There is more than one reason why the Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST) is considered to be pioneering. Of the four instruments on JWST, it's the only one that observes in the mid-infrared range, from 5 to 28 microns; the other three are near-infrared devices with a wavelength range of 0.6 to 5 microns. To reach these wavelengths, MIRI had to be kept the coldest of any instrument on JWST, meaning it essentially set the requirements for the telescope's cooling system.
The stunning images taken by MIRI are a testimony to the remarkable engineering feats that went into it, feats that were achieved by overcoming formidable challenges through meticulous transatlantic teamwork and coordination.
Making MIRI
"I remember being told in the early days that the instrument will never be built. Some people at NASA looked at the block diagram of our management structure and said it will never work," professor George Rieke, who leads the science team of MIRI, recalled.
MIRI was jointly built by the Jet Propulsion Lab and a European consortium involving several institutions. While the control software and detector electronics were developed at JPL in the US, major subsystems of the instrument were developed in the UK, France, Germany, Belgium, Netherlands, Denmark, Sweden, Ireland, Spain, and Switzerland.
Although everything eventually fell into place, there were moments when professor Gillian Wright, who is the European principal investigator for MIRI, harbored some nervousness. One of them was about the possibility of budget cuts in the US impacting the project. "Since it was a 50-50 partnership, there were some things the US was required to provide. There were times when I thought, 'I hope they really do,'" she said.
Wright also said the US government's International Trade in Arms Regulations (ITAR) restrictions created some hurdles, especially in the early days. "By definition, space hardware falls under [ITAR]. We would have liked a bit more insight into the things the US was providing. But it was a struggle due to ITAR restrictions," she added.
The team also faced other challenges related to military uses, starting with MIRI's imaging detectors, which convert mid-infrared light into electrical signals. "We were using a detector type that was developed in the US for military purposes. By the time we started developing MIRI, the military had moved to other types. So it wasn't strongly supported," Rieke recalled.
He said the MIRI team had to work with the manufacturer to recover a key step in building the detectors. "To get these detectors when they were sidelined by the manufacturer was a scary part," he said.
Keeping things cool
The second challenge was to ensure the detectors worked properly by achieving a temperature of 7 kelvin (266º C below freezing). It may not sound like it, but this is much lower than the 37 kelvin (-236º C) achieved by the radiative coolers on JWST.
According to Wright, the cooler had the potential to put the MIRI project at risk. Initially, the MIRI team had designed a thermos-like container filled with liquid hydrogen to keep the instrument cool. However, this system, which could cool MIRI for five to 10 years, weighed a lot. "The observatory was over its mass budget. One way of saving the mass was to take away this system and replace it with an active cooling mechanism," Wright said.
This decision posed a different set of problems. "It was a significant change happening late after the MIRI design had been confirmed. Though the active cooling technology had been in development for other future missions, it hadn't been designed for JWST and MIRI until then. It was a risk because the technology development started about five years behind the rest of the telescope," Wright said.
However, the cloud of uncertainty was removed due to what Wright termed "JPL's superb job."
"The quality of the cooler and the time in which they developed it was extraordinary," Wright said.
According to Pierre-Olivier Lagage of the French Alternative Energies and Atomic Energy Commission, when the system's design was conceived in the late 1990s, it wasn't completely obvious that MIRI would be part of JWST.
"Besides the challenge of actively cooling MIRI, there weren't a lot of astrophysicists pushing for such an instrument. That's because, at that time, they were used to doing observations in the near-infrared range from ground based observatories. Thus, we had to convince NASA and ESA that MIRI was both technically feasible and scientifically interesting," Lagage, who is one of the co-principal investigators of the MIRI European consortium, said.
Ahead of its time
Despite these challenges, MIRI was the first instrument to be delivered, almost a year before the others. "It was inspirational to see the cooperation involving so many countries and doing it seamlessly. It was the first instrument delivered because people worked so well on it together," Rieke said.
Wright said that the team was very committed, had a common goal, and just got on with the job. "That's one very good thing about the system in Europe. For us, it's about science and not about a contract from NASA or ESA. 'I am not going to solve this problem unless somebody gives me some extra money' was not our attitude. Such commercial attitudes can sometimes create additional problems which can slow things down," Wright said.
Despite the successful launch of JWST into orbit in December 2021, Rieke, Wright, and Lagage refused to celebrate before seeing MIRI's first images. "You can't not have doubts about such a complex instrument," Wright remarked.
The suspense surrounding MIRI was exceptionally high, as it was the last instrument to be turned on due to the low temperature the cooler needed to achieve. "You see other instruments working well and have to trust that ours will not become the only instrument to have problems," Wright said.
So when Wright saw MIRI's first test image of the Large Magellanic Cloud on July 12, she could finally celebrate. "It was a very emotional moment... a mix of relief and excitement. The image was utterly amazing. It was so sharp. You could see the dust in between the stars. That was one of the reasons we built MIRI," she said.
Lagage also felt a surge of emotions after seeing MIRI's first images. "I have been waiting for this moment for 24 years," he said.
For Lagage, Wright, and Rieke, the successful working of MIRI marks the end of one chapter and the beginning of another as scientists. They are looking forward to the prospect of studying the atmospheres of exoplanets as well as star formation and galaxy evolution with MIRI.
