Chandrayaan-3 in Moon's orbit: The trajectory to lunar glory

By Girish Linganna | First Published Aug 7, 2023, 6:45 PM IST

The mission's next phase involves separating the Landing Module, Vikram, containing the rover Pragyan, from the Propulsion Module. The lander will then undergo de-orbiting manoeuvres, concluding with a controlled powered landing on the Moon's surface around August 23 or 24. Girish Linganna explains

Chandrayaan-3 spacecraft has successfully entered the Moon's orbit. If everything continues as planned, it will safely land near the Moon's south pole between August 23 and 24. Since its launch on July 14, ISRO has been moving the spacecraft farther away from Earth into different orbits. After successfully raising Chandrayaan-3's orbit on Sunday night, ISRO announced that the next step is to perform a manoeuvre to reduce the orbit on August 9. This means they will adjust the spacecraft's path, bringing it closer to the Moon for its upcoming landing. It is like adjusting the route of a car to get closer to a destination.

As per ISRO's Twitter post, the spacecraft successfully executed a planned manoeuvre by firing its engines in the opposite direction, which brought it closer to the Moon's surface. Now, it is at a height of about 170 kilometres (105 miles) above the Moon's surface and its path around the Moon stretches to about 4313 kilometres (2682 miles). It is like flying at a certain height above the ground and travelling in a circular path around the Moon, covering a long distance in that circular journey.

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The next plan to bring the spacecraft even closer to the Moon's surface is set for August 9, 2023, between 1 PM and 2 PM IST. They will make another manoeuvre, similar to what they did before, to adjust the spacecraft's path and move it even nearer to the Moon.

The Journey So Far

* On July 14, the LVM3 M4 vehicle successfully launched Chandrayaan-3 into orbit.

* On July 15, the Chandrayaan-3 spacecraft underwent its first orbit-raising manoeuvre, called "Earthbound firing-1", at ISRO's facility in Bengaluru. This involved firing the spacecraft's engines to make it go higher above the Earth. After the manoeuvre, it reached a maximum distance of 41,762 kilometres from Earth and came as close as 173 kilometres. This helps the spacecraft stay on a proper trajectory around the Earth.

* On July 17, the Chandrayaan-3 spacecraft performed its second orbit-raising manoeuvre. At that time, the spacecraft's position in space was 41,603 kilometres at its farthest point from Earth and 226 kilometres at its closest point.

* On July 22, the Chandrayaan-3 spacecraft completed its fourth orbit-raising manoeuvre, also known as 'Earth-bound perigee firing'. At that moment, the spacecraft's position in space was 71,351 kilometres at its farthest point from Earth and 233 kilometres at its closest point. During this manoeuvre, the spacecraft's engines were fired when it was at its closest point to Earth, which is called 'perigee'. They do this by firing at perigee to take advantage of the gravitational boost from Earth, helping the spacecraft to increase its velocity and raise its orbit more efficiently. This allows the spacecraft to achieve the desired trajectory and reach the Moon with better efficiency and precision.

* On July 25, an orbit-raising manoeuvre was performed.

* On August 1, Chandrayaan-3 was placed into the translunar orbit. At that time, the achieved orbit was 288 kilometres at its closest point to the Moon and an impressive 369,328 kilometres at its farthest point.

When we say Chandrayaan-3 was inserted into the translunar orbit, it means that the spacecraft's trajectory was carefully adjusted so that it is now on a path that takes it towards the Moon. This is a crucial milestone in the mission because reaching the translunar orbit is necessary for Chandrayaan-3 to travel from Earth to the Moon.

ISRO executed a slingshot manoeuvre that successfully propelled the spacecraft from Earth's orbit towards the Moon. In simpler terms, a slingshot manoeuvre is like using a swing to gain speed and move to a higher position. It helped the spacecraft speed up and head towards its destination. ISRO used the slingshot manoeuvre, also known as a gravity assist or swing-by, to increase the spacecraft's speed and change its trajectory more efficiently. By utilizing the gravity of a celestial body, typically a planet (earth), the spacecraft can gain momentum without using additional fuel.

This helps save fuel and enables the spacecraft to reach its destination, in this case, the Moon, with less energy expenditure. The slingshot manoeuvre is a cost-effective and practical way to propel the spacecraft on its intended path with the help of celestial bodies' gravitational forces.

By achieving this orbit, the spacecraft can now start its journey towards the Moon and prepare for the upcoming phase of the mission, which includes getting closer to the lunar surface and eventually attempting a soft landing. The successful insertion into the translunar orbit marks a significant step towards achieving the mission's objectives and exploring the Moon.

* On August 5, Chandrayaan-3 successfully entered the lunar orbit. The orbit achieved is at a distance of 164 kilometres at its closest point to the Moon and 18,074 kilometres at its farthest point, just as planned.

When we say Chandrayaan-3 was inserted into the lunar orbit, it means that the spacecraft is now circling around the Moon. The achieved orbit specifies how far Chandrayaan-3 is from the Moon's surface at its closest and farthest points.

The lunar orbit insertion is a highly critical manoeuvre for the Chandrayaan-3 mission. It marks a major milestone as it allows the spacecraft to move around the Moon and get into position for the upcoming tasks, such as conducting scientific observations, mapping the lunar surface, and preparing for the soft landing attempt.

This manoeuvre requires precise calculations and precise timing to ensure that the spacecraft is captured by the Moon's gravity and placed in the correct orbit. Successfully entering the lunar orbit is crucial for the mission's success, as it enables Chandrayaan-3 to accomplish its objectives and contribute valuable data to our understanding of the Moon.

What's Next for Chandrayaan-3?

On Sunday night, August 6, Chandrayaan-3 conducted another manoeuvre at 11 PM. Over the following days until August 17, three more operations were performed. After that, the Landing Module Vikram, which carries the rover Pragyan inside, separated from the Propulsion Module. Subsequently, the lander underwent de-orbiting manoeuvres before finally descending to the Moon's surface with its powered landing, which was expected to occur on either August 23 or 24.

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