This time, there it is! the telescope James Webb Space Telescope, or “Webb”, is in Kourou and will be launched by a rocket Ariadne 5 the December 24 [la date de lancement peut encore changer, NDLR]. Yes, it is with a delay of more than 10 years compared to the first estimates! Yes, its cost has been multiplied by 10! But the scientific harvest promises to be exceptional.
The Webb will make it possible to probe areas of the cosmos virgins of observations, thanks to its mirror 6.5 meters in diameter, the largest ever deployed in space, and its four instruments observing in theinfrared : NIR Cam, NIRISS, NIRSpec and MIRI (“IR” sounds come from “infrared”).
Explanation of the Miri instrument. © CNRS
Webb, flagship mission of the Nasa and European space agencies (ESA) and Canadian (CSA), will take over from Hubble Space Telescope to observe further into theUniverse. Speed of light obliges, he will thus look earlier in history, until the moments 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 sum, all fields of astrophysics will benefit from it. We are one of more than 1,200 scientists from 14 countries to have contributed to the development of the JWST. In France, we mainly participated in the development of the Miri instrument, the only one of the four instruments that operates in the so-called “thermal” infrared domain. Observing in the wavelengths between 5 and 28 micrometershe will be best able to observe the 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 gas and dust in which they form. Miri will also be the essential complement to NIRCam to identify the first galaxies in the Universe.
The Epic of the Webb Telescope
Initially, it was far from certain that an instrument for thermal infrared would be part of the JWST instrument suite (called ” next generation space telescope at the 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 ofastrophysicists who campaigned in Europe and the US for such an instrument.
That was… in the late 90s. The launch of Webb Telescope was then planned for 2007. But the Webb’s launch has been pushed back many times and the epic of the Miri instrument in fact illustrates well the reasons for these successive delays.
The Webb will be in orbit 1.5 million kilometers from Earth, i.e. 4 times the Earth-Moon distance. It will not be possible to go and repair it in the event of a problem, as has been done for Hubblewhich orbits “only” 570 kilometers from the Earth: when Hubble was put into operation, the quality of the images proved to be very disappointing, but the installation by astronauts of an optical corrector made it possible to restore the expected image quality.
For Webb, we have no room for error — hence the importance of pre-launch design and testing!
Miri, a cutting-edge instrument for exoplanets
Miri consists of two main parts: an “imager”, which allows you to take pictures (this is the part called “Mirim”and one spectrometerwhich makes it possible to study the light received as a function of wavelength – and thus, for example, to determine which chemical elements are present in the object being observed (this is the “MRS”). The performances of these instruments placed at the foyer of the largest space telescope in operation will be unprecedented.
In a sense, for the study of exoplanets, Webb’s launch delays are good news. Indeed, this field has exploded in recent decades and we currently have a wealth of exoplanets to observe, including rocky planetswhich were not known in 2007.
We are now studying a lot exoplanets by the method known as transits » : we scrutinize the tiny variations of the brightness of a distant star due to the passage of an exoplanet which would surround it. Miri has therefore been “enhanced” to use this method of transits. It is a question of reading only a small part of the detector, in order to do it very quickly without saturating the detector. Basically, we “divert” a little the primary purpose of the Webb, designed to observe faint or very distant objects, to take advantage of its great sensitivity.
Miri also has ” coronagraphs “. Historically used to observe the corona of the Sun by hiding the disc that is too bright which prevents the surrounding details from being seen, coronagraphs have been adapted to observe the stars, and thus distinguish any exoplanets that may be nearby. Miri carries a classic coronagraph (known as “de Lyot”) and three coronagraphs with phase mask “, very efficient, and which will be sent into space for the first time.
From cradle to takeoff
After several years of preliminary studies, it was in 2004 that the French contribution to Miri was approved by the CNESthe CEA and the CNRS.
The Mirim imager flight model was assembled and tested at CEA Paris-Saclay in 2008 and 2009; a test bench that can reproduce the vacuum and cold conditions that Mirim will encounter once in space has been developed especially for the occasion. In 2010, Mirim was sent to Rutherford Appleton Laboratory in England to be coupled with the other part of Miri, the MRS spectrometer, and then tested in a vacuum chamber large enough for the complete instrument.
In 2012, Mri was sent to Goddard Space Center NASA, near Washington, where it was coupled with the three other JWST instruments. Three rounds of cryogenic tests followed between 2012 and 2016.
The 18 hexagons of the telescope’s primary mirror 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 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 the tests when Hurricane Harvey fell down. More fear than harm ; just a few nights in the laboratory without being able to return to the hotel and a car completely drowned!
Assembly of the telescope (golden mirrors and instruments, including MIRI) and heat shield (which looks like foil or plastic and is deployed at 0:28 in the video). © NASA Goddard
Once the tests were finished, we “dropped” Miri for his trip to the premises of the company Northrop Grumman, in California, where he arrived in early 2018. There, the telescope was coupled with the satellite and the large thermal screens that will prevent the rays of the Sun, the Earth and the Moon to reach the telescope. This will then be able to passively reach a temperature of around 45K (-228 ℃), which is necessary so as not to interfere with observations in the infrared.
Finally, at the end of September 2021, Webb left California for Kourou where he arrived after a 16-day boat trip that took him through the Panama Canal (blocked a few months earlier!).
Ready for take-off… and to begin 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, scientific exploration will be able to begin, after three decades of development.
A small part of the observation time is reserved for astrophysicists who have participated in the instrumental development. In this context, we coordinate the observations which will be devoted to exoplanets, Supernova 1987a, and two photodominated regions.
Most of the observing time will be “open”: each year during the 5 to 10 year lifespan of Webb, several calls for the use of the Webb are scheduled. The first call took place in 2020. More than 1,000 applications were submitted, involving more than 4,000 astrophysicists around the world. The number of hours of observation requested is much higher (4 to 5 times) than the number of hours available and the selection was made by committees of scientists. It’s satisfying to see that Miri is the second most requested instrument. We did well to insist that he “come on board” the Webb!
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