Landing on the Far Side of the Moon
A few days later, this time in our own celestial neighborhood, another historic event took place. For the first time in the history of the space explorations, an unmanned robotic lander, successfully landed on the far side of the Moon.
Chang’e-4 (named after the goddess of the Moon) is the latest mission in the China lunar exploration program. After successfully orbiting and landing on the familiar and near side of the Moon, China took a step forward and successfully landed its lander on a crater called Von Kármán, inside the South Pole–Aitken basin which is one of the largest impact craters in the solar system.
This mission is an important one. For the first time, the Chinese space program is accomplishing a task that never has been done before by any other nations. No one ever landed on the far side of the moon. This mission also has scientific importance.
The lander accompanied by a small moon rover called, Yutu-2, equipped by several scientific instruments and a small bio-box to test the growth of seeds and insects in low gravity.
The Far Side or the Dark Side
The moon's orbital periods and its rotation are tidally locked, in another word, the moon will rotate around its axis in the same time that it takes to complete one rotation around the Earth. As a result, from the Earth, we only can see one side of the Moon and the other side is hidden from us (except a small area that could be visible because of phenomena called libration).
That is why we call it the far side of the moon. Sometimes the far side of the moon is also called the dark side of the moon. It is a popular name (especially after the famous Pink Floyd album), but it is not accurate.
The far side of the moon is not always dark. It is experiencing days and nights like the near side of the moon but both days and nights of the far side are happening behind the moon and far from our eyesight.
The far side of the moon also has a different appearance from the near side.
There are no signs of the seas there (the flat areas in the front side of the moon that seems a little darker or greyish and you can see them any night that the moon is shining in the night sky.) The surface of the far side of the moon is more similar to the surface of Mercury and covered with many ancient, and new, craters.
It is a strange and undiscovered land, waiting to be discovered.
Calling Home from the Far Side of the Moon
One of the challenges in conducting any mission behind the moon are the communications problem. If you are on the surface of the far side of the moon or in a low orbit—like the command module of Apollo missions during their orbit around the Moon, the body of the Moon blocks your line of sight of the Earth. Which means there is no way to talk directly to the Earth from the surface or when you are in a low orbit behind the moon. During the Apollo missions, the communications with the command module (CM) disconnected for few minutes while the CM was orbiting the far side of the moon.
There are two major ways to keep in touch with Earth from the far side of the Moon.
First, to put a series of communications satellite around the Moon in medium height orbits. These multi propose satellite network can relay the communications and also play the role for some kind of positioning systems and mapping the surface. It is probably going to be the case in the future when we will establish proper infrastructure there.
The second way is the one that Chinese chose. They put a satellite in a specific point behind the Moon. This point called halo orbit or second Lagrangian point in the Earth-moon system. This is the place that sum of gravitational forces allows us to put an object there and it will keep its position fixed in regard to the Moon.
L2 is about 65,000 km behind the moon. In that distance, any satellite sees not only the surface of the far side of the moon, but also the Earth will be on its eyesight and therefore the line of communications is open with the lunar surface and the Earth at the same time.
China Space Agency has sent an orbiter, Quèqiáo, to this point a year earlier. In Chinese folklore, Quèqiáo means the bridge formed by birds to make it possible for the seventh daughter of the goddess of heaven to reach to her earthling lover. Quèqiáo is now the bridge between the Earth and its messenger on the moon.
This could be a first step toward the lunar communication networks that would be critical for any long-term project on the moon.