January 6, 2024
Halo-Orbit Insertion (HOI) of its solar observatory spacecraft, Aditya-L1 was accomplished 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 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 Sun, Earth and a spacecraft.
This specific halo orbit is selected to ensure a mission lifetime of 5 years, minimising station-keeping
manoeuvres and thus fuel consumption and ensuring a continuous, unobstructed view of sun.
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).
The insertion of Aditya-L1 into this Halo orbit presents a critical mission phase, which demanded
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 realised at UR Rao Satellite Centre (URSC) with participation from
various ISRO centres. 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 L1 point. Five liquid engine burns (LEB) were executed during Earth orbit phase; gradually
raised 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, 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 performance of all the payload is confirmed to be satisfactory.
The picture below shows the Halo orbit insertion process graphically in a two dimension picture. The Aditya-L1 spacecraft was moving from Earth towards the L1 point in the direction of Sun. The TCM1 &2 firing oriented the spacecraft toward the Halo Orbit, making it reach the HOI condition (which is a minimum fuel consumption condition) as on 6th January 2024 (Marked by the red dot). The final firing was done at this point, making the spacecraft align with the Halo Orbit. If the HOI manoeuvre was not conducted as done today, the spacecraft would have moved in the direction marked (Without HOI).
Halo Orbit Insertion in L1 Rotating Frame