Thrilling moments of the launch of James Webb Telescope

Science & Technology

You may never have believed it would happen, but the James Webb Space Telescope has launched, and it no, its rocket didn’t explode in a fireball on the way up. But that doesn’t mean we are all good to go just yet, the telescope has some risky months ahead of it.

Why is that? Well, James Webb has to make it to the Earth’s L2 Lagrange point, all the while performing the delicate operation of unfolding itself before it can unfold our understanding of the universe.
Credit to NASA / JPL-Caltech.

The Ariane 5 rocket is what engineers planned the James Webb Space telescope around, the rocket being one of the European Space agency’s big contributions to the mission. The Ariane 5 launches by first igniting the liquid fuel main stage and then the supplementary solid-fuel boosters. Ariane 5 has two of these boosters, weighing 277 tons, and those combined with the liquid fuel main stage can take the rocket to the edge of space by themselves.

Solid fuel is often used initially as while it can’t be controlled so easily, it is a cheaper fuel type and good for that brute force initial lift-off. Once the boosters are expended at about 75km up, they are jettisoned from the main stage, leaving just the liquid fuel engine burning. Liquid fuel is useful because it can be throttled if needs be.

About a minute later, the rocket is beyond the Karman line of 100km, and the fairing surrounding James Webb can be safely jettisoned. At this altitude, there is virtually no atmosphere anymore, so the telescope no longer needs the fairing’s protection. After about 8 and a half minutes after lift-off, the main liquid fuel stage is finally expended, and it too separates from the upper stage.

The main stage, like the fairings and the boosters before it, will all end up in the ocean. With all that weight gone, the upper stage ignites and gives Webb the boost it needs to get to the Earth’s L2 point. Initially, the altitude doesn’t increase, in fact, it even dips, as this part of the process is about building speed, and gravity helps with that a bit.

However, this is already a potentially risky phase of the trip, as the Sun’s rays are directly interacting with the telescope here. Although it is folded up, the sun shield is not deployed, and sunlight can reach Webb’s delicate instruments here, warming them up. Until the sunshield fully deploys, it’s not protected.

To distribute heat as evenly as possible, and to keep the delicate parts in shadow, this upper stage rotates back and forth by 30°, like a baby in a cradle. Once in position, the flight trajectory takes it up into space. Its speed still increases even as it combats Earth’s gravity due in part to the fact that the rocket is getting lighter with every passing second as the fuel is burning.

At 25 minutes in, the upper stage cuts off, and there is a 2-minute coasting phase. Here, the rocket rotates around to perform a collision avoidance maneuver. Webb then separates from the upper stage, and from then it is all by itself.

It is very interesting to me that this view of it leaving the upper stage is the last view we will ever have of the telescope itself, and seeing as this has already happened, here’s a beautiful shot of the very first thing Webb did, which is its thrusters firing, and then its solar panels deploying, which was performed slightly earlier than expected.

During separation, usually some rot, action is introduced on the payload, but this time there was hardly any rotation at all, and the thrusters had to do very little to keep James Webb from rotating too much, meaning it was safe to deploy the solar panels almost immediately.
Webb liftoff on Ariane 5Credit to ESA

From here, James Webb provides its thrust. The cradling maneuver continues using these thrusters, regulating the telescope’s temperature. To protect the mirrors as much as possible, these thrusters are only located on the sun-facing side of the spacecraft.

A day and a half after launch, the high gain antenna is deployed, and James Webb passes the moon’s orbit. At this point, James Webb has already slowed considerably, as it’s going directly against the gravity of Earth. Just short of three days into the mission, the sun shield palettes begin to deploy.

This is a very slow process to ensure nothing breaks, and so Webb doesn’t start spinning around uncontrollably from the palette’s movement. Almost 5 days into the mission, the tower separating the telescope and the sun shield extends. Next is the aft momentum flap.

This is a pretty clever fuel-saving idea, this flap will offset the tiny amount of solar pressure pushing on the sunshield’s membrane, meaning Webb won’t need to use as much fuel to combat this pressure and stay aligned. Still on day 5, the sun shield protectors will unroll and open. Now for the delicate operation. The center booms will extend, and with them, they will gently pull the sun-shield membrane open.
Webb’s journey to L2Credit to ESA

If you recall, this was one of the tests that failed back in 2018, as the membrane ripped in deployment. These membranes are thinner than human hair, and once fully extended, are the size of a tennis court. When stretched, these membranes will be tensioned, finally fully protecting James Webb from the Sun’s harsh rays.

The sun-facing side of the telescope will get very hot, almost to a boiling temperature, whereas the side with all the instruments will be kept to a cool -230°C thanks to this sunshield. 10 days after launch, the instruments are cooling down by passively radiating their heat into space.

From here, the secondary mirror deploys, locking in place. Next is the aft radiator, a passive cooling system that will radiate the instrument's own heat into space. This is crucial to maintain cool temperatures.

On day 12, the side mirror wings will rotate and latch into place15 days in, the telescope is fully deployed, and it's simply a matter of waiting for the telescope to reach the L2 point and cool into its final cryogenic temperature state. The last operation to complete before James Webb will be operational is the alignment of the primary mirror segments.

Each can be adjusted to within nanometre precision, allowing the light collected by Webb to be turned into crystal clear images. After 29 days, there is one final correction using the onboard thrusters to put Webb into its halo orbit around the Earth’s L2 point.

From this orbital point, the bright Sun, Earth, and Moon will always be behind the sun shield, meaning the brightest thing in Webb’s view will only ever be Jupiter.

Roughly 160 days after launch, the instruments will have cooled down enough to be operational, and any adjustments that need to have been made will have already been completed. From here on out, the James Webb Space Telescope will be collecting science and unfolding the universe.

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