Chandrayaan-3 enters lunar orbit: What happens next

Chandrayaan-3, India’s third Moon mission, launched two weeks ago from Sriharikota has been successfully inserted into the lunar orbit, says the Indian Space Research Organisation. This means that the spacecraft is now on the final stretch of its journey and will soon reach its destination. What happens next? -- Bengaluru-based Defence & Aerospace analyst Girish Linganna explains.

Bengaluru: Following its launch on July 14, the Chandrayaan-3 mission to the Moon underwent a series of five sequential orbit raises to gradually elevate the spacecraft’s orbit. It has completed about two-thirds of its journey and is now headed towards its next destination, the Moon, with the Lunar-Orbit Insertion (LOI) taking place at 7:15 pm on Saturday (August 5, 2023), according to the Indian Space Research Organization (ISRO).

• Chandrayaan-3 Mission:
  “MOX, ISTRAC, this is Chandrayaan-3. I am feeling lunar gravity 🌖”
  🙂
  
  Chandrayaan-3 has been successfully inserted into the lunar orbit.
  
  A retro-burning at the Perilune was commanded from the Mission Operations Complex (MOX), ISTRAC, Bengaluru.
  
  The next… pic.twitter.com/6T5acwiEGb

  — ISRO (@isro) August 5, 2023 " class="align-text-top noRightClick twitterSection" data="

  Chandrayaan-3 Mission:
  “MOX, ISTRAC, this is Chandrayaan-3. I am feeling lunar gravity 🌖”
  🙂
  
  Chandrayaan-3 has been successfully inserted into the lunar orbit.
  
  A retro-burning at the Perilune was commanded from the Mission Operations Complex (MOX), ISTRAC, Bengaluru.
  
  The next… pic.twitter.com/6T5acwiEGb

  — ISRO (@isro) August 5, 2023

  ">

  Chandrayaan-3 Mission:
  “MOX, ISTRAC, this is Chandrayaan-3. I am feeling lunar gravity 🌖”
  🙂
  
  Chandrayaan-3 has been successfully inserted into the lunar orbit.
  
  A retro-burning at the Perilune was commanded from the Mission Operations Complex (MOX), ISTRAC, Bengaluru.
  
  The next… pic.twitter.com/6T5acwiEGb

  — ISRO (@isro) August 5, 2023

Earlier, on August 1, the spacecraft fired its engines to enter the Moon’s sphere of influence, after a critical slingshot manoeuvre was executed successfully, propelling the Chandrayaan-3 spacecraft from Earth’s orbit towards the Moon. This trans-lunar injection (TLI) allowed the spacecraft to break free from Earth’s gravitational pull and set it on course towards the Moon’s vicinity, which means it is now under the Moon’s gravitational pull. Since it has successfully performed the LOI, it is now in orbit around the Moon.

Leaving Earth’s Gravity

The Trans-Lunar Injection (TLI) manoeuvre has propelled Chandrayaan-3 into a type of trajectory called the Lunar Transfer Trajectory that takes a spacecraft from Low Earth Orbit (LEO) to the Moon. The TLI burn is usually performed by a chemical rocket engine, also known as a Perigree Engine. This increases the spacecraft’s velocity by about 3 km/s. This increase in velocity is enough to escape Earth’s gravity and reach the Moon.

The TLI burn is typically timed so that the spacecraft reaches the Moon’s sphere of influence at a time when the Moon is in the correct position. This ensures that the spacecraft is captured by the Moon’s gravity and begins to orbit it. The TLI is a critical part of any mission to the Moon. It is the next important step in the journey to the Moon, and it is essential for ensuring that the spacecraft reaches its destination safely.

What happened today?

Around 7:15 pm, it performed a critical manoeuvre called the Lunar Orbit Insertion (LOI) to enter the orbit around the Moon. It is typically performed after the spacecraft has left Earth for the Moon, and involves firing the spacecraft’s engines to slow its velocity and place it into a stable orbit around the Moon. If the LOI manoeuvre is not performed correctly, the spacecraft could miss the Moon altogether, or it could enter an unstable orbit that would eventually decay and cause the spacecraft to crash.

The time taken for LOI after the TLI can vary depending on the specific mission. A few possible reasons for this are:

• The spacecraft may need to wait for the right launch window: The Moon’s orbit around Earth is not perfectly circular, so there are certain times when it is easier to reach the Moon. The spacecraft may need to wait for one of these launch windows in order to conserve fuel and ensure a successful mission
• The spacecraft may need to adjust its trajectory: After the TLI, the spacecraft’s trajectory may not be perfectly aligned with the Moon’s orbit. The spacecraft may need to make a few course corrections to ensure that it arrives at the Moon at the correct location
• The spacecraft may need to perform other manoeuvres: In addition to the LOI, the spacecraft may need to perform other manoeuvres, such as a mid-course correction or a lunar flyby. These manoeuvres can add time to the overall mission.

So, what happens next?

Now that the spacecraft is in the Moon's sphere of influence, it will continue to orbit the Moon for a few days to further adjust its trajectory and get closer to the Moon’s surface, namely the 100-km circular orbit above the Moon’s surface. This ensures a proper landing for the spacecraft by carefully positioning itself before touching down on the lunar surface.

August 17 is the next big day when ISRO separates the landing module from the propulsion module. The landing module, Vikram, carries the rover Pragyaan. Vikram will then aim to land safely on the Moon’s surface on August 23. This part of the operation is critical as it allows the landing module to navigate independently and make a precise landing on the Moon.

Approximately four hours after a successful soft landing, rover Pragyaan is scheduled to detach from lander Vikram. Both Vikram and Pragyaan will then conduct in situ experiments on the Moon’s surface. In-situ experiments refer to tests and analyses being carried out directly on the Moon’s surface without the need for bringing back samples to Earth. These experiments will provide valuable data and insights regarding the lunar environment, composition and other scientific measurements, helping expand our understanding of the Moon and its potential for future exploration.

The propulsion module, with its array of scientific instruments, will continue orbiting the Moon. This means that, after Vikram successfully touches down and Pragyaan is deployed, the propulsion module will keep orbiting the Moon instead of landing on the lunar surface. By staying in orbit, the propulsion module will keep gathering valuable data and conducting scientific tests. This will make a comprehensive exploration of the Moon’s environment possible and enable further study of the Moon’s gravitational field and mapping of the lunar surface, besides carrying out other scientific measurements. This data will help us understand the Moon’s geological composition, its evolution and help future missions and explorations by humans.