Context: 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 of the moon.
What are the moon’s ‘near’ and ‘far sides’ and is there a ‘dark’ side?
- Near side: The near side refers to the portion of the moon — about 60% — that is visible from Earth. This side of the Moon always faces the Earth and it is the side that humans have visited during various lunar missions.
- Far side: The far side of the Moon is the side that is not visible from Earth. The reason it is less visible from Earth is due to a phenomenon called “tidal locking”. The phrase “dark side of the Moon” does not refer to “dark” as in the absence of light, but rather “dark” as in the unknown. In reality, both the near and far sides receive (on average) almost equal amounts of light directly from the Sun.
- The ‘dark side’ was mysterious and its various topographical features were 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.
- All crewed and uncrewed soft landings had taken place on the near side of the Moon, until 3 January 2019 when China’s Chang’e 4 spacecraft made the first landing 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.
Hence, both the near and far sides experience sunlight and darkness as they orbit the Sun, but the far side has historically been less explored and observed from Earth due to the Moon’s tidally locked rotation.
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, 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.
What is 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. 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.
- Tidal locking, also known as gravitational locking or synchronous rotation, is a phenomenon that occurs when a celestial body’s rotation period matches its orbital period around another body. This phenomenon is most commonly observed in natural satellites like moons, but it can also occur between planets and their stars.
- In the case of the Moon, its rotation period is nearly the same as its orbital period around the Earth, which is about 27.3 days as well. This tidal locking has resulted in the Moon always showing the same face, the near side, to the Earth.