Explainer: Why studying samples from the far side of the moon matters

Editor's note: Lunar samples collected by China's Chang'e-6 mission from the far side of the moon will be displayed at the 15th China International Aviation and Aerospace Exhibition, also known as Airshow China, in November. Wang Zhaokui, a professor at Tsinghua University's School of Aerospace Engineering and head of the Department of Aeronautics and Astronautics Engineering, explains how new technologies enabled the Chang'e-6 mission and why the sampling mission to the far side of the moon is historic. The article reflects the author's opinions and not necessarily those of CGTN.

Why is the far side of the moon so mysterious?

The far side of the moon has limited visibility from Earth, as the moon is "tidally locked" with Earth, meaning the same side of the moon always faces us. This positioning is the result of the dissipation of the moon's rotational energy over billions of years of magnetic interaction with Earth's oceans.

It wasn't until 1959 that the Soviet Luna 3 spacecraft captured the first images of the moon's far side, revealing a starkly different landscape with more craters and fewer maria. Maria are large, dark, basaltic plains.

In 2019, China's Chang'e-4 became the first spacecraft to land and conduct surveys on the moon's far side. The Chang'e-6 returned with 1,935.3 grams of samples from the far side of the moon in late June this year.

One reason for the slower pace of exploration is that it is difficult to reach spacecraft, landers and rovers with direct communication technologies because of the barrier of the moon itself.

China has developed technologies to facilitate communication around the moon's curvature, effectively enabling contact with the moon's far side.

What new technologies have made the Chang'e-6 mission possible?

A key breakthrough that has enabled real-time communication with the far side involves the deployment of the Queqiao ("magpie bridge") relay satellite, a challenging task in itself. One of the primary complexities of the Queqiao satellite involves positioning it in a stable halo orbit around the Earth-Moon L2 Lagrangian point.

Lagrangian points are positions in space where the gravitational forces of two large bodies precisely balance the centripetal force felt by a smaller object. At these points, a satellite can theoretically maintain a stable position relative to the Earth and the moon.

This strategic positioning allowed Queqiao to act as a communication bridge, continuously relaying signals to and from Earth, thus facilitating immediate data transfer and decision-making that are critical for the success of lunar operations.

However, the L2 point is particularly challenging because it is dynamically unstable, meaning that any small deviation from this point requires corrective maneuvers to maintain the satellite's position. Additionally, deviations are bound to happen due to various influences like solar pressure and other perturbative forces. However, it is stable enough that the satellite won't drift far and can be kept near the intended point with limited corrections.

Essentially, Queqiao was designed to perform complex maneuvers to stay within the right vicinity, navigating the delicate balance of forces at play. Another key innovation has been the "half-ballistic jump re-entry" technique, also known as the "stone skipping" technique, which ensures that spacecraft return safely to Earth.

A stone-skipping re-entry uses Earth's atmospheric drag to decelerate the spacecraft. Upon first contact with the atmosphere, the spacecraft uses atmospheric drag to slow down and "bounce" out again, returning briefly to space.

After this bounce, the spacecraft re-enters the atmosphere at a reduced speed and thermal load, allowing for a safe landing. Both Chang'e-5 and Chang'e-6 have used this technique, which is similar to throwing a stone and hitting the water so that it skips along the surface.

What could we learn from the materials brought back?

Moon rocks can offer a wealth of knowledge. For instance, many scientists are eager to analyze samples from the South Pole-Aitken Basin, which could shed light on the "late heavy bombardment theory."

The South Pole-Aitken Basin is among the oldest and largest impact craters in the solar system. It was formed approximately 4.6 billion years ago, not long after the formation of the Solar System, when it's believed that leftover planetesimals frequently collided with the newly formed planets.

This period of intense impacts is thought to have been so severe that it could have stripped away enough material from Earth to contribute to the formation of the moon. By around 3.8 billion years ago, some scientists suggest that the rate of these impacts had declined to a level similar to what we observe today.

The theory has shaped our understanding of when life originated on Earth, as scientists previously believed that all water had vaporized during the early period, making the planet uninhabitable for life as we know it. However, recent evidence from meteorites and new data from space missions have led experts to question this theory.

Resolving this debate could have significant implications for answering key questions, such as when life first emerged and what conditions were like on early Earth. It could also yield many other exciting and important insights.

For instance, recent discoveries suggest that the moon may contain water and other volatile substances, which would greatly facilitate future lunar exploration and potential colonization. Analyzing moon rocks could help determine the presence, distribution and origin of these substances.

Could manned missions be next?

China is already planning a manned landing on the moon before 2030. However, there are currently no plans for a manned mission to the far side of the moon. Tsinghua University has conducted many research projects centered on manned lunar missions and hopes to contribute to future space exploration.

What extra challenges might an astronaut face on the far side of the moon?

An astronaut on the far side of the moon would face several unique and significant challenges. One of these is the terrain, which is more rugged and heavily cratered than the "relatively smooth" near side.

Navigating this challenging landscape would require advanced autonomous systems and robust training for the astronauts. Additionally, isolation due to communication delays in sending signals around the moon could lead to significant psychological stress. The astronauts taking on that challenge will need resilience and very effective support systems to cope.

What's next for the moon mission?

The moon is the closest celestial body to Earth, with a communication delay of less than two seconds on the near side. This proximity makes it an ideal initial step for deeper space exploration.

Before venturing to more distant bodies like Mars, mastering lunar exploration and operations is crucial. This foundational step to the far side of the moon will allow for the development of technologies and strategies essential for longer missions.

It has also been more than 50 years since the Apollo moon landings, and the next phase of lunar exploration is set to go beyond mere visits.

Future missions will likely construct functional lunar bases. These bases could facilitate lunar mining operations, likely utilizing resources directly on the moon rather than transporting them back to Earth, making space operations more sustainable. This also positions the moon as a key "stepping stone" for ventures to other celestial bodies.

(Cover: Chang'e-6 lunar soil samples in numbered glass vessels. /CFP)