ISRO's Aditya-L1 solar observatory spacecraft accomplished Halo-Orbit Insertion (HOI) at 16.00 Hrs (approx) on January 6, 2024 (IST). The final phase of the maneuver involved firing of control engines for a short duration.

The orbit of the Aditya-L1 spacecraft is a periodic Halo orbit which is located roughly 1.5 million km from Earth on the continuously moving Sun–Earth line with an orbital period of about 177.86 Earth days. This Halo orbit is a periodic, three-dimensional orbit at L1 involving the Sun, Earth, and a spacecraft. This specific halo orbit is selected to ensure a mission lifetime of 5 years, minimizing station-keeping maneuvers and thus fuel consumption and ensuring a continuous, unobstructed view of the sun.

The Aditya-L1 mission is an Indian solar observatory at Lagrangian point L1 for “Observing and understanding the chromospheric and coronal dynamics of the Sun” in a continuous manner. Placing the Aditya-L1 in a halo orbit around the L1 point has advantages as compared to placing it in a Low Earth Orbit (LEO):

• It provides a smooth Sun-spacecraft velocity change throughout the orbit, appropriate for helioseismology.
• It is outside of the magnetosphere of Earth, thus suitable for the "in situ" sampling of the solar wind and particles. 
• It allows unobstructed, continuous observation of the Sun, and view of earth for enabling continuous communication to ground stations.

Halo orbit insertion

The halo orbit insertion process commenced as the spacecraft crossed the XZ plane in the Sun-Earth- L1 rotating system, with the required orbital state. The insertion maneuver is essential to nullify the X and Z velocity components and attain the required Y-velocity in the L1 rotating frame for the required Halo orbit. The targeted Halo-orbit for Aditya-L1 is Ax: 209200 km, Ay: 663200 km, and Az: 120000 km (The semi-axes of the 3-dimensional Halo orbit-refer figure).

Halo Orbit Insertion in L1 Rotating Frame

The insertion of Aditya-L1 into this Halo orbit presents a critical mission phase, which demands precise navigation and control. A successful insertion further involved constant monitoring along with the adjustment of the spacecraft's speed and position by using onboard thrusters. The success of this insertion not only signifies ISRO's capabilities in such complex orbital manoeuvres, but it but gives confidence to handle future interplanetary missions.

Aditya-L1 was designed and realized at UR Rao Satellite Centre (URSC) with participation from various ISRO centers. The payloads onboard Aditya-L1 were developed by Indian scientific laboratories, IIA, IUCAA, and ISRO. The Aditya-L1 spacecraft was launched by PLSV-C57 on September 2, 2023, from SDSC SHAR, into an elliptical parking orbit (EPO) of 235.6 km by 19502.7 km. 

From here, Aditya-L1 embarked on an extraordinary journey towards the Sun-Earth-L1 Lagrange point, with the help of the onboard propulsion system, increased its orbital size progressively and moved toward the L1 point. Five liquid engine burns (LEB) were executed during the Earth orbit phase; gradually raising the apogee of the EPO in order to attain the desired trajectory with the fifth burn, known as the trans-L1 injection (TL1I) maneuver. The maneuver strategy is carefully devised to minimize incremental velocity addition (ΔV) for reaching the target L1 halo orbit while restricting the number of perigee passes to minimize the spacecraft's exposure to the high radiation Van Allen radiation belts. 

To address errors during, the TL1I phase, a short burn of the engines, called TCM-1 was conducted on October 5, 2023, and another TCM-2 on December 14, 2023, to ensure compliance with Halo orbit insertion condition parameters. The spacecraft underwent a cruise phase lasting approximately 110 days to achieve the present condition prior to HOI targeted on January 6, 2024.

All the payloads were tested during the pre-commissioning phase and the performance of all the payloads is confirmed to be satisfactory.

Click here to learn more about ISRO's Aditya-L1 Mission.