The JWST needs extremely cold temperatures to operate properly – that’s the point of its huge sun shield. Without sunlight to heat it, many of the telescope’s instruments will be able to cool passively to between 34 and 39 kelvins (between -239°C and -234°C, or less than 40°C warmer than the coldest possible temperature).
But one device on JWST – the Mid-Infrared Instrument or MIRI – needs to be at seven kelvins (-266°C) before it can do its job, collecting data in the mid-infrared range.
“At higher temperatures, any signal that may be detected from the sky is lost beneath the signal from its own internally generated ‘dark current’,” explains Alistair Glasse, a Webb-MIRI instrument scientist at the Astronomy Technology Centre, UK, and Macarena Garcia Marin, a MIRI instrument and calibration scientist, at the European Space Agency.
Why develop an instrument with such a fussy temperature range? It’s all to do with the range of light it can see with.
“Mid-infrared light, as seen by MIRI, can pass through 20 times thicker clouds than visible light,” says Klaus Pontoppidan, a Webb project scientist at the Space Telescope Science Institute, US.
“Because young stars are formed quickly (by cosmic standards, anyway) – in as little as a few 100,000 years – their natal clouds have not had time to disperse, hiding what is going on in this critical stage from visible view.
“Webb’s infrared sensitivity allows us to understand what happens at these very first stages, as gas and dust are actively collapsing to form new stars.”
MIRI needs more help than passive cooling to get cold enough – and the cooling process is a source of some anxiety for Webb researchers.
“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. Soon, the cryocooler is about to experience the most challenging days of its mission,” says Konstantin Penanen and Bret Naylor, cryocooler specialists at NASA’s Jet Propulsion Laboratory, in California.
“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 seven kelvins.
“But first, the cryocooler must make it through the ‘pinch point’ – the transition through a range of temperatures near 15 kelvins, when the cryocooler’s ability to remove heat is at its lowest.”
This process will need several quick and very carefully timed switches in valves and compressors, balanced so that MIRI neither cools too much, nor starts warming. But the cryocoolers have tested the process extensively on Earth, so they’re fairly confident it will work.
In a few month’s time, when MIRI and the other instruments have cooled sufficiently, the real astronomy work can begin.
“MIRI will be the last of Webb’s four instruments to open its eyes on the universe,” say Glasse and Garcia Marin.