Aditya-L1 was the first Indian spacecraft to enter a halo orbit around the…

Concept: The Sun–Earth system has five Lagrange points — positions where the gravitational pulls of the two bodies and the centripetal force needed to travel along with them balance out. L1 is the one that lies between the two: it sits on the Sun–Earth line roughly 1.5 million km from the Earth on the sunward side, which gives a continuous, eclipse-free view of the Sun. L1 is dynamically unstable, so a spacecraft is never parked at the point itself: it is flown in a halo orbit, a large periodic loop about the point that is maintained by small station-keeping burns. Reaching such an orbit therefore has two separate milestones, months apart — the launch that leaves the Earth, and the orbit-insertion burn performed on arrival.

Application: Aditya-L1 is India's first dedicated solar observatory, and its journey to that orbit ran in this sequence:

1. 2 September 2023 — lift-off aboard PSLV-C57 from the Satish Dhawan Space Centre, Sriharikota, which placed the spacecraft in an elliptical orbit around the Earth.

2. September 2023 — a series of earth-bound orbit-raising manoeuvres, followed by the trans-Lagrangian insertion burn that sent the spacecraft out of the Earth's sphere of influence.

3. Roughly four months of cruise along the Sun–Earth line towards the L1 region, about 1.5 million km from the Earth.

4. 6 January 2024 — ISRO fired the spacecraft's control engines for the Halo-Orbit Insertion, binding Aditya-L1 into a halo orbit about the Sun–Earth L1 point.

Cross-check: The distinction to hold on to is launch year versus arrival year. Because the cruise took about four months, lift-off (September 2023) and halo-orbit insertion (January 2024) fall in different calendar years; ISRO later reported the completion of the first full halo revolution on 2 July 2024. The years before the launch belong to the mission's ground-preparation, payload-delivery and spacecraft-integration phases.

Answer: Aditya-L1 entered its halo orbit around the Sun–Earth L1 point in 2024, becoming the first Indian spacecraft to occupy such an orbit.