It took 30 years of development and almost $ 9.7 billion in investment. But after years of delay, the initial launch scheduled for 2007, the James Webb Space Telescope (JWST) is finally ready. Installed in the fairing of an Ariane 5 ECA rocket, it should lift off from the Guyana Space Center (CSG) on December 22. A historic moment for the US space agency (NASA), as well as for its European and Canadian partners. “This is the most complex mission that, on December 22, will put Kourou at the center of the world”, summarizes almost placidly Pierre Ferruit, scientific director of the telescope of the European Space Agency (ESA).
A few days before the start, the pressure is necessarily increasing. Because the Americans had never entrusted the launch of one of their most expensive technological gems to a foreign power, in this case the ESA and ArianeGroup. “The choice was made in 2003 because our rocket had the highest payload capacity available: a 5.4 meter diameter fairing to transport the 6.2 ton machine”, specifies Hervé Gilibert, Technical Director of ‘ArianeGroup. If the decision had been made today, surely Space X could have won the bet. But the choice is still relevant because, in addition to its high capacity, the Ariane 5 has proven its reliability (111 launches in all versions combined, 5 failures including 3 partials) and is powerful enough to propel JWST to its final destination. ” , Lagrange point 2 “, 1.5 million kilometers from our planet (against 384,400 kilometers from the Moon). However, failure is not an option: it would be catastrophic for the entire European aviation industry.
Diagram of the five Lagrange points, which are stable orbits for small objects. Lagrange point 2 (L2), located behind the Moon, was chosen because it is the farthest from the Sun. The James Webb Telescope will rotate around this point as it follows the Earth’s rotation.
NASA / WMAP Science Team
“We are preparing with special concentration and always with the same objective: to launch ourselves in full consciousness, that is to say with the certainty of having done the maximum when the final” go “is given, continues Hervé Gilibert. The same occurs with the National Center for Space Studies (CNES) that manages the Kourou base: “The JWST arrived here two months ago,” says Olivier Bugnet, Ariane’s deputy director for the French agency. under a ‘hood’, not only in a clean room but also in a specific tent to prevent the slightest dirt from settling there, particularly at the height of the main mirror and on its instruments ”.
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Prevention that did not prevent an incident when it was necessary to mount the telescope on site: a strap came loose and vibrated the main structure, fortunately without consequences. But NASA had to carry out a series of controls that postponed the departure for a few days. Soon, the telescope will reach the top of the Ariane 5 before being meticulously “encapsulated” within the fairing thanks to a millimeter guidance system. Finally, the assembly will move to the launch pad the day before launch.
The James Webb Space Telescope, here under the EPCU-S5 “hood,” at the Kourou launch center, French Guiana, on November 5, 2021.
Jody Amiet / AFP
The most complex display in space history
Takeoff will obviously be a crucial moment in the mission. The engineers worked to minimize vibrations inside the fairing, then in the ventilation: during the climb, there is an inevitable depressurization that will be managed by valves to make it as smooth as possible. But it is above all the sequel that promises serious sweats to the engineers. Because after takeoff, the telescope, detached from the Ariane 5, will begin “the most complex sequence of deployments ever attempted during a single space mission,” stresses Anthony Boccaletti, deputy director of the Laboratory for Space Studies and Instrumentation in Astrophysics (Meudon). And for good reason, the “monster” James Webb (22 meter wingspan when unfolded) had to fold up like a giant origami to fit on the French rocket. Once you are in space, the reverse process will begin.
“There are 344 points of failure, about 80% of them [275, NDLR] are associated with deployment, “warned Mike Menzel, the Webb mission principal engineer for NASA, during a press conference in November. In other words, if one of these” points “fails, the entire telescope will fail. it will involve in particular 144 trigger mechanisms that should work perfectly. Enough to make any rocket engineer pale. “When I started my studies, we were told in class that only when ‘we were designing a space mission, we had to try to have zero mechanisms! We have more than a hundred there ”, points out Olivier Berné, astrophysicist at the National Center for Scientific Research and head of one of the JWST scientific programs. The operation will be so complex that specialists plan to distribute it in 29 days. (see video below), the time it takes for the JWST to reach its final destination, Lagrange point 2.
The first stage will begin thirty-one minutes after launch, when the telescope deploys its solar panels that provide its power. 1h30 later, it will be the turn of its communication antenna with the Earth. After three days of travel, you will begin to unroll and separate the five different layers of your heat shield. This main element will cool down the telescope instruments, which should remain at – 233 ° C, when it will be at + 80 ° C on the other side of the shield, heated by the sun. The most sensitive MIRI instrument will even benefit from a “cryocooler,” an active cooling system that keeps it at -266 ° C. “It’s only 7 ° C above absolute zero,” insists Anthony Boccaletti, who participated in the design. of this tool. Cooling is a fundamental process because heat emits infrared emissions. However, the JWST instruments will accurately scan deep space in the infrared spectrum. In other words, the telescope cannot afford to heat up, otherwise it will contaminate its own observations.
The five overlapping layers of James Webb’s heat shield.
NASA / Goddard / Chris Gunn
“JWST without a heat shield is like a human being looking at the sky at night, but under a lamppost”, imagines Pierre Ferruit to describe the capital role of this sail, as big as a tennis court. “It raised some problems during the first tests on the ground,” recalls Olivier Le Masle, director of the CNES Universe Sciences program. Often these are not critical parts (detectors, electronics) but mechanical parts that were difficult to develop. ”
“Without risk, there is no science”
The technical epic doesn’t stop there, as JWST will still have to unfold its two mirrors. The larger one will reflect the light back to the second, which in turn will reflect it back to the center of the main mirror. “The primary, the largest, will have to deploy its 18 independent segments, even if they are mounted on the same structure, details Anthony Boccaletti. Above it, the secondary mirror, supported by three arms, will have to be perfectly aligned to correctly reflect the light back to the optical system and the instruments! No space telescope has a segmented mirror, unless the military has already, but in astrophysics, in civil science, it’s a great first, it’s crazy! ”
The 18 hexagonal elements of the main mirror are made of beryllium and coated with gold, which reflect infrared light particularly well.
NASA / Chris Gunn
In the face of such complexity, scientists cannot completely hide their apprehension, especially since Lagrange point 2 is too far away to hope that astronauts can come and repair it, as they had with Hubble. “I am necessarily a little worried, but also full of hope and, above all, I have confidence in NASA engineers, I know that they will know how to control the entire deployment,” explains Nicole Nesvadba, CNRS researcher in the laboratory. JL Lagrange in charge of a JWST science program aimed at better understanding the interactions between supermassive black holes and the galaxies around them.
The precautions of the American engineers also explain, in part, the accumulated delays: with each new defect discovered, they drew up the necessary plans for its elimination. Some mechanisms could be reset, for example, by sending a digital command. Zero risk does not exist. “Without a certain daring, we could not do science”, analyzes Nicole Nesvadba. A vision totally shared by his colleague Olivier Berné. “The complexity of this mission is tied to its scientific ambition, which is to revolutionize astrophysics and our understanding of the universe, he says. To achieve this goal, we must take a certain amount of risk. ”
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And even if the deployment apparently goes perfectly, just five months later JWST will make its first observations, at which point NASA must carry out a battery of tests ensuring the correct operation of the four main instruments: the NIRCam camera, the generator of NIRISS images, the NIRSpec spectrometer and MIRI. Everyone will look at space in the infrared spectrum, which is essential for space exploration, since it allows in particular … to go back in time! In fact, the more light travels in space, the older it gets, the more it “blushes.” Thanks to its infrared vision, JWST will go back to -13.5 billion years (Ma), or a few hundred million years after the Big Bang (-13.8 Ma)! It will then scan the “dark ages” of the Universe, when the stars first shone. It will also be able to scrutinize the processes of formation of the first stars, but also of the first galaxies and their relationship with the supermassive black holes that host at their center. You will continue to observe the Orion Nebula, a “nursery” of stars where stars comparable to our Sun come to life. Finally, you should be able to analyze the atmosphere of exoplanets or even, perhaps, detect water or methane molecules that, if they do not justify the presence of life alone is essential for its appearance. The first results should be published between the end of June and the beginning of July 2022 … If all goes well.
Diary of a Liberal
Diary of a Liberal
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