Four weeks after the launch with the Ariane 5 rocket, the JWT has been fully deployed and arrived at its final observation point "L2" without any problems. But what is this "L2" point all about? Today it gets physical!

The "L" stands for Lagrange, more precisely: Joseph-Louis Lagrange, a great mathematician who lived in the 18th century. He wondered whether there were points in a system with the sun and the earth (or another planet) at which all the forces cancelled each other out. This is not at all self-evident. Since Isaac Newton, we have known that there is such a thing as "gravity". So if you were to place a lump of rock somewhere in our solar system by chance, for example, sooner or later this lump would be attracted by some planet or by the sun because of gravity and would move there. You always think that out in space there is "weightlessness" and you are floating around. But that is not the case. The gravity emanating from a planet or even from the giant sun has an infinite effect. The further away the lump is and the smaller it is, the weaker the gravity is, but it is there and the lump "feels" it.

Lagrange was now able to calculate that there are 5 points between the sun and the earth (or another planet) at which such a lump of stone would move with the earth and would neither rush towards the earth nor towards the sun. These stable points are called "Lagrange points". At these points, all gravitational and other forces cancel each other out. Three of these points are on the Sun-Earth axis, i.e. one between the Earth and the Sun (L1), one behind the Sun (L3) and one behind the Earth (L2). Two other points are located along the Earth's orbit around the Sun, one in front and one behind (L4 and L5).

So why does the JWT now fly to this L2 point, i.e. the point behind the Earth and the Sun? At this fixed point, the telescope (like a lump of stone) always moves with the Earth around the Sun and is therefore close to the Earth, even if the point is 1.5 million kilometres away. Because of its proximity to the Earth, good communication with the telescope is always possible. In addition, it is far from the Sun and is thus well shielded from the Sun's thermal radiation.

This L2 point is very popular. Several other telescopes and probes have already been parked there, e.g. the "Herschel" and "Planck" telescopes, or the "Gaia" probe, which will measure over 1.6 billion stars in the Milky Way.

However, the L1, L2 and L3 points are not particularly stable. With every small deflection, the probes and telescopes are pushed away from this point and fly out of the point somewhere. These deflections can originate, for example, from the Moon or Mars, which also exert gravitation. That is why the JWT has to fly small correction manoeuvres again and again. It needs its own fuel for this. The cost of fuel at this L2 point is very low, but the availability is also limited. Unfortunately, this limits the telescope's lifetime. But according to current estimates, the JWT should last 10 years. Enough time for many great images and groundbreaking discoveries.

On 25 December 2021, the time had finally come: the James Webb Telescope (JWT) was successfully launched into space on an Ariane 5 rocket from the European spaceport Kourou. The JWT is a joint project of the European Space Agency ESA, NASA and the Canadian Space Agency CSA and was supposed to be completed in 2007. But the technical challenges were enormous. It is the most complex and powerful telescope ever built. It weighs 6.5 tonnes, about as much as a school bus, and is, along with the Space Shuttle and the large particle accelerator at CERN, the most complex technical device ever built. The development and construction costs amounted to 8.8 billion euros.

What will the JWT observe / Part 1

Unlike the Hubble Space Telescope, it will not take pictures in visible light, but in the infrared range. We cannot see infrared light, but we can feel it when we lie in the sun, for example. It is therefore also called thermal radiation. The Hubble Telescope was able to take a picture of the most distant galaxy. The light travelled from there about 13.4 billion light years to us. It is one of the first galaxies after the Big Bang. But what was before that? We cannot "see" the first stars that were formed after the Big Bang. They are too small for that. But with the JWT, we can measure the thermal radiation they emitted after their formation today. At least that is what the scientists hope to achieve, among other things. To do this, they need an infrared telescope. And a big one, a very big one. Because the thermal radiation that has travelled over 13 billion light years to reach us only reaches us very weakly. The mirror of the JWT therefore has a diameter of 6.5 metres. This has never been seen before in space. The aim is to better understand what happened about 100,000 to 300,000 years after the Big Bang. Exactly what JWT will see and how far back the view will reach is not yet known. The expectations of the scientists are correspondingly high.

When will the first pictures come?

There won't be any real pictures, as we can see, because the JWT measures in the infrared range. But the images will probably be coloured, just like a thermal imaging camera on Earth. But that will take time. After the launch, the JWT will be on the road for about 4 weeks at its observation point, which is 1.5 million km away from Earth. During this time, the telescope has to unfold completely. A critical phase, because everything has to work. Over 170 so-called release points are set in motion so that everything can unfold. Among them is the heat shield, which is the size of a tennis court. It shields the sensitive devices from the heat radiation of the earth and the sun. Otherwise, this radiation would interfere with the measurements. On one side of the JWT it will be about plus 83 degrees, on the other side with the mirror about minus 233 degrees. The individual mirror segments will also be unfolded. Everything has to work. The JWT cannot be repaired (unlike the Hubble telescope back then). It is too far away for that. So, fingers crossed.