India's Chandrayaan-3 performs second Lunar orbit reduction

New Delhi, India

India's third lunar spacecraft Chandrayaan-3 has successfully completed its second Lunar orbit reduction. This is a process of firing the spacecraft's engines, in a manner that would slow down the craft and bring it into an orbit that's closer to the moon.

The firing carried out between 1-2 pm Wednesday, August 9 (Indian Standard Time), reduced the craft's orbit from 170 km x 4313 km to 174 km x 1437 km. This Lunar orbit reduction is part of a gradual process that will eventually lead the craft to the Lunar soft landing that is scheduled for 23rd August. 

"Even closer to the moon’s surface. Chandrayaan-3's orbit is reduced to 174 km x 1437 km following a manoeuvre performed today. The next operation is scheduled for August 14, 2023, between 11:30 and 12:30 Hrs IST," ISRO said in a tweet.

"When the satellite 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. 

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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 a retro-firing (opposite firing), the engine firing is done in an opposite direction and thereby reduces the velocity of the craft and lowers its orbit. These are the fundamental differences between orbit raising manoeuvres and orbit reduction manoeuvres.

ISRO intends to perform two more Lunar orbit reduction burns (retro-firings) of the craft. The next retro-firing, which is also the third such would be carried out on 14th August between 11:30 and 12:30 noon (IST). The final and fourth retro-firing to reduce the orbit is expected on 16th August.

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The journey so far...                       

Chandrayaan-3 was launched on 14th July from India's spaceport, Satish Dhawan Space Centre, Sriharikota. Sixteen minutes from 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 Earth's orbit. 

On the intervening night of 31st July and 1st August, 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.

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On 6th August, the spacecraft performed its maiden Lunar orbit reduction manoeuvre.

The journey ahead...                     

ISRO has said that the craft's next operation would be the third reduction of orbit around the moon. It is scheduled for August 14th at noon, ISRO said. 

"When the lander separates from the propulsion module(on August 17th), the craft will be in a 100x100km circular orbit around the moon. The craft would be in that orbit for 4-5 days and we would use that period to test the crucial systems, and sensors that would be used for the lunar landing(because this is the first time we would be testing them in space)," M. Sankaran, Director of ISRO's UR Rao Satellite Centre, had told WION.

Thereafter, the craft would be brought to 100x30km orbit and then a process of Rough braking, Attitude hold, Fine braking, and velocity reduction would be carried out, in the run-up to lunar landing. ISRO has planned the Lunar landing at 5:47 pm Indian Standard Time, on August 23.

Objectives of Chandrayaan-3...                

India's 3rd Lunar spacecraft comprises a propulsion module, lander and rover. The combined mass of them is 3.9 tons. 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.