JWST | The most complicated deployment in the history of satellites

Ariane 5 has already carried out a variety of missions, including the launch of the ATV cargo ship to refuel the International Space Station, the launch of the Rosetta probe to a comet and the launch of the Herschel and Planck satellites to the second Lagrange point, known as L2, where the JWST will soon be going. But Ariane 5 is first and foremost a launcher designed to send satellites into geostationary transfer orbit. This elliptical orbit is characterised by a perigee of a few hundred kilometres and an apogee of 36,000 kilometres. This altitude of 36,000 kilometres corresponds to the altitude of satellites in geostationary orbit, which includes telecommunications and meteorological satellites. This launch will therefore not be a routine launch, especially as launches remain high-risk events where failure can never be ruled out. This has been the case for Ariane 5's Russian competitor, the Proton launcher, which has suffered several major failures over the past decade. 

The L2 Lagrange point, one of the equilibrium points in the Earth-Sun system, is located 1.5 million kilometres from the Earth on the side opposite the Sun. This distance corresponds to one hundredth of the distance from the Earth to the Sun. Although the trip to L2 will only take about a month, the JWST will not be operational for another six months while the mirrors are aligned and calibrated.   

During the first month of the mission, many steps will have to be successfully completed. First of all, it should be noted that the launch is a very short stage in the life of a satellite. Ariane 5 will operate for about ten minutes and the JWST will separate from the rocket after 30 minutes and set off on a direct trajectory towards L2. From that moment on, what is probably the most complicated deployment in the history of satellites will begin.   

First, the 6-metre solar panels will be deployed immediately to provide the electrical power needed for the satellite to function properly, followed by the deployment of the telecommunication antenna. On the first day, the first of three small propulsive manoeuvres, known as mid-course correction manoeuvres, will take place to ensure that the satellite is in the correct orbit on arrival. The other deployment operations will start 2.5 days after launch and will take place over several days. The critical point will be the deployment of the JWST's solar shield, which consists of five layers of Kapton, each a few hundredths of a millimetre thick and the size of a tennis court. As the telescope observes in the infrared, it will have to be cooled to extremely low temperatures (-220 degrees Celsius) and the role of the solar shield will be to protect it from external light sources such as the sun, the earth and the moon, but also from the heat emitted by the satellite itself. The telescope with its primary and secondary mirrors can then be deployed. 

A total of 50 elements will need to be deployed including the solar panels, telecommunication antennas, mirrors, solar shield and a flap to balance the solar radiation pressure on the satellite. This will require the use of just under 200 deployment mechanisms and 400 pulleys.  

Once the satellite is launched, the die will be cast. More than 300 elements have been identified as single-point of failure, which means that if one of these elements fails, there is no backup; the mission is in great danger. For example, if the sun shield fails to deploy properly, it will be impossible to cool the telescope to the required temperature, resulting in images that are likely to be unusable.   

This kind of mishap happened to the JWST's predecessor, the Hubble Space Telescope. Soon after launch, engineers and scientists realised that the primary mirror had an aberration that severely affected image quality. Astronauts therefore installed a correction system during the first servicing mission for Hubble. But unlike Hubble, which orbits the Earth at an altitude of 550 kilometres, the JWST will be 1.5 million kilometres away, making this kind of mission unfeasible. 

JWST will be a revolution in science, especially astrophysics, and will produce images of unprecedented accuracy and quality. But JWST also represents one of the greatest technological challenges in space to date. This has resulted in an explosion of the original budget and a delay of several years. Firstly, the size of the satellite with its solar shield is enormous, while everything has to be folded up at launch (solar panels, antennas, solar shield, mirrors). Secondly, the cooling of the telescope at very low temperatures and the design of the mirror in several segments also represented major technological challenges for the engineers.