On the James Webb Space Telescope, temperatures are far more frigid. Dispatched to deep space on Christmas Day 2021 to study the formation of the most distant galaxies and exoplanets, Webb has had to perform ultra-fine precision adjustments to its mirrors and other instruments. Detectors inside each scientific instrument convert infrared heat signals into electrical signals. These signals are processed to generate the images astronomers see here on Earth. Because its own hardware produces infrared heat energy, any data gathered becomes useless if that energy isn’t suppressed.
That’s why Webb is fixed with an electrically-powered cryogenic cooler that chilled its onboard Mid-Infrared Instrument (MIRI) to a final temperature of 6.4 kelvins on April 7. The kelvin is a unit of temperature that names zero as the absolute coldest point. This is the temperature at which all atomic motion is suppressed. So, 6.4 kelvins is unimaginably cold—equivalent to -448.15 degrees Fahrenheit. That’s just a bit warmer than the coldest, emptiest parts of space, which are -455 degrees Fahrenheit. At this temperature, MIRI’s camera and spectrograph can accurately carry out its assignment: to see the infrared light of distant galaxies, young stars, faint comets, and objects in the Kuiper Belt—a belt of small rocky objects beyond Neptune that orbits the sun.
Webb’s location about one million miles from Earth lets it escape the sun’s light and heat. From there, it can deeply examine the youngest galaxies born after the Big Bang, 13.5 billion years ago. Yet it still needs a five-layer, tennis-court sized sunshield, deployed shortly after launch, to provide even more shade. Each layer sandwiches a vacuum that acts as an insulator. The specially-coated, reflective, silvery layers block some heat and deflect the rest. Passive cooling using just the sunshield brought Webb down to about 35 kelvins (-396.67 degrees Fahrenheit), the equivalent of “rejecting all but one part in a million of all the sun’s energy,” according to Jane Rigby, a Webb operations project scientist who described the cooling process during a press conference in January.
To protect its sensitive instruments from detecting their own heat radiation, Webb’s MIRI has to get even chillier; even the vibration of atoms in the detectors is forbidden. This atomic-level “dark current” contributes to misleading data and can mask the real signals from far-away stars. Temperature is a measurement of atom vibration speed in the detector, according to a NASA news release, so reducing the temperature is imperative to dial down the dark current. MIRI detects longer infrared wavelengths than the other instruments on Webb, so dark current could really mess up its readings. In fact, for every degree MIRI’s temperature goes up, the dark current increases by a factor of about ten.
Hence, a “cooler team” was dedicated to plunging all radiation levels down to the “pinch point.” This point signifies the drop to less than 7 kelvins. Engineers designed a cryogenic cooler, or a “closed cell refrigerator,” about the size of a household fridge. Its pumps and valves shoot helium gas throughout MIRI. The helium absorbs heat inside the instrument and then releases it away from Webb.
Since it’s a “closed” system, the cryogenic cooler does not consume coolant like an ice chest full of ice. Instead, it recirculates helium gas through 67 feet of thin tubing. To make ultra-cold helium, the cooler lets compressed helium expand, because expanding gas absorbs heat. The cooler captures and directs the heat away from Webb and uses the cooled helium before recompressing it. Similarly, a home air conditioning unit compresses refrigerant with a pump and then expands it to cool the air. As the refrigerant expands, a pump takes it away, and a radiator dumps its absorbed heat. The MIRI cooler system is fundamentally similar, but uses a more complex, four-stage process to chill gas to progressively lower temperatures.
Now that it’s at optimal temperature, Webb’s MIRI is ready to provide some of the coolest cosmic images Earth has ever seen.
