The James Webb Space Telescope
D. Indumathi, The Institute of Mathematical Sciences, Chennai
The James Webb Space Telescope (JWST) has recently been launched into space. As the name suggests, it is a telescope that will stay in space and will observe the Universe from above the Earth.
Hubble Space Telescope
Long ago, in 1990, the Hubble Space telescope was launched into space. We have learned many things about our Universe, its galaxies and stars, from observations made by Hubble. Some of the most important discoveries of Hubble is the measurement of the age of the Universe. We know that the Universe began in a big bang, about 10-20 billion years ago. After Hubble was launched, the age of the Universe is known to be precisely 13.7 billion years. In addition, physicists discovered that the Universe is expanding; it is not a static one. The rate of expansion of the Universe was also measured by Hubble. It also established that most galaxies have a massive black hole in their centre.
Now, the James Webb Telescope (JWST) is a much improved telescope which was launched on Dec 25, 2021. JWST was launched on a European Space Agency's Ariane 5 rocket from Kourou, French Guiana. The launch was successful and it is being tested to ensure that it is in proper working condition.
The telescope has been developed by the American National Aeronautics and Space Administration (NASA) in collaboration with the European Space Agency (ESA) and the Canadian Space Agency (CSA). It is the most powerful telescope ever launched into space. It has been designed primarily to conduct infrared astronomy. It will use infrared measurements to view distant objects which are too faint to be seen by the Hubble Telescope.
Why observe distant galaxies?
One of the most exciting discoveries of Einstein and other scientists is that nothing can travel faster than the speed of light. This speed of light is constant in vacuum (air) and is 3 lakh km/sec. For instance, light from our Sun takes 8 minutes to reach the Earth. So when we "see" the Sun, we are actually seeing it as it was 8 minutes earlier. This was when the light that we are seeing was emitted. If you think for a moment, you can see light from a far away star or galaxy has taken thousands of years to reach us. Since the age of the Universe is fixed, this means that far away stars that we see were formed early during the evolution of the Universe. Even though they may be old today, we are viewing the light they emitted when they were much much younger.
So observing distant stars actually gives us information about the early Universe and how it was about 13.5 billion years ago. It will give us information on how the first galazies formed, how stars first began to shine, and perhaps even tell us if there are other habitable earth-like planets in other galaxies.
The telescope
Like any other telescope, it has a primary and secondary mirror that helps to focus the light falling on it. One peculiar feature due to the expansion of the Universe is that space itself has stretched due to this expansion. So light emitted by these distant galaxies and starts in the ultraviolet and visible region are so stretched out that their wavelengths are now in the infra-red region! This is called the “redshift” of light. The Webb telescope is therefore designed to be maximally sensitive to (red and) infra-red light.
JWST's primary mirror consists of 18 hexagonal mirror segments made of gold-plated beryllium which combine to form a 6.5-meter diameter mirror. In contrast Hubble was only 2.4 m. Also, remember both Hubble and Webb are out in space: it is in a solar orbit near the Sun-Earth L2 Lagrange point (see Box for an explanation of Lagrange points), about 1.5 million kilometers from Earth and cannot be repaired by humans! This huge increase in size and complexity made the construction and deployment of Webb a huge technological challenge. It will take six month just to commission it so it can start taking data.
Cooling the telescope
Infra-red light is what hot objects emit. So if you want to observe objects by measuring their infra-red radiation, you must be careful to remove other local sources which could be emitting such radiation. These local sources could be small electrical instruments on the telescope, all the way to the Sun, Moon and the Earth! To achieve this, the entire telescope is kept at an incredibly low temperature of below 50 K (−223 °C). This is done by a five layer kite-shaped massive sunshield that protects it from infra-red light from all these objects. But the telescope had to be launched on a rocket into space, and there was no space to put such a huge sun-shield. The engineers working on Webb came up with an amazing idea: The sunshield was folded into five layers, each roughly the size of a tennis court, and each layer as thin as a human hair with a membrane made of Kapton to reflect back the heat falling on it. Within 10 days of launch, the sunshiled was fully spread open, to a size of 14.6 meters wide by 21.1 meters long.
Unfolding the mirror
The mirrors are very big and were stowed away during launch. These have also to be moved into place one by one. First the secondary mirror was moved into place. The secondary mirror plays an important role in reflecting the light from the primary mirror to where the instruments sit, behind the primary mirror. It must be aligned to within 1.5 mm!
After this is done the primary mirror is unfolded, in 6 sections. Webb's mirrors are covered in a microscopically thin layer of gold, which optimizes them for reflecting infrared light. Within two weeks of launch, Webb's mirrors were unfolded. Next, the 18 mirror portions had to be carefully aligned so that they would act like one big mirror, smoothly reflecting the light that fell on them. This will take a few months and is still being done.
Moving into position
The observatory then travelled to an orbit about 1.5 million km away from Earth till it reached the special Lagrange L2 point (see box). For the next few months it will undergo extensive testing and alignment of mirrors before it moves into the final phase of taking data. The world is eagerly waiting to find out what it will see and what new mysteries it will uncover.
BOX: What is a Lagrange point?
We know that two massive bodies are attracted to each other by the force of gravity. The Sun attracts the Earth towards itself. At the same time these objects are rotating; the Earth goes around the Sun in a nearly circular orbit. We have also learned that when an object is undergoing circular motion, it experiences the centrifugal force as a reaction force according to Newton's third law. When the gravitational force and centrifugal force cancel one another, the forces are balanced at that point. These are points of equilibrium and are called Lagrange points. When one body is much larger than the other body, then the point of balance is closer to the lighter body.
Systems such as the Sun-Earth or Earth-Moon system can be analysed for their Lagrange points. There are five such points, numbered L1, L2, L3, L4, L5, as seen in the figure. We see that L1 and L2 lie on the line joining the centres of the two bodies. At L1, the gravitational force of the Sun (bigger body in figure) is equal and opposite to that of the Earth (smaller body). Hence, a third object placed at L1 will not feel any force of gravity. At the L2 point, the gravitational pulls of the Sun and Earth do not cancel. But if you place a small body at L2, the centrifugal force it experiences is exactly cancelled by the gravitational force due to the two larger bodies. Hence L2 is a stable point, and is where the James Webb telescope is located. L3 is the point symmetric with L2, on the other side of the Sun. An object placed at any of these points will have the same oribital period as the Earth.
The L4 and L5 points lie at the third corners of the two equilateral triangles formed by the two heavy bodies, making an angle of 60° with the line joining the two. Any object placed at these points is in stable equilibrium. So it is very common to find parts of comets or other space debris at these points. The L1, L2 and L3 points are unstable so no space junk accumulates at these points. A small amount of power is needed to keep space ships such as Webb in small orbits around these points.
END OF BOX
Credits: Pictures from NASA website https://jwst.nasa.gov/index.html and Wikipedia.