All four instruments of heat detect infrared light, which requires a lower temperature – slightly longer than the wavelengths visible to the human eye. Distant galaxies, stars hidden in dust clouds and planets outside our solar system all emit infrared light. But other hot appliances, including Webb’s own electronics and optical hardware, do just that. Cooling the detectors of the four instruments and the surrounding hardware suppresses that infrared emission. MIRI detects longer infrared wavelengths than the other three instruments, which means it must be even cooler.
Another reason why web heat detectors are cold is to suppress the so-called dark current or electricity generated by the vibrations of the atoms in the detectors. Dark current reflects the actual signal on the detectors, giving the false impression that they are being struck by light coming from an external source. Those false signals can drown out the real signals astronomers want to find. Since temperature is a measure of how fast the atoms in a detector vibrate, lowering the temperature indicates a lower vibration, which implies a lower dark current.
The ability of MIRI to detect long infrared wavelengths is highly sensitive to dark current, so it must be cooler than other instruments to completely eliminate that effect. With each degree the temperature of the instrument increases and the dark current increases about 10 times.
Once MIRI reached a cold 6.4 Kelvin, scientists began a series of tests to confirm that the detectors were working as expected. Like a doctor looking for any symptoms of the disease, the MIRI team looks at the data describing the health of the instrument and then gives a series of commands to see if the instrument can perform the tasks correctly. This milestone, in addition, is the pinnacle of the work of scientists and engineers in many organizations