The James Webb Space Telescope Explained By Those Who Did

This time, there it is! The James Webb Space Telescope, or "Webb", is at Kourou and will be launched by an Ariane rocket on 22 december. 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 the infrared: NIR Cam, NIRISS, NIRSpec and MIRI (the consonances in "IR" come 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. Speed ​​of light requires, it will look thus 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 go and probe the atmospheres of exoplanets, stars and planetary systems in formation, the evolution of galaxies ...

Fomalhaut is the brightest star in the constellation Pisces Austral. It is surrounded by a disc of debris and dust. On the left, the image obtained by the Spitzer space telescope, on the right, a simulation of a typical observation expected with the Webb.
Andras Gaspar, Provided by the author

In short, all areas of astrophysics will benefit from it.

We are among more than 1 scientists from 200 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 wavelengths between 14 and 5 micrometers, it will be best able to observe 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 of 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 was part of the JWST instrument suite (called the “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 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 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 was done for Hubble, which orbits "only" 570 kilometers from Earth: when Hubble is put into operation, the quality of the images is reduced. was very disappointing, but the installation by astronauts of an optical corrector made it possible to restore the expected image quality.

Improvement of the optical quality of the Hubble Space Telescope thanks to the installation of corrective optics in 1993… when Hubble was already in space. Before correction on the left, after correction on the right.
NASA

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 a spectrometer, which makes it possible to study the light received as a function of the wavelength - and therefore, for example, to determine which chemical elements are present in the object that we observe (this is the "MRS"). The performance of these instruments placed at the focus of the largest space telescope in operation will be unprecedented.

Supernova 1978A seen by the Spitzer Space Telescope on the left, and a simulation of what you would expect with MIRI on the right. The image illustrates the improvement in the quality of observation, in particular in terms of angular resolution.
Patrice Bouchet, Provided by the author

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 exoplanets a lot by the so-called "transit" method: we examine the tiny variations in the luminosity of a distant star due to the passage of an exoplanet which would surround it. MIRI has therefore been “improved” to use this transit method. 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 coronographic mode tests carried out at Saclay: we see in the image on the right that when we position the source just in the center of the four quadrant coronagraph, we “turn off” the source, clearly visible on the left.
Pierre-Olivier Lagage, Provided by the author

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 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 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, and then tested in a vacuum chamber large enough for the complete instrument.

The MIRI instrument after assembly of MIRI and MRS.
Rutherford Appleton Laboratory

In 2012, MIRI 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 testing when Hurricane Harvey hit. 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). Source: NASA Goddard.

Once the tests were finished, we “let go” MIRI for its trip to the premises of the company Northrop Grumman, in California, where it arrived in early 2018. There, the telescope was coupled with the satellite and the large thermal screens which will prevent the rays of the Sun, the Earth and the Moon from reaching the telescope. This can then passively reach a temperature of about 45K (-228 ℃), necessary so as not to interfere with infrared observations.

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!).

MN ship Colibri arrived at the Port of Pariacabo, on the Kourou River, in French Guiana, on October 12, 2021. In a specially designed container is the James Webb Space Telescope.
Chris Gunn / NASA

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 sky tests which will last 6 months. Then, at the end of June 2022, it is scientific exploration that will be able to begin, after three decades of developments.

A small part of the observation time is reserved for 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 observation time will be "open": each year during the 5 to 10 years of Webb's lifespan, several calls for the use of Webb are scheduled. The first call took place in 2020. More than 1000 applications were filed, involving more than 4000 astrophysicists around the world. The number of hours of observation requested is much greater (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 he "get on board" the Webb!


MIRI is an instrument co-developed by a consortium of European space laboratories, which dealt with the opto-mechanical aspects, assembly and overall testing of the instrument, and the JPL center of NASA, which has supplied the detector arrays and the cooling system for MIRI.

Pierre-Olivier Lagage, CEA researcher at the Astrophysical Laboratory, instrumentation, modeling of the CEA, CNRS, University of Paris; Alain Abergel, Professor Paris-Saclay University, Astrophysicist at the IAS, Paris-Saclay University; Anthony Boccaletti, CNRS Research Director at LESIA, Paris-PSL Observatory, CNRS, University of Paris; Christophe Cossou, CEA engineer, developer for the JWST / MIRI instrument at the Astrophysical Laboratory, instrumentation, modeling of the CEA / CNRS, University of Paris; dan dicken, Project Scientist, Paris-Saclay University et Patrice Bouchet, Project Manager of the MIRI / JWST Center of Expertise, Department of Astrophysics, Atomic Energy and Alternative Energies Commission (CEA)

This article is republished from The Conversation under Creative Commons license. Read theoriginal article.

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