Webb, the largest space telescope ever built, told by the French scientists who made it

This time, there it is! the telescope James Webb Space Telescope, or “Webb”, is at Kourou and will be launched by a rocket Ariane 5 the December 24 [la date de lancement peut encore changer, NDLR]. Yes, it is more than 10 years behind 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 untouched by observations, thanks to its mirror 6.5 meters in diameter, the largest ever deployed in space, and its four instruments observing in theinfrared : NIRCam, NIRISS, NIRSpec and MIRI (the consonances in “IR” come from “infrared”).

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 the Universe. Speed ​​of light obliges, he will thus look earlier in the story, until the moments when the first galaxies and the first stars were formed. But it will also relay the Spitzer infrared space telescope to go and probe the atmospheres ofexoplanets, stars and planetary systems in formation, the evolution of galaxies …

In short, all fields of astrophysics will benefit from it. We are among more than 1,200 scientists from 14 countries who have contributed to the development of JWST. In France, we have mainly participated in the development of the Miri instrument, the only one of the four instruments that operates in the field of so-called “thermal” infrared. Observing in the wavelengths between 5 and 28 micrometers, he will be the most able to observe the gas and dust in objects much colder than stars like our Sun. It will, for example, make it possible to see young stars still deeply buried in the cloud gas and dust in which they are formed. 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 a thermal infrared instrument would form 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.

It was … at the end of the 90s. The launch of Webb Telescope was then scheduled for 2007. But the Webb launch has been postponed many times and the epic of the instrument Miri in fact illustrates well the reasons for these successive delays.

The Webb will be in orbit 1.5 million kilometers from Earth, 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 to Hubble, which orbits “only” 570 kilometers from Earth: when Hubble was put into operation, the quality of the images was very disappointing, but the installation by astronauts an optical corrector made it possible to restore the expected image quality.

For Webb, we can’t afford to make mistakes – hence the importance of pre-launch design and testing!

Miri, a cutting-edge instrument for exoplanets

Miri is made up of two main parts: an “imager”, which makes it possible to take pictures (this is the part called “Mirim”, and one spectrometer, which makes it possible to study the light received as a function of the wavelength – and thus, for example, to determine which chemical elements are present in the object that we observe (it is the “MRS”). The performance of these instruments placed at foyer of the largest space telescope in operation will be unprecedented.

In a sense, for the study of exoplanets, the delays in launching Webb are good news. Indeed, this field has exploded in recent decades and we currently have a wealth of exoplanets to observe, including rocky planets, which were not known in 2007.

We now study a lot exoplanets by the so-called ” 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 “improved” 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 “hijack” the primary purpose of Webb, designed to observe faint or very distant objects, to take advantage of its great sensitivity.

Miri also has ” coronographs “. Used historically to observe the corona of the Sun while hiding the too bright disc which prevents to see the surrounding details, coronographs have been adapted to observe the stars, and thus distinguish possible exoplanets which would be nearby. Miri carries a classic coronograph (called “de Lyot”) and three coronographs ” phase mask », Very efficient, and which will be sent into space for the first time.

From the cradle to take off

After several years of preliminary studies, it was in 2004 that the French contribution to Miri was approved by the CNES, CEA and CNRS.

The Mirim imager flight model was assembled and tested at CEA Paris-Saclay in 2008 and 2009; a test bench which makes it possible to reproduce the vacuum and cold conditions that Mirim will encounter once in space has been specially developed for the occasion. In 2010, Mirim was sent to Rutherford Appleton Laboratory in England to be paired with the other part of Miri, the MRS spectrometer, then tested in a vacuum chamber large enough for the complete instrument.

In 2012, Mri was sent to Goddard Space Center from NASA, near Washington, where it was paired with the other three JWST instruments. Three series 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!