Day: December 27, 2021

The world’s largest and most powerful space telescope yet built – the James Webb Space Telescope (JWST) – finally made its way into space on Christmas Day, December 25th, 2021, marking the start of a mission almost 30 years in the making.

That mission is multi-part in its scope, encompassing as it does looking back to the origins of the universe and the galaxies around us, together with gaining a greater understanding of the nature and formation of galaxies, stars and planetary systems, and learning more about the nature of worlds beyond our own solar system, as well as seeking signs of the potential origins of life. It is a mission that has been plagued by technical and other issues that have repeatedly delayed its launch – and high winds along its path of ascent to orbit caused one final delay, pushing the launch back from Christmas Eve to Christmas day.

Final countdown commenced several hours ahead of lift-off, with the Ariane 5 launch vehicle igniting its engines as scheduled at 12:20 UTC, rising into the sky over the European Spaceport near Kourou, French Guiana, carrying the US $10 billion telescope on the first leg of a journey to its operational destination that will take it almost a month to complete. Along the way it will go through a series of complex activities along the way, each one vital to its operational success.

The first three of these activities came just half-an-hour after lift-off, with the separation of the telescope from its Ariane upper stage after the latter had boosted it onto the start of its 1.6 million kilometre journey away from Earth. Almost at the same time, JWST deployed the solar array vital for supplying it with electrical power. This was followed two hours later by the deployment of the high gain communications antenna and, 12 hours after launch, JWST completed the first “mid-course” correction to its trajectory, steering itself more closely towards its final destination.

This destination lies close to the Earth- Sun L2 Lagrange point, 1.6 million km further out from the Sun than Earth’s orbit, but which orbits the Sun in the same period of time as Earth. It’s a location selected for JWST’s operations for a number of reasons, including:

• It effectively puts the Earth, Moon and Sun “behind” the telescope, affording it uninterrupted views of the solar system and all that lies beyond it.
• It is a semi-stable position in space that orbits the Sun at the same time as Earth. This both allows for continuous direct-line communications, and reduces the amount of propellants JWST would otherwise require for basic operations such as station-keeping and orbital corrections.

Even so, operations at the position will not be straightforward. As the L2 position is a point of gravitational equilibrium, JWST will operate in an orbit 800,000 km wide around it. Whilst relatively stable, this orbit will require JWST to make small periodic adjustments every 23 or so days. Given it can only carry a finite amount of propellants (168 kg) for these adjustments, the telescope effectively has an operational “shelf life”: it’s primary mission is set at just 5 years – although it is hoped it has sufficient propellants for at least 10 years worth of controlled observations.

Having been launched in a “packed” form that allowed it to fit inside the payload fairing of its launch vehicle, JWST will spend the next two weeks gradually “unfolding” itself, as per the video below, with a number of firings of its thrusters to fine-tune its flight to its intended orbit.

All of these activities are vital to JWST being able to perform its desired mission, but perhaps the two most important are the deployment of the telescope’s secondary and primary mirrors, and that of its incredible and delicate heat shield.

The optics deployment will see the booms supporting the secondary mirror that reflects light gathered from the primary back to where it can be delivered by a third mirror to the instruments deep inside JWST. The second part comes with the unfolding of the “table flap” elements of the primary mirror, allowing it to reach its full 6.5 metre diameter, almost 2.5 times the diameter of the primary mirror on the Hubble Space Telescope. (HST), and with potentially 100 times its power.

JWST is primarily intended to operate in the infrared, but in order to do so, its instruments and science systems must be kept very cold. If any of them exceed 50ºK (-223.2ºC), the heat they generate will be registered in the infrared; potentially overwhelming the telescope’s ability to capture the infrared light of stellar objects. Given that JWST will be in permanent sunlight, maintaining such an incredibly low temperature this is a considerable challenge – hence the vital role of JWST’s remarkable heat shield.

This comprises 5 layers of Kapton E polymide formed into sheets as thin as a human hair and then covered on both sides with a thin membrane of aluminium, this shield is carried folded within two “pallets” that also need to be unfolded to form the “base” of the telescope.

Once these pallets have unfolded, booms can be extended on either side of JWST, allowing the 5 layers of the heat shield to be unfurled like the sails of a ship, and then tensioned off. This will provide an area of shadow the size of a tennis court within which the instruments and optics of the telescope will sit, while radiators behind the main mirror will circulate the heat absorbed by the shield and radiate it back into the cold shadow without impacting telescope operations.

Continue reading “Space Sunday: JWST and a touch of SpaceX” →