The phrase “back side of the Moon” often suggests a perpetual shadow, but this is a misunderstanding of lunar mechanics. The Moon does not have a permanently dark side; every part of its surface receives sunlight during the lunar day, which lasts about 29.5 Earth days. The scientifically accurate term for the hemisphere we never see from Earth is the Far Side, and it is just as illuminated as the Near Side.
Defining the Far Side and Orbital Mechanics
The reason we only ever see one face of the Moon is due to tidal locking, also known as synchronous rotation. This stable orbital relationship was achieved through gravitational interaction with Earth. Tidal forces slowed the Moon’s rotation until the time it takes to rotate on its axis became equal to the time it takes to complete one orbit around Earth, approximately 27.3 days.
This synchronization means the Moon always presents the same hemisphere toward us, keeping the Far Side perpetually out of view. Because the Moon’s orbit is not perfectly circular, a slight wobble, known as libration, allows observers on Earth to occasionally glimpse a marginal sliver of the Far Side’s edges. However, about 82 percent remains permanently hidden.
The Stark Visual Differences of the Lunar Far Side
When the Far Side was revealed, it presented a stark visual contrast to the familiar Near Side. The most striking difference is the near-total absence of the dark, smooth plains known as lunar maria, which were formed by ancient volcanic eruptions. These vast basaltic plains cover roughly 31 percent of the Near Side, giving it characteristic features.
In comparison, the Far Side is a rugged, mountainous landscape heavily pockmarked with impact craters across its entire surface. Only about one percent of the Far Side is covered by maria, resulting in a paler, more uniformly gray appearance. This hemisphere is dominated by the bright, ancient crustal material known as the lunar highlands.
How We Mapped the Surface
The Far Side remained completely unknown until the dawn of the Space Age. The Soviet Union’s Luna 3 probe was the first spacecraft to successfully photograph the hidden hemisphere in October 1959, capturing the first grainy images.
Following this initial glimpse, comprehensive mapping was conducted by the American Lunar Orbiter program in the mid-1960s. The astronauts of the Apollo 8 mission in 1968 were the first humans to visually observe the Far Side directly as they orbited the Moon.
Recent Exploration
China’s Chang’e 4 mission achieved the first-ever soft landing on the Far Side in January 2019, landing within the massive South Pole-Aitken Basin. To communicate with Earth, this mission required a dedicated relay satellite, Queqiao, positioned beyond the Moon to transmit signals that would otherwise be blocked by the lunar body.
Unique Geological Characteristics
The reason for the striking visual difference between the two hemispheres lies in a fundamental asymmetry in the Moon’s internal structure. Data showed that the Far Side crust is significantly thicker than the Near Side crust, estimated to be approximately 20 kilometers thicker on average.
This thicker crust acted as an insulating barrier, inhibiting magma from the lunar mantle from easily reaching the surface after large impact events. Consequently, while impacts on the Near Side often allowed magma to fill basins and form maria, the Far Side’s thicker crust prevented this extensive volcanism.
Another element is a greater concentration of heat-producing elements, such as Potassium, Rare Earth Elements, and Phosphorus (KREEP), found on the Near Side. This localized concentration of radioactive material likely lowered the melting temperature of the underlying mantle, creating more magma and sustaining volcanic activity for a longer period in that hemisphere.