The Moon presents a constant face to Earth, a celestial arrangement that has long fueled speculation about the hidden hemisphere. That other side, known as the far side, is not some perpetually dark place, but a portion of the lunar surface simply never visible from our planet. This hidden half is geologically distinct from the familiar near side and holds a unique value for future astronomical study.
Why the Far Side is Never Visible
The reason we only ever see one side of the Moon is due to a phenomenon called synchronous rotation, often referred to as tidal locking. This condition means that the Moon takes the exact same amount of time to complete one rotation on its axis as it does to complete one orbit around Earth, about 27.3 Earth days. Earth’s gravity gradually slowed the Moon’s rotation until it reached this synchronized state.
A common but incorrect assumption is that the far side is the “dark side” of the Moon. This misnomer suggests it receives no sunlight, but the Moon’s rotation ensures that both the near and far sides experience a full cycle of day and night over the course of a lunar month. When the near side is experiencing a full moon, the far side is in the midst of its two-week-long night; conversely, during the new moon phase, the far side is fully illuminated by the Sun.
Because the Moon’s orbit is not a perfect circle and its axis is slightly tilted, the Moon appears to wobble slightly as it orbits Earth, a motion known as libration. This slight rocking allows observers on Earth to actually peek around the edges and see about 59% of the lunar surface over time. Despite this libration, the central portion of the far side remains permanently hidden from terrestrial view, shrouded by the bulk of the Moon itself.
The Striking Geological Contrast
The first glimpse of the far side in 1959 immediately revealed a stark geological difference from the near side, creating a long-standing lunar mystery. The familiar face of the Moon is dominated by vast, dark, smooth plains known as maria (Latin for “seas”), which are the solidified remnants of ancient lava flows. The far side, in contrast, is nearly devoid of these large, dark plains, presenting a uniform, heavily cratered appearance.
The far side is covered in a dense concentration of impact craters, evidence of billions of years of bombardment. The far side’s crust is also significantly thicker and has a higher elevation than the near side crust.
One leading scientific explanation for this asymmetry relates to the concentration of certain heat-producing elements. The near side contains a large region rich in Potassium (K), Rare Earth Elements (REE), and Phosphorus (P), collectively known as KREEP. These elements generate heat, which is thought to have caused the prolonged volcanic activity and extensive lava flows that formed the near side’s maria.
Scientists hypothesize that a massive impact event, such as the one that created the South Pole-Aitken basin on the far side, may have triggered a thermal plume that redistributed this KREEP material to the near side. This concentration of heat-generating material on one side may be the reason why the near side experienced volcanic eruptions that formed the dark plains, while the far side remained a rugged, crater-pocked highland.
How We Mapped and Explored the Unseen Surface
The far side remained completely unknown until the Space Age, when the Soviet Union’s Luna 3 spacecraft returned the first grainy photographs in October 1959. This mission provided the initial evidence of the hemisphere’s distinct, heavily cratered topography, surprising scientists who expected a similar surface.
Later, the Apollo 8 crew became the first humans to see the far side directly as they orbited the Moon in 1968. Subsequent orbital missions by various nations have since mapped the far side in high detail, revealing features like the immense South Pole-Aitken basin.
However, the most significant step in far side exploration occurred in January 2019, when China’s Chang’e 4 mission achieved the first soft landing on the far side. The lander and its Yutu-2 rover explored the Von Kármán crater within the South Pole-Aitken basin, providing the first direct, on-the-ground data from this unique hemisphere.
The Far Side’s Unique Value to Science
The far side of the Moon offers a singular advantage for a specific type of astronomy due to its unique location. The Moon’s mass acts as a physical shield, blocking out the constant barrage of radio frequency interference (RFI) emitted by Earth’s communication systems and terrestrial broadcasts. This shielding creates a radio-quiet zone, the only truly pristine environment of its kind in the inner solar system.
This environment is ideal for low-frequency radio astronomy, which seeks to detect extremely faint signals that would be completely drowned out by Earth-based RFI. Scientists are particularly interested in observing the low-frequency radio waves from the universe’s “Dark Ages,” a period before the first stars and galaxies began to form. Placing radio telescopes on the far side, such as the planned Lunar Surface Electromagnetics Experiment (LuSEE-Night), allows astronomers to listen for these ancient cosmic signals, providing a window into the universe’s earliest moments.