Lagrange points

Also called: L1, L2 · Topic: Origin and Evolution of the Earth and the Solar System · NCERT: Beyond NCERT

Meaning

Lagrange points are five points in space, set by two large bodies such as the Sun and the Earth, where a small object stays in the same place compared with both bodies. This works because the pull of the two bodies together gives the small object exactly the force it needs to move around with them. They matter because a spacecraft parked at one of these points needs little fuel to hold its place. India's Aditya-L1 solar observatory works at the Sun–Earth L1 point [1].

Explanation

How a Lagrange point works

• Two big bodies pull together
• A small object near the Sun and the Earth feels the gravity of both.

• At five special points, these two pulls together give exactly the force the object needs to go around the Sun along with the Earth [3].

• Result: the object keeps its place

• The craft moves around the Sun with the Earth and keeps the same position compared with both.

• It does not have to keep burning fuel to stay there. So it needs little fuel.

• Gravity is not zero at these points. The pulls do not cancel out to nothing. Together they supply the exact force needed for the orbit.

The five points: L1 to L5

Point	Position	Stability
L1	On the Sun–Earth line, between the Sun and the Earth	Unstable [3]
L2	On the Sun–Earth line, beyond the Earth (on the side away from the Sun)	Unstable [3]
L3	On the Sun–Earth line, behind the Sun	Unstable [3]
L4, L5	Tips of two equilateral triangles, with the Sun and the Earth at the other two corners	Stable [3]

• Three points lie on one line. L1, L2 and L3 all sit on the Sun–Earth line.
• Two points sit off the line. L4 and L5 make equal-sided triangles with the Sun and the Earth.

Stable and unstable points

• Unstable points (L1, L2, L3)
• If a craft is left alone, it slowly drifts away. At L1 and L2 this takes about 23 days [3].

• So the craft fires small engines now and then to stay in place. This is called station-keeping (small, regular engine burns that keep a craft in its planned position) [3].

• Stable points (L4, L5)

• The Coriolis force (an apparent force caused by the rotation of the system) keeps objects circling the point [3].

• So objects there stay put without help.

• Halo orbit

• A craft at L1 does not sit exactly on the point. It goes around it in a looping path called a halo orbit (a loop around the Lagrange point).
• Aditya-L1's halo orbit was chosen because it needs fewer station-keeping burns. This saves fuel [1].

Where it occurs

• Sun–Earth L1: Aditya-L1 (India)
• It is ISRO's first solar observatory in space.
• It was launched on 2 September 2023 on PSLV-C57 from SDSC SHAR (Satish Dhawan Space Centre, Sriharikota, Andhra Pradesh) [1][2].
• It was first put into an elliptical parking orbit of 235.6 km × 19,502.7 km [1].
• Five engine burns raised the orbit step by step. The fifth burn, the trans-L1 injection, sent it towards L1 [1].
• It entered its halo orbit on 6 January 2024, at about 16:00 IST [1].
• It is about 1.5 million km from Earth, on the Sun–Earth line, towards the Sun [1].
• One trip around its halo orbit takes about 177.86 Earth days [1].
• The orbit was planned for a 5-year mission life [1].

• Why L1: the Sun is always in view, with no eclipses and no night-time gaps [1].

• Sun–Earth L1: SOHO (world)

• NASA–ESA's SOHO (Solar and Heliospheric Observatory) works here. It has a view of the Sun that is never blocked [3].

• Sun–Earth L2: James Webb Space Telescope (world)

• The JWST sits at L2.
• At L2, the Sun, Earth and Moon all stay behind the craft. So it can point at deep space and still talk to Earth easily [3].

Don't confuse with

• L1 vs L2: L1 lies between the Sun and the Earth, so it is used to watch the Sun (Aditya-L1, SOHO). L2 lies beyond the Earth, away from the Sun, so it is used to look at deep space (JWST).
• Unstable points (L1, L2, L3) vs stable points (L4, L5): crafts at L1, L2 and L3 drift away in time and need station-keeping. Objects at L4 and L5 are held by the Coriolis force [3].
• Halo orbit vs sitting at the point: Aditya-L1 loops around L1 in a halo orbit. It does not sit fixed exactly on the point [1].
• Lagrange point vs zero-gravity point: gravity is not zero at a Lagrange point. The two pulls together give exactly the force needed to move along with the two bodies.

Prelims Hooks

• There are five Lagrange points. L1, L2 and L3 are unstable. L4 and L5 are stable [3].
• L1, L2 and L3 lie on the Sun–Earth line. L4 and L5 form equilateral triangles with the Sun and the Earth [3].
• Spacecraft and their points: Aditya-L1 and SOHO → L1. JWST → L2.
• Aditya-L1: launched on PSLV-C57 on 2 September 2023 from Sriharikota → halo orbit on 6 January 2024 → about 1.5 million km from Earth → halo period about 177.86 days [1].
• Trap: a craft left alone at L1 or L2 drifts away in about 23 days. That is why it needs station-keeping [3].
• Trap: the Coriolis force is what keeps objects stable at L4 and L5, not at L1 [3].

Mains Points

• Lagrange points as strategic space assets (GS-III, science and technology)
• A craft at these points keeps its place with little fuel, so missions can last longer.

• L1 gives a view of the Sun that is never blocked. L2 gives a clear view of deep space with easy contact with Earth [3].

• Aditya-L1 and space weather

• Aditya-L1 watches the Sun from L1 without a break [1].
• This helps space-weather forecasting (warning of storms from the Sun), which protects satellites, power grids and communication links.

• It places India among the few space agencies working at a Sun–Earth Lagrange point [1].

• Smart orbit design saves fuel

• Choosing a halo orbit around L1 cut the number of station-keeping burns needed.
• This saved fuel and supports a 5-year mission life [1].

Related concepts

• Planetesimal
• Accretion
• Dwarf planet
• Kuiper Belt

Read more

• Formation of the planets and the solar system today