Moon exploration
- Key People:
- Eugene Cernan
- Harrison Schmitt
- Related Topics:
- Chang’e
- Apollo
- Lunar Prospector
- Surveyor
- Luna
Moon exploration, the investigation and study of the Moon by robotic and crewed spacecraft.
Exploration by spacecraft
First robotic missions
Following the launch in 1957 of the U.S.S.R.’s satellite Sputnik, the first spacecraft to orbit Earth, the next major goal of both the Soviet and the U.S. space programs was the Moon. The United States quickly prepared a few robotic probes, most of which failed and none of which reached the Moon. The Soviet Union had more success, achieving in 1959 the first impact on the lunar surface with Luna 2 and the first photograph of the Moon’s far side with Luna 3.
After NASA was founded in 1958, the U.S. program became more ambitious. Initial spacecraft investigations were geared toward studying the Moon’s fundamental character as a planetary body by means of seismic observation, gamma-ray spectrometry, and close-up imaging. Scientists believed that even limited seismic data would give clues toward resolving the question as to whether the Moon was a primitive, undifferentiated body or one that had been heated and modified by physical and chemical processes such as those on Earth. Gamma-ray measurements would show whether the Moon’s interior was heated by radioactive decay and would give some information on the chemical composition of the surface. Close-up imaging would reveal features too small to be seen from Earth.
Among nine U.S. Ranger missions launched between 1961 and 1965, Ranger 4 (1962) became the first U.S. spacecraft to strike the Moon. Only the last three Rangers, however, avoided the malfunctions of their predecessors. Ranger 7 (1964) returned thousands of excellent television images before impacting as designed, and Rangers 8 and 9 (both 1965) followed successfully. The impact locale of Ranger 7 was named Mare Cognitum for the new knowledge gained.
After failures in the mid-1960s, the Soviet Union scored several notable achievements: the first successful lunar soft landing by Luna 9 and the first lunar orbit by Luna 10, both in 1966. Pictures from Luna 9 revealed the soft, rubbly nature of the regolith and, because the landing capsule did not sink out of sight, confirmed its approximate bearing strength. Gamma-ray data from Luna 10 hinted at a basaltic composition for near-side regions.
The United States carried out its own soft-landing and orbital missions. In 1966 Surveyor 1 touched down on the Moon and returned panoramic television images. Four more successful Surveyors followed between 1966 and 1968; they provided not only detailed television views but also data on the composition and properties of lunar soil. Also, during 1966–67, five U.S. Lunar Orbiters made photographic surveys of most of the lunar surface, providing essential mapping for the Apollo missions.
Apollo
After Soviet cosmonaut Yuri Gagarin became the first human to travel into space in April 1961, U.S. Pres. John F. Kennedy established the national objective of landing a man on the Moon and returning him safely before 1970. Apollo was the result of that effort.
The Soviet Union and the United States were heavily engaged in a race to launch crewed flights to the Moon. At the time, the Soviets did not publicly acknowledge the full extent of their program, but they did launch missions between 1968 and 1970 under the generic name Zond, using spacecraft based of their crewed Soyuz design. Some of the Zond flights brought back color photographs of the Moon’s far side and safely carried live tortoises and other organisms around the Moon and back to Earth.
In parallel with these developments, Soviet scientists began launching a series of robotic Luna spacecraft designed to go into lunar orbit and then land with heavy payloads. This series, continuing to 1976, eventually returned regolith samples to Earth and also landed two robotic rovers, Lunokhod 1 and 2 (1970 and 1973).
In December 1968, acting partly out of concern that the Soviet Union might be first in getting people to the Moon’s vicinity, the United States launched Apollo 8 to take three astronauts—Frank Borman, James Lovell, and William Anders—into lunar orbit. After circling the Moon three times, the crew returned home safely. Apollo 9 and 10 completed the remaining tests needed for landing on and ascending from the Moon. On July 20, 1969, Apollo 11 astronauts Neil Armstrong and Buzz Aldrin set foot on the Moon while Michael Collins orbited above them. Five more successful crewed landing missions followed, ending with Apollo 17 in 1972. A total of 12 astronauts had set foot on the Moon.
Return to the Moon
After Apollo, scientists continued to conduct observations from Earth and study the returned samples. During Galileo’s flybys of Earth and the Moon in December 1990 and 1992 en route to Jupiter, the spacecraft imaged the Moon in several wavelength ranges to gather geochemical data. After a long hiatus, orbital mapping of the Moon resumed with the flights of the Clementine and Lunar Prospector spacecraft, launched in 1994 and 1998, respectively.
In the first decade of the 21st century, interest in exploring the Moon was revived among the major spacefaring countries. The United States had an ambitious exploration program, with three uncrewed satellites—the Lunar Reconnaissance Orbiter (launched in 2009), the Gravity Recovery and Interior Laboratory (launched in 2011), and the Lunar Atmosphere and Dust Environment Explorer (launched in 2013).
Exploration of the Moon was a key part of an Asian space race in which probes to the Moon were launched by Japan (Kaguya, launched in 2007), China (Chang’e 1, 2007), and India (Chandrayaan-1, 2008). Chang’e 1 and Chandrayaan-1 were each their respective country’s first satellite to leave Earth orbit. All three of these probes orbited the Moon.
Chang’e 2 (launched in 2010) also orbited the Moon and after eight months left orbit for the second Lagrange point (L2) and a flyby of the asteroid Toutatis. Chang’e 3 launched in 2013 and landed on the Moon, the first such landing since Luna 24 in 1976. It carried a robotic rover, Yutu, which operated for almost three years. Chang’e 4 also carried a rover, Yutu-2, and on January 3, 2019, Chang’e 4 became the first spacecraft to land on the Moon’s far side.
India’s next lunar mission, Chandrayaan-2 (launched in 2019), was unsuccessful. However, Chandrayaan-3 (launched in 2023) became the first spacecraft to land in the Moon’s south polar region. Chang’e 5 (launched in 2020) returned about 1.7 kg (3.7 pounds) of material to Earth. Japan’s Smart Lander for Investigating Moon (SLIM) probe landed on the Moon in January 2024, making Japan the fifth country to land a spacecraft on the Moon.
Future lunar missions include a planned return of American astronauts to the Moon as part of the Artemis project, beginning with the Artemis II flight around the Moon in 2025 and the crewed landing of Artemis III in 2026. As part of Artemis, NASA plans to build a space station, Gateway, in orbit around the Moon. China is planning a robotic research station at the lunar south pole in the 2020s.
Mission results
The Apollo program revolutionized human understanding of the Moon. The samples collected have continued to be studied into the 21st century. Analyses of samples from the Luna missions have also continued and are valuable because they were collected from areas far from the Apollo sites.
Analysis of the first lunar samples confirmed that the Moon is an evolved body with a long history of differentiation and volcanic activity. Unlike Earth’s crust, however, the lunar crust is not recycled by tectonic processes, so it has preserved the records of ancient events. Highland rock samples returned by the later Apollo missions are nearly four billion years old, revealing that the Moon’s crust was already solid soon after the planets condensed out of the solar nebula.
The mare basalts, though they cover a wide range of ages, generally show that the basin-filling volcanic outpourings occurred long after the formation of the highlands and are believed to have originated from later internal radioactive heating within the Moon.
The Clementine and Lunar Prospector spacecraft, operating in lunar polar orbits, used complementary suites of remote-sensing instruments to map the entire Moon, measuring its surface composition, geomorphology, topography, and gravitational and magnetic anomalies. The topographic data highlighted the huge South Pole–Aitken Basin, which, like the other basins on the far side, is devoid of lava filling. Measuring roughly 2,500 km (1,550 miles) in diameter and 13 km (8 miles) deep, it is the largest impact feature on the Moon and the largest known in the solar system; because of its location, the basin was not confirmed to exist until the Lunar Orbiter missions in the 1960s. The gravity data collected by the spacecraft, combined with topography, confirmed the existence of a thick, rigid crust, giving yet more evidence that the Moon’s heat source has expired. Both spacecraft missions hinted at the possibility that water ice exists in permanently shadowed polar craters.
In 2009 Chandrayaan-1 used its spectrometer to find water molecules on the Moon. The water molecules are mixed in with the lunar soil in small quantities; one ton of surface material contains about 1 kg (2 pounds) of water. The strongest signals of water came from the lunar poles. However, water was also detected over much of the lunar surface. Observations by the U.S. EPOXI spacecraft seemed to show that water was formed by hydrogen ions in the solar wind interacting with oxygen atoms in the lunar soil. Also in 2009, the U.S. LCROSS spacecraft crashed a Centaur rocket stage into the permanently shadowed floor of Cabeus, a crater near the Moon’s south pole. Analysis of the plume thrown up by the impact showed that the lunar soil at the bottom of Cabeus was 5.6 percent water ice.