Next stop, the Moon!: Chandrayaan 3 executes Trans Lunar Injection (TLI)

Chandrayaan 3 has successfully executed the Trans Lunar Injection (TLI) maneuver and has begun its departure for the Moon.

Chandrayaan 3 is ISRO’s third lunar exploration mission, and it is a follow-up to the Chandrayaan 1 and Chandrayaan 2 missions launched in 2008 and 2019 respectively. The spacecraft which was launched on 14 July 2023 aims to demonstrate a soft landing on the lunar surface to make India one of the only four countries to do it alongside Russia, the United States of America, and China. 

Apart from making a soft landing on the lunar surface, specifically in the south polar region of the moon, Chandrayaan 3 also aims to achieve various objectives using the instruments on board it such as Spectro-polarimetry of Habitable Planet Earth (SHAPE) to study the spectro-polarimetric signatures of the earth in the near-infrared wavelength range which can be used in in the search for life on exo-planets beyond the solar system, Chandra’s Surface Thermophysical Experiment (ChaSTE) which will measure the thermal conductivity and temperature of the lunar surface, Alpha Particle X-Ray Spectrometer (APXS) which will derive the chemical composition and infer mineralogical composition of the lunar surface and other various in-situ experiments. 

Chandrayaan 3 was launched on 14 July 2023 at 2:35 PM IST  from Satish Dhawan Space Centre in Sriharikota using the GSLV-M3 (Geosynchronous Satellite Launch Vehicle Mark 3). The spacecraft after its launch sequence completed five orbit-raising maneuvers between the 15th and 25th of July 2023. On the 1st of August 2023, the spacecraft executed the Trans Lunar Injection(TLI). The maneuver was executed from ISRO Telemetry, Tracking, and Command Network, or ISTRAC for short, based in Bengaluru. The spacecraft is expected to land on the Moon by 23 August 2023.

Orbit raising maneuver of Chandryyaan 3

 Looking at the mission duration you may wonder why Chandrayaan 3 mission is taking 40 days to reach the Moon. When missions like the Luna 9, the first spacecraft to soft land on the lunar surface took about 3 days to land on the Moon in 1966 and Apollo 11, the first lunar mission to land humans on the Moon took about 4 days in 1969. Well, the simple answer is save cost on fuel required to reach the moon, the total budget of Chandrayaan 3 is about 615 Cr Rupees while it may seem like a lot, it’s far lesser than what is spent by NASA on each Apollo mission. 

ISRO was able to achieve this by using something called the orbital maneuver where the Chandrayaan 3 uses the Earth’s gravitational force to assist in its launch towards the Moon. When Chandrayaan 3 completed its launch sequence on 14 July, it performed 5 earth-raising orbit maneuvers between 15th and 25th July

In the above table ‘Apogee’ refers to when the spacecraft is furthest from Earth and ‘Perigee’ refers to when the spacecraft is closest to Earth in an elliptical orbit. After the launch Chandrayaan 3 entered an orbit of 41,762 km apogee and 173 km perigee on 15th July, then on 17th July the spacecraft fired its thrusters to raise its orbit and attain an orbit of 41,603 km apogee and 226 perigee, the spacecraft repeated this maneuver 3 more time to achieve a final orbit of 1,27,609 km apogee and 236 km perigee. This enabled Chandrayaan 3 to escape Earth’s gravity by using a concept called gravitational assist or slingshot maneuver.

Gravitational assist or slingshot is a type of maneuver where a spacecraft uses the gravity of a planet or a celestial body to alter its trajectory and speed to save cost and fuel. Gravitational assist can also be used to decelerate and redirect the path of a spacecraft. The assist is provided by the orbital angular momentum when the planet’s gravity pulls on the spacecraft and swings it around, later a calculated firing of the spacecraft’s thruster is executed to either raise the orbit or escape the planet’s gravity. The maneuver was also used by Voyager 1 and Voyager 2 to do fly-by’s around Jupiter and Saturn.

Trans Lunar Injection of Chandrayaan 3

At 12:15 am midnight on 1st August 2023 Chandrayaan 3 fired up its thruster once again to execute the Trans Lunar Injection where it finally escaped Earth’s gravity and is heading for the Moon. Chandrayaan 3 will fly into space toward the Moon and will then be caught by the Moon’s gravitational force and begin Lunar Orbit Insertion maneuver.

Lunar Orbit Insertion and Deceleration 

Chandrayaan 3 is expected to reach the Lunar Orbit by 5th August 2023 and begin Lunar Orbit Insertion maneuver to insert itself into lunar orbit. Upon entering the lunar orbit Chandrayaan 3 stack which consists of a Propulsion module, a Lander and a Rover will be parked in a 100 km x 100 km orbit around the moon. On 17th of August 2023, the Propulsion module and the Lander along with Rover are expected to separate and make a touchdown on the south polar region of the Moon by 23 August 2023