India’s first solar mission, Aditya-L1 spacecraft, recently completed its first halo orbit around the Sun-Earth L1 point. The Aditya-L1 mission – a solar observatory at Lagrangian point L1 – was launched on September 2, 2023 and was inserted in its targeted halo orbit on January 6, 2024.

As the first space mission, Aditya-L1 is dedicated to studying the Sun from a distance of 1.5 million kilometers (approximately 1% of the distance between the Earth and the Sun). It is Indian Space Research Organization’s (ISRO) second mission in the astronomy observatory class, following AstroSat (studying celestial sources in X-ray, optical and UV spectral bands simultaneously) in 2015.

But how does Aditya-L1 help in understanding the Sun? Why was it placed in a halo orbit around Lagrange point 1? What are Lagrange points and halo orbit?

Aditya-L1 mission

Aditya-L1 mission was launched using the Polar Satellite Launch Vehicle (PSLV-C57) rocket and reached its destination in 127 days. This PSLV (the Workhorse of ISRO) has been instrumental in launching a variety of missions, including India’s Chandrayaan and Mangalyaan.

The Aditya-L1 mission is designed to place a satellite in a halo orbit around the Lagrange point 1 (L1) of the Sun-Earth system. Lagrangian points (L-point) are named after the mathematician Joseph-Louis Lagrange, who first described these points in his work on the three-body problem in 1772.

In a three-body system, there are five specific positions where the gravitational forces of the two larger bodies balance the centripetal force felt by a smaller third object, allowing it to remain in a stable or semi-stable position relative to the two larger bodies. . In other words, it is a position in space where the gravitational forces of two large celestial bodies, such as the Earth and the Moon, or the Earth and the Sun, balance with the centripetal force of a satellite. This creates a point of equilibrium where the small object can maintain a stable position relative to the two large bodies.

What are L-Points or Lagrange points

There are five L-Points for the Sun-Earth system, each with unique properties, where a satellite can maintain a stable orbit within the gravitational influence of the Sun and Earth.

L1: It is located between the Earth and the Sun, where their gravitational forces balance with the centripetal force of a satellite. A satellite placed at L1 Lagrange point can maintain a stable orbit while continuously observing the Sun. The L1 point offers the significant advantage of providing an uninterrupted view of the Sun without any occultation or eclipses.

L2: It is located on the opposite side of the Earth from the Sun, where the gravitational forces of the Earth and the Sun balance with the centripetal force of a satellite. This position allows a spacecraft to maintain a stable orbit with a continuous view of deep space.

L3: It is located on the opposite side of the Sun from the Earth, directly behind the Sun from Earth’s perspective. It is not typically used for space missions, as it is always hidden from Earth’s view by the Sun, making communication and observation difficult.

The five Lagrangian points for the Sun-Earth system. L1 and L2 are on the line joining the Sun to the Earth. (Image courtesy ISRO)

L4: It is located 60 degrees ahead of the smaller body (like the Moon in the Earth-Moon system or Earth in the Sun-Earth system) along its orbit around the larger body. In the case of the Earth-Sun system, it forms an equilateral triangle with the earth and sun. L4 is considered a stable Lagrange point, meaning that if an object is slightly disturbed from this point, it will tend to orbit around the L4 point rather than drift away. This makes L4 an interesting location for placing satellites, space-based observatories, or even serve as potential locations for future space colonies.

L5: It is located 60 degrees behind the Earth along its orbit around the sun. It is located at the other stable point that forms an equilateral triangle with the Sun and the Earth. Similar to L4, the L5 point is a stable equilibrium point. If an object is slightly perturbed from this point, it will orbit around L5 rather than drift away. It has been proposed as a location for space colonies, space stations, and other long-term missions.

These five points are mirrored in other pairs of bodies. For example, the Earth-Moon system has its own set of Lagrange points. L1 is between Earth and the moon, L2 is on the far side of the moon, L3 is on the opposite side of Earth to the moon, and L4 and L5 are 60 degrees in front of and behind the moon on its orbit around Earth.

Due to their stability, the L4 and L5 points are ideal for long-term, low-maintenance space missions or potential colonisation. This stability contrasts with the other Lagrange points (L1, L2, and L3), which are characterized by unstable equilibrium and require continuous propulsion to maintain their positions.

Significance of the L1 location

A halo orbit is a type of three-dimensional periodic orbit that exists near the five L-points. Unlike simple 2D circular or elliptical orbits, halo orbits trace a 3D loop around these points, resembling a halo (extends above and below the orbital plane). In such an orbit, a spacecraft can maintain its position around an unstable Lagrange point with minimal use of thrusters (fuel) for station-keeping.

Placing Aditya L1 in a halo orbit (around L1-point) allows continuous, and uninterrupted observation of the Sun without Earth’s shadow interference, providing a stable point for solar monitoring. In contrast, a Low Earth Orbit (LEO) would subject the spacecraft to periodic eclipses and limited viewing windows, reducing observation efficiency and data continuity.

The L1 location significantly reduces the influence of Earth’s atmosphere and magnetic field, thereby improving the quality of solar data collected compared to LEO (just a few hundred kilometers away from the Earth). Being located at the L1 point allows Aditya-L1 to monitor solar activity before they reach Earth. This early detection capability is crucial for space weather forecasting, providing advanced warnings that can protect satellites, power grids, and communication systems on the Earth.

A satellite in LEO would not be able to provide early detection of solar events as effectively. Thus, the mission benefits from continuous solar observation, minimal atmospheric interference, stable thermal conditions, and enhanced ability to monitor and predict space weather events. SOHO (Solar and Heliospheric Observatory) is in halo orbit (around L1 point), allowing it to continuously observe the Sun. In addition, the James Webb Space Telescope (JWST) is placed in a halo orbit (around L2 point), giving it an uninterrupted view of deep space.

The Aditya-L1 mission is designed to observe and analyze various aspects of the Sun’s outermost layers, particularly the corona (the outermost layer of the Sun’s atmosphere), photosphere (visible surface of the Sun), and chromosphere (second layer between the photosphere and the corona) using a satellite placed in a halo orbit around the Lagrange point 1 (L1) of the Sun-Earth system.

Post Read Question

What is the Aditya-L1 mission? Why was it placed in a halo orbit around L1?

What are the Lagrangian points for the Sun-Earth system?

Aditya-L1 spacecraft recently completed its first halo orbit around the Sun-Earth L1 point? Discuss its findings.

(Amit Kumar is a doctoral candidate at IIT Delhi. In the second part of the article, he will discuss the significance of the Aditya-L1 mission, its payloads and the way forward. )

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