Lunar surface experiments

The Moon has been studied from Earth and from lunar orbit, but the most detailed studies must be done on the lunar surface. The United States and the Soviet Union successfully landed eighteen spacecraft on the lunar surface, with many of these missions carrying multiple science payloads.

Overview

The Soviet Union’s Luna 1 spacecraft was the first to visit the vicinity of the Moon, flying past it on January 4, 1959. Later missions by both the United States and the Soviet Union crashed into the Moon, collecting photographic and other data on the way. On February 3, 1966, the Soviets’ Luna 9 became the first spacecraft to soft-land on the lunar surface. Luna 9, however, had only cameras and a radiation detector as scientific instruments. Luna 13 landed later that year, carrying instruments to measure the density and strength of the lunar regolith (soil) and to study cosmic-ray reflections from the lunar surface. Also in 1966, on June 2, the United States’ Surveyor 1 spacecraft soft-landed on the lunar surface. It, too, carried no more than cameras and landing radar with which to study the Moon but still returned useful data on the nature of the lunar surface. Four other Surveyor probes landed on the Moon over the next two years. All carried cameras, and two carried robotic arms to scrape the surface to determine its consistency and to move the regolith into a better position to photograph it. The final three Surveyors also carried alpha-scattering surface analyzers used to measure the abundances of the many elements making up the lunar regolith.

The Soviet Union successfully landed seven Luna spacecraft on the Moon, with the last one being Luna 24, which touched down on August 18, 1976. Three Luna missions (16, 20, and 24) returned small samples of lunar regolith to Earth. Luna 17 and Luna 21 each carried remote-controlled roving vehicles called Lunokhod. From November 1970 until September 1971, Lunokhod 1 traveled 10.5 kilometers and returned nearly twenty thousand images. It carried an X-ray spectrometer to study the composition of the lunar soil. Lunokhod 2 traveled thirty-seven kilometers across the lunar surface from January 16 to June 4, 1973, returning about eighty thousand images and performing mechanical tests of the lunar regolith.

The uncrewed lunar missions have proved very important to our understanding of the Moon, but some of the most important science missions were deployed by astronauts during six Apollo landings from 1969 to 1972. Many experiments were conducted by several of the missions. In addition to conducting experiments on the lunar surface, the Apollo missions collected nearly 382 kilograms of lunar material to bring back to Earth. These samples included rocks as well as samples of the lunar regolith. The landing sites for the Apollo missions were chosen to provide samples from a variety of different geological features to maximize the impact of only a few sample sites. Scientific instruments carried by Apollo 11 in July 1969 were solar-powered. However, the Moon’s slow rotation means that the lunar night lasts for two weeks. The remaining Apollo missions carried radiothermal generators to supplement the power for the instrument packages. Instruments for Apollo 12 through Apollo 17 continued operations until the science stations were shut down on September 30, 1977.

All of the Apollo missions carried seismographic equipment with which to study the Moon below its surface. Most of the seismographs were passive systems monitoring moonquakes. However, Apollo 14 and Apollo 16 both carried active seismographs. These used mortars to fire explosive shells some distance away from the landing site to produce seismic waves that could be used to study lunar geology. A similar seismic system was also deployed by Apollo 17 in December 1972, except that it used explosive charges carefully placed by the astronauts rather than mortars. In all three cases, the explosives were not detonated until after the astronauts had left the lunar surface.

Apollo missions also carried lunar dust detectors designed to study the dust disturbed by the liftoff of the lunar lander’s ascent stage. Later versions of the detector also included provisions to study the long-term degradation of solar panels exposed to the harsh radiation environment of the Moon. Astronauts also set up foils to capture solar wind particles to return to Earth for study. However, in addition to returning particles for study, ion detectors monitored solar wind and cosmic radiation on the lunar surface, and Apollo 12 deployed a spectrometer to measure the composition of the solar wind. These detectors continued to operate for several years after the end of the Apollo program and returned useful data on variations in solar activity. Ion detectors also were used to monitor gas molecules near the lunar surface. These gases constitute what planetary scientists refer to as the lunar atmosphere. Apollo 17 carried a mass spectrometer to measure the composition of the lunar atmosphere.

Supplementing instruments designed to study the Moon, Apollo 16 carried a far ultraviolet telescope and spectrograph. This was the only astronomical instrument placed on the Moon. Among other targets, it was used to study the Earth and the Large Magellanic Cloud.

To study the geophysics of the Moon, the later Apollo missions also carried magnetometers to study the Moon’s residual magnetic field, and Apollo 17 carried an experiment to measure the Moon’s surface gravity and monitor it for any variations over time. Heat-flow experiments to measure the amount of heat flowing from the lunar interior were set up on each of the last three Apollo missions. However, astronaut John Young tripped over the cable for the experiment, breaking it. The cable could not be repaired by the astronauts, so the instrument returned no data.

In addition to experiments carried to the Moon to be performed there, several missions carried mirrored corner reflector arrays that were positioned to point back toward Earth. These corner reflectors, completely passive systems, were designed to reflect light, striking it back in the direction from which it came. Powerful lasers fired from Earth at the sites of the reflectors are reflected back to Earth, where they can be detected. Careful measurements of the time that it takes for the light to get to the Moon and back are used to determine the lunar distance and variations in the lunar orbit. The astronauts of Apollo 11 deployed the first corner reflector array. Apollo 14 carried another array, and Apollo 15 deployed a much larger array. In addition to the Apollo arrays, both Lunokhod rovers carried such arrays. With the exception of Lunokhod 1’s array, which has not reflected lasers since 1971, these corner reflector arrays are still used in lunar ranging experiments.

Knowledge Gained

Though the Moon is our nearest neighbor in space, it is still a long way away from astronomers on Earth, and detailed studies of its surface were not possible until the advent of spacecraft capable of traveling to the Moon. One of the first, and very important, steps in studying the Moon was photographic studies of the surface characteristics. Many of the early surface experiments, particularly those of the Surveyor program, were designed to study the physical properties of the lunar surface to determine if the regolith would be able to support a heavy spacecraft such as the planned crewed missions that were to follow. However, the experimental science stations set up by the Apollo astronauts returned important data about the nature of the Moon itself.

Prior to the surface investigations of the Moon, it was thought that the surface material was largely volcanic in nature. However, the lunar regolith has been found to be composed primarily of impact ejecta from meteorite impacts. Some speculation had been that micrometeorite impacts might have ground the surface of the Moon into a vast ocean of dust, unable to support the weight of a spacecraft landing on it. However, that idea was soundly dismissed by the early lunar landings. A layer of fine dust does exist on the lunar surface, but it is more compact than had been thought.

Other surface studies confirmed some ideas put forth after orbital observations that the lunar seas were ancient lava fields that resulted from massive impacts on the lunar surface. Seismological data suggest that the basalts that flooded the impact basins to form the seas did not come all at once but over a period of time early in the Moon’s history. However, a surprising finding was that the impacts that caused these basins occurred primarily near the end of a period of intense bombardment on the Moon rather than randomly distributed in time, as had been suspected.

The experiments performed on the Moon by both crewed and uncrewed missions, along with the studies of lunar samples returned to Earth by the Apollo missions and three Luna missions, have shown the Moon to be an alien world but with some familiarities. The Moon has a much lower density than Earth, and it has a very tiny core. Most of the Moon is composed of material similar to Earth’s crust and mantle. Though a few minerals were found on the Moon that did not have counterparts on Earth, most lunar samples were composed of minerals found on Earth. However, the lunar samples were much richer in refractory minerals than Earth rocks, and they contained very few volatile minerals.

Astronomers had assumed that liquid water may have once flowed on the lunar surface, but surface experiments showed a near-total lack of water and no indication that water had been present on the Moon. Furthermore, surface investigations suggest that many of the rocks of the lunar highlands formed from a global magma ocean present soon after the Moon’s formation.

These findings have revolutionized our understanding of how the Moon and Earth are related to each other. It is now believed, based on the Apollo and Luna findings, that the Moon formed when a giant planetesimal, perhaps the size of the planet Mars, collided with Earth very early in the history of the solar system, perhaps even before the Earth had cooled. The debris from the collision coalesced to form the Moon. Thus, understanding the Moon helps us to understand Earth.

Context

Much of the early research on the Moon was done in the heat of the space race between the United States and the Soviet Union during the 1960s. However, after initial successes, the political will to continue the study of the Moon faded. The last spacecraft to land on the Moon was Luna 24 in 1976. In the early twenty-first century, however, interest in the Moon revived, and several nations developed plans to land spacecraft on the Moon and once again begin studies on the lunar surface.

Experiments to be done on the Moon in the future will build on the work done in the 1960s and 1970s. Scientific theories about the Moon based on the findings of these earlier experiments have led to several theories of the Moon and its evolution, and future missions will carry experiments to test those theories.

In 2009, scientists at the University of Tennessee uncovered new evidence that there may be small amounts of water on the lunar surface. According to scientists, the water is endogenic or originating from the Moon and not other sources such as comets. In 2012, scientists at Vrije Universiteit in Amsterdam suggested that the Moon's temperature is cooling, which might result in the eventual return of active volcanoes on the planet. Should these missions come to fruition, the door will open for new lunar surface experiments.

Bibliography

Beattie, Donald A. Taking Science to the Moon: Lunar Experiments and the Apollo Program. Baltimore: Johns Hopkins UP, 2001.

Bond, Peter. Distant Worlds: Milestones in Planetary Exploration. New York: Copernicus Books, 2007.

Boyle, Rebecca. “NASA's Return to the Moon Is Off to a Rocky Start.” MIT Technology Review, 9 Jan. 2023, www.technologyreview.com/2023/01/09/1065143/nasa-artemis-return-to-moon. Accessed 15 Sept. 2023.

Crawford, I.A. "Back to the Moon: The Scientific Rationale For Resuming Lunar Surface Exploration." Planetary & Space Science. 74.1 (2012): 3–14. Print.

Freedman, Roger A., and William J. Kaufmann III. Universe. 8th ed. New York: W. H. Freeman, 2008.

Hamblin, W. Kenneth, and Eric H. Christiansen. Exploring the Planets. New York: Macmillan, 1990.

He, Linfeng. "Effect of Surface Roughness On Microwave Brightness Temperature From Lunar Surface: Numerical Analysis With A Hybrid Method." Advances in Space Research. 51.1 (2013): 179–187. Print.

Heiken, Grant, and Eric Jones. On the Moon: The Apollo Journals. New York: Springer, 2007.

Kaydash, V. "Structural Disturbances of the Lunar Surface Caused by Spacecraft." Solar System Research. 46.2 (2012): 108–118. Print.

Mackenzie, Dana. The Big Splat: Or, How Our Moon Came to Be. Hoboken, N.J.: John Wiley & Sons, 2003.

Orloff, Richard W., and David M. Harland. Apollo: The Definitive Sourcebook. New York: Springer, 2006.