Chandrayaan-3 craft to slingshot itself out of Earth's influence, set course for Lunar capture

After spending 17 days in orbit around the Earth, India's third Lunar spacecraft Chandrayaan-3 is prepared to bid goodbye to its orbit around the home planet and set course for its destination - Earth's Moon. By performing a Trans-Lunar Injection(TLI), a long duration burn of its engines (around 20 minutes), Chandrayaan-3 will completely slingshot itself out of earth's orbit and influence, thereby putting it on a path that will eventually lead it to the moon. This crucial manoeuvre is expected to be performed on the intervening night of Monday and Tuesday (31st July and 1st August) between midnight hour and 1:00 am (IST).

So far, during its orbit around the earth, as per commands from ISRO's tracking, telemetry and command facility (ISTRAC), the spacecraft fired its onboard engines on five pre-planned occasions. During each of these engine firings, the craft slingshot itself into a higher orbit and moved further away from the Earth.

To get its spacecraft to destinations far away from the Earth (Moon, Mars etc), the Indian space agency has been using the Slingshot method, scientifically known as Hohmann Transfer Orbits. This is an energy-efficient technique to get a spacecraft to its destination, in the absence of powerful and heavy-lifting rockets. With extremely powerful rockets(such as America's Space Launch System), it is possible to propel a massive craft (weighing close to 40 tonnes) to the moon in less than a week. Given that India's LVM3 rocket has around only a fraction of the lifting power of global super-heavy rockets, the Indian space agency is using a time-consuming method (which takes more than a month) to get its 3.9-tonne craft to the moon, using available resources.

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Having heavier and more powerful rockets means the ability to carrymore capable spacecraft. The combination of heavy rockets and heavy spacecraft will ensure that the craft is able to develop the velocity and departure energy required to rapidly exit the earth's influence and set course for the moon (Trans Lunar Injection).

Once the TLI burn is complete, the craft will travel around five days to reach a pre-determined point in space, where the moon will also be at the time. Remember, the moon is also circling the earth and the spacecraft is also circling the earth, so the craft and the moon will have to meet at a certain ideal time and place. Once that meeting happens, the craft will be captured by the moon's gravity and it would begin to orbit the moon.

Thereafter, during the ideal time and day, the Lunar craft is allowed to de-boost itself and progressively get closer to the moon's surface, slow itself down gradually and prepare for a lunar landing. ISRO has planned Chandrayaan-3's Lunar landing for5:47 pm (IST) on August 23.

With a mission life of one Lunar day (14 Earth days), Chandrayaan-3 is meant to land near the Lunar south pole (approximately at 80 degrees latitude).

The mission's 'Vikram' lander must touch down on the Lunar surface at the start of the Lunar day in order to make use of available sunlight (using solar panels and batteries) and operate its science payloads, sensors, gather data and communicate it back to earth. ISRO's Lunar spacecraft is meant to land a few hundred kilometres away from the Lunar south pole. The Lunar south pole is a region that has largely remained unexplored, owing to the near-nil sunlight illumination there. It is believed that many mysteries of the moon could be unravelled by performing in-situ probing of this region.

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