Explained: What's This 'Lagrange Point 1' Where India’s Aditya-L1 Is Meant To Park And Probe The Sun?

India’s destination with its third lunar mission, Chandrayaan-3, was straightforward: the surface of the Moon in the south pole region.

However, the same cannot be said about its space-based solar mission, Aditya-L1.

To be fair, Aditya-L1’s home in space is not complicated, just probably unfamiliar.

India’s Sun-studying observatory is set to make the “Lagrange point 1” its new home in space. It will get to this point, located 15 lakh kilometre away, in about four months after lift-off this Saturday (2 September).

The Lagrange point 1, or L1, by its very name suggests that there are other Lagrange points and that, perhaps, there is someone or something that goes by the name Lagrange.

Named after the eighteenth-century, Italian-born mathematician and astronomer Joseph-Louis Lagrange, the Lagrange point is a spot in space where gravity from the Sun and Earth balance the orbital motion of a satellite.

“Technically at Lagrange point, the gravitational pull of the two large bodies equals the necessary centripetal force required for a small object to move with them,” the Indian Space Research Organisation (ISRO) explains in its Aditya-L1 booklet.

So, a spacecraft at this point stays in a fixed position relative to the Sun and Earth. It’s a kind of a gravitational sweet spot. Here, it won't take a lot of energy, fuel, to keep a spacecraft there.

Lagrange arrived at this “point” by solving what is called the “three-body problem,” wherein three objects orbit each other while also remaining in the same place relative to each other.

He found that there are five solutions to the problem and, therefore, five Lagrange points in all.

Not all Lagrange points are alike, though. Whereas the L4 and L5 are considered stable Lagrange points (provided a certain mass ratio is met), L1, L2, and L3 are considered ‘unstable’.

At L4 and L5, if an object, such as a spacecraft, happens to be nudged, “an effective restoring force” kicks in and the object returns to its spot, explains a BBC Science Focus article.

But at L1, L2, and L3, such a nudge could lead the object to break loose from its orbit and get lost to interplanetary space.

“The L1 and L2 points are unstable on a time scale of approximately 23 days,” NASA says, “which requires satellites orbiting these positions to undergo regular course and attitude corrections.”

While L1, L2, and L3 lie along the line connecting the two large masses, L4 and L5 form the apex of two equilateral triangles that have the large masses at their vertices.

These five Lagrange points exist for any gravitational system involving celestial bodies like a star or a planet.

Incidentally, while L1 will be inhabited by Aditya-L1, L2 has been home to the incredible James Webb Space Telescope since early last year.

The Webb has settled down for its life of science at L2, which accords a great spot from where to view deep space.

To remain in its orbit, the Webb requires adjustments, leveraging thruster fire, about once every three weeks.

The Webb, to be more precise, does not occupy the prized L2 spot, nor will Aditya-L1 take up the L1 seat. Aditya-L1 will be in a “halo orbit” around the point L1 of the Sun-Earth system.

“A satellite placed in the halo orbit around the L1 point,” ISRO says, “has the major advantage of continuously viewing the Sun without any occultation/ eclipse.”

The outpost provides the advantage of continuous, unobstructed viewing of the solar processes.

Further, it is “a good position from which to monitor the Sun since the constant stream of particles from the Sun, the solar wind, reaches L1 about an hour before reaching Earth,” the European Space Agency (ESA) explains.

This is why an orbit around L1 is already home to the Solar and Heliospheric Observatory Satellite, or SOHO, since 1996.

According to ISRO, using the special vantage point of L1, four of the seven scientific instruments on board Aditya-L1 will view the Sun, while the remaining three instruments will carry out in-situ studies of particles and fields at L1.

Although L1 is great, it isn’t perfect.

“Another Lagrange point known as L5 is a good vantage point for studying the Earth directed CME (coronal mass ejection) events and assessing the space weather,” ISRO says.

At 15 lakh kilometre, the distance between Earth and L1 is a mere 1 per cent of the distance to the Sun, at 1,500 lakh kilometre. But it still takes some getting. Reaching L1 will be a roughly four-month journey for Aditya-L1.

A polar satellite launch vehicle, PSLV XL, will be launched with Aditya-L1 on board from the Satish Dhawan Space Centre-Sriharikota Range (SDSC-SHAR) at 11.50 am on Saturday, 2 September. 

After the spacecraft has been placed in a low-Earth orbit, its path will begin to trace out increasingly elongated ellipses before it is slingshot towards L1 using onboard propulsion.

Earth will pull back on Aditya-L1 with its gravity as the spacecraft heads towards its destination, but a time will come when it exits Earth’s gravitational sphere of influence.

Thereafter, India’s solar probe will cruise before being injected into a large halo orbit around L1.

Then, solar science will begin!