Launched on December 25, 2021, the James Webb Space Telescope is undergoing its cooling process to allow it to reach the proper temperature for full capacity operation. A new statement released on NASA’s official website explains this part of the mission in a little more detail.
The instrument, known as “MIRI” (acronym for “Middle Infrared Instrument”), is said to operate at a temperature between -239.15ºC (Celsius) and -234.15ºC, according to the US space agency. However, its photonic detectors – essentially the “eyes” of the telescope that can see the light – need to be even cooler. This is where the “cryocooler” comes into play.
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MIRI, one of the James Webb Space Telescope’s main instruments, requires extreme cooling to function (Image: NASA/JPL-Caltech/Handout)
“For the past two weeks, the cryo-fridge has been circulating chilled helium past MIRI’s optical line, which will allow us to cool it down to -258.15°C. The cryocooler will soon face the most difficult days of its mission. Operation of the cryogenic valves diverts the helium and forces it into a flow restriction,” said Konstantin Penanen and Bret Naylor, refrigeration specialists at NASA’s Jet Propulsion Laboratory (JPL-Caltech). “As the gas expands as it exits the boundary, it gets colder, bringing the operating temperature of the MIRI detectors below -266.15°C.”
In fact, the photonic detectors are effectively “blind” at any temperature above that. Because they are tuned to mid-infrared frequencies, there is no other option for their operation. For this reason, the MIRI was installed outside of the metal grid that houses the telescope lenses – these get incredibly hot from absorbing sunlight, but this way the instrument is thermally isolated.
“Cooling down this instrument is one of the last major challenges Webb faces before our team can truly relax, and getting past that point is an incredibly important step in that challenge.” When the time comes, the cooler will have siphoned off almost all of the heat left over from MIRI’s 100kg of metal from the tropical day it was launched,” said Alistair Glasse and Macarena Garcia Marin, both MIRI calibration experts.
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