The James Webb Space Telescope

The L2 point is great for space observation as it puts the Earth, Moon, and Sun behind the telescope providing a clear view of deep space, whilst still allowing easy communication with Earth, however, the L2 point orbit is unstable, and this means that the orbit will decay over time and the JWST will 'fall' into an unwanted orbit. To keep the JWST in the L2 orbit it's required to perform an orbital correction burn roughly once a month, unfortunately, this gives the JWST a limited life span in the L2 orbit as the fuel it consumes is finite .{2}

In its month-long journey to the L2 point the JWST will have to complete the so-called 14 days of terror. This refers to the 14 days over which the JWST will unfold its self, but first, approximately 3 minutes after detaching from the Ariane 5 launch vehicle the JWST will 'automatically' unfold its solar array. We say automatically as this action is one of only 2 actions the JWST will ever perform on its own, without commands being sent from a control centre on Earth. The second on these automatic actions occur approximately 90 minutes later, at this time the gimbaled high gain antenna will deploy this is how the JWST can receive its next instructions from Earth.

Now that the JWST can receive data from Earth commands can be sent for it to begin its unfolding processes.

First approximately two and a half days after launch the JWST will unfold its forward sunshield boom, and approximately 10 hours later the aft sunshield boom will also deploy, followed by the port and starboard mid-booms. The sunshield, which will face towards the Sun, Earth and, Moon at all times will not only enable to JWST to block light from our star, making it easier to observe fainter signatures in deep space but will also act as a buffer from the Suns' intense heating effects.

There are 5 layers to the sunshield once it is fully operational and correctly tensioned, and this means that while the sun side of the JWSTs' heat shield will be approximately 383K (109C), the other side (the cold side) of the sunshield where all the instruments are will be approximately 36K (-237C) thanks to the way the sunshield is designed this incredible temperature delta (346C) happens over less than 1.8 meters.

Another two and a half days will pass before all the sunshields membranes are released and tensioned correctly, the sunshield will then be fully operational.

A further day will pass before anything else happens. At this point approximately 6 days post-launch, the secondary mirror support structure will be deployed, 3 long booms extend the secondary mirror out in front of the primary mirror, this secondary mirror enables light collected by the primary mirror to be reflected and focused onto the telescope's instruments. The process of deploying the secondary mirror takes around 7 hours.

The final part of the telescope to be deployed over a week into the journey is the primary mirror segments. In its folded configuration the JWST has to reduce the cross-section of its mirror, and this has been achieved by using 18 gold-plated hexagonal segments which when deployed can fit together to produce a larger mirror analogue, the benefit of using smaller segments is that they are each individually controllable allowing for far more accurate reflection of the incoming light onto the secondary mirror.

The deployment of the primary mirror begins with the port side segments unfolding into place, followed by the starboard side, each wing holds 3 mirror segments and takes approximately 3 hours to completely unfold. over the coming week the JWST will continue its journey to L2 and the individual mirror segments with be manoeuvred into place to make sure that they work correctly.

Along the journey the JWST has to perform 3 mid-course correction manoeuvres, these manoeuvres are to enable to JWST to reach L2 at exactly the right speed and trajectory to quickly establish its self in orbit, the first of these manoeuvres must occur between 12.5 and 20 hours after launch and is the most important, it a continuous burn on the onboard thruster which could last a few hours.

The second burn fine-tunes the trajectory after launch and its duration will depend on the performance of the Ariane 5, this second burn is performed around 2 and a half days after launch.

The third and final manoeuvre burn is performed 29 days after launch with the JWST fully deployed this burn is an insertion burn into L2.

During this whole period the JWST cannot turn towards the Sun as it would damage the delicate onboard instrumentation. Even without turning around for the first few weeks orbiting in L2 the JWST will just be cooling down in a controlled manner using electrical heaters situated at certain important parts of the instrumentation, as well as turning on all the equipment which hasn't already powered up, the following 5 months will be spent correctly aligning the optics and calibrating the instruments. {5}

The James Webb will enable us to explore a massive range of questions, because the JWST will mostly be making use of the infrared spectrum, which humans cannot naturally see, but, will also be able to operate in the red (and some yellow) parts of the visible spectrum, and this will enable to JWST to 'see' light emitted by the very first luminous objects in the early universe.

It is able to identify this old light as it has become 'redshifted' and we now see what was once visible light as infrared, this 'red shifting' occurs when an object is moving away from the observer. The light 'stretches' increasing the wavelength shifting it towards the red end of the spectrum, coincidentally we can use this same method to identify that the universe is expanding.