In the previous part of this Space Telescope Series, we discussed about the basic principle on which James Webb Space Telescope (JWST) is based i.e. Infrared astronomy. Today we will discuss the challenges faced in launching space telescopes keeping in mind JWST in particular and space telescopes in general.
Size : How to fit it in rocket?
The James Webb Space Telescope is the largest, most powerful space telescope ever built. Details are as follows:
Payload mass: Approx. 6200 kg
Diameter of Primary Mirror: 6.5 m
Clear aperture of primary Mirror: 25 m2
The Webb telescope is as tall as a 3-story building and as long as a tennis court! The people at NASA have come up with a solution — to fold it! It was folded in origami style to fit inside the rocket to launch. The telescope unfolded, sun shield first, once in space.
Temperature: Maintain optimum range
Webb primarily observes infrared light, which can sometimes be felt as heat. Because the telescope will be observing the very faint infrared signals of very distant objects, it needs to be shielded from any bright, hot sources. This also includes the satellite itself! The sun shield serves to separate the sensitive mirrors and instruments from not only the Sun and Earth/Moon but also the spacecraft bus.
The telescope itself will be operating at about 225 degrees below zero Celsius (minus 370 Fahrenheit). The temperature difference between the hot and cold sides of the telescope is huge – one could almost boil water on the hot side, and freeze nitrogen on the cold side!
To have the sun shield be effective protection (which gives the telescope an equivalent of SPF one million sunscreens) against the light and heat of the Sun/Earth/Moon, these bodies all have to be located in the same direction.
This is why the telescope will be out at the second Lagrange point.
Location: Maintaining Right Balance-Direction
At Lagrange points, the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them. The L1, L2, and L3 points are all in line with each other – and L4 and L5 are at the points of equilateral triangles.
It is easy for an object (like a spacecraft) at one of these five points to stay in place relative to the other two bodies (e.g., the Sun and the Earth). In fact, L4 and L5 are stable in that objects there will orbit L4 and L5 with no assistance. Some small asteroids are known to be orbiting the Sun-Earth L4 and L5 points. However, L1, L2, and L3 are metastable so objects around these points slowly drift away into their own orbits around the Sun unless they maintain their positions, for example by using small periodic rocket thrust. This is why L1, L2, and L3 don’t “collect” objects like L4 and L5 do.
The balance of the combined gravitational pull of the Sun and the Earth at the L2 point means that Webb will keep up with the Earth as it goes around the Sun. The gravitational forces of the Sun and the Earth can nearly hold a spacecraft at this point so it takes relatively little rocket thrust to keep the spacecraft in orbit around L2.
As we see, the above three are the main considerations of the numerous parameters which need to be kept in mind while placing a telescope in space.