Aditya-L1 completes first halo orbit around L1 point

Context: India’s Aditya-L1 solar mission spacecraft completed its first halo orbit around the Sun-Earth L1 point on July 2. The Lagrange Point 1 (L1) of the Sun-Earth system is approximately 1.5 million km away from the Earth, which is about 1 per cent of the Earth-Sun distance. Aditya-L1, weighing about 1,480.7 kg, is the first spacebased Indian observatory to study the Sun. It was launched on September 2, 2023 and was inserted in its targeted halo orbit on January 6, 2024.

What is halo orbit?

• The orbit of Aditya-L1 spacecraft is a periodic halo orbit which is located roughly 1.5 million km from Earth on the continuously moving Sun–Earth line with an orbital period of about 177.86 earth days.
• This is a periodic, three-dimensional orbit at L1 involving Sun, Earth and a spacecraft.
• This halo orbit is with a size of six lakh km in one dimension, over two lakh km in another dimension and one lakh km in one more dimension, forming an egg-shaped orbit.
• This specific halo orbit is selected to ensure a mission lifetime of five years, minimising station-keeping manoeuvres and thus fuel consumption.
• A satellite in a halo orbit around the L1 point has the major advantage of continuously viewing the Sun without any occultations/ eclipses. This will provide a greater advantage in observing solar activities and their effect on space weather in real time.

Orbital station-keeping manoeuvres

• Aditya-L1 spacecraft in the halo orbit takes 178 days to complete a revolution around the L1 point.
• During its travel in the halo orbit, Aditya-L1 spacecraft will be subjected to various perturbing forces that will cause it to depart from the targeted orbit.
• Aditya-L1 underwent two station-keeping manoeuvres on February 22 and June 7, respectively, to maintain this orbit. The third station-keeping manoeuvre on July 2 has ensured that its travel continued in
the second halo orbit path around L1.

What are Lagrange Points?

• For a two body gravitational system, the Lagrange Points are the positions in space where a small object tends to stay, if put there. These points in space for two body systems such as Sun and Earth can be used by spacecraft to remain at these positions with reduced fuel consumption.
• At Lagrange Points, the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them. These points in space can be used by spacecraft to reduce fuel consumption needed to remain in position.
• Lagrange Points are named in honour of Italian-French mathematician Joseph-Louis Lagrange.
• Of the five Lagrange points, three are unstable and two are stable. The unstable Lagrange points – labelled L1, L2, and L3 – lie along the line connecting the two large masses. The stable Lagrange points – labelled L4 and L5 – form the apex of two equilateral triangles that have the large masses at their vertices.

• The closer an object is to the Sun, the faster it will move. So, any spacecraft going around the Sun in an orbit smaller than Earth’s will soon overtake our planet. However, there is a loophole: if the spacecraft is placed directly between the Sun and Earth, Earth’s gravity pulls it in the opposite direction and cancels some of the Sun’s pull.
• With a weaker pull towards the Sun, the spacecraft needs less speed to maintain its orbit, so it can slow down. If the distance is just right – about a hundredth of the distance to the Sun – the spacecraft will travel slowly enough to keep its position between the Sun and the Earth.
• This is L1, and is a good position from which to monitor the Sun since the constant stream of particles from the Sun, the solar wind, reaches L1 about an hour before reaching Earth.

• L2 is located 1.5 million kilometres directly behind the Earth as viewed from the Sun. L2 is a great place from which to observe the larger universe.
• A spacecraft here does not have to orbit Earth and so is spared from sweeping in and out of our planet’s shadow, heating up and cooling down, and distorting its view.

• L3 lies behind the Sun, opposite Earth, just beyond our planet’s orbit. Objects in L3 cannot be seen from Earth. It offers the potential to observe the far side of the Sun.
• A spacecraft at L1, L2, or L3 is ‘meta-stable’, like a ball sitting on top of a hill. A little push or bump and it starts moving away, so a spacecraft must use frequent rocket firings to stay in so-called ‘halo orbits’ around the Lagrangian point.

L4 and L5

• L4 and L5 lie at 60° ahead of and behind the Moon in its orbit with respect to the Earth. Each one of them is a third point of an equilateral triangle with the base of the line defined by those two bodies.
• These Lagrangian points are stable for the Earth-Moon mass ratio. These Lagrangian points represent remarkable positions to host astronomical observatories or space stations. Unlike the other Lagrange points, L4 and L5 are resistant to gravitational perturbations.
• Because of this stability, objects such as dust and asteroids tend to accumulate in these regions.

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