The Indian Space Research Organisation (ISRO) executed with precision a crucial manoeuvre when the propulsion system on board Chandrayaan-2 fired for 1,738 seconds (less than 30 minutes) from 9.02 a.m. on August 20 and the spacecraft slid into its orbit around the moon. Tension had filled the Mission Operation Complex at the ISRO Telemetry, Tracking and Command Network (ISTRAC) in Bengaluru and only when it became clear that the moon’s gravity had captured Chandrayaan-2 in its orbit that ground controllers relaxed.
Chandrayaan-2 started going round the moon in an orbit of 114 km x 18,072 km. This was precisely the orbit that ISRO had planned to achieve. The successful execution of this tricky and risky manoeuvre demonstrated at once ISRO’s ability to navigate the spacecraft around the moon. If the firing of the propulsion system had gone wrong, the spacecraft would have gone past the moon. If the spacecraft’s velocity had not been reduced, Chandrayaan-2 with the lander Vikram and the rover Pragyaan could have crashed on the moon.
ISRO Chairman K. Sivan told mediapersons in Bengaluru, “Today, as the time of the lunar burn came closer, our heartbeats increased although we have done a lunar insertion earlier [for Chandrayaan-1]. And during the 30 minutes, our hearts almost stopped till the LOI [lunar orbit insertion] was done.” (The Hindu, August 21).
Around 3 p.m. on August 19, Chandrayaan-2 reached the vicinity of the moon and its velocity increased due to the moon’s gravity. “The latest LOI manoeuvre was essential in order to reduce its speed from 2.4 km per second to 2.1 km per second. If we had not done it or done it improperly, we would have lost the spacecraft as it would have gone astray,” Sivan explained.
S. Ramakrishnan, former Director, Vikram Sarabhai Space Centre, Thiruvananthapuram, called the LOI “a crucial manoeuvre which was executed precisely”. He said: “The computation and execution of this manoeuvre was precise. The spacecraft’s propulsion system fired for about 30 minutes and it fired very well.” To add to the confidence of ISRO, the second lunar-bound orbit manoeuvre on August 21 went off as planned, using the spacecraft’s on-board propulsion system. The duration of the manoeuvre lasted 1,228 seconds. The orbit achieved was 118 km x 4,412 km. The spacecraft’s parameters were normal, ISRO said in a statement. On August 14, when the spacecraft’s propulsion system fired for about 20 minutes, Chandrayaan-2 escaped from the earth’s orbit and headed towards the moon.
There will be five lunar burns; two on August 20 and 21 were successful. These manoeuvres “will shrink the spacecraft’s orbit so that it will go into an orbit of 100 km by 100 km around the moon,” Ramakrishnan said. “We will shrink it further to 30 km x 100 km,” he added. On September 2, the mission will cross a major milestone when the lander separates from the spacecraft. Vikram’s circular orbit of 100 km will be reduced to 30 km x 100 km. In this orbit, Vikram’s various parameters and health will be observed for four days (Frontline, August 16).
The acid test of the mission will be on September 7. On that day, when the lander is at a height of 30 km from the moon’s surface, it will start descending towards the moon’s surface. Five throttleable engines on board Vikram will fire and control its descent in a calibrated manner so that it lands softly in the South Polar region of the moon. The descent and landing will take 15 minutes. After four and a half hours, a door on the lander will open and a ramp will unfold. Pragyaan will emerge from the lander, slide down the ramp, reach the moon’s surface and drive about there for one lunar day or 14 earth days.