The James Webb Space Telescope continues to cool down out at its location at Lagrange Point 2, about 1.5 million kilometers from Earth. Since JWST is an infrared telescope, it needs to operate at extremely low temperatures, less than 40 K (-223 degrees Celsius, -369.4 degrees Fahrenheit). But one instrument needs to be even colder.
To operate at peak efficiency, Webb’s Mid-Infrared Instrument (MIRI) must be cooled to a chilly 7 K (-266 C, -447 F). And it will need a little help to reach those frigid temps.
Most of the telescope and its instruments rely on JWST’s massive sunshield as well as passive cooling, taking advantage of the frigid temperatures in deep space. The near-infrared instruments (NIRCam, NIRSpec, FGS-NIRISS) have now reached their target range from 34 to 39 K by cooling passively.
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MIRI carries detectors that need to be at less than 7 K to be able to detect longer wavelength photons of infrared light. This temperature is not possible on Webb by passive means alone, so Webb carries an innovative cryocooler, dedicated to the task of cooling MIRI’s detectors so that it can see farther into the infrared than the other instruments.
Previous infrared missions, such as the Spitzer Space Telescope, used a tank of cryogenic liquid helium that acted as a coolant by producing a freezing vapor that cooled the entire telescope assembly. But the vapor was vented to space, and once the supply helium was gone, the ability to cool the telescope was over. Spitzer launched in 2003 and the mission ended in 2020.
But MIRI’s cooler reuses its helium just like the refrigerator in your kitchen continuously recycles its own coolant.
“Over the last couple weeks, the cryocooler has been circulating cold helium gas past the MIRI optical bench, which will help cool it to about 15 kelvins,” explained Konstantin Penanen and Bret Naylor, cryocooler specialists, NASA JPL, in a JWST blog post. “Soon, the cryocooler is about to experience the most challenging days of its mission. By operating cryogenic valves, the cryocooler will redirect the circulating helium gas and force it through a flow restriction. As the gas expands when exiting the restriction, it becomes colder, and can then bring the MIRI detectors to their cool operating temperature of below 7 kelvins.”
This type of recycling cooler also means the lifetime of MIRI instrument, as well as the entire JWST could be even longer than Spitzer’s 16 years. Webb engineers have mentioned a possible 20-year lifetime, or even longer.
Once MIRI reaches the final temperatures, engineers can begin the final phase of commissioning the telescope.
“Getting this instrument cold is one of the last major challenges faced by Webb before the MIRI team can truly relax,” wrote Alistair Glasse, Webb-MIRI Instrument Scientist, UK Astronomy Technology Centre and and Macarena Garcia Marin, MIRI Instrument and Calibration Scientist, ESA. They said the cryogenic cooler will “pull out almost all of the heat left in MIRI’s 100 kilograms (220 pounds) of metal and glass from that tropical launch day morning, three months ago. MIRI will be the last of Webb’s four instruments to open its eyes on the universe.”
You can read more details about how the cryogenic cooler works here. More information about MIRI is available at this NASA website. You can see the temperatures of all the instruments and progress on Webb’s commissioning at the Where’s Webb site.