The controlled descent of the Vikram lander of Chandrayaan-3 made it one of the closest approaches of a lunar mission to the moon’s South Pole. However like most of the lunar-landing missions before, Vikram too landed on the near side, making the Chinese Chang’e 4 mission the only one to have landed on the far side.
What are the moon’s ‘near’ and ‘far sides’ and is there a ‘dark’ side?
The near side refers to the portion of the moon — about 60% — that is visible to us. It is always the same side that is visible from Earth because the moon takes the same time to rotate about its axis as it does to circle around the Earth. However this doesn’t imply that the half the moon is in perpetual darkness.
The ‘new moon’ or when the moon is invisible from Earth is the time when the other ‘far side’ of the moon is bathed in sunlight and continues to receive light for nearly a fortnight. The ‘dark side’ is thus dark only in the sense that it was mysterious and its various topographical features hidden until the Soviet spacecraft Luna 3 in 1959 photographed it and the Soviet Academy of Sciences released an atlas of these images. Astronauts aboard the Apollo 8 mission of 1968 were the first humans to see the far side of the moon.
Is the dark side very different from the near side?
The major difference between the two sides is that the near side is relatively smoother and has many more ‘maria’ or large volcanic plains compared to the far side. On the far side however, there are huge craters, thousands of kilometres wide, which have likely resulted from collisions with asteroids. While both sides of the moon in its formative phase were similarly bombarded, the crust on the near side is thinner because of which, over millions of years, the volcanic lava in the lunar crust has flowed more extensively into the thinner side and filled up its craters. The resulting plains that have thus formed are far more conducive to space missions because they provide a relatively flat terrain for landers and rovers. Chandrayaan-3 identified an area 2.4 km wide and 4.8 km long that had spots of 150 m spaces that would be conducive to a safe descent. China’s Chang é-4 lander remains the only one to have successfully landed on the far side. This vehicle landed on the Von Karman crater situated within a larger 2,500 km wide crater called the South Pole Aitken basin.
What’s special about the Chandrayaan-3’s landing?
The Chandrayaan-3 mission, while still on the near side, has managed to land Vikram the closest ever to the lunar South Pole. The coordinates of Chandrayaan-3 at 69.36 S and 32.34 E make it about 600 km away from the South Pole. The choice of being as close as possible to the South Pole was to get closer to a “permanently shadowed region” or where no sunlight ever reaches, A.S. Kiran Kumar, former Chairman, Indian Space Research Organisation, told The Hindu. This would mean increasing the chances of encountering frozen water-ice along with several “interesting deposits” that can reveal more about the moon and its harvestable resources. The Vikram lander “wasn’t exactly in a shadowed region” as it was necessary to shine sunlight on the lander and rover to charge their solar batteries to keep them powered. The mission’s guiding purpose was to execute a successful controlled or ‘soft landing’ and the chances of doing that best while being near the South Pole were best served by keeping it in the near side, said Mr. Kumar. Crucially, landing on the far side would have meant no direct, line-of-sight communication with the Earth, necessary for regular near-real-time updates. “In such a situation you will need a relay — something that will communicate with the rover and then transmit to the Earth (and vice versa). While the Chandrayaan-2 orbiter (from the 2019 mission) is still functional you would have to reorient its orbit to function as a relay. This would not only mean moving it further away from the moon in a different elliptical orbit but also delays of up to half a day in transmitting and receiving information. It’s always the objectives of the mission that determine the choice of landing locations,” he added.