Aditya-L1, ISRO’s first mission to study Sun, in final orbit; another landmark, says PM Modi

L1, or Lagrange 1 point, is one of the five locations in the Earth-Sun system where the gravitational effects of the Earth and the Sun, as also the centripetal force on any satellite placed there, roughly cancel each other out. It is a relatively stable point for a spacecraft to be parked, and observe the Sun. The L1 point is about 1.5 million kilometres from the Earth. This is just 1 per cent of the total distance between the Earth and Sun.

ISRO Chairman S Somanath said Saturday’s event was about only placing the Aditya-L1 in the precise ‘halo’ orbit. “It was moving towards the ‘halo’ orbit but we had to do a little bit of corrections to put it at the right place. So something like a 31 metres per second velocity had to be given cumulatively to the satellite to keep it at the right orientation,” PTI quoted him as telling reporters.

“So in this new orbit, it had to be placed very precisely. If you don’t do the correction today then it can escape from this point (L1). We will not allow it to escape. There are some contingencies but I am telling you mathematically it can escape. So it has been done very precisely,” he added.

What the ISRO scientists have achieved is exact placement based “on our measurement and very correct prediction of the velocity requirement.” The space agency further said the orbit of Aditya-L1 spacecraft is a periodic one, on the continuously moving Sun-Earth line, with an orbital period of about 177.86 Earth days.

“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 the Sun… The success of this insertion not only signifies ISRO’s capabilities in such complex orbital manoeuvres, but it gives confidence to handle future interplanetary missions,” ISRO said.

Placing the Aditya spacecraft in the ‘halo’ orbit around L1 point was not as simple as putting a satellite in an orbit around Earth, or any other planetary body like the Moon or Mars, which ISRO and other space agencies routinely do.

“Usually, satellites are put into orbits around physical bodies like Earth, Moon, Mars etc. In this case, at L1, there’s no physical object. It is an empty space, just a location. Besides, closed orbits around celestial bodies are either circular or elliptical. The orbit around L1 is more complex. It is almost perpendicular to the line joining the Earth and the Sun. As such, the insertion into this orbit required a very challenging manoeuvre. Even after the insertion, ISRO would need to carry out correction manoeuvres periodically to remain in the orbit,” A N Ramaprakash, a senior professor at Pune-based Inter-University Centre for Astronomy and Astrophysics (IUCAA) who has worked on one of the main payloads on-board the Aditya mission, said.

The success of Saturday’s manoeuvre was made public by Science and Technology Minister Jitendra Prasad, which prompted a congratulatory message from Prime Minister Narendra Modi, who is known to take a keen interest in India’s space activities.

“India creates yet another landmark. India’s first solar observatory Aditya-L1 reaches its destination. It is a testament to the relentless dedication of our scientists in realising among the most complex and intricate space missions,” Modi wrote on his ‘X’ handle.

“I join the nation in applauding this extraordinary feat. We will continue to pursue new frontiers of science for the benefit of humanity,” Modi said.

ISRO is only the third space agency, after NASA and the European Space Agency, to station a spacecraft at the L1 point to study the Sun.

The entry into the orbit happened 127 days after the launch of Aditya-L1 on September 2 last year, within days of Chandrayan-3 making the historic moon landing.

“This insertion into the halo orbit at L1 is something that ISRO has never done before,” Aditya-L1 mission director Nigar Shaji had told The Indian Express earlier.

The four-month journey to its destination was also the second longest that an Indian spacecraft had travelled in space to reach its destination. The Mars Orbiter Mission had travelled about 11 months before entering the orbit around the red planet.

“It (the insertion) was something similar to a car speeding on the highway and then making a manoeuvre to enter into a side lane. A change of speed and direction was involved. It was great that ISRO managed to do it in the first attempt itself. If not, more attempts could have been made, but that would have involved spending more fuel. The challenge is to enter into the orbit with minimal manoeuvring and firing of thrusters,” Professor Dibyendu Nandi of the Indian Institute of Science Education and Research at Kolkata said.

The spacecraft is carrying seven payloads, many of which were switched on for some time during its journey to the L1 point.

“Initial data show that they are all working as per expectations. Some of the payloads have already started detecting and monitoring activities on the Sun like solar flares. The most spectacular one has been the so-called New Year fireworks on the Sun, a flare that happened on the solar limb on December 31,” Prof Ramaprakash, one of the lead developers of the SUIT (Solar Ultraviolet Imaging Telescope) payload, said.

After the insertion, the payloads would be switched on sequentially. SUIT, seated on the top deck of the Aditya-L1, will be among the first to get operational once the spacecraft reaches L1.

“All the instruments will be checked for their health and calibration performed. Since these instruments are highly complex, the commencement of actual science observations may take another three months or so,” Dipankar Banerjee, director of Nainital-based Aryabhata Research Institute of Observational Sciences, said.

Prof R Ramesh of the Indian Institute of Astrophysics, Bengaluru, explained why the instruments would not begin their observations immediately. “When thrusters are fired to correct the trajectory of a spacecraft, or in this case to insert it into a desired orbit, some dust is generated in the vicinity. One of the reasons to study the Sun from space is to ensure that there is no distortion from dust particles. This is why the entrance aperture protecting the primary mirror of the main payload called VELC is still closed. ISRO will wait for the dust to settle for around two weeks before opening the aperture,” he said.

He said the other payloads will undergo calibration in space during this time and the first results should start coming in by the end of the month. “Since the aperture for VELC will be opened two weeks later, it will take some more time to calibrate it and start getting information,” he said.

Ramesh said one of the most challenging aspects of the mission would be to keep the spacecraft in the desired configuration through the rest of its life.

“When we talk about the missions to Moon or Mars, the orbiters can take images of the celestial body from any direction. However, when it comes to VELC, the Sun has to be perfectly lined up behind the occulting disk. The solar corona is several times fainter than the surface of the Sun, and if the photosphere is even a little bit visible as a ring around the occultor, the instrument will not be able to study the corona,” he said.

The range of experiments and observations that are planned to be carried out by the instruments on board Aditya are expected to provide completely new information about the dynamics inside the Sun, Banerjee said.

“The combination of instruments onboard Aditya are going to be operational for the first time ever at L1. No similar instruments have been present during previous missions (by other space agencies). Aditya has unique and different instruments with better capabilities… Aditya’s coronagraph has the ability to look very close to the Sun. It has an X-ray spectrometer. In addition, there are instruments capable of performing multi-slit spectroscopy, spectro-polarimetry and taking full-disc near ultraviolet images. All of these are new elements on a solar mission,” he said.

Aditya has seven scientific payloads of which the Visible Emission Line Coronograph (VELC), Solar Ultraviolet Imaging Telescope (SUIT), Solar Low Energy X-ray Spectrometer (SoLEXS), High-energy L1 Orbiting X-ray Spectrometer (HEL1OS) are designed to directly track the Sun. The in-situ (on site) measuring instruments include the Aditya Solar Wind Particle Experiment (ASPEX), Plasma Analyser Package for Aditya (PAPA), Advanced Tri-axial High Resolution Digital Magnetometers.

Anjali Marar works with the Raman Research Institute, Bengaluru.