JWST’s Mid-Infrared Instrument, or MIRI, uses a cryocooler to reach its operating temperature.
MIRI observes mid-infrared wavelengths from roughly 5 to 28 microns. Its detectors use arsenic-doped silicon, a material that must be cooled below about 7 kelvin to work properly. Webb’s sunshield can passively cool most of the observatory, but it cannot make MIRI cold enough on its own.
A dedicated mechanical cryocooler therefore circulates helium through a multistage cooling system. It brings MIRI’s detectors to approximately 6.7 kelvin, only a few degrees above absolute zero, or about −448 degrees Fahrenheit. Keeping the instrument this cold reduces its own thermal glow, which could otherwise overwhelm the faint mid-infrared signals from distant astronomical objects.
The other three main instruments—NIRCam, NIRSpec, and NIRISS—operate at warmer temperatures and rely primarily on passive cooling from Webb’s sunshield. MIRI’s cryocooler is consequently one of the telescope’s distinctive engineering features, as well as a key reason it can study cool dust, molecules, young stars, and distant galaxies in the mid-infrared.