India’s first space mission to study the Sun, Aditya-L1 spacecraft, made its fourth Earth-bound manoeuvre on Friday. The next manoeuvre for a send-off from the Earth is scheduled for September 19 around 02:00 IST, ISRO said in its latest update. The third manoeuvre was completed on September 10.
India successfully launched its maiden space-based observatory to study the Sun from the Satish Dhawan Space Centre in Sriharikota on September 2. Aditya-L1 on its 1.5 million km journey will be placed in a halo orbit around the Lagrange point (L1) of the Sun-Earth system, which has the advantage of viewing the Sun without any eclipses. This will provide a greater advantage of observing the solar activities continuously.
Also Read: Aditya-L1 is one of the many solar missions — check out the other probes into the Sun’s secrets
A total of five manoeuvres will be carried out throughout the spacecraft’s 16-day voyage around the Earth, during which it will build the velocity required for its subsequent journey to L1.
These manoeuvres are pivotal during the 16-day trajectory around Earth, facilitating the spacecraft in gaining the necessary velocity for its journey toward the L1 Lagrange point.
What is an orbital manoeuvre
In spaceflight, an orbital manoeuvre (otherwise known as a burn) is the use of propulsion systems to change the orbit of a spacecraft. For spacecraft far from Earth (those in orbits around the Sun) an orbital manoeuvre is called deep space manoeuvre.
A manoeuvre is needed to raise the orbit and increase the velocity of the spacecraft so that it can move to its destination. The interplanetary missions like Chandrayaan, Mangalyaan or Aditya L1 require taking the spacecraft outside the Earth’s sphere of influence.
This requires the spacecraft to have additional velocity which can be provided directly by the launch vehicle or can be achieved by successive firing of the spacecraft propulsion system. The direct insertion by the launch vehicle can result in much faster transfer trajectories but requires powerful rockets.
Raising the orbit
The manoeuvres are done essentially to optimise the mission achieve its goals. These are planned well in advance as part of the mission,” says Anil Bhardwaj, director Physical Research Laboratory (PRL), which developed and designed one of the most important instruments — ASPEX on board.
Bhardwaj said that if the velocity is increased when the spacecraft comes close to the Earth, the orbit becomes more elliptical. “The Earth Burn or the manoeuvres as they are better known are done to put the spacecraft into the correct orbit and gain velocity. This is a novel way that saves a lot of propellant.”
Santosh Vadawale, an astrophysicist at the PRL involved with the payload ASPEX said that the multiple Earth bound orbits also provide the opportunity for any corrections that are needed, resulting in better control over the final trajectory.
The final orbital manoeuvres are arrived after detailed calculations and simulations to optimise the final mass and the duration.
Mylswamy Annadurai, director of India’s first Moon mission, said that PSLV is a proven cost-effective launcher but it is not possible to place Aditya L1 spacecraft of 1475 kg mass by PSLV-C beyond 235x19500 km around the Earth.
“From this orbit to go to the Sun-Earth-L1 point of 1.5 million kms from the Earth the fuel optimal mission plan requires few earth-bound manoeuvres by the spacecraft propulsion system as has been done in Chandrayaan 1, Chandrayaan 2, Chandrayaan 3 and Mangalyaan.”
Dittos Ajay Lele, space expert and consultant, Manohar Parrikar Institute for Defence Studies and Analyses. “To reach its destination point L1 it has to travel a distance of 15 lakh km. To reach such a long distance, the first step is to reach to an orbit from where the craft would be shifted to transfer orbit towards L1 and for purpose four earth-bound orbital manoeuvres are required to incrementally raise the orbit of the spacecraft,” he says.
Raising the velocity
At the surface of the Earth, if you throw anything more than 11.2 km/sec it will escape the Earth’s gravity and will not come back. At 235 km altitude, where the PSLV injected Aditya-L1, it is around 10.8 km/s., says an ISRO scientist.
“Some space centres have powerful rockets that can directly push with that velocity. But in our case, the spacecraft was injected at 9.4 km/sec. In order to gain velocity we have to go for manoeuvres, which means burning the propellants using propulsion systems inside the satellite,” he says.
Each time the manoeuvre happens, there is a gain in the velocity and increase in apogee height(furthest point to the Earth) When the velocity becomes 10.8 km/sec it is injected into the Sun bound orbit. Typically, the number of manoeuvres depends on how heavy the spacecraft is and a balance between a deficit in velocity and the amount of propellent carried. There are other reasons also for carrying out these ‘burns’
M S Pannirselvam, former Range Operation Director Mas Mission and former Chief General Manager at the Satish Dhawan Space Center, Sriharikota says that Aditya L1 during its journey towards the L1 point has to go past Moon’s orbit around the earth.
“If the Moon is anywhere near then it will affect the path of Aditya. We need to ensure that the Moon is far away. As the new moon happened on September 14 (the day when the Moon comes in between the Earth and the Sun), the manoeuvre was needed to ensure that when Aditya crosses the Moon’s orbit, the Moon will be behind the Earth,” he explained.
The spacecraft was kept in the Earth-bound orbit initially and the orbit was raised step by step till the designated time for the L1 insertion.
Pannirselvam maintains that when Aditya L1 leaves Earth’s sphere of influence and enters the Sun's sphere of influence, Earth’s equator plane and its orbit plane around the Sun should be aligned.
“This happens twice a year on March 21 and September 21. Therefore, the Lagrangean insertion date and time is fixed based on this. Otherwise, a lot of fuel is needed to take it towards the L1 point,” he adds.
Halo orbit
Former ISRO scientist T N Suresh Kumar maintains that when the spacecraft is put from the rocket it is launched in an elliptical orbit. The closest point to the Earth is called perigee and the furthest point is called apogee.
“We have to fire the engines on the spacecraft so that the orbit is raised step by step. The initial apogee is 19,500 km and we have to increase it gradually in four steps through these firings and manoeuvres. The last manoeuvre that happened on September 15 at 02.15 hours took the apogee to 1.21 lakh km. There will be one final manoeuvre on September 19 after which Aditya will go out of the earth’s influence of gravitation, he said.
Essentially the Earth-bound manoeuvres are needed to raise the orbit height so that it does not rotate back to the Earth P J Bhat, former Deputy Director at the UR Rao Satellite Centre, ISRO says that the launcher PSLV put the spacecraft in an elliptical orbit around earth with perigee 235 km and Apogee 19500 Km.
“But the ultimate objective is to put the spacecraft into a halo orbit around Earth-Sun Lagrange point L1 which is around 1.5 million km from earth. The manoeuvres are required to change the orbit in multiple steps in such a way that the apogee reaches near L1, so that a special manoeuvre can be conducted to establish it in the halo orbit around L1,” he says.
At L1, the spacecraft would remain quite stable with minimum propulsion energy requirements for corrections in its orbit, such that one side of Aditya always faces the Sun and the collected data can be transmitted to the Earth from the Earth-facing side.
(Edited by : C H Unnikrishnan)
First Published: Sept 17, 2023 6:08 PM IST
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