Half a century after the Apollo 11 moon landing, the mission and its successors have led to significant insights for understanding not only the moon, but also Earth and the rest of the solar system.
But a single step — or rather, a series of them, since the Apollo program carried a dozen astronauts to the lunar surface over four years — is not enough to fully comprehend the nature of our lunar neighbor. Scientists, engineers and explorers around the world are looking forward to returning to the moon.
This week, the journal Science published a special commemorative issue that looks both backward and forward in time to see what we have learned from exploring the moon, and how much more it has to offer.
Related: Apollo 11 at 50: A Complete Guide to the Historic Moon Landing Mission
Humans have always looked to the surface of the moon, but not until the dawn of space exploration could satellites and astronauts see the lunar surface up close. The samples carried home by Apollo astronauts were even more astounding, examined by scientists at the time and later re-examined with newer, more powerful instruments long after the Apollo program closed.
One of the biggest changes that the Apollo program brought to our understanding of the moon came from insights into the birth of Earth's sibling. Prior to Apollo, scientists thought that planets grew by gentle accretion at low temperatures, giving them a relatively cold start. Lunar samples quickly revealed that a large portion of the lunar surface was once molten, leading to what would become known as the magma ocean model, to explain how the lunar layers solidified.
Mysteries remain, such as why some of the signature elements are unevenly distributed, but a lunar magma ocean answered many questions about the rocks returned by Apollo astronauts. "The basic magma ocean model explains many characteristics of the lunar samples," Richard Carlson, of the Carnegie Institution for Science in Washington, D.C., wrote in a review of how the moon impacted our understanding of planet formation and evolution.
The lunar samples were also surprisingly similar to rocks on Earth. Different planets, and even meteorites, have subtle isotopic differences due to the layout of elements in the solar system prior to planet formation. But those signatures are the same for Earth and the moon, suggesting an unexpected relationship and a surprising origin for the moon.
Young Earth was born in a violent neighborhood: Collisions between early planets and leftover debris were constant. Less than 100 million years after the formation of the solar system, a planetary embryo crashed into our world, sending terrestrial rocks exploding into space. The ejected material eventually fell together to form a new sister, the moon.
Today, that's the leading theory for lunar formation, although scientists are still working to understand exactly how the process captured so much terrestrial material and so little of the impactor. They also wonder how the moon retained so much light material, like water and gases, that should theoretically have escaped during the collision.
The scarred surface of the moon can be mapped from Earth, allowing scientists to determine the relative ages of lunar features. The math is simple — more heavily cratered regions are older, since they've had time to collect more collisions, while more lightly cratered regions are younger, having experienced fewer impacts. What couldn't be understood from Earth was the absolute ages of any of these regions.
Apollo samples changed that. By studying the composition of lunar rocks, astronomers were finally able to date the surface of the moon. To their surprise, they found most of the regions visited by the astronauts had roughly the same age — about 3.9 billion years old. This led to the theory of a Late Heavy Bombardment, an uptick in collisions experienced by the moon. Only in recent years, with more detailed analysis of the lunar surface and samples, has that uptick come under question.
Because it lacks water or an atmosphere, the moon maintains a relatively pristine surface, changed predominantly by impacts (and by volcanism in its past). This ancient surface told scientists not only how often material crashed into the moon, but also how often such objects collide with Earth and the other planets. Lunar samples even allowed planetary scientists to date features on Pluto, which lies much farther out in the solar system.
Lunar samples opened themselves to researchers in the 1970s, and they continue to unveil mysteries about the moon today. But the samples raised questions that can only be answered with more moon samples; the United States isn't the only country working to understand our nearest neighbor.
China's lunar exploration program
Since the beginning of the 21st century, more than a dozen satellites launched by countries around the world have turned toward the moon. One of these countries, China, has an ambitious program in the works to not only explore the lunar surface but also to build a permanent lunar scientific research station.
In 2004, the Chinese Lunar Exploration Program (CLEP) developed a robotic lunar exploration program, the Chang'e project, named for the Chinese goddess of the moon. CLEP has a total of 14 key issues that guide the agency's blueprint for exploring the moon. These include understanding the atmosphere, topography and ionosphere of the moon, as well as its internal structure and thermal evolution.
"These science questions are fundamental in lunar exploration missions," Chunlai Li, of the Chinese Academies of Sciences in Beijing, wrote in a review for Science.
From 2004 to 2020, CLEP has worked to gain a global and comprehensive understanding of the moon with a series of Chang'e missions that Li called "iterative and intertwined." Chang'e-1 and Chang'e-2 orbiters created detailed lunar global image and topographic maps, the first global analysis of the moon's microwave radiation, and the acceleration of protons at the lunar day-night line.
The Chang'e-3 lander and rover, which touched down in December 2013 and became the first spacecraft to land on the moon since 1976, discovered a new type of lunar basaltic rock and used the surface as an observational platform to observe the stars.
On January 3, 2019, Chang'e-4 became the first lander mission to explore the lunar farside. The robotic craft touched down within the South Pole-Aitken basin, a region currently targeted by NASA for human exploration. Chang'e-4 is studying the far side of the moon up close, providing visible and near-infrared observations of impact ejecta and analyzing the electromagnetic environment.
In 2015, China proposed a follow-up plan of three missions to complete before 2030. Chang'e-6 would provide a sample return from the south polar area, Chang'e-7 would study the environment and resources around the south pole, and Chang'e-8 would verify key technologies, such as 3D printing for construction on the lunar surface.
"Through these missions, a robotic scientific research station prototype will be built on the moon," Li wrote.
After 2030, China hopes to continue to develop capabilities for both robotic and human exploration. The goal is for the lunar research station to turn into a long-term lunar base that astronauts could visit for short but increasing stays.
China is hoping they won't complete these missions alone. "International cooperation is an important element in China's strategy of lunar and deep space exploration," Li wrote. Chang'e-4 carries experiments from Germany, Sweden and the Netherlands; Chang'e-6 has similar international-cooperation opportunities.
Multi-nation teamwork goes beyond scientific inclusion. China has signed lunar exploration agreements with Russia and with a group that includes the United Nations Office for Outer Space Affairs, Turkey, Ethiopia and Pakistan.
"China is also open to cooperation with NASA on lunar exploration," Li wrote. The nation "looks forward to exploring more opportunities to cooperate with NASA to preserve the space environment for generations to come."
In April 1972, Apollo 16 astronauts set up the first observatory on another world. The gold-plated telescope allowed astronomers to confirm the existence of interstellar molecular hydrogen and reveal the first-ever ultraviolet images of distant galaxy clusters, the solar wind and Earth's ionosphere and aurorae. Since that time, astronomers have continued to eye the moon as an ideal location for telescopic observations.
While optical telescopes would gain from the lack of lunar atmosphere, the cost of landing an instrument on the moon instead of placing it in orbit around the Earth seemed to argue against such a telescope. But for radio astronomers, the far side of the moon is the perfect place to position an instrument.
Radio astronomy relies on the radio waves also used by TV and radio transmitters. At its lowest frequencies, radio astronomy's observations are easily disrupted by Earth's ionosphere – an atmospheric layer of charged particles that can refract, disperse or block radio waves that can provide insights about the early universe.
"I've been thinking about this for 35 years and we're finally seeing some traction," astronomer Jack Burns, of the University of Colorado in Boulder, told Science. "The feeling is that the moon isn't that hard to do anymore."
In 2008, Burns formed a NASA-funded team called LUNAR to work out how to build a lunar radio telescope. The collaborators designed an array of lunar telescopes that would include hundreds of simple dipole antennas that autonomous rovers could lay flat on the ground.
Another NASA-funded program, the Network for Exploration and Space Science (NESS), is working on lunar telescopes that could study exoplanets as well as the early universe. Planets with a magnetic field emit low-frequency radio waves that could be detected by a telescope, offering clues about its interior and habitability. While an Earth-based instrument can detect the radio emissions of exoplanets the size of Jupiter or larger, a lunar observatory could pick up the fainter signature of a rocky world.
NESS is working on two programs. The Dark Ages Polarimeter Pathfinder (DAPPER) satellite would orbit the moon. While on the far side, shielded from Earth's noisy radio interference, DAPPER will try to detect the radio fingerprint of the early universe. The spacecraft should be able to map radiation in enough detail to allow scientists to understand how dark matter tugged ancient clouds of hydrogen into the clumps that would form the first stars.
Down on the moon's surface, the Farside Array would use the techniques pioneered by the LUNAR team, with small rovers laying out 128 antennas across a 6.2-mile (10-kilometer) region.
Chang'e-4 communicates with Earth via China's Queqiao satellite. The probe also carried a radio experiment that will begin its hunt for the early universe's radio fingerprint later this year. Although not completely shielded from Earth, the spacecraft serves as a trial run for a future giant radio telescope array.
The European Space Agency also recently released a 10-year strategy for science on the moon, including testing low-frequency radio receivers on the far side.
Radio astronomers face a sense of urgency as other groups discuss ways to explore and exploit the moon. Mining could toss up dust that would interfere with observations while creating tremors that would undermine the stability required for sensitive telescopes. Human exploration would bring the same radio noise that blocks radio astronomers on Earth.
The prospect has radio astronomer Heino Falcke, of Rabound University in the Netherlands, concerned. "If the far side gets spoiled, I don't know where we would go," he told Science.
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