Special mission

Indian scientists have so far observed the Sun through telescopes on the ground, and relied on data from solar missions launched by the United States, Europe, the United Kingdom, and Japan.

Credit: ISRO

“All these years, India has been observing the Sun using only ground-based telescopes, which have grown old now. As India lacked a large-scale modern observational facility, we were dependent on other sources for solar data. Aditya-L1 presents a unique opportunity to not only address the existing gaps but also complement with newer data to address the unsolved problems in solar physics,” Prof Dipankar Banerjee, director of the Aryabhatta Research Institute of Observational Sciences, Nainital, said.

S Seetha, former director of ISRO’s Space Science Programme Office, said astronomy-based space missions are gaining importance “due to the new findings, and inspiration to the youth and general public it can provide”.

“Science missions also take longer to develop, since technological development can be demanding,” Dr Seetha said.

Eyes in space

Disturbances in the form of solar flares, Coronal Mass Ejection, or solar winds directed towards Earth, can adversely impact space weather; studying the Sun is, therefore, of paramount importance.

While AstroSat, India’s first dedicated astronomy mission aimed at studying celestial sources in X-ray, optical, and UV spectral bands simultaneously, remains operational almost eight years after its launch, Aditya-L1 can potentially pave the way for future Indian astronomy missions.

AstroSat, weighing 1,515 kg, lifted off with five in situ instruments; Aditya-L1, weighing 1,475 kg, will carry seven payloads, four of which will directly look at the Sun. The other three will perform in situ (on site) studies of particles and magnetic fields at and around the L1 point.

“Solar physics now demands multiwavelength astronomy. It will be important how data from various instruments on Aditya-L1 are effectively combined and put to use to make sense of a solar event, its source, causes, local conditions, etc. This will require coordinate observations taken across different instruments,” Prof Banerjee said.

The four remote sensing instruments will probe the solar sources and try to identify the source regions with greater focus — an edge over all predecessor solar missions. This could help better understand the origins of solar eruptions or flares.

Space weather alerts

The mission hopes to generate user-friendly information that can help safeguard a range of satellite-dependent operations such as telecommunications, mobile-based Internet services, navigation, power grids, etc. Once tested, tailormade information obtained from the data can be used to issue space weather alerts.

“A recommendation was to come up with novel ways to utilise data from Aditya-L1 to extract space weather information and predict space weather. One way is to issue space weather alerts, which will be tested during the initial few months after the successful insertion of the satellite in the desired orbit,” solar physicist Dr Dibyendu Nandi, Chair of the Space Weather and Monitoring Committee of Aditya-L1, said.

Dr A N Ramaprakash, one of the two principal investigators who led the team that designed and built the Solar Ultraviolet Imaging Telescope (SUIT), one of the payloads on board, said: “We can also get information about the environment around the L1 point, which is key for understanding space weather.”

L1 and afterward

Aditya-L1 will travel for nearly 100 days to cover the 1.5 million km distance to L1. This is a shorter voyage than Mangalyaan, which took 298 days to reach the Martian orbit in 2014.

Like the Chandrayaan-3 mission, Aditya-L1 too, will undergo multiple apogee-raising orbital manoeuvres, and is expected to exit the Earth’s orbit on the fifth day after launch.

“After leaving Earth’s gravity, it will get into a heliocentric path, and this is crucial. Later, getting into the orbit around L1 is the most crucial aspect. L1 is not an object, just a location in space, which also co-moves with Earth around the Sun,” Dr Seetha said.

Six of the mission’s payloads — VELC, SUIT, SoLEXS, HEL1OS, PAPA, and MAG — will remain in the ‘off’ mode until around January 6, 2024, when the spacecraft is expected to be inserted into a ‘halo’ orbit near L1. The Aditya Solar wind Particle Experiment (ASPEX), built by the Physical Research Laboratory, will turn on while in transit.

“During the cruise phase, ASPEX will turn on and start performing in situ measurements of solar particles and ions,” Dr Sreejith Padinhatteeri, who was part of the team at Inter-University Centre for Astronomy and Astrophysics (IUCAA) that built SUIT, said.

Designed to image the Sun in the 200-400 nanometre (nm) of the ultraviolet band, SUIT’s imager will continuously record the entire disk of the Sun through 11 filters. SUIT’s images of these layers could help improve our understanding of the Sun’s immediate atmosphere.

By early 2024, scientists are hopeful of being able to commence a series of experiments lasting 2-3 months towards calibrating the instruments before high quality scientific data begin to roll out.

(Anjali Marar works with the Raman Research Institute, Bengaluru)

L1: the MISSION DESTINATION

The place between the Sun and Earth, where the spacecraft will park itself, is called L1, or Lagrange Point 1 — one of the five Lagrange Points that exist between any moving two-body system in space. The destination is the reason the mission is called ‘Aditya-L1’.

Lagrange points, named after mathematician Josephy-Louis Lagrange who discovered them, are positions in a moving two-body system where forces acting on a third body of smaller mass cancel each other out. A spacecraft placed between the Earth and Sun, and wanting to move with them, would feel the gravitational pull from each side as well as the centripetal force by virtue of moving in a circular or elliptical orbit. There are five points in any two-body system like this, where the net of all these forces is zero (see illustration).

Being positioned at a Lagrange point makes sense because the spacecraft requires very little energy to just stay put and make continuous observations. At any other place, the spacecraft would feel additional force, and would need to expend energy to remain stationary relative to both the Earth and Sun.

Among the five Lagrange points, L1 is the most favoured to get an unhindered view of the Sun. L2 is located behind the Earth, and thus obstructs the view of the Sun, while L3 is behind the Sun which is not a great position to communicate with Earth. L4 and L5 are good and stable locations, but are much farther from Earth compared to L1, which is directly between the Sun and the Earth.

AMITABH SINHA