After a successful launch, subsequent deployment of its sunshield and mirrors, and arrival at its orbit around the Second Lagrange Point, the goal now for the James Webb Space Telescope is to align the optical components of the telescope to the required accuracy. This is not a simple task – each of the segments of the main mirror has to be aligned to within a fraction of a wavelength of the light it will observe (in the near-infrared part of the electromagnetic spectrum) – and it will take several months to complete. However, we did hear yesterday that the telescope has now seen “first light”, in the sense that the first photons have landed on its detectors. The first images to be formed will be blurry and distorted, but these will be used to adjust the components until they reach the required sharpness.
For more details of this process see here.
Incidentally, it is worth saying a little bit about L2, the second Lagrange point of the Earth-Sun system. As the diagram below shows, this orbits the Sun at a greater distance from the Sun than the Earth. According to Kepler’s Laws, and ignoring the Earth’s gravitation, a test particle placed in a circular orbit at this radius would move more slowly than the Earth and would not therefore hold a fixed position relative to the Earth and Sun as it went around. The effect of the Earth’s gravity however is to supply an extra force to speed it up a bit, so it can keep up and thus remain in a fixed configuration relative to both Earth and Sun.
The opposite applies to L1: an object placed here would, in the absence of Earth’s gravity, move more quickly and thus pull ahead of the Earth. Having the Earth there holds it back by just the right amount to maintain a fixed position in the rotating frame.
The interesting thing about L1 & L2 is that while they are both equilibrium points, they are both unstable to radial perturbations. An object placed at either of these points would move away if disturbed slightly. JWST does not therefore just sit passively at L2 – it moves in a so-called halo orbit around L2 a process which requires some fuel. It’s not that there’s an actual mass at L2 for it to orbit around, but that its motion produces a Coriolis Force that keeps it from moving away. It’s very clever, but does require a bit of energy to keep it in this orbit.
Unlike L1 & L2, the Lagrange Points L4 & L5 are stable and therefore attract all kinds of space junk, such as asteroids, cometary debris, and preprints by Avi Loeb.
Another interesting Lagrange Point is that Joseph-Louis Lagrange was born in 1736 in Turin, but that does not mean that he was Italian. At that time Italy did not exist as a political entity; in 1736 Turin was part of the Kingdom of Sardinia. Although born in the part of the world now known as Italy, he was never an Italian citizen. In fact he lived most of his life in Berlin and Paris and died in 1813, long before the Kingdom of Italy was founded (in 1861).
In the Dark 