Zahlen, bitte! 41 percent mysterious: the dark side of the moon
The moon's far side, facing away from Earth, has received little attention. Now, it is attracting scientists interested in lunar and space exploration.
The moon is a faithful companion of mankind: twelve astronauts have trudged around on it - but only on the side facing the earth. While this side has been familiar since time immemorial, a total of 41 percent of the moon's surface would remain hidden from observers on Earth forever if it weren't for space travel.
We have only known what the far side of the moon looks like for almost 55 years: in 1959, the Soviet space probe Lunik 3 provided the first blurred images of the far side of the moon. The crew of Apollo 8 were the first humans to see the far side of the moon as they orbited the Earth's satellite. More recently, Chinese probes in particular have provided new insights.
The surface raises many questions because it is completely different from the side facing the Earth. And it has unique properties that could make it worthwhile to live "behind the moon".
Earth-bound rotation due to gravitational pull
But why does the moon only show us the same side? The reason is its bound rotation. The gravitational forces of the earth and moon act on each other. The tidal forces of the moon cause ebb and flow on Earth, for example. Tidal friction also slows down the Earth's rotation. However, you have time before you have to adjust the clock: This slows down the Earth's rotation by one second in about 100,000 years. However, as the Earth is 80 times more massive than the moon, tidal friction also has a much stronger effect on the Earth's companion, to the point of slight deformation.
(Image: NASA/GSFC/Arizona State University)
The gravitational force also decreases with distance, so that the side facing the Earth is attracted more strongly than the center of the moon. As a result, the Earth's gravitational pull slows down the Moon's own rotation in such a way that its own rotation corresponds to its orbital period around the Earth. The elliptical orbit and the inclination to the ecliptic (the apparent orbit of the sun in the sky) ensure that about 59 percent of the surface can be seen from the earth with an apparent wobble over the course of the year – the fluctuations are called librations.
Back of the moon brighter than Pink Floyd thought
The British band Pink Floyd created a monument to the mysteriously hidden far side of the moon with the album "The Dark Side of The Moon". The side facing away from the earth is certainly mysterious, but not dark: due to the rotation of the moon, there are lunar days and lunar nights just like on the side facing the earth. At new moon, the moon stands between the earth and the sun: the side facing away from the earth is illuminated. At full moon, the far side is in deep darkness.
The far side of the moon is even brighter than the side facing the Earth: while 31.2 percent of the side facing the Earth is formed by the maria (seas) – large, dark plains of solidified remnants of molten magma – these make up only 2.6 percent of the far side. These dark regions take their name from the fact that they were once thought to be seas in astronomy. However, the title of the album with the world-famous prism is only a metaphor anyway.
(Image: CC BY-SA 3.0, Orion 8)
The reason why the side facing away from the earth looks so different from the side facing the earth is a mystery to science. It is assumed that the crust on the side facing away from the Earth is thicker, making it more difficult for the liquid magma to reach the surface.
Radioactive back of the moon
The remoteness was still a problem for the Apollo program: the permanent radio shadow prevented serious consideration of the far side as a landing site. The moon has since become a target for space missions again and now the properties of the far side are of particular interest: The shielding, which still prevented a moon landing over 50 years ago, protects against interference from the permanently radiating Earth and makes the far side an interesting location for a radio telescope.
As several space powers are interested in new moon missions, astronomers are already warning that the new race to the moon could jeopardize the unique conditions of the far side. They fear that the more than 200 planned lunar missions could disrupt the optimal conditions. To prevent this, the space probes would have to be ideally shielded against unwanted emissions.
(Image: NASA/EPIC)
China with several landing missions
When it comes to exploring the far side of the moon, China is at the forefront: Chang'e 4 was the first probe to land on the far side of the moon on January 4, 2019. The probe released a lunar rover with the euphonious name Jade Rabbit 2.
Conspiracy theorists were ecstatic when the probe transmitted an image to Earth showing a distinctive object on the horizon that resembled a hut. Closer examination revealed that it was just a rock. It was another example of a pareidolia, similar to the face of Mars, which on closer inspection turned out to be a whimsical rock formation created by nature rather than a man-made stone face.
The next milestone was achieved by China with the Chang'e 6 probe: it not only landed on the far side of the moon, but also recently brought back moon rocks on a return mission. The Chinese also have plans for a manned moon base on the far side.
(mawi)
