction, and develop test plans and procedures. Based on the design, they will build ETUs and perform qualification-level testing of the ISIM prior to committing to building a flight unit. This is intended as a key element of the risk reduction plan. After building the flight unit, only acceptance testing of the flight ISIM is required before proceeding to integration and testing.