Webb will probe areas of the cosmos that have never been observed, thanks to its 6.5-meter diameter mirror, the largest ever deployed in space, and its four instruments observing in the infrared : NIRCam, NIRISS, NIRSpec and MIRI (the consonance in "IR" comes from "infrared").
Webb, the flagship mission of NASA and the European (ESA) and Canadian (CSA) space agencies, will take over from the Hubble Space Telescope to observe further into the universe. Because of the speed of light, it will look earlier in history, to the time when the first galaxies and the first stars were formed. But it will also relay the Spitzer infrared space telescope to probe the atmospheres of exoplanets, stars and planetary systems in formation, the evolution of galaxies...
In short, all fields of astrophysics will benefit.
We are among the more than 1200 scientists from 14 countries who have contributed to the development of the JWST. In France, we have especially participated in the development of the MIRI instrument, the only one of the four instruments that operates in the so-called "thermal" infrared range. Observing in wavelengths between 5 and 28 micrometers, it will be the most capable of observing gas and dust in objects much colder than stars like our Sun. It will allow for example to see young stars still deeply buried in the cloud of gas and dust in which they form. MIRI will also be the indispensable complement to NIRCam to identify the first galaxies of the Universe.
The epic of the Webb telescope
Initially, it was far from being a given that a thermal infrared instrument would be part of the instrumental suite of the JWST (called "next generation space telescope" at that time). NASA and ESA had to be convinced of the scientific importance and feasibility of such an instrument. One of us (Pierre-Olivier Lagage) was part of the small group of astrophysicists who campaigned in Europe and the US for such an instrument.
That was... at the end of the 90s. The launch of the Webb Telescope was then planned for 2007. But the launch of Webb has been postponed many times and the epic of the MIRI instrument illustrates well the reasons for these successive delays.
The Webb will be in orbit at 1.5 million kilometers from Earth, which is 4 times the distance between Earth and Moon. It will not be possible to go and repair it in case of a problem, as was done for Hubble, which orbits at "only" 570 kilometers from the Earth : when Hubble was launched, the quality of the images was very disappointing, but the installation by astronauts of an optical corrector made it possible to re-establish the expected image quality.
For Webb, there is no room for error - hence the importance of pre-launch design and testing !
MIRI, a state-of-the-art instrument for exoplanets
MIRI consists of two main parts : an "imager", which allows us to take pictures (this is the part called "MIRIM", and a spectrometer, which allows us to study the light received as a function of wavelength - and thus, for example, to determine which chemical elements are present in the object we are observing (this is the "MRS"). The performance of these instruments placed at the focus of the largest space telescope in operation will be unprecedented.
In a sense, for the study of exoplanets, the delays in the Webb launch are good news. Indeed, this field has exploded in recent decades and we now have a wealth of exoplanets to observe, including rocky planets, that were not known in 2007.
Exoplanets are now studied by the method known as "transits" : we scan the minute variations in the luminosity of a distant star due to the passage of an exoplanet that would surround it. MIRI has therefore been "improved" to use this method of transits. It is to read only a small part of the detector, in order to do it very quickly without saturating the detector. Basically, we "divert" a little the first goal of the Webb, designed to observe faint or very distant objects, to take advantage of its high sensitivity.
MIRI also has "coronographs". Historically used to observe the corona of the Sun by hiding the disk too bright which prevents to see the details around, the coronographs were adapted to observe the stars, and thus to distinguish possible exoplanets which would be in the vicinity. MIRI carries a classical coronograph (called "Lyot") and three "phase mask" coronographs, very powerful, and which will be sent for the first time in space.
From cradle to liftoff
After several years of preliminary studies, the French contribution to MIRI was approved in 2004 by CNES, CEA and CNRS.
The flight model of the MIRIM imager was assembled and tested at CEA Paris-Saclay in 2008 and 2009 ; a test bench that reproduces the vacuum and cold conditions that MIRIM will encounter once in space was developed especially for the occasion. In 2010, MIRIM was sent to the Rutherford Appleton Laboratory in England to be coupled with the other part of MIRI, the MRS spectrometer, then tested in a vacuum chamber large enough for the complete instrument.
In 2012, MIRI was sent to NASA’s Goddard Space Center near Washington, D.C., where it was coupled with the other three JWST instruments. Three rounds of cryogenic testing followed between 2012 and 2016.
The telescope’s 18 primary mirror hexes were also assembled at Goddard Space Center from November 2015 to February 2016. The instruments were mounted on the back of the telescope’s primary mirror and the entire assembly was sent in 2017 to Houston for testing, as the test station at Goddard Space Center was not large enough to accommodate the telescope. The CEA team was on site for testing when Hurricane Harvey hit. More fear than harm ; just a few nights in the lab without being able to get back to the hotel and a car completely drowned !
Assembly of the telescope (golden mirrors and instruments, including MIRI) and the heat shield (which looks like aluminum or plastic foil and is deployed at 0 :28 in the video).
© NASA Goddard.
Once testing was complete, we "dropped" MIRI off for its journey to the Northrop Grumman facility in California, where it arrived in early 2018. There, the telescope was coupled with the satellite and the large thermal shields that will prevent the Sun’s, Earth’s and Moon’s rays from reaching the telescope. This one will then be able to passively reach a temperature of about 45K (-228 ℃), necessary not to interfere with infrared observations.
Finally, at the end of September 2021, Webb left California for Kourou, where it arrived after a 16-day boat trip that took it through the Panama Canal (blocked a few months earlier !).
Ready for liftoff... and to start the scientific tests and observations
The space adventure will then begin on December 22, 2021, with the series of tests on the sky which will last 6 months. Then, at the end of June 2022, it is the scientific exploration which will be able to start, after three decades of developments.
A small part of the observation time is reserved for the astrophysicists who participated in the instrumental development. Within this framework, we coordinate the observations which will be devoted to exoplanets, to Supernova 1987a, and to two photodominated regions.
Most of the observing time will be "open" : each year during the 5-10 years of Webb’s lifetime, several calls for the use of Webb are scheduled. The first call was in 2020. More than 1000 applications were submitted, involving more than 4000 astrophysicists worldwide. The number of observing hours requested is much higher (4 to 5 times) than the number of hours available and the selection was made by committees of scientists. It is satisfying to see that MIRI is the second most requested instrument. We did well to insist that it "goes up" on the Webb !
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MIRI is an instrument co-developed by a consortium of European space laboratories, which took care of the opto-mechanical aspects, assembly and overall testing of the instrument, and NASA’s JPL Center, which provided the detector arrays and cooling system for MIRI.