Chandrayaan-2 successfully entered into an elliptical orbit around the moon on Tuesday as India officially ‘reached the moon’ for the second time, 11 years after its first mission that discovered water on the lunar surface.
Tuesday’s ‘heart-stopping’ move was described as a ‘major milestone’ by the Indian Space Research Organisation (ISRO) chairperson K Sivan.
A successful soft landing, scheduled for 1.55 am on September 7, will make the Indian mission the first ever to land near the moon’s south pole at a latitude 70 degrees south of the equator.
It will also make India the fourth country after the US, the former USSR and China to land on the lunar surface.
‘The unique requirement Chandrayaan-2 mission that no other country had was to achieve the lunar orbit with an inclination of 90 degrees. This is to ensure that the spacecraft is in place to land near the south pole of the moon. The trans-lunar injection manoeuvre on August 14, and the lunar orbit insertion today were very important for that,’ Sivan told reporters.
All other lunar landings so far have happened in areas about 30 to 40 degrees from the lunar equator.
Entering lunar orbit
It was highly precise trans-lunar injection and lunar orbit insertion manoeuvres that ensured the spacecraft reached the intended inclination, Sivan said. ‘To give you perspective, for a spacecraft moving at a velocity of 10.9 km/second, a deviation of 10cm per second during the trans-lunar injection would have resulted in a 7 degrees change in the inclination in the lunar orbit,’ he explained.
At 9.02am, ISRO scientists fired the propulsion system on-board the spacecraft for 1,738 seconds (nearly 29 minutes) to achieve the highly elliptical orbit of 114×18,072 km and an inclination of 88 degrees. In four manoeuvres over the next 12 days, the orbit will be reduced to a circular 100x 100km, and the inclination of 90 degrees will be achieved.
‘For 30 minutes today, our heart was almost stopping,’ Sivan said.
A trans-lunar injection manoeuvre on August 14 slung the Chandrayaan-2 composite module (Orbiter, Lander and Rover) in a path towards the moon with a velocity of 10.9km per second.
This manoeuvre increased the apogee, or the point on the orbit farthest from Earth, to 418,000km. On August 19, at about 3pm, when the moon came closer to the spacecraft while it was returning from the apogee, Chandrayaan-2 entered into the moon’s sphere of influence.
‘Once this happened, Chandrayaan-2 started gaining speed under the influence of the moon’s gravity. To ensure it gets into the orbit around moon, the manoeuvre reduced the velocity from 2.4km per second to 2.1 km per second,’ Sivan said.
In the past, moon missions such as the United States’s Ranger 3 in 1962 and Explorer 33 in 1966, and the USSR’s Luna 6 in 1965 failed and flew past the moon at this point.
Separation and landing
The next big milestone for the mission will be on September 2, when the Lander-Rover separates from the Orbiter.
There will be a small manouevre to check the system on September 3, following which the space agency will check various parameters of the Lander. ‘Once we have confirmed that the system is alright on September 7, at 1.40am the powered descent will start and at 1.55am the lander will touchdown at the site,’ Sivan said.
‘The landing will be a terrifying moment for us. Till now, we have not operated the systems on-board the lander, especially the propulsion system. This is the phase, including the powered descent, that we will be doing for the first time, whereas, the lunar orbit insertion we have already done once,’ he added, pointing out that only 37% of attempted lunar landings have succeeded.
Earlier this year, Israel’s Beresheet mission crashed onto the surface of the moon due to engine failure.
‘The landing site [on September 7] will be autonomously selected by the Lander-Rover by comparing the images taken by it with on-board images. The main considerations will be that the slope of the landing site should be less than 12 degrees and free of boulders to prevent the lander from toppling,’ said Sivan.
The Lander-Rover is expected to land in a high plain between two craters, Manzinus C and Simpelius N. This region was chosen because the craters here have been untouched by the sunlight for billions of years, offering undisturbed record of the creation of the solar system. The permanently shadowed craters are also estimated to hold 100 million tons of water.
The position and availability of resources such as hydrogen, ammonia, sodium, methane, mercury, and silver at the South Pole also make it an ideal pit stop for future space exploration missions
‘This is first time an Indian mission would land on a non-terrestrial surface, and that too, the South Pole of the moon. Isro did not want to go to an area which has already been explored as it does not add to our scientific understanding,’ said Rajeswari Rajagopalan, head of the nuclear and space policy initiative, Observer Research Foundation.
‘Hereafter, our space programme will attempt more complex missions, but this remains a very complex mission and we have to wait and see what happens on the day of the landing,’ she added.