I’James Webb Space Observatory (JWST) is a telescope space developed in cooperation between the Nasa and the canadian space agencies and European aircraft whose entry into service is scheduled for June 2022 after a successful launch on board a Ariadne 5 in December 2021. Originally, it was intended to replace the hubble telescope. But the exceptional longevity of the latter will allow them to work together during observation campaigns. Like Hubble in its time, James-Webb will allow us to see theUniverse in an unprecedented way. Originally designed to operate for at least five years, the JWST is expected to operate for at least 10 years thanks to the lalmost perfect launch of Ariane 5.
See the light from the first stars that formed shortly after the Big Bang
Unlike Hubble, which makes observations in the light seen, the spectrum ultraviolet and the near infraredJames-Webb works only in the wavebands near infrared and mid-infrared. In these areas, he will be able to see the first detectable objects in the Universe, those that formed after the dark ages or dark, a period in the history of the Universe which begins after the diffusion of cosmic radiation, when it appeared hot and opaque, and before the formation of the first luminous structures consisting ofstars and of galaxies from 200 million years after the big Bang.
Four state-of-the-art scientific instruments
James-Webb embarks a series of four instruments, designed to observe the most distant objects, exoplanets with unprecedented precision, very little studied phenomena and allow major advances in many areas of theastrophysics and astronomy. These four instruments are housed inside the ISIM which also houses many subsystems. The MIRI and NIRSpec instruments are provided by Europe as part of its participation in the project:
- NIR Cam (Near-InfraRed Camera) : wide-field camera operating in the near-infrared wavelength range of 0.6 to 5 µm;
- NIRSpec (Near-InfraRed Spectrometer) : spectrometer multi-object operating in the near-infrared wavelength range of 1 to 5 µm,
- MIRI (Mid InfraRed Instrument) : camera and spectrometer operating in the near infrared wavelength range from 5 to 28 µm.
- NIRISS: set of four instruments with two spectrographs large field with grism and single-object with another grism as well as a non-redundant pupil masking interferometer between 3.8 and 4.8 microns and an imager with widescreen filters bandwidths between 1 and 5 microns in a 2.2′ x 2.2′ field.
This space observatory’s main technological characteristics are to be equipped with the largest mirror ever launched into space, to use a heat shield as large as a tennis court and to operate 1.5 million kilometers from the Earth.
The largest mirror ever sent into space
James-Webb’s 6.5-meter primary mirror is the largest ever sent into space. It is made up of 18 hexagonal segments made up of beryllium and coated with gold which, once in space, will unfold and align with an accuracy of 1/10,000 to form a single primary mirror of remarkable quality. Compared to the 2.4 meters of Hubble, the telescope of the James-Webb Observatory will have a collecting surface seven times larger. However, with a reclusion of 0.1 second of arcits images will be as sharp as Hubble’s, but much more sensitive, allowing it to image objects that Hubble cannot see, including exoplanets and details inside some galaxies.
The choice of a folded mirror at the time of its launch is explained by the fact that it is impossible to launch a mirror of such a large size in one piece. To date, no launcher in service has a fairing capable of carrying a payload of more than 6.5 meters in diameter. The largest mirror launched in one piece was that ofHerschel in 2009 (3.5 meters).
A heat shield as big as a tennis court
Since the JWST is intended to make observations in theinfrared only, and not in theultraviolet and the visible like Hubble, its temperature must be as low as possible to limit thermal noise, guarantee the proper functioning of the instruments and also reduce thermal variations. This is why the telescope and its instruments will be maintained at a permanent operating temperature of -253°C!
Despite its remoteness Sun and although located 1.5 million kilometers from Earth, the JWST needs a heat shield to be cooled to -253°C.
The size of this shield is puzzling. 22 meters long and 10 meters wide, it is almost as big as a tennis court! Due to its size, the shield was launched folded and unfolded in space. Indeed, unlike traditional shields, that of the James-Webb has the particularity of being made up of five layers of Kapton (a polyimide). With an area of approximately 150 square meters, each layer provides thermal protection in a wide range of temperatures (from – 237 to + 377 degrees). Each layer acts as a dissipator of heat, so that the temperature difference between the hottest and the coldest layer reaches 240°C! The thermal models show that the maximum temperature of the outermost layer is 109 degrees and that the cold side, the one placed in deep space, will be exposed to a temperature of the order of -237.5 degrees centigrade.
James-Webb will be located 1.5 million kilometers from Earth
Unlike Hubble, positioned some 570 kilometers from Earth, James-Webb is located about 1.5 million kilometers from Earth (four times the distance from Earth to the Moon) at the Lagrange point 2. A distance that makes it impossible to plan a repair or maintenance mission, like the five that punctuated the life of Hubble and which made it possible to correct its lack ofaberration optics as well as the replacement of numerous equipment and instruments.
From a first workshop in 1989 to its launch in 2021, a look back at the main dates that have marked the history of the JWST
1989: the Space Telescope Science Institute (STScI) of Baltimore, Maryland, and NASA are hosting the workshop Next Generation Space Telescopewhere engineers and astronomers discuss the scientific and technical capabilities of a space observatory that would succeed the Hubble Space Telescope.
1996: first formal recommendation that this future NGST should operate in the wavelengths infrared and have a primary mirror of more than four meters.
2002: NASA selects the consortia in charge of the construction instruments and satellite.
2004: the construction of the James-Webb Observatory begins.
2005: James-Webb will be launched by Ariane 5. The European launcher was provided by the European Space Agency (ESA) as part of its participation in the James-Webb program. In addition to launch services, ESA contributes to two of the four scientific instruments and provides the personnel necessary for mission operations.
2011: the 18 segments of the primary mirror are built and their compliance with the required specifications proven by tests. This year is also the year in which the program is threatened with abandonment by the American Congress because of the explosion of its cost from a small billion to more than six billion in 2011. Its cost is approximately 10 billion dollars. today.
2013: start of construction of the heat shield. An absolute necessity to guarantee the proper functioning of the instruments, whether the sensors or the optics which will have to remain extremely cold. The size of this shield is puzzling. 22 meters long and 10 meters wide, it is almost as big as a tennis court! Due to its size, the shield will launch folded and unfold in space. The challenge was therefore to design a mirror folded at launch, then deployed in orbit.
2017: start of satellite assembly and integration operations with its payloads and services. The James-Webb Observatory is taking shape.
2019: the observatory is fully assembled. Beginning of environmental, electrical, functional and communication tests which will last until Webb be bent one last time in view of its launch.
2021: JWST is launched on December 25 aboard an Ariane 5.
Summer 2022: commissioning of the James-Webb observatory.
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