Chandrayaan-3 was a very complex mission, calling for difficult maneuvering through delicate descent and soft-landing on the lunar surface, right?

Indeed, it was, but Chandrayaan-3 is nothing compared with ISRO’s next mission, Aditya L-1.  

ISRO has announced that the Aditya L-1 spacecraft would be launched on September 2, at 11:50 am, by a PSLV rocket. 

Aditya L-1 is by far the most complex mission ever undertaken by ISRO. It not only requires the spacecraft to be taken as far as 1.5 million km from the Earth, but also positioning it carefully at the Lagrange-1 point and keeping it there.  

A mission as complex as Aditya L-1 needs a lot of demystifying. Here are some questions and answers to help demystify Aditya L-1. 

1. What exactly is Aditya L-1? 

Aditya L-1 is a spacecraft. It is a space observatory which has space telescopes and other instruments. It keeps looking at the sun 24x7, studying it. 

2. Is it a satellite, like the other satellites that we launch, that keeps circling the Earth? 

No. This one won’t orbit around the Earth. It will stay at a point that is 1.5 million km from the Earth along the straight line between the Earth and the Sun – at a point called Lagrange-1.  

To understand Aditya L-1, one must first know about Lagrange points. 

All about Lagrange points

3. What is Lagrange-1, or L-1? 

Lagrange points are spots between any two celestial bodies (like Earth-Sun or Earth-Moon). An object, like a spacecraft, placed at a Lagrange point will remain stable there, essentially because it is subject to equal pull from both the bodies. There could be several such Lagrange points between any two bodies. For example, for the Earth-Sun pair, there are five such points. 

4. So a Lagrange point lies between the two bodies? 

Not necessarily ‘between’. They may lie anywhere. For instance, for the Earth-Sun pair, only L-1 is between the Earth and the Sun. L-2 (where NASA’s James Webb telescope has been placed) lies on the other side of the Earth. L-3 lies on the other side of the Sun. You can draw a straight line connecting L-3 (extreme left), the Sun, L-1, the Earth, and L-2 (extreme right). L-4 and L-5 lie on either side of the line connecting the Earth and the Sun. If you draw a line between the Earth, the Sun, and L-4 (or L-5), you will get a triangle. 

5. But if L-2 and L-3 lie beyond the Earth and the Sun, how is an object kept there stable? Won’t it be subject to double pull, by both Earth and the Sun, because they are both pulling the object in the same direction rather than in opposite directions? 

No. Here lies a nuance. The stability spacecraft at L-2 or L-3 is not only subject to the gravitational pull of the Earth and the Sun but is also influenced by an interplay of multiple forces acting on it—such as its own orbital velocity as it moves and its revolution around an invisible center at the Lagrange point. You see, as the Earth moves around the Sun, the entire Lagrange system moves in tandem. While an object placed at a Lagrange point, say L-3, is seen as stationary relative to the Earth, it is also moving in sympathy with the Earth’s orbit around the Sun. 

6. Oh, then you can place a spacecraft at a Lagrange point and forget about it, because it will always remain there, right? 

Unfortunately, not so. An object at a Lagrange point, like Aditya at L-1, is also subject to gravitational perturbation caused by the movements of celestial bodies in the sky. For example, when the Moon comes between the Earth and Aditya, it will exert its pull on the spacecraft. All celestial bodies, such as other planets and asteroids, affect the stability of the spacecraft—requiring some deft station-keeping from Earth-based controllers. 

7. So, the Aditya L-1 space observatory remains fixed at a spot in L-1, but because it is subject to multiple pulls from the innumerable celestial bodies in space, it could swerve from its path and when it does, people sitting in a control room would have to steer it back to its path? 

Yes. However, it may not be ‘people sitting in a control room’ all the time, because some amount of automation is built into the system. And the spacecraft does not ‘remain fixed’ at a spot in L-1. It keeps orbiting an invisible center. 

8. Oh, is it going round and round in a circle there? 

Exactly. It is orbiting an invisible center at a speed slightly over 1 km per second. 

9. But why? Can’t it remain rooted to a point at L-1? 

It can. But it helps if it is orbiting. An orbiting body is more stable and is easier to maneuver. The biggest challenge in Aditya L-1 mission is in ‘placing’ the spacecraft so carefully at a chosen point from where it will begin to orbit there like a merry-ground. 

10. But, if it is circling around an invisible center, how can it be facing the Sun all the time? 

Because it is orbiting ‘vertically’ -- the orbit is perpendicular to the plane of revolution of the earth around the sun. The orbit is ‘north-south’ rather than ‘east-west’. The spacecraft will always be facing the sun.

11. But for an object to orbit, don’t you need a mass at the center? For example, the Sun is at the center of planets’ orbits and the planets are at the centre for their moons. 

It is true that usually there is a mass to create ‘gravity well’ for an object to orbit around it is not necessary for a mass to be there. As for orbits of a spacecraft placed at Lagrange points, the interplay of forces causes the spacecraft to keep circling around. 

To give a crude parallel—not scientifically accurate, but just to simplify—a coil moves round and round in a motor due to the magnetic force exerted by multiple magnets around it. The orbit of Aditya L-1 spacecraft at the L-1 point is something like that. 

The Mission 

12. Moving on, why are we studying the Sun? What is the whole point of the Aditya L-1 mission? By the way, how much does the mission cost? 

Aditya L-1 objectives are two-fold—short-term and long-term. The short-term objective is to keep a sharp eye on the Sun for any bursts throwing up matter (called ‘coronal mass ejections’) or charged particles, which, if they reach here, could damage our satellites and electrical grids. Aditya L-1 is a sort of an early warning system against any ‘tsunami’ of radiation from the Sun. Today, India itself has space assets worth about ₹50,000 crore that need to be protected from the Sun’s caprice. 

The long-term objective is to study the Sun, to add to mankind’s knowledge of the star. The ultraviolet radiation from the Sun is known to impact Earth’s climate. Knowing the Sun better might help. 

As for costs, at the time of writing this, ISRO has not revealed the exact number, but it is said to be around ₹400 crore. 

Studying the Sun

13. How will Aditya learn about the Sun? 

To learn about anything from a distance, you need some sort of a communication from the object of learning to you, right? For example, if you want to learn about a person, it could be a letter from him or his voice or the sight of his appearance, etc. Likewise, if you want to learn about a celestial object from a distance, the only way to learn is to study stuff that is coming from the object and reaching you. Mostly, it is light and other electromagnetic radiation. 

As for the Sun, it keeps sending (a) electromagnetic radiation and (b) charged particles. Aditya L-1 has instruments to pick up and analyze both. Instruments that analyze electromagnetic radiation are called ‘remote sensing’ instruments. Those that analyze particles are called ‘in-situ’ instruments. 

As you know, electromagnetic radiation is the propagation of energy through the electromagnetic field that pervades the Universe. In the decreasing order of wavelength (distance between two peaks of a wave), or correspondingly, in the increasing order of frequencies (number of peaks per second), the range of electromagnetic radiation is as follows: radio waves (used for radio broadcast and communication with satellites), microwaves (used in microwave communication and in cooking), far and near infra-red, visible light starting from red to violet, near and far ultra-violet, low energy (soft) X-rays, high energy (hard) X-rays and gamma rays. 

Some instruments onboard the Aditya spacecraft are designed to capture near-infrared, visible light, near-ultraviolet, and soft and hard X-rays. Other instruments capture the charged particles from the sun that stream into the telescope. Both—the radiation and the particles—are analyzed. One instrument also checks the orientation of the infrared waves (like, are they slated this way or that way). This, called ‘Spectro polarimetric observation’, gives an idea about shifts the Sun’s magnetism. 

14. Why have multiple instruments to study the Sun? Won’t just a good camera or an optical telescope do the job? 

Well, just training an optical telescope on the Sun won’t be much help because it can at best tell you what is happening now (or, what happened eight minutes earlier, because it takes 8 minutes for light from the Sun to reach the Earth). But to forecast what the Sun is likely to do next, you need to read the tell-tale signs the Sun gives.

Each instrument is meant to read a sign. If you put all the signs together, you can make a prediction. 

But before we proceed, it is necessary to know that the Sun, a burning gas ball and not a solid like the Earth, is made up of three distinct layers – photosphere (the visible, light-emitting part), chromosphere (the layer around the photosphere, visible only during eclipses) and corona (the outermost layer, which burps out charged particles and is characterised by magnetic fields). 

Here is what each instrument of Aditya L-1 is meant to do. 

Remote sensing instruments

Solar Ultra-violet Imaging Telescope (SUIT): This instrument, designed by scientists at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, headed by Dr Somak Raychaudhary (now, Vice-Chancellor of Ashoka University, Delhi), picks up ultraviolet radiation from photosphere and the chromosphere. 

Visible Emission Line Coronagraph (VELC): Also developed at IUCAA, the VELC studies the corona. It picks up both white light as well as near infrared. This instrument does spectro-polarimetric measurements, analyzing the orientation of the waves which speaks about sun’s magnetism. The VELC can measure temperature, density and changes in magnetism in the corona. 

Soft and Hard X-ray spectrometers: These spectrometers analyze X-rays coming from the Sun and can tell the temperature and composition of the X-ray emitting region. 

In-situ payloads

Aditya Solar Wind Particle Experiment: analyzes the solar winds, which are streams of charged particles—mainly, electrons and protons—from the sun. 

Plasma Analyzer: analyzes the plasma coming from the sun. 

Advanced Tri-axial High-resolution Digital Magneto Meter: is an instrument that measures the magnetic field that arrives with the particles. 

Now, each instrument generates data. When you correlate the data you get a good idea about the cause and effect of phenomena. For example, you may observe a certain change in the photosphere of the sun regularly leads to a shift in the magnetism of the corona, which in turn might throw up a solar flare later. Hence, all the instruments together are capable of providing alerts about upcoming events in the Sun. 

15. So, once Aditya L-1 is up there, are we well protected from the Sun? 

You can’t bet against something like the sun. Aditya L-1 will be of help, but it is not guaranteed protection from the Sun. 

16. Won’t these instruments melt in the heat of the Sun? 

They won’t, because the L-1 is ‘just’ 1.5 million km from the Earth in the direction of the Sun, which is 1 per cent of the average distance between the Earth and the Sun. Of course, temperatures will be very high L-1, but only about a few hundred degrees celsius, rather than a few thousands or millions. Some protective measures can take care of this.  

However, there is a difference between ‘temperature’ and ‘heat’. In common language we use both words to convey the same sense, but in science, temperature refers to the kinetic energy of charged particles while heat is the ‘transfer of energy’ from one body to another. To illustrate this, tea becomes piping hot inside a micro-oven, but you can comfortably put your hand in to pick up the cup. Inside the oven, it is high temperature but low heat.  

In December 2021, NASA’s Parker solar probe actually ‘touched’ the corona of the sun, where the temperatures are of the order of millions of degrees. It did not melt. Do check out this NASA resource about ‘The science behind why it won’t melt’.  

In space, it could be thousands of degrees hot but you won’t ‘feel’ the heat. 

<The article has been corrected to reflect the correct orbit of the spacecraft around L-1, under Question No.10>