Launch of the James-Webb telescope by Ariane 5: an extraordinary scientific and technical mission
The Ariane 5 rocket on its way to its launch pad.
© Simon Rozé, RFI
Text by: Simon Rozé Follow
10 mins
This Saturday, December 25, an Ariane 5 launcher is due to launch from the Guyanese Space Center.
On board, a space telescope awaited for decades by the scientific community and the most expensive satellite ever sent to space.
Advertising
Read more
From our special correspondent in Kourou,
The birds of the Guyanese jungle continue their songs, unruffled.
Yet an impressive scene takes place right next door.
A strange team moves slowly between the trees: an Ariane 5 rocket 55 meters high, placed on an imposing launching table.
The assembly weighs 1,500 tonnes and has just left the launcher's final assembly building to head for the launch pad.
“
We have a very high cruising speed, 4 km / h,
” smiles Bruno Gérard, director of Arianespace in Guyana.
As impressive as it is, this operation is nevertheless routine and takes place each time the machine is launched.
The difference this time is at the top, well protected under the rocket fairing:
the James-Webb Space Telescope.
“
I started working on it in the early 2000s,
” recalls Daniel de Chambure, Ariane 5 manager at the European Space Agency (ESA).
It is indeed a project whose age can be counted today in decades.
In 1990, NASA launched Hubble, a space telescope that revolutionized astronomy with the power of its 2.4-meter mirror and the quality of its images.
As early as 1989, however, the scientific community began to work on a follow-up;
these projects take so long to complete that it requires a good dose of anticipation.
Over the years, the successor becomes clearer: it will be bigger, and will look at something else.
See the invisible
Hubble indeed observes in the visible part of the electromagnetic spectrum.
In short: he can see the same thing as our eyes.
It gave us wonderful shots of distant galaxies, interstellar dust clouds or planets in the solar system.
Unfortunately, there are very many items that are not visible in this lineup and Hubble could not answer all of the questions.
The decision was then taken to design a machine that could look in the infrared, invisible to our eyes.
"
This infrared vision consists of looking at this color which is inaccessible to us
", explains Pierre Ferruit, the scientific manager of the European side of the mission, at ESA.
“
It is really the color of choice to go and observe the first galaxies in our universe which appeared more than 13 billion years ago.
"
James-Webb will also have other targets than these primordial galaxies: exoplanets, orbiting stars other than the Sun.
This is all the more astonishing as the first of them was not discovered until 1995, that is to say after the first design works of the telescope.
“
Infrared is also very interesting for studying their atmosphere and their composition.
All the molecules that interest us, water, carbon monoxide, carbon dioxide, methane… all these molecules are visible in the infrared
”, continues Pierre Ferruit, who also mentions the study of black holes.
►Also listen:
La fabrique des astronautes: behind the scenes of a grueling training
But now, infrared has a very big drawback: anything hot emits in this range.
Measures therefore had to be taken to cool the telescope, otherwise the emissions of its own heat would interfere with its observations.
Webb will therefore have to be lowered to extremely cold temperatures: between - 230 and - 266 degrees Celsius depending on its instruments.
To achieve this, it was necessary to develop a system to protect it from the heat of the Sun, but also of the Moon and the Earth, as well as its own.
It is for this reason in particular that Ariane 5 must send it into orbit around the point of Lagrange L2.
1.5 million kilometers from Earth, this is an area of space that allows Webb to always turn his back on all of these heat sources.
Its back, in fact, is a gigantic heat shield that will deploy in space to the size of a tennis court.
On the Sun side, the temperature will be around + 125 degrees.
On the other side of the shield, it will be - 235 degrees.
An additional cooling system will lower the temperature of the instruments to the desired level.
An engineer nightmare
Infrared is not the only specificity of James-Webb: “
Beyond that, it's a large telescope.
This means that it will be able to collect more light and we will be able to see weaker objects.
Webb will go beyond the limits of Hubble,
”explains Pierre Ferruit.
Its main mirror, 6.5 meters in diameter, is thus almost three times the size of Hubble's.
Composed of 18 hexagonal panels covered with gold, it is kept folded in three during the launch and will deploy once in space: no rocket would indeed have been large enough to carry it.
A fold-out mirror, thermal protection the size of a tennis court, ultra-sensitive instruments… All these elements made the construction of James-Webb an engineering nightmare for Northrop Grumman, the industrialist selected by NASA to develop the telescope.
This is how Webb has become the satellite of all records in terms of exceeded deadlines and exploding costs.
It is today the most expensive in the world with a price over 10 billion dollars.
It is this object worthy of all superlatives that Ariane-5 will have to carry out successfully this December 25.
The European launcher was indeed very quickly chosen by the American space agency to fulfill this mission.
“
At the time, with the end of the shuttle program, the United States simply did not have a capable rocket and SpaceX did not yet have the capacity,
” recalls Daniel de Chambure.
“
Ariane 5 is also a very reliable launcher capable of propelling a satellite with excellent precision.
"
►Also read: Space: a reusable European mini-launcher signed ArianeGroup to compete with SpaceX
Extraordinary launch campaign
This is how Webb arrived at the port of Pariacabo in Kourou on October 12 for an extraordinary launch campaign.
For example, the satellite had to be prepared in a room with cleanliness criteria never seen before at the Guyanese Space Center.
"
NASA came with gigantic filters making it possible to create a very clean atmosphere
", details Béatrice Roméro, project manager for Arianespace.
“
It was absolutely necessary to avoid any human pollution.
Operators wore coveralls covering all parts of their bodies.
We only saw the eyes which were not allowed to be made up!
Perfume was also prohibited.
"
This level of requirement is essential for the proper functioning of the telescope.
The slightest dust deposited on the main mirror can cause a disturbance in the image capture, like a small insect that would land on a pair of glasses.
For perfume, this time it was molecular contamination that had to be avoided.
“
These are all the molecules released, like perfumes.
They are invisible but can react chemically with different components of the telescope.
This can damage the thermal properties of certain blankets and radiators, which will then not be able to dissipate heat properly
”.
Several adjustments also had to be made to the flight path that will be followed by Ariane 5 after take-off.
Webb doesn't like heat, and being exposed to the sun could damage him.
When it leaves the atmosphere and the fairing is released, the upper stage will thus begin to oscillate: “
this is called thermal roll, or in English the 'saw tooth', the tooth of saw,
”explains Béatrice Roméro.
“
There is a part of the telescope that cannot be exposed to the Sun.
"
It will therefore be oriented on the shadow side.
"
But the other party can't stay too long exposed either!"
We will therefore rotate, like a barbecue spit, to distribute the heat.
It is a maneuver specific to this mission
.
"
The Jupiter control room at the Guyanese Space Center.
© Simon Rozé, RFI
All these maneuvers will be followed from a very specific location.
A giant screen sits at its center, in front of which rows of computers are lined up like seats in an amphitheater.
In front of one of them, in the middle, a plaque: DDO.
Jean-Luc Voyer is the director of operations, in the Jupiter control room.
He will coordinate the launch, and he will take place a little over ten hours before the launch.
All information concerning the launch is sent to the DDO.
He must ensure that all the systems are green: the launcher on his launch pad must be in good condition and communicate with the ground;
same criteria for the satellite under the fairing;
the radar monitoring stations must be operational and finally weather conditions compatible with the launch.
“
There is a last weather report at less than 10 minutes,
” explains Jean-Luc Voyer.
"
If all goes well, at minus 7 minutes, I announce:" start of the final thrower sequence
. "
Ariane 5 is then considered "
alive
".
Unless there is a reverse order, it will automatically apply its flight program and take off at H0.
“
In the final count, the last fifteen seconds will be very emotional for everyone I think.
The last few seconds before Webb takes off from the Guyana Space Center, Europe's spaceport
.
"
The path to space
When the count reaches 0, Ariane-5 will ignite the Vulcain engine on its main stage.
The launcher will stay grounded for 7 seconds while it revs up.
At that point, its two booster boosters, the boosters, will fire, setting their 240 tons each of explosive powder on fire.
Ariane will then begin her ascent.
Two minutes and 21 seconds later, these will be empty and dropped.
At 3 minutes 26, it will be the fairing's turn and the thermal roll maneuver will begin.
Webb and Ariane will then be at an altitude of 110 kilometers, higher than the so-called Karman line which marks the border with space.
The main stage will continue to push until 8:47 minutes. Its almost 200 tonnes of cryogenic fuel will be used up and it will be dumped in turn.
►Also read:
Is the space battle launched?
The second stage will then take over and continue to propel the telescope up to 27 minutes and 7 seconds after takeoff.
James-Webb will be detached at that time and will take advantage of the momentum given by Ariane 5 to go to the point of Lagrange L2, 1.5 million kilometers further.
It will take 29 days for the telescope to reach its destination.
During this journey, he will have to face a very large number of critical stages.
Barely released by Ariane 5, it will deploy its solar panels to start generating electricity.
Then in the following days, he will have to unfold his heat shield, his secondary mirror, then his main mirror.
So many phases essential to the success of the mission, but of rare complexity.
Once all this has been done and arrived in position, it can then begin to cool down.
There will then be a phase of several months where the instruments will be switched on and tested one by one to verify that they have withstood the violence of the launch as expected.
The first calibration observations can then begin and its first image should reach us in six months.
This will be the start of a very big series: James-Webb is designed to run a minimum of 5 years in the worst case, but the community expects to receive his findings for at least 10 years.
Newsletter
Receive all international news directly in your mailbox
I subscribe
Follow all the international news by downloading the RFI application
google-play-badge_FR
Technologies
France
Space
our selection