ISRO’s Aditya L-1 mission, the Indian space agency’s most complex mission ever, which is scheduled to launch by the end of August or early September, is unique in many ways.  

For the first time, India is building a ‘space observatory’ — the spacecraft that will be peering at the Sun all the time, checking out the ball of fire 24×7. 

India has never put a spacecraft at a Lagrange point, which is a point between two or more massive objects (like the Sun and the Earth) where the massive objects exert equal pull over the spacecraft so that it “stays” right there. Placing a spacecraft precisely at a point in space 1.5 million km away from Earth (between Earth and Sun), calls for extreme deftness in ‘steering’ the spacecraft to its slot. Keeping it there is even tougher.  

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There are five Lagrange points in the Sun-Earth system; Aditya is going to be positioned at Lagrange-1. 

And the two principal instruments onboard Aditya L-1 — SUIT and VELC — are completely home-made — designed and built by Indian scientists. Furthermore, the VELC will do ‘spectropolarimetric measurements’ to study the magnetic field of the Sun — for the first time by any country from space. As such, the data it generates will contribute a lot to science. 

But first, why the interest in the Sun? 

The Aditya L-1 spacecraft is essentially a space telescope. Broadly, the Aditya L-1 mission has two purposes — long term (scientific quest) and short term (protecting our satellites).  

The mission had its genesis in 2006, when a group of scientists from the Indian Institute of Astrophysics and the Astronomical Society of India made a presentation to ISRO, underscoring the need to protect satellites from ‘things’ coming out of the Sun. Back then, the idea was to put up a small satellite in the Low Earth Orbit, which would monitor the Sun, imaging it. But Prof U. R. Rao, a former Chairman of ISRO, suggested that the scope of the mission be expanded, and the spacecraft placed at Lagrange-1 point.  

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The idea was to monitor the Sun constantly so as to provide an early warning against solar storms that can damage our satellites and electrical grids. Solar storms can take many forms, such as coronal mass ejections (or billions of tons of matter flung out of the Sun, which can shoot off anywhere including towards the Earth) and solar flares, which are sudden bursts of energy, often in the form of tongues of fire thousands of kilometres long that can spew X-rays, electromagnetic waves, or high-energy particles all across space and can disrupt radio communications and harm astronauts in space). Imagine GPS going out of whack! Aditya L-1 is a sort of an early warning system. 

As for the long term, it is understood that ultraviolet rays from the Sun can impact climate on the Earth and the ozone layer in the atmosphere. UV radiation of wavelengths between 200 and 310 nanometres is absorbed by the oxygen and ozone in the Earth’s atmosphere. UV radiation above 310 nm pierces through the atmosphere. We need to know what kind of UV the Sun is likely to emit. Changes in UV radiation can influence cloud formation, water vapor content and temperature patterns in the Earth’s lower atmosphere. It is important, therefore, to study the behaviour of the Sun to see its impact on the Earth’s climate. 

Why Lagrange-1 point? 

Illustration of Lagrange points of the Sun-Earth system.

As shown in the picture, the L-1 point lies between the Sun and the Earth, affording a spacecraft placed there an excellent view of the Sun. L-1 (along with L-2 and L-3) are ‘halo orbits’, where a spacecraft placed there keeps going round an invisible centre. An object kept there is very unstable, because the spacecraft is subject to constant pulls and pushes in space. Imagine keeping a pin stable between two magnets — that is how difficult it is. While taking the spacecraft to that ‘parking slot’ is tough, keeping it there is tougher, because all celestial objects cause ‘gravitational perturbations’ on the spacecraft and ground controllers on the Earth would have to make small orbital adjustments to counteract the perturbations. Still, L-1 is preferred because it is the best vantage point to observe the Sun. If you want to build a ‘watchtower’ in space to observe the Sun 24×7, L-1 is where you should build it.  

How does Aditya L-1 study the Sun? 

Aditya L-1 houses seven instruments; some study the Sun from afar while the others analyse the particles from the Sun that stream into the spacecraft. But mainly there are two instruments — the Solar Ultraviolet Imaging Telescope (SUIT) and the Visible Emission Line Coronagraph (VELC) — both designed and built in India. 

Locations of Aditya-L1 payloads on the spacecraft. R, P and Y indicate the Raw, Pitch and Roll axis of the spacecraft. ASPEX Payload Consists of SWIS & STEPS.

The SUIT will be looking at the disc of the Sun, which comprises the inner photosphere and the outer chromosphere, while the VELC will peer into the rim (corona). The SUIT will capture the near-ultraviolet rays (200-400 nm wavelength) coming from the Sun; VELC will pick up the near-Infra red radiation from the Sun. Both the instruments were built at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune. 

“These are very unique instruments, completely built in India,” says Dr Somak Raychaudhary, who was involved in the development of SUIT. Raychaudhary, who is now the Vice Chancellor of the Ashoka University, Delhi, explained to businessline that since both SUIT and VELC look at the Sun at the same time, it would be possible to see the effect of any changes in the Sun’s photosphere and chromosphere on the corona—giving a better picture of how the star behaves.  

The Sun is not solid like the Earth is, but a huge ball of gas with different layers, all surrounded by the corona. Each layer spins at a different speed. SUIT will simultaneously map different parts of the Sun — photosphere and chromosphere of the Sun using 11 filters sensitive to different wavelengths and covering different heights in the solar atmosphere. “This will help in the understanding of the processes involved in the transfer from mass and energy from one layer to the other,” according to a 2017 paper published by IUCAA scientists. 

The VELC will study the corona. It will do both photograph (optical imaging) and spectrograph, which is splitting of light into its constituent wavelengths — a study of the spectrographic lines can tell a lot about the light emitter, which, in this case, is the Sun. Dr. Dipankar Banerjee, who was involved with the development of the VELC in IUCAA, explains that the instrument can spectropolarimetric observation. Polarimetric measurements refers to the orientation of electromagnetic waves — sort of slanting this way or that way — which “carries information about the magnetic field of the Sun,” says Banerjee. “This is a unique experiment, because this has never been done by anybody from space,” Banerjee told businessline. The magnetic field is the “main culprit” responsible for all the dynamics of the Sun, so understanding the magnetic field is useful. 

Aditya-L1 trajectory from Earth to L1.

Then the VELC can investigate the red and green spectroscopic lines, which give a peek into the temperature of the region of the Sun from where the light has come. 

The other five instruments pick up and analyse X-rays and particles from the Sun. So, the seven instruments on Aditya L-1 cover the entire gamut of electromagnetic radiation — near infra-red, visible light, near ultraviolet and X-rays as well as particles bursting out of the Sun — all from a vantage point at L-1. If the mission is successful, ISRO can claim to have the Sun in its pocket.