The moon is one of the most familiar sights in Earth’s night sky. For millennia, humans have gazed upon its pockmarked surface, watching it wax and wane through its monthly cycle of phases. But despite the moon’s proximity and prominence, it hides a secret – there is an entire half of the moon that is perpetually turned away from Earth, forever hidden from view. This far side of the moon remained a mystery until the Space Age, when the first spacecraft traveled around the moon and sent back images of previously unseen lunar landscapes.

Synchronous Rotation Keeps One Side Facing Earth

The reason we see only one face of the moon is due to a phenomenon called synchronous rotation or tidal locking. The moon rotates on its axis in the same amount of time that it takes to orbit the Earth – about 27.3 days. In other words, the moon’s rotation period matches its orbital period precisely. As a result, the same side of the moon is always oriented towards Earth.

This tidal locking is caused by the gravitational interaction between the Earth and moon. The Earth’s strong gravitational pull creates a tidal bulge on the moon, distorting its shape into a slight oval with the long axis pointed towards Earth. This bulge essentially acts as a handle for Earth’s gravity to grab onto, slowing the moon’s rotation over millions of years until its rotation period matched its orbital period. At this point, the bulge became permanently aligned towards Earth, and the moon’s rotation became locked in this configuration.

Interestingly, the moon has not always been tidally locked to Earth. When the moon first formed around 4.5 billion years ago, it rotated much faster. But the relentless tug of Earth’s gravity acted as a brake, gradually slowing the moon’s spin through tidal friction until synchronous rotation was achieved about a billion years later. Most of the large moons in our solar system have also become tidally locked to their parent planets through this same process.

Libration Reveals a Bit More Than Half

Although tidal locking keeps the same lunar hemisphere perpetually facing Earth, libration allows us to peek around the edges and see a little bit more than 50% of the total lunar surface over time. Libration is a wobbling motion caused by the moon’s slightly elliptical orbit and its axial tilt.

As the moon’s orbital speed varies throughout its elliptical path, its rotational speed remains constant, so the side facing Earth appears to wobble back and forth, revealing an extra 9% of its surface – about 6° along the eastern and western edges. Additionally, the moon’s axial tilt of about 5° relative to its orbital plane allows us to see a bit over the lunar north and south poles at different points in the moon’s orbit. Cumulatively, libration lets Earth-bound observers see about 59% of the moon’s surface.

The Far Side Revealed

Until the advent of the Space Age, no human had ever laid eyes on the far side of the moon. It was a complete mystery whether the hidden hemisphere was similar to the familiar near side or something entirely different. That all changed in 1959, when the Soviet Luna 3 spacecraft looped behind the moon and photographed the far side for the first time, beaming back grainy but historic images.

Surprisingly, the photos revealed that the far side looks quite different from the near side. The most striking difference is the almost complete absence of the dark, flat lunar maria (Latin for “seas”) that cover nearly a third of the near side. Instead, the far side is dominated by rugged, heavily cratered highlands. The terrain is also more varied, with towering mountains and deep craters not found on the side facing Earth.

Later missions mapped the entire far side in much greater detail. In the 1960s, NASA’s Lunar Orbiters imaged the whole moon from orbit, and the Apollo missions gave astronauts an up-close look at swaths of the far side as they looped behind the moon. More recently, spacecraft like NASA’s Lunar Reconnaissance Orbiter have created stunningly detailed global maps and elevation models of the entire lunar surface.

Theories for the Two-Faced Moon

The dramatic differences between the near and far sides of the moon have intrigued planetary scientists for decades. The prevailing theory suggests that the disparity can be traced back to a massive collision early in the moon’s history.

According to the giant impact hypothesis, the moon formed from the debris left over after a Mars-sized protoplanet slammed into the early Earth. Computer simulations indicate that this collision could have created an initially molten moon that quickly solidified into a magma ocean. As this lunar magma ocean cooled and crystallized, low-density minerals floated to the top to form the bright lunar highlands while denser minerals sank to form the mantle.

Some scientists propose that the far side highlands may have formed sooner and cooled more quickly than the near side crust. Alternatively, the tidal influence of Earth’s gravity on the still-molten near side may have triggered volcanic eruptions, flooding the basins with dark basaltic lava to form the maria. The far side, being shielded from this tidal heating, would not have experienced the same volcanic resurfacing.

Another theory suggests that the chemical composition of the far side may be fundamentally different, with a thicker crust and fewer radioactive elements to fuel volcanism compared to the near side. However, the true reasons for the moon’s two-faced appearance are still being debated, and future exploration of the far side may help resolve this lunar mystery.

Exploring the Far Side

Despite being hidden from view, the far side of the moon is a scientifically intriguing place for exploration. Its ancient, pristine surface records a longer and more complete history of asteroid impacts than the resurfaced near side. The far side also provides an ideal location for radio telescopes, as it is perpetually shielded from the radio chatter emanating from Earth.

In 2019, China made history by landing the first spacecraft on the far side. The Chang’e-4 lander and Yutu-2 rover touched down in the Von Kármán crater, carrying cameras and science instruments to study the local geology. The mission has operated for several years, sending back fascinating images and data from this unexplored region.

Future missions may continue to explore the far side in the coming years and decades. NASA’s Artemis program aims to return humans to the moon and establish a sustainable presence, potentially including bases on the far side. Other nations like Russia and Japan are also planning lunar missions that could target the far side for scientific study. As we continue to explore Earth’s celestial companion, we may finally unravel the secrets of the moon’s hidden hemisphere.

Summary

Earth’s perpetual view of only one side of its cosmic neighbor is a consequence of the moon’s synchronous rotation, tidally locked in place by our planet’s gravitational grip. But libration and lunar exploration have revealed that there is more to the moon than meets the eye from Earth. The far side, with its unique topography and composition, provides a new frontier for scientific discovery that is only beginning to be unveiled. As we continue to study and explore the moon, both from orbit and on the surface, we will surely learn more about the formation and evolution of Earth’s steadfast companion, and gain a more complete picture of the familiar yet alien world in our night sky.