How Miri Became Webb S Coolest Instrument

					11/08/2022 585 views 18 likes

The NASA/ESA/CSA James Webb Space Telescope is widely referred to as the successor to the NASA/ESA Hubble Space Telescope. In fact, it is the successor to much more than that. With the inclusion of the Mid-Infrared Instrument (MIRI), Webb also became a successor to infrared space telescopes such as ESA’s Infrared Space Observatory (ISO) and NASA’s Spitzer Space Telescope. At mid-infrared wavelengths, the Universe is a very different place than what we are used to seeing with our eyes. Spanning from 3 to 30 micrometers, the mid-infrared reveals celestial objects with temperatures from 30 to 700ºC. In this regime, objects that appear dark in visible light images now glow brightly. For example, dust clouds in which stars form tend to be at these temperatures. Additionally, molecules tend to be easily visible at these wavelengths. “It’s such an exciting wavelength range in terms of the chemistry you can do and how you can understand star formation and what happens in the cores of galaxies,” says Gillian Wright, the principal investigator for the European Consortium behind by the MIRI instrument. . MIRI peered into the heart of M74, the Phantom Galaxy to reveal the delicate filaments of gas and dust in the galaxy’s spiral arms. Our first real glimpses of the mid-infrared universe came from ISO, which operated between November 1995 and October 1998. Arriving in orbit in 2003, Spitzer made further progress at similar wavelengths. Both the ISO and Spitzer discoveries highlighted the need for a mid-infrared capability with a larger collecting area for better sensitivity and angular resolution to advance many big questions in astronomy. Gillian and others began to dream of an instrument that could see the mid-infrared in vivid detail. Unfortunately for them, ESA and NASA saw the shorter wavelengths of the near-infrared as the primary target for Webb. ESA would take the lead on a near-infrared spectrometer, which became NIRSpec, and NASA set its sights on an imager that became NIRCam. Not to be deterred, when ESA issued a call for proposals to study the near-infrared spectrometer instrument, Gillian and her colleagues saw an opportunity. “I led a team that had a rather cheeky response. He said we’re going to study the near-infrared spectrograph, but we’re also going to have an extra channel that’s going to do all that mid-infrared science. And we presented the scientific case for why mid-infrared astronomy would be fantastic at Webb,” he says. Although her team did not win that particular contract, the bold move helped raise the profile of mid-infrared astronomy in Europe, and she was invited to represent those scientific interests in another ESA study looking at the European industry’s ability to manufactures infrared instruments. With the help of academic institutions from across Europe, part of this study looked at mid-infrared instruments. The results were so encouraging, as were those of parallel US-led studies, that the appetite for such an instrument became even greater. Assembling in Europe an international collaboration of scientists and engineers willing and able to design and build the instrument – ​​and raise the substantial funds to do so – Gillian and her colleagues encouraged and gradually convinced ESA and NASA to include it in the Webb. MIRI and NIRCam reveal a landscape of star-forming mountains and valleys in the Carina Nebula. Large consortia are not an uncommon way of building spacecraft instruments in Europe. ESA often builds the spacecraft or telescope and then relies on consortia of academic and industrial institutions to raise funds from their national governments to build the instruments. But it’s unusual in the US, where NASA usually funded the instruments as well. Extending European leadership in this working method into the realm of international cooperation with the US, on a flagship NASA mission where the instrument-making culture is so different, was not a guaranteed recipe for success. “The biggest fear was that this complexity would be the biggest threat to the instrument,” says Jose Lorenzo Alvarez, MIRI Instrument Manager for ESA. But the gamble paid off, as Jose explains, “It was surprising to see the change in attitude between people with completely different work cultures. In the early years we were on a learning curve. Ultimately, MIRI, which was organizationally more complex, was the first instrument to be delivered.” In addition to raising their own money, the consortium had another caveat: the instrument could have no effect on Webb’s operating temperatures and optics. In other words, the telescope would remain optimized for near-infrared instruments, and MIRI would receive what it could. This would limit the instrument’s performance beyond ten micrometers, but it was a small price to pay for Gillian. “I never saw it as a compromise because it would be even better than anything we’d ever seen before,” he says. MIRI and NIRSpec observed the five Stefan Quintet galaxies to reveal the large gravitational forces at work between interacting galaxies and the star formation that sparks within them. One of the biggest technological hurdles to overcome was that MIRI had to operate at a lower temperature than the near-infrared instruments. This was accomplished with the cryocooler mechanism provided by NASA’s Jet Propulsion Laboratory. To be sensitive to mid-infrared wavelengths, MIRI operates at about 6 Kelvin (–267°C). This is lower than Pluto’s average surface temperature, which is about 40 Kelvin (–233°C). Coincidentally, this temperature is where the other instruments and the telescope operate. Both are extremely low temperatures, but because of this difference, heat from the telescope would still leak into the MIRI once attached to the telescope, unless the two were thermally isolated from each other. “To minimize thermal leakage we had to choose some pretty weird and pretty exotic harness materials to minimize heat conduction from one side to the other,” says Brian O’Sullivan, MIRI System Engineer for ESA. Another challenge was the limited space available for the instrument in the telescope. This was made even more difficult since MIRI was to be essentially two instruments in one, an imager and a spectrometer. It required some clever design work. “We have a mechanism, and not only do we use the light shining from one side, but we also use the other side of it, just to minimize the number of mechanisms we use and the space we take up. It’s a very interesting and very solid visual design,” says Brian. The instrument uses one light path for its image and another for its spectrometer. Even after the instrument was completed and delivered to NASA for integration with the rest of the telescope, there were more challenges for the team to face. The highly complex telescope took longer to complete than anyone had imagined, and this meant that MIRI and the other instruments would need to survive on the ground for much longer than originally planned. Designed to stay on Earth for about three years before launch, it took nearly another decade before the spacecraft reached orbit. To ensure the health of the instrument, the MIRI was stored in strictly controlled conditions and checked periodically. Then, on Christmas Day 2021, an ESA Ariane 5 rocket carried the spacecraft into orbit in a perfect launch. In the weeks and months that followed, ground teams prepared the telescope and its instruments and delivered them to the scientists. MIRI’s fantastic view of the Pillars of Creation in the Eagle Nebula brings the dusty pillars to vivid if eerie life. Within each tiny “finger” protruding from these large pillars, an entire solar system is formed. Along with the other instruments, MIRI is now sending back the kind of data scientists have been dreaming of. “Yes, these first few months in particular have been quite surreal,” says Sarah Kendrew, MIRI instrument and calibration scientist, ESA. “We did so much preparatory work with simulated data, so in a sense we knew what the data would look like. So you could look at it and think it all looks very familiar, but at the same time, it’s like, but it came from outer space!” MIRI data featured heavily in the very first images released by Webb, including the “mountains” and “valleys” of the Carina Nebula, the Stephan Quintet Interacting Group of Galaxies and the Southern Ring Nebula. Subsequent images continued to raise the bar in terms of both beauty and science. However, because MIRI is such a big step ahead of any previous mid-infrared instrument, the bar has also been raised in terms of image interpretability. “MIRI gives us a lot of new things that are harder to interpret, just because MIRI is such a big difference from what was there before,” says Sarah. But this is the essence of cutting-edge science, and astronomers are already scrambling to develop more detailed computer models that can tell them more about the various physical processes that lead to mid-infrared measurements. “There is huge potential for new understanding with MIRI, particularly in star formation and the properties of dust and galaxies. It may take a little longer to interpret, but I think the new science to come out of MIRI is…