On Friday, China announced that its Chang’e-4 probe had successfully transmitted back images from the far side (also known as the dark side) of the Moon. Chang’e-4 is the first probe ever to land on that side. What is the dark side of the Moon, and why have none of the previous landings been made there?
Far side, near side
Over billions of years, Earth’s gravitational pull has brought the Moon’s spin into sync with its orbit. It takes exactly 28 days for the Moon to complete one rotation, and the same time to make one orbit around Earth. This leads to a phenomenon called “tidal locking”. With the Moon’s rotation and orbit keeping it forever in step with the Earth, only one part of it is visible from this planet at any time. The unseen part is the “far side of the Moon”.
Although it is also called the “dark side of the Moon” — a description that owes much of its popularity to a Pink Floyd album — this is actually a misnomer. Viewed from Earth, half the Moon is sunlit at any time; and during a new moon, the near side is dark while it is the far side that is fully lit. The far side of the moon is also lighter in colour.
Why far side is difficult
All previous Moon landings, manned and unmanned, have been on the near side. This has been primarily because the Moon would have blocked radio communication between its far side and Earth. To work around this problem, the Chinese mission has used a “relay satellite”, called Queqaio (Magpie Bridge) and launched in May 2018. It is in orbit around a strategically selected point, called L2. Signals between the far side and Earth are transmitted via the relay satellite.
While Chang’e-4 is the first spacecraft to actually land on the far side, its images of that side are not the first. On October 7, 1959, the Soviet spacecraft Luna 3 clicked a number of photographs of the far side, from over 60,000 km away. On its way back to Earth, it relayed a set of poor-quality photographs on October 18.
Why far side is important
Chang’e-4 landed on January 3 in the Von Kármán Crater in the South Pole-Aitken Basin on the far side of the Moon. The SPA Basin (2,500 km wide, 13 m deep) and the Von Kármán Crater (180 km) are both large impact craters. A study of the Moon’s craters will seek to establish their compositions and ages, a history of collisions between Earth and the Moon, and various other aspects of the early Solar System.
Chang’e-4 landed at an altitude of minus 6,000 m. “The information from the depths of the Moon will be one of our focuses in the exploration,” PTI quoted Li Chunlai, commander-in-chief of the ground application system of Chang’e-4, as saying.
The first images
One of the published images is a 360° panorama pieced together from 80 photos from Chang’e-4. “From the panorama, we can see the probe is surrounded by lots of small craters, which was really thrilling,” Li was quoted as saying. Previously, the Chinese spacecraft Chang’e-3 had landed on the near side. Compared to that landing site, fewer rocks can be found in the area surrounding Chang’e-4, indicating that the landing area of the new mission might be older.