The James Webb Space Telescope, launched on December 25, 2021, is going through its cooling process so that it reaches the right temperature to operate at full capacity. A new statement posted on the official NASA website explains this part of the mission a bit more.
According to the US space agency, the instrument known as “MIRI” (acronym for “Middle Infrared Instrument”) should operate at a temperature between -239.15 ºC (Celsius) and -234.15 ºC. However, its photonic detectors — basically, the telescope’s “eyes” that will be able to see the light — must be even cooler than that. This is where the “cryocooler” comes in.
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“For the past two weeks, the cryorefrigerator has been circulating chilled helium past the MIRI optical line, which will allow us to cool it down to -258.15°C. Soon, the cryocooler will face the most difficult days of its mission. By operating the cryogenic valves, it will redirect the helium and force 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 leaves the restriction, it will become colder, bringing the operating temperature of the MIRI detectors below -266.15°C.”
Indeed, at any temperature above this, the photonic detectors will be effectively “blind”. As they are tuned to operate at mid infrared frequencies, there is no other option for their operation. For this reason, the MIRI was installed outside the metal grid where the telescope lenses are also located – these get incredibly hot by absorbing sunlight, but this way the instrument is thermally insulated .
“Cooling this instrument down is one of the last big challenges Webb faces before our team can really relax, and getting past that point is an incredibly important step in that challenge. When the time comes, the cooler will have siphoned off nearly all of the heat from MIRI’s 100kg of metal, leftover from the tropical day of its launch,” said Alistair Glasse and Macarena Garcia Marin, both MIRI calibration experts.
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