Chandrayaan 2: Many changes made to flight plan to ensure 7 September landing on the Moon

The 20-hour countdown to launch of the GSLV MkIII from Sriharikota will begin at 6.43pm on Sunday.


After the planned launch date for the Chandrayaan 2 mission was postponed by a week due to a technical fault in a fuel connecting joint in the GSLV-MKIII rocket, it was also announced that there won't be any change in the planned landing date of the mission on the Moon.

As per the original mission timeline, the first 17 days after lift-off were dedicated to the 3,850-kg spacecraft entering its Earth-bound phase. This was when the orbiter-lander-rover composite would have been injected into an "Earth Parking Orbit" (170 x 38,000 km) to perform orbital manoeuvres crucial to the spacecraft escaping Earth's gravity. As this orbit-raising continued, the spacecraft will have needed to perform a "trans-lunar injection burn" (TLI) to send the spacecraft on its way to the Moon. The spacecraft will have made a total of five orbit-raising burns to reach its desired 150 x 1,41,000 km orbit.

Chandrayaan 2: Many changes made to flight plan to ensure 7 September landing on the Moon

An illustration of the spacecraft's Earth-bound orbital manoeuvres. Image: ISRO

The spacecraft would have then begun its coasting phase into an elliptical orbit about the Earth with an apogee (its furthest orbiting distance around the Earth) roughly equal to the radius of the Moon's orbit. The translunar injection burn of the engines will need to be measured and timed to precisely target the Moon and match its revolution around the Earth for this movement.

A five-day cruise would have then followed when the Chandrayaan 2 composite travels the bulk of the distance between the Earth and Moon. Starting day 22 was a 3-day plan for lunar capture, with four Moon-bound burns of the engines to lower its orbit from the translunar 150 x 18,000 km to a 100 x 100 km lunar orbit, where it will orbit for 28 continuous days collecting data.

Former and revised launch sequence for the GSLV MkIII. Image courtesy: ISRO

Former and revised launch sequence for the GSLV MkIII. Image courtesy: ISRO

Former (left) and current (right) approach plan for Chandrayaan 2. Image: ISRO

Former (left) and current (right) approach plan for Chandrayaan 2. Image: ISRO

After spending four weeks (28 days) in lunar orbit, the Vikram lander was to separate from the orbiter and enter a 30 x 100 km orbit, maintaining this orbit for four days. On D-Day (where D=descent), Vikram performs one final deorbit burn to kick off its descent, beginning its 15-minute-long pre-programmed landing sequence.

Quite a few adjustments have been made to the approach:

  • the Earth-bound phase is six days longer than the earlier plan,
  • the transfer from Earth to the Moon's orbit is now extended by two more days
  • the lunar-bound phase has been cut short dramatically by 15 days
  • the deboosting of the Vikram lander and rover after separating from the orbiter has been extended by one day

While a revised landing date hasn't been announced, it can be deduced from the stated information in the press release that the new landing date is 8 September. The Chairman of ISRO, Dr K Sivan, said that the agency intends to make up for some of the lost time in the orbit-raising segments of the mission, in an interview with the New Indian Express.

"The integrity of the mission is intact. Lander Vikram will touch down on Moon's surface as originally planned on September 6, which is crucial since the mission life of Vikram and rover Pragyan is only one Lunar Day (14 Earth days)," Sivan told the Express. "So, the travel time to Moon will be brought down from 54 days to 47 days. The mission has enough scope to make such an adjustment."

The 20-hour countdown to launch of the GSLV Mk-III from Sriharikota will begin at 6.43pm on Sunday.

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