James Webb Space Telescope Reaches Operational Temperature, Just Shy of Absolute Zero

3 min readApr 15, 2022

by Ryan Whitwam

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It seemed like there was a new bit of news on the James Webb Space Telescope (JWST) every few days earlier this year, but we haven’t heard as much from the revolutionary observatory lately. That’s because it’s been chilling out — literally. The telescope can’t start doing science until its instruments reach the correct operating temperature. NASA now reports that Webb’s coldest instrument has reached its desired temperature, marking yet another step toward being fully operational.

Unlike Hubble, the JWST was designed to peer into the mid-infrared part of the electromagnetic spectrum. Any errant heat, either from terrestrial sources or the sun, could wreck such observations. That’s why the spacecraft was deployed at the Earth-Sun L2 Lagrange point, out beyond the orbit of the moon. A few months ago, Webb successfully deployed its sunshield, allowing the internals to cool to their desired temperature just shy of absolute zero.

NASA says that Webb’s Mid-Infrared Instrument (MIRI) reached a temperature of just 7 kelvins (-266 Celsius, -447 Fahrenheit). Initially, MIRI and the telescope’s other instruments hit 90 kelvins just on the strength of the tennis-court-sized sunshield. To get it all the way down to where it currently sits, the team activated the spacecraft’s electric cryocooler. To get there, the cooler had to successfully pass the “pinch point,” a range around 15 kelvins where the cooler’s ability to remove heat is at its lowest. Several time-critical valve and compressor operations were triggered in quick succession to bridge the gap and get the observatory where it is now.

All four of Webb’s science instruments lean into the infrared, so they need to be kept chilly. However, MIRI is special. Because it is designed to scan longer infrared wavelengths, it is more susceptible to an effect known as dark current. This is a minuscule electric current generated by atomic vibration. Dark current can appear as a legitimate signal in detectors, but keeping them cool mitigates the effect. That’s why MIRI needs to be colder than the rest of the telescope, and at 6.4 kelvins, it’s right…


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