Fifty years ago this week, on 17 February 1965, NASA launched the Ranger VIII robotic spacecraft toward the Moon. Three days later, the vehicle provided the world’s first high-resolution glimpse of what would become Tranquility Base—the landing site of Apollo 11, where humans first stepped onto another planetary surface.
Together, the Ranger VIII results and subsequent Apollo 11 mission illustrate the tremendous value of an integrated robotic and human space exploration program. Uncrewed craft snapped pictures for mission planning and tested landing technologies. However, only Apollo’s human crews could perform the intensive field work that paved the way for that era’s scientific legacy.
These lessons may help guide the international scientific community as it considers future plans for lunar exploration. They point the way to a new series of lunar missions in which robotic spacecraft and humans could work together to solve the most pressing mysteries surrounding our solar system’s formation.
Beginning with Impact
The U.S. government started the Ranger program in 1959 to conduct lunar science and compete with the Soviet Union’s Luna program. However, in 1962, when President Kennedy announced plans to safely land astronauts on the Moon and return them to Earth, NASA redirected Ranger to support this effort, dubbed the Apollo program.
The Ranger program’s primary objective involved characterizing the fine-scale structure of the lunar surface and thus determining if robotic and human missions could land on the surface safely [Trask, 1970]. The Ranger spacecraft did so by flying toward the Moon, taking photographs at ever-lower altitudes until they hit the lunar surface.
Ranger VIII was the second successful mission in a series of nine spacecraft. It targeted one of the Moon’s dark plains—formed by ancient lava flows since cooled into basalt—called Mare Tranquillitatis, Latin for the “Sea of Tranquility.” Scientists studying lunar photographs made using telescopes on Earth found the plain alluring in part because it was relatively flat terrain close to the equator—attributes that made it more accessible for the first attempt at landing a crew on the Moon. The spacecraft carried six television cameras with different exposure times, fields of view, lenses, and scan rates.
In the 23 minutes before crashing into the lunar surface, Ranger VIII continually transmitted back to Earth full scans from its wide-angle camera A and narrow-angle camera B, providing 60 and 90 frames, respectively, with about 5 seconds between frames on each camera. Camera B pointed farther south than camera A and captured several pictures of the terrain that would become Tranquility Base from an altitude as low as 229 kilometers. The spacecraft hit the lunar surface 68 kilometers north-northeast of what would later become Tranquility Base.
Scientists used the images to make a series of shaded relief charts with depths of impact craters estimated using shadow lengths. In parallel and with the same data, the U.S. Geological Survey (USGS) generated a series of geologic maps for the Moon.
Using Ranger VIII images, Wilshire  completed a preliminary map of the Sabine Region, including the area where Apollo 11 would land, at a 1:250,000 scale. However, work on Ranger-based maps eventually slowed as personnel began shifting to NASA’s next phase of robotic lunar exploration—sending craft to orbit and land on the Moon.
Orbiters and Landers
NASA began the Lunar Orbiter (LO) program in 1964, launching five spacecraft specifically designed to photograph potential Apollo landing sites. From the Lunar Orbiter images, officials selected eight sites for detailed study. The USGS assigned Grolier [1970a, 1970b] the task of unraveling the geology of the Mare Tranquillitatis site, also known as Apollo Landing Site 2. He based his map principally on seven high-resolution images that LO II acquired in November 1966.
The final phase of NASA’s robotic Apollo preparations was to demonstrate landings themselves and test surface conditions. To do this, NASA initiated the Surveyor program while lunar photogeologic studies were underway. Within a year of the LO II flight, Surveyor V landed in a small 9 × 12 meter crater 25 kilometers northwest of what would become Tranquility Base.
Surveyor V survived three lunar nights (14 Earth-day periods without sunlight), finally succumbing and going dark after about 107 Earth days. Surveyor V, the first lunar lander to carry an alpha particle backscattering instrument, produced the first estimates of the lunar surface’s chemical composition.
Excited, geologists confirmed that the maria were composed of basalt, meaning the Moon was a differentiated body with a crust derived by partial melting of a mantle with a low silica content.
Choosing Tranquility Base
After Surveyor V and the success of Apollo 8, the first human mission around the Moon, researchers still had not decided on the site for the first lunar landing. The Ranger VIII, LO II, and Surveyor V spacecraft had provided important precursor data for Apollo Landing Site 2, increasing its favorability. In addition, in 1966, astronaut Harrison Schmitt (who later flew on Apollo 17) had used the last image from camera B on Ranger VIII to develop a hypothetical moonwalk route around a hypothetical lander in Mare Tranquillitatis [Schmitt, 1966].
This mapping exercise provided a measure of mission reality that did not exist for any other landing site. Schmitt used it to argue that the simulated landing of the lunar module (LM) in the upcoming Apollo 10 mission should be above that site. Schmitt won his case, and Apollo 10 performed a dress rehearsal for Apollo 11 landing over Apollo Landing Site 2.
In June 1969, map makers with the U.S. Army delivered to NASA 116 charts and geologic maps, complete with NASA’s robotic images and the USGS’s geologic interpretations [U.S. Army Topographic Command, 1969]. The Apollo 11 astronauts carried these with them when they launched the following month.
The charts and photogeologic maps of the landing area included maps at 1:100,000 and 1:25,000 scales that mapped out the anticipated landing zone and the boulders and craters that Apollo 11’s lander would have to dodge.
The Eagle Has Landed…Where?
As Apollo 11’s “Eagle” LM descended toward the lunar surface on 20 July 1969, carrying Neil Armstrong and Edwin “Buzz” Aldrin, a cascade of factors knocked the Eagle off course. These included the initial jolt of undocking from Apollo 11’s Command Service Module (CSM), a series of test firings of the Eagle’s thrusters, and variations in the Moon’s gravity field encountered during the descent [Mission Evaluation Team, 1971].
Although the Eagle was flying over the geology mapped on the 1:25,000 charts, the crew did not know where they were on those charts. Occupied by resolving multiple alarms during the descent, they did not have an opportunity to monitor the landscape until they found themselves less than 600 meters above the surface [Mission Operations Branch, 1969]. By that point, Armstrong and Aldrin were not where they expected to be, and their trajectory would take them beyond the LO II high-resolution photographic coverage that had been used to certify safe landing sites.
Armstrong and Aldrin landed several kilometers west and south of their target, about 1.5 kilometers beyond the geology mapped on the 1:25,000 charts created from LO II data. They ended up in the older of two mare surfaces shown on the 1:100,000 geology map. At the time, however, they had no landmarks to place themselves. Michael Collins in the orbiting CSM could not locate the Eagle, either.
A Scientific Bonanza
Despite the initial uncertainties in the LM’s position, the value of a sample return mission involving a crew soon became obvious from the scientific bonanza that followed. This value only grew with each subsequent Apollo mission.
Analysis of the returned samples showed Mare Tranquillitatis to be volcanic, composed of at least two relatively iron- and titanium-rich basalts that were very old (3.6–3.9 billion years old). The regolith—lunar “soil”—also contained igneous dust and loose rocks characterized by plagioclase feldspar. Distant impact cratering events in the highlands likely kicked up this anorthositic material.
Petrological and geochemical analysis of this material led to the completely novel idea that an extensive magma ocean covered the early Moon and that the lunar crust formed from material floating to the top of this rapidly solidifying ocean.
Finally, several key studies of Apollo 11 samples and photography strongly suggested that most craters on the Moon formed from impacts, not from volcanic eruptions.
Geologists generated these big ideas of the Moon’s evolution after analyzing samples collected from only 2.2 hours of astronauts’ field work. In addition, during that time, Armstrong and Aldrin also managed to deploy a television camera, an experiment to measure solar wind composition, seismic monitoring instruments, a lunar dust detector, and a mirror for Earth-based laser ranging experiments, the latter of which remains in use.
To the Moon Again and Beyond
Future exploration of the Moon—including future sample return missions—will address fundamentally important scientific questions involving the origin and evolution of the entire solar system, according to a broad international consensus of scientists [e.g., National Research Council, 2007], while also providing a credible path to carry humanity beyond low-Earth orbit [e.g., International Space Exploration Coordination Group, 2013].
Toward that goal of renewed lunar exploration, scientists have accumulated new insights from a new generation of robotic spacecraft sent to the Moon. These spacecraft include the United States’ Clementine, Lunar Prospector, Lunar Reconnaissance Orbiter, Gravity Recovery and Interior Laboratory (GRAIL), and Lunar Atmosphere and Dust Environment Explorer (LADEE); Europe’s Small Missions for Advanced Research in Technology (SMART-1); China’s Chang’e series of orbiters; Japan’s Kaguya; and India’s Chandrayaan-1.
Unfortunately, the world no longer has the capability to land crew on the Moon’s surface. Until new lunar landers are designed, researchers hoping to nonetheless accomplish sound science are working around this limitation by combining robotic and human capabilities.
In such a plan, astronauts orbiting the Moon in, for example, the Orion spacecraft could remotely operate landers on the Moon. These landers could collect samples and bring them to Orion before it returns to Earth. Such integrated robotic and human missions could be the first of many giant leaps that enhance our ability to explore space.
For more details on such joint robotic-human missions, as well as potential landing sites, read the companion story in Eos, “Human and Robotic Missions: To the Moon Again and Beyond.”
The author thanks two anonymous reviewers. The work was supported, in part, by NASA through the Lunar and Planetary Institute (LPI) and the LPI-JSC Center for Lunar Science and Exploration. This is LPI contribution 1833.