The Indian Space Research Organisation (ISRO) launched its Aditya-L1 solar observatory from Satish Dhawan Space Centre in Sriharikota Range (SDSC SHAR), India, at 11:50 IST on 2 September 2023.

Aditya-L1 is an ambitious mission that will generate vast quantities of science data as the spacecraft balances in an unstable orbit. With its global network of deep space ground stations and experience flying similar missions, ESA has just the right infrastructure and expertise to provide support.

Aditya-L1 will be the first Indian satellite mission to study the Sun. After launch, the spacecraft will travel to its new home – the first Lagrange point (L1) of the Sun-Earth system.

From there, its seven instruments will be used to investigate open questions about our dynamic and turbulent star. Four of them will view the Sun directly, while the other three will carry out in-situ measurements to explore the nature of the space weather that the Sun generates in interplanetary space.

ESA Support To Aditya-L1

ESA is supporting Aditya-L1 in two ways: the Agency is providing deep space communication services to the mission, and, last year, ESA assisted ISRO with the validation of important new flight dynamics software.

Communication is an essential part of every space mission. Without ground station support, it’s impossible to get any science data from a spacecraft, to know how it’s doing, to know if it is safe or even to know where it is.

“ESA’s global network of deep space tracking stations and use of internationally recognised technical standards allows us to help our partners track, command and receive data from their spacecraft almost anywhere in the Solar System,” says Ramesh Chellathurai, ESA Service Manager and ESA Cross-Support Liaison Officer for ISRO.

“For the Aditya-L1 mission, we are providing support from all three of our 35-metre deep space antennas in Australia, Spain and Argentina, as well as support from our Kourou station in French Guiana and coordinated support from Goonhilly Earth Station in the UK.”

ESA is the main provider of ground station services for Aditya-L1. ESA stations will support the mission from beginning to end: from the critical ‘Launch and Early Orbit Phase’, throughout the journey to L1, and to send commands to and receive science data from Aditya-L1 for multiple hours per day over the next two years of routine operations.

Lagrange Point 1 – A Perfect Home For Solar Explorers

The 5 Lagrange points of the Sun-Earth system.Aditya-L1 will operate from a halo orbit around L1

When one large mass orbits another, their gravitational forces and orbital motion interact to create five equilibrium points where a spacecraft can operate for a prolonged period of time without having to use a lot of fuel. These locations are known as Lagrange points.

The first Lagrange point, L1, is located between Earth and the Sun, roughly one percent of the distance to the Sun. It’s a great location for solar explorers such as Aditya-L1, as it allows for an unobstructed view of the Sun that is never eclipsed by Earth. At L1, Aditya-L1 will join spacecraft such as the ESA/NASA Solar Heliospheric Observatory (SOHO), which has been at L1 since 1996.

Spacecraft that are designed to look outwards at the outer Solar System and far beyond, such as the NASA/ESA/CSA James Webb Space Telescope or ESA’s Euclid and Gaia telescopes, instead travel to L2. L2 is an opposite to L1, located the same distance from Earth but on the other side of the planet, as seen from the Sun. At L2, these spacecraft always have the brightness of the Sun and Earth behind them as they gaze outwards at faint objects hiding in the darkness of the Universe.

Getting There

Aditya-L1 will not travel to L1 directly from launch. Instead, ISRO operators will need to perform a ‘transfer manoeuvre’ similar to the one that ESA performed recently to take its Euclid telescope to L2.

This manoeuvre will be performed soon after launch, as the amount of fuel required to achieve the necessary trajectory grows quickly with time. Aditya-L1 will first perform manoeuvres to adjust its orbit around Earth after launch, before performing a transfer manoeuvre to L1. The spacecraft will reach L1 approximately 100 days after launch.

Staying There

L1 is one of the ‘unstable’ Lagrange equilibrium points. 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.

An inability to perform these monthly manoeuvres can be a big problem. In June 1998, the SOHO mission experienced an issue and failed to carry out its station keeping. The error in its orbit grew so quickly and so unpredictably that contact was lost with the spacecraft, and it began drifting into the void.

A combined team of NASA and ESA experts set to work to safely recover the spacecraft, eventually finding it far from its expected position and re-establishing contact. 25 years later, SOHO is still in orbit around L1 and returning valuable scientific data.

It's Not Just Orbits That Come Full Circle

For some of ESA’s flight dynamics experts, this exercise felt familiar. As ESA prepared to launch its own early deep space missions, it faced similar challenges to those ISRO faces today. ESA reached out to a team from NASA’s Jet Propulsion Laboratory (JPL) to help validate the interplanetary orbit determination software for ESA’s Mars Express mission and for the comet chaser, Rosetta, both of which were then successfully navigated by ESA. The exercise was similar in scope and goal to the one carried out by ESA and ISRO for Aditya-L1 last year.

The International Space Community

ESA’s two-pronged support to Aditya-L1 demonstrates the value of international spaceflight collaboration. ESA’s ground station network (known as ‘Estrack’) and flight dynamics expertise have been built up over decades of flying the most challenging spacecraft missions and are now cornerstones of the Agency’s support to its partners.

On Earth, Estrack is undergoing an expansion. Construction is underway on ESA’s fourth deep space antenna, as the Agency prepares to meet the rising demand for communication bandwidth from its own deep space and space safety missions and from support to an increasing number of partners.

Meanwhile, in space, Aditya-L1 will be the newest member of the fleet of solar explorers, including ESA’s Solar Orbiter, SOHO, NASA’s Parker Solar Probe and others, on humankind’s shared mission to unravel the mysteries of our star.