The James Webb Space Telescope

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The James Webb Space Telescope

What will the next-gen Webb telescope unveil? How is it different from Hubble? What will it enable us to see in the farthest reaches of the universe?

The story so far: The James Webb Space Telescope (JWST), hurled into space by the Ariane 5 rocket from European Space Agency’s Spaceport in French Guiana, South America, on December 25, at 7:20 am EST (5:50 pm IST), is en route to its destination. After a 29 day-long journey, the spacecraft will arrive at a point in space called ‘Lagrange point 2’, also known as L2. Costing $9.7 billion, this joint project of the National Aeronautics and Space Administration (NASA), ESA (European Space Agency) and the Canadian Space Agency is billed as the next-generation telescope. It is slated to unveil unseen distant parts of the universe and help fathom the mysteries of the cosmos.

THE GIST

  • The James Webb Space Telescope (JWST), hurled into space by the Ariane 5 rocket will now be able to observe the farthest reaches of the universe without any atmospheric turbulence.
  • After a 29 day-long journey, the spacecraft with the telescope will arrive at a point in space called ‘Lagrange point 2’, also known as L2. At this point, the Earth-Sun system’s gravitational forces, and the spacecraft’s orbital motion would balance each other. Therefore, the spacecraft positioned at L2 will orbit the Sun, tagging along with the Earth in 365 days. It will also give the telescope the required temperature to detect faint signals from distant stellar objects.
  • As the JWST’s diameter is larger than that of Hubble’s it will collect more photons than Hubble’s 2.4 metres mirror. JWST will have about seven times as much light-gathering capability as Hubble. Therefore, the JWST would observe fainter stellar objects that Hubble cannot detect.

What awaits the telescope in the next part of the journey?

The successful launch is just the beginning of an arduous nearly one-month journey. The telescope was intricately folded like an origami toy before it was launched. Now in space, hundreds of moving parts will need to fall in place precisely as designed for the telescope to unfold. In the next few weeks, step by step, various components have to be deployed before the telescope reaches its final configuration. A single misstep can reduce the whole telescope into space junk.

Speeding towards its destination, as of now, the JWST has successfully deployed its solar array, released its antenna and completed its mid-course correction burn. Before day 7, the sun-shield would be deployed. On day 10, nerves will be on edge as the engineers hoist its secondary mirror to its position. The rest of the mirror segments would be unfurled, step by step, before day 14. On day 29, the JWST is expected to fire its thrusters to enter its predestined orbit around L2. After it arrives at its destination, the 18 telescope mirror segments will have to be aligned flawlessly; this will take about 10 days. This will be followed by weeks of testing and calibration. The first image from the telescope is at least six months away.

Why are telescopes in space?

The thermal turbulence of the Earth’s atmosphere hinders telescopic observation of the universe. Stars twinkle, light from the faint stellar objects are absorbed by the thick lower atmosphere, and part of the spectrum, such as infrared rays from space, hardly reach the ground. By placing the telescopes on a high mountain top, we avoid as much atmosphere as possible. Yet the atmospheric turbulence hinders the super-sharp images of objects in space.

Telescopes in space altogether avoid the atmospheric disturbance and provide us with a clear, sharp and more profound vision of the farthest reaches of the universe. While the most giant ground-based telescopes revealed galaxies over 5 billion light-years away, the Hubble space telescope has identified the farthest known galaxy located at whooping 13.4 billion years in the past.

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Why is JWST an infrared telescope?

The telescope mounted on the JWST is an infrared telescope. The invisible magical rays that change channels in our TV remote are infrared. They are like the visible light and radio waves, part of the electromagnetic spectrum, but of different wavelengths. Why take an infrared telescope rather than a telescope that can see the visible spectrum? The answer to this resides in the Big Bang.

After the Big Bang, galaxies, stars and planets evolved. Since the Big Bang, the universe has been in a constant state of expansion. As the universe expands, space stretches. As the light travels far in space, the wavelength elongates. Aged light turns redder. The light from the earliest massive young stars and nascent galaxies was predominately visible and ultraviolet. However, traversing the vast stretches of the expanding space, they turn into infrared rays before reaching the Earth. An infrared telescope is apt to observe the ancient, early universe, which is the primary goal of the JWST.

What is the temperature at L2?

Primarily, the JWST is designed to detect faint signals from distant stellar objects in the infrared region of the electromagnetic radiation. The IR telescope must be cooled to -220C or lower. About 15 million kilometres from Earth, four times the distance of the Moon, L2 is a bone-chilling cold place. At this point in space, the second Lagrange point (L2), the Earth-Sun system’s gravitational forces, and the spacecraft’s orbital motion would balance each other. Therefore, the spacecraft positioned at L2 will orbit the Sun, tagging along with the Earth in 365 days.

While satellites orbit around the Earth, they periodically move from sunlight into Earth’s shadow and back and are subject to thermal fluctuations. Placed at L2, JWST is permanently behind Earth. When seen from space, Sun and the spacecraft will appear on the opposite sides of Earth.

Even at L2, the Sun-facing side of the telescope will be at +85C. However, on the opposite side, the temperature is -233 C. Using the tennis court-sized sunscreen, the telescope would be protected from the Sun’s heat and glare.

Will JWST see better than Hubble?

Suppose we keep two tubs, one smaller radius and the other larger radius, in the open. During rain, the larger tub will collect a lot more rainwater than the smaller one during a given time. Likewise, the JWST telescope’s 6.5 metres in diameter will collect more photons than Hubble’s 2.4 metres mirror. JWST will have about seven times as much light-gathering capability as Hubble. Therefore, the JWST would observe fainter stellar objects that Hubble cannot detect. Farther a thing is, fainter it is. The JWST would see objects much farther in the universe with a bigger collecting area than Hubble. With its sharp eye, JWST can see details on a twenty-five paise coin (penny) held at a distance of 40 kilometres.

The average time for light to reach Earth from the Moon is about 1.282 seconds. This means the Moon shining bright is 1.282 seconds old. As the light takes nearly eight minutes from the Sun to reach the Earth, the image of the Sun we see is about eight minutes old. The Red giant star Betelgeuse (Thiruvathirai Nakshatra) is 548 light years away. Therefore, the light from this star must have commenced its journey roundabout when Azhagan Perumal Jatavarman Parakrama Pandyan coronated himself as king in 1473 AD. Take-home message: By looking far away, we look back in time.

How far can JWST peer into the past?

Let us imagine the time from the Big Bang to now as a year-long calendar. In this cosmic calendar, the Big Bang occurred precisely at midnight on January 1. In this timeline, right now is December 31 at midnight. The JWST would let us see the universe as it was all the way back to January 6. That was when the earliest stars started to shine. Literally, the JWST would take us on time travel to the unimaginable ancient past.

A telescope can detect an object and show how it looks. The spectroscope, a key instrument mounted on the telescope, can analyse the light rays and tell us what is there. From the spectral image, we can understand the elemental composition, the temperature of the stellar object and much more. Unlike the Hubble, JWST carries the spectrascope, which is expected to unravel the elemental composition of early stars and galaxies.

T.V. Venkateswaran is Scientist F at Vigyan Prasar, Dept of Science and Technology

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