Chandrayaan-3 was launched on July 14 from India's spaceport, Satish Dhawan Space Centre, Sriharikota

India's Chandrayaan-3 spacecraft has moved closer to the moon after performing its third lunar orbit reduction. This means that the craft has been gradually moved from a highly-elliptical orbit (174 km x 1437 km) to a near-circular orbit (150km x 177km) around the Moon. In an elliptical orbit, there is a huge difference between the craft's nearest point to the moon (perilune) and farthest point to the moon (apolune). In a near-circular orbit, there is a very minor difference between perilune and apolune.

"Orbit circularisation phase commences. Precise maneuvre performed today has achieved a near-circular orbit of 150 km x 177 km. The next operation is planned for August 16, 2023, around 0830 hrs IST," ISRO posted on X, formerly known as Twitter.

ISRO's next target is to bring the craft to a circular orbit where the craft is 100 km above the lunar surface. This is what ISRO calls the orbit circularisation phase. When this orbit is achieved, the spacecraft will be at a constant 100 km distance above the moon, at any period of time in that orbit.

"When the craft is circling around the moon, a command is issued to re-orient the craft by 180 degrees. This re-orientation will ensure that the craft's engine(s) can be used as a brake. Such engine firings are known as retro-firings. In retro-firing, the engine is not pushing the craft forward. Instead, it is pushing the craft in a direction opposite to its original direction of movement and it will slow down the craft," Dr Mylswamy Annadurai, former director of ISRO's UR Rao Satellite Centre, explained to WION.

When the craft was circling the earth, commands were issued to fire the engines and push the craft further away from the earth (orbit-raising manoeuvres). In those cases, the engine was fired to propel the craft forward and impart more velocity to it. In retro-firing (opposite firing), the engine firing is done in an opposite direction, thereby reducing the velocity of the craft and lowering its orbit. These are the fundamental differences between orbit-raising manoeuvres and orbit-reduction manoeuvres.

The Journey So Far

Chandrayaan-3 was launched on July 14 from India's spaceport, Satish Dhawan Space Centre, Sriharikota. 16 minutes after lift-off, the craft was placed in a highly elliptical orbit around the Earth. Thereafter, by performing five engine burns on as many occasions, the craft gradually raised its orbit and slingshot itself further away from Earth, while continuing to remain in the home planet's orbit.

On the intervening night of July 31 and August 1, Chandrayaan-3 performed its Trans-Lunar injection burn. By this long-duration firing of its engines, the craft was pushed out of Earth's gravitational influence and hurled towards lunar gravitational influence.

On August 6, the spacecraft performed its maiden Lunar orbit reduction manoeuvre. The second Lunar orbit reduction happened on August 9.

The Journey That Lies Ahead

ISRO has said that the fourth orbit reduction manoeuvre is scheduled for 8:30 am IST, August 16.

"At the time of the lander separating from the propulsion module, the craft will be in a 100 km x100 km orbit. Many systems that are required to work for landing will be utilised and tested at that point. The craft will be in the orbital phase for 4-5 days," Dr M. Sankaran, director of ISRO's UR Rao Satellite Centre, said.

"So, we will carry out tests of the various systems. Four 800N throttleable engines (that are meant to help in slowing down the craft for landing) have to be test fired in different combinations, sensors on the lander will be oriented towards the propulsion module and tested for various performance parameters by getting a reflection back from it."

By the time the engine testing is complete, the craft will be brought to a 100 km x 30 km orbit, he added.

Thereafter, a process of Rough braking, Attitude hold, fine braking, and velocity reduction would be carried out in the run-up to the lunar landing. ISRO has planned the Lunar landing at 5:47 pm IST on August 23.

ISRO Chairman Dr S Somnath has assured that the Vikram lander will be able to make a soft landing on the Moon's surface, come August 23. He added that the landing is certain even if all the sensors and two of its engines do not work. The Vikram lander is expected to descend on the moon on August 23.

"If everything fails, if all the sensors fail, nothing works, still it (Vikram) will make a landing. That's how it has been designed -- provided that the propulsion system works well," Somnath said.

Objectives of Chandrayaan-3

India's third lunar spacecraft comprises a propulsion module, lander and rover. The combined mass of them is 3.9 tonnes. The key goal of the craft is to perform a lunar soft-landing and perform in-situ analysis near 70 degrees latitude of the Lunar surface. The mission is expected to last 14 Earth days or one lunar day, during which the six-wheeled rover and the lander will perform their respective experiments.