The James Webb Space Telescope (JWST) is in orbit of the second Lagrange point (L2) with 3 of its 4 main instruments aligned up & ready to-go but one remaining.
The most un-explored frontiers of space are in the mid-infrared wavelengths. And Webb’s Mid InfraRed Instrument (MIRI) is poised to fill that empirical blind spot. The reason for this can be chiefly because of the way our atmosphere interferes with our ability to observe the universe from ground — where most telescopes have operated since the time of Galileo Galilei.
“Dramatic results in the mid-infrared come from telescopes in the vacuum of space, where they are cooled to cryogenic temperatures,” said Webb’s deputy project scientist Jonathan Gardner of NASA’s Goddard Space Flight Center, in a release on blog post from the agency.
But it’s already very cold in space, and there’s no atmosphere way-out beyond the moon orbit. Then why does MIRI need to be cryogenically cooled?
How the JWST cools its MIRI instrument
Unlike the JWST first three sets of instruments, MIRI will take longer to be fully ready because it uses a different type of sensor that requires incredibly low temperatures to function. So low that a cooler & heater are required on onboard to keep-it in the target temperature range.
The other 3 instruments have reached their operating temperature between 34 & 39 Kelvin, but for MIRI to work, it has to be reduced to 7 Kelvin. This is achieved with a special cryocooling system. “For the past few weeks, the cryo cooler has been circulating cold helium gas past MIRI’s optical bench, which will help cool it to about 15 Kelvin,” said Bret Naylor & Konstantin Penanen in a joint statement from the Jet Propulsion Laboratory, in agency post.
“Soon the cryo cooler faces the most challenging days of its mission,” continued Naylor & Penanen. “By operating the cryogenic valves, the cryo cooler will re-direct the circulating helium gas and forces it through a flow restriction. As the gas expands when exiting the restriction, it gets colder and can then bring MIRI detectors to their cold operating temperature below 7 Kelvin.
Why Webb’s MIRI instrument must be kept below 7 kelvins
The James Webb telescope’s MIRI instrument is highly sensitive, so sensitive that it must be extremely cold to suppress infrared background noise, including the heat of the instrument itself. “The detectors inside each scientific instrument that convert infrared light signals into electrical signals in order to process them into images must be cold to work properly,” says another NASA blog page.
In general, the longer the wavelength of infrared light, the cooler each allocated detector need to be execute this conversion, without itself producing too many random “noise” electrons. MIRI can see mid-infrared light at wavelengths from 5 – 28 microns, meaning its arsenic-doped silicon detectors must be kept below 7 Kelvin to function properly.
This outside space temperature is normally 2.7 Kelvin, but in Webb, where the instruments & computers are active, it can jump a little above that, hence “passive cooling” (the way other instruments are kept cool from exposure to space alone) just isn’t enough. Therefore, the James Webb telescope specifically uses a cryo cooler to cool its MIRI mid-infrared instrument down to sufficient temperatures, below 7 Kelvin.