Why the space telescope is still far from delivering its first images


It will be necessary to activate more than 150 mechanisms from Earth without any possible misstep. « Only » so that the James Webb Telescope (JWT) can be deployed once it arrives in space, which will embark the folded Ariane 5 rocket.

This step alone shows the complexity of launching the largest and most powerful telescope ever to be sent into space. A sun visor as big as a tennis court, a primary mirror 6.5 meters in diameter, made up of 18 hexagonal segments made of beryllium and covered with a gold film … The JWT is « without doubt the machine. the most complicated that humans have built « , launches German astronomer Markus Kissler-Patig, chief » science « director of the European Space Agency (ESA), one of the three involved in this mission with the CSA (the Canadian) and especially Nasa (the American), which is in charge.

A jewel of technology worth 10 billion euros in the hands of Europeans

It is this Saturday that we will have to hold our breath. After several postponements, this is the last date set by NASA for the launch of the JWT, encapsulated in the fairing at the very top of the European Ariane 5 rocket which will send it from the Guyanese Space Center (CSG) in Kourou. Take off at 9:20 a.m. local time, 12:20 p.m. on the Greenwich Mean Time.

A formality for the European rocket? Ariane 5 is in any case far from being a novice with its 112 flights on the clock, which make it « the world leader in heavy launchers, » indicates Daniel de Chambure, engineer and responsible for the adaptation of Ariane 5 for the JWST mission. . A little more after its last flight, for which Ariane 5 sent more than ten tonnes of payloads to geostationary transfer orbit. A record. « 

With the JWT, we are on 6.5 tonnes of payloads. But what a payload! The space telescope, presented as the successor of Hubble, has been imagined since 1989 and required $ 10 billion in investment. And the expectations are colossal. With its ability to see in the near and medium infrared, the James Webb represents « such a leap forward » for astronomy « that it will inevitably be at the origin of discoveries and that the most beautiful are those that we do not imagine yet ”, slipped in June Perre Ferruit, co-responsible for the ESA of the mission. This gives an idea of ​​the pressure weighing on the shoulders of the more than 100 experts present at the CSG in recent weeks to prepare for this launch.

Already several hot shots

The heat waves will not start on Saturday. Daniel de Chambure quotes a first: the transport by boat of the telescope from Long Beach (California), from where it left on September 26, to Kourou, where it arrived on October 12. A cruise of around 10,000 km, passing through the Panama Canal, which the JWT carried out in a container specially designed for it. Safe from the slightest contamination, whether dust, organic molecules or clothing fibers. In short, anything that is likely to be deposited on one of its two mirrors (the secondary and the primary with 18 hexagonal segments) and which could adversely affect the quality of its observations.

Daniel de Chambure made this « need to permanently maintain the James Webb Telescope in an ultra-clean atmosphere » one of the complexities of the mission. Not just during his transport by ship, but also throughout his preparation in the Final Assembly Building (BAF), where he spent his last weeks on Earth. To the point of having placed it « under an overpressurized atmosphere in a sort of tent delimited by curtains. [de 12 mètres de haut] and in which the interveners enter by an SAS and in full suit, ”says the engineer from ESA. In other words, a clean room in the clean room, the BAF having itself been prepared to be free of all visible dust and organic molecules in suspension before welcoming the JWT.

In the Final Assembly Building, the JWT was placed in a sort of tent bounded by curtains 12 meters high. In other words, a clean room in the clean room. – ESA-Manuel Pedoussaut

Folded and encapsulated to within 8 mm in the rocket fairing

Cleanliness was not the only challenge in recent weeks. Another, just as colossal, was being able to fit the telescope into the launcher. No choice: it had to be folded. “Like an origami”, we describe at ESA. Until it only forms a rectangle 10.66m high and 4.5m wide. “It occupies the entire fairing with space margins that did not exceed 8 mm,” says Daniel de Chambure. This encapsulation was successfully completed last Friday. A week earlier, the telescope had found itself suspended from a cable 40 meters high before being delicately posed to the top of the Ariane 5 rocket. Another very delicate operation.

The James Webb Telescope and its launcher will be transferred to the launch pad on Wednesday, said the ESA, to then stand still until takeoff on Saturday. “The launch phase will be short, precisely 27 minutes,” resumes Daniel de Chambure. This step includes takeoff, separation of the rocket with its thrusters, then the capsule with the rocket, and finally the telescope with its capsule. The first signals of the JWT received from Earth are expected within the 30th minute after take-off.

Timeline of the launch of the James Webb Telescope, scheduled for Friday, December 24.
Timeline of the launch of the James Webb Telescope, scheduled for Friday, December 24. – ESA

One month to get to your destination … and deploy

Will it finally be time for the teams from the three space agencies to take a break? Not at all. The JWT will then only be at the very beginning of a long journey of 1.5 million kilometers to the point of Lagrange L2, where it is expected to orbit the Sun. Allow about four weeks, during which the James Webb Telescope will not have to just move forward. It will also have to deploy. We then come back to the 150 mechanisms that must be triggered without a false note. Starting with the solar panels, 31 minutes after takeoff, which will power the telescope instruments.

Then, “the sequence will begin with the deployment of the sun visor,” describes Catarina Alves de Oliveira, ESA’s scientific operations manager. First of the two structures that protect it, then of the sun visor itself. Before, finally, to ensure the right tension [l’étirement] and the separation of the five layers of insulation that compose it. Then it will be time to tackle the two mirrors. First « by deploying the support of the secondary mirror, then of the two side wings that make up the secondary mirror », continues Catarina Alves de Oliveira.

Fully operational in six months

All these operations are at risk, concedes the scientist from Esa, “but they were prepared and meticulously repeated on the ground,” she assures us. And, once the Webb is in space, we will proceed with caution, taking all of our time. « Once at destination and deployed correctly, we can then say that he did the hardest part. It will still be necessary to calibrate the telescope and its instruments by exercising it on several targets. A phase which should last five months. In other words: the first images of a fully operational James Webb are not expected for six months.

Hopefully they are the first in a very long series, with Hubble’s big brother scheduled to run for five years. At least at least because it takes fuel to operate at least double.



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