Aditya-L1 was launched on September 2 and commenced its journey to its final destination, the Sun-Earth Lagrange’s Point 1 (L1), on September 18.The L1 is a region of stability between Earth and Sun where the gravity of the two bodies and the centrifugal force balance out.
Diagram from Isro’s earlier situational analysis showing orbital positions of various operational spacecrafts at Sun-Earth L1 point with the expected location of Aditya-L1 (not to scale). Credit: Isro
At L1, India’s satellite will join four operational probes. Three of these belong solely to NASA: WIND, Advanced Composition Explorer (ACE) and Deep Space Climate Observatory (DSCOVER). The fourth, the Solar and Heliospheric Observatory (SOHO), is a joint mission by NASA and the European Space Agency (ESA).
UR Rao Satellite Centre (URSC) director M Sankaran told TOI: “The final manoeuvre will be a short one using a group of thrusters. Aditya-L1 has 12 thrusters and we’ve not decided which ones will be used. A final decision will be taken on whether to use LAM (liquid apogee engine) or other thrusters, based on the spacecraft’s position on Saturday.”
Aditya-L1 carries 7 instruments to study the Sun and solar storms, with a planned 5 year mission and L1 offers an unobstructed view of the Sun.
If reaching L1 is a challenging journey, staying there is also tricky. To ensure it gets to its destination and stays safely in orbit, Isro needs to know exactly where their spacecraft “was, is and will be”. This tracking process, called ‘orbit determination,’ involves using mathematical formulae and specially developed software by Isro’s URSC.
Isro chairman S Somnath told TOI: “Once it reaches there, we will perform periodic manoeuvres to keep the spacecraft in the intended orbit.”
Staying at L1
According to the European Space Agency (ESA), L1 is one of the ‘unstable’ Lagrange points and keeping a spacecraft exactly at the L1 point is practically impossible. Instead, spacecraft enter orbit around L1 “as if the Lagrange point were an ‘invisible planet’”.
“Even so, due to the instability of this orbit, small trajectory errors will grow quickly. As a result, spacecraft must perform ‘station keeping’ manoeuvres roughly once a month to keep them in the correct orbit.”
Somanath had said earlier that while L1 is an unstable point, the instability is very mild and spread over a long period of time making it still the best place to be for a spacecraft. “L3 and L4 are much more difficult, for instance,” he had said, adding that if Isro isn’t careful with regard to orbit determination then the spacecraft can diverge.
