Why is Aditya L1 mission lodged at the Lagrange’s point?
Space Nuggets #8: Uncovering the mysteries of equilibrium and Unstability for solar observatory missions
Hi everyone, welcome to the overall 8th edition and the first edition in 2024 of the fortnightly series of Space Nuggets! Space Nuggets are bite-sized pieces on the latest astronomy events and the phenomenon behind them, intending to share the marvels of the world around us and the Universe with one and all!
I am elated to start it off with the success of India’s maiden mission to study the Sun; I hope you enjoy reading about it!
Just days into the New Year, the Indian Space Research Organization (ISRO) created history when its first ever solar observatory — Aditya L1 — was successfully placed in its final destination of orbit on January 6th, 2024! This is the tweet shared by ISRO on their Twitter page —
The tweet talks about a point called Lagrange Point L1 — some 1,500,000 km away from the Earth (that’s 15 lakh km or 1.5 million km away), which the spacecraft will call its home for the lifetime of its mission.
But what even is a Lagrange Point? What does it mean when we say L1 — are there more than 1 Lagrange Points? Why is this a good place for any space observatory on a mission to observe the Sun? Wouldn’t a place closer to the Sun, and probably in the Sun’s orbit in some way be a better place? Let’s find out!
What is a Lagrange’s Point?
Let’s consider the scenario in front of us.
- We have 2 huge massive bodies — the Sun and the Earth and one body of negligible mass relatively — the solar probe.
- The Sun and the Earth are not at rest but rather in motion — the Earth is revolving around the Sun, or to be more accurate: both the Sun and the Earth are revolving around a common centre of mass called a barycenter: the Sun has a much much smaller orbit because of its extremely high mass relative to the Earth. So the motion looks something like this:
3. Our aim: The solar probe should ideally be placed in an orbit such that it requires minimum fuel for orbit corrections and does not fall into the Sun or the Earth.
This can be considered a three-body problem of physics, with a special case of 2 massive bodies and the third one with negligible mass, to find the equilibrium for these 3 bodies to exist together in the system.
These equilibrium points — where the body of negligible mass can exist — so that the combined gravitational force on it by the 2 massive bodies provides the exact centripetal force(force towards the center of circular motion) needed for the body of negligible mass to stay in an orbit for circular motion — are called Lagrange’s points.
There are a total of 5 Lagrange’s points in any system — and a solar probe can exist at any of these points and use minimum fuel for orbit corrections because the force needed by it to go around the Sun is provided by the combined gravitational forces of the Sun and the Earth.
Imagine if you were sitting in this solar probe, with the aim to stay in one place and study the Sun, Lagrange’s point is the perfect place to be — the equilibrium.
That said, not all Lagrange’s points are the same; they have slightly different properties and each point has its significance.
Why was Lagrange’s point L1 chosen by Aditya-L1 mission as its home?
In an analysis presented by ISRO, they mention how L1, L2, and L3 lie in the plane of the 2 massive bodies and hence are points of unstable equilibrium(an object here tends to fall out of its place) compared to L4 and L5.
Imagine if you are at Lagrange’s Point L1. Go any closer to the Sun crossing the Lagrange’s point, you will be pulled into an orbit around the Sun. This orbit will keep becoming smaller and smaller in radius until you fall into the Sun (the heat for sure will get to us and the spacecraft even before that). And if you move any closer to the Earth from the Lagrange’s point, the Earth will pull you back with its gravitational force and you will end up falling on the Earth instead of the Sun.
Unstable equilibrium is the reason you will tend to fall towards the Earth or the Sun if are at point L1.
That said, L1 is the closest point to Earth, it has an uninterrupted view of the Sun and the Earth, which means good communication and continuous observation opportunities for any solar probe.
This is the reason so many of the other well-known solar probes like SOHO(Solar and Heliospheric Observatory), and DSCOVER (Deep Space Climate Observatory) sit at L1 to study the Sun’s atmosphere, solar winds, their effect on Earth, etc. On the other hand, the famous James Webb Space Telescope and European Space Agency’s Euclid sit at L2, to protect themselves from solar winds and the Sun’s heat, while at the same time being out of the Earth’s shadow enough to be able to use solar power for their systems to study the deeper mysteries of space.
Does a solar probe remain at rest at Lagrange’s point L1?
The short answer is, no.
Nothing is at absolute rest in the Universe, and Lagrange’s points L1 and L2 lie in the direct line of the 2 massive bodies — so they are slightly unstable points. But the interesting thing is, there are quasi-stable periodic orbits around these points themselves — which are called halo orbits.
The halo orbit movement will look something like this —
So finally, all spacecraft at points L1, L2, and L3 are in halo orbits, whereas L4 and L5 are where a lot of natural objects can be found orbiting the system. For instance, Jupiter has a lot of “trojan asteroids” at points L4 and L5 of the Sun-Jupiter system and since these are points of stable equilibrium, once the asteroids wandered in, they have stayed here naturally on their own.
It’s amazing how so many not-so-well-known yet fascinating decisions are made while determining the course of a space mission, picking orbits being one of the most crucial ones among them! For now, we know that the Aditya-L1 mission has reached its meticulously-picked position and is ready to begin the study of the chromosphere, the photosphere, the outermost layer of the Sun (yes, it is called the corona), using its electromagnetic, particle and magnetic field detectors. This will play a critical role in our understanding of how solar winds and other solar activities as a part of the Sun’s lifecycle affect the Earth’s atmosphere and its climate patterns and we hope Aditya-L1 keeps enchanting us with exciting discoveries on these in the course of its mission!
That’s all for this edition of Space Nuggets! Stay tuned for the next one a fortnight later, and till then, remember that we all have a little of cosmic dust inside us, so you are nothing short of stellar yourself! Happy skywatching 🔭
Do you have any suggestions for questions, phenomena, and topics you would like to see in the Space Nuggets? Reach out to me with all of these and any other feedback you have on this series 😁
