This is the first question that comes to mind when thinking about space telescopes.
The main reason is that we want to get rid off the atmosphere. Acting as a filter, and as the main reason why we can live on Earth, the atmosphere stays just between the observer and the observed. However, due to its composition, physical phenomena prevent a ‘true’ observation of Space, and therefore we need to get out to obtain better results. Remember, James Webb Space Telescope will not just ‘look’ as we would do with our eyes: a lot of sophisticated instruments are onboard and the scientific reasons behind the need of atmosphere absence will be detailed, but this is the general overview of why it needs to avoid atmospheric perturbations.
Once it has been decided that it had to be put outside our atmosphere, the next question was: where exactly? Space is huge…
The answer is simple but, as often happens, simple answers hide complex processes: Lagrangian Points. Without jumping into maths (if you want to see it let us know: get in touch!), our reduced System can be limited at the proximity of Earth’s gravitational sphere of influence. It means: look at the biggest celestial bodies around our planet and see how their gravity influences objects nearby.
It happens that we can consider three actors in the play: the Sun, Earth and the Moon. By considering their motion and their mass, five points are determined.
As we can see from the figure (and assured by Maths…), they are disposed in a way that gravity from each celestial body perfectly balances. While L4 and L5 are unstable points, L1, L2 and L3 are basically stable. An important fact has to be remembered: this is an instantaneous representation, like a photography, but Earth and the Moon are continuously moving and rotating along their orbits, respectively around the Sun and around the Sun-Earth system. This means that reaching those points will be very energy demanding, and energy means rocket fuel.
Webb has been placed in L2. As said it is a stable point, which means that once an object is located onto it, only minor adjustments are necessary to keep it in position. This will benefit in particular the energy required to keep Webb alive and in place, saving amounts of fuel and electricity.
Let’s now give a closer look to its orbit.
Launched from the Sun-facing side of the Earth, Webb travelled about 1,609,344 km to enter its halo orbit around L2 point.
Last but not least, this orbital configuration with the design of the spacecraft, allows it to keep observing the cosmos with its back side towards the Sun. In this way, the precious scientific instruments located on the front are kept cool and protected from solar radiation.
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Some has already been published about this topic, and more will be in the future!
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