The most expensive and perhaps most complex folding work ever has to be completed today.

It is without a doubt the most important part of the James Webb telescope: the 6.5 meter wide primary mirror. No wonder we look forward to this afternoon with somewhat unhealthy tension. Because then NASA hopes to fully unfold this primary mirror. It’s the last – and perhaps the most nerve-racking – step to make James Webb into a functioning space telescope.


James Webb’s main mirror is made up of 18 smaller segments covered in real gold. Prior to launch, the six segments located on either side of the main mirror have been gently folded back. This was necessary, because otherwise the main mirror would not fit in the Ariane 5 rocket.


It’s now been two weeks since James Webb freed himself from that cramped launcher and it’s time to restore the main mirror by folding the six mirrors forward. A first important step in that direction was already taken yesterday. Then the left side panel – showing three of the six rearward-folded mirror segments – was successfully folded forward. And today follows the side panel with the last three mirrors, which is located on the right side.


It may all sound quite simple. But it is quite a complex and time-consuming undertaking that – based on what we saw yesterday – will certainly take several hours. First of all, engineers need to make sure that the mechanisms that keep the side panel folded backwards are released. If successful, motors on board the telescope will be instructed to fold the side panel forward. That takes about five minutes. It then takes the engineers another two hours to secure the side panel. And all of this is happening while James Webb is more than 1 million kilometers from Earth and moving through space at more than 1400 kilometers per hour.

Exciting two weeks

It’s a feat. And there is not much room for mistakes or setbacks; technicians or astronauts cannot travel to James Webb to fix a mistake or do some repair work, as was the case with the Hubble telescope, for example. It makes every step taken to make James Webb a functioning space telescope incredibly exciting. In fact, anyone who cares about the mission has actually been in suspense ever since it was launched, exactly two weeks ago. Because it wasn’t just the main mirror that had to be folded to fit the Ariane 5 rocket; countless other parts also collapsed and folded prior to launch, and over the past two weeks, they were all unfolded and unfolded piece by piece. Solar panels, an antenna, the sun shield, the secondary mirror; we’ve seen the telescope gradually take shape over the past few days. The unfolding of the primary mirror is the last major step in that extremely complex plan and that means that James Webb – if all goes well – could still be fully unfolded today.


And you don’t have to miss any of that, because from this afternoon 3 pm (Dutch time) NASA will broadcast the unfolding of the last side panel of the main mirror live from. Spectacular images made from the telescope should not be expected (see box). But you’ll no doubt get a peek into the control room – where the tension naturally runs high – and there are animations of the process.

Why can’t we watch?
It’s been a common question in recent weeks: why aren’t there any cameras on James Webb that can record the unfolding of the various parts for us? Of course, the team behind James Webb did consider adding such cameras to the telescope. But that was eventually abandoned. And there are several good reasons for that. First of all, we must not forget that James Webb is a large telescope; the main mirror is 6.5 meters wide and the sun shield is even as big as a tennis court. To properly follow the unfolding of such parts, you therefore need a camera with a large field of view. The use of this would yield few detailed images and is therefore not very interesting for engineers. The use of many cameras with a limited field of view is more useful, but a lot more complex. And for the use of both variants, the required wiring would in turn create additional risks. In addition, Webb is very shiny, so especially on the side that faces the sun, you would hardly see anything in images. And on the side that does not face the sun, it is too dark for good images. The temperatures are also so low – thanks to that sun shield – that ‘normal’ cameras can’t actually function there. Of course, the absence of cameras doesn’t mean engineers have no idea what’s going on aboard the telescope. “Webb’s built-in sense of touch (e.g. buttons and various mechanical and electrical sensors and temperature sensors) provides us with much more useful information than surveillance cameras,” said Paul Geithner, member of the James Webb team.

When the panel on the side of the main mirror is unfolded, James Webb is officially finished. But the first images of the most powerful space telescope ever built are still some time away. Because James Webb is not yet at his destination: it is expected that the telescope will need another two weeks to reach Lagrange point 2. And then it takes another five months or so to calibrate the instruments and align the mirror segments so that they work together as one large mirror. That time is also necessary to allow the telescope to cool down further; the telescope must be very cold to be able to observe the infrared light from faint, distant objects. So just a little patience!