Figure 1.1. View of the waxing gibbous Moon generated from LRO WAC images. The dark, smooth plains (maria) are basaltic lava flows, mostly erupted before three billion years ago. The rough, heavily cratered highlands (terrae) are the remnants of the original lunar crust. Bright spots are fresh craters. (Credit 1.1)
In 1892, Chief Geologist of the US Geological Survey Grove Karl Gilbert, intrigued by the craters of the Moon, spent many nights studying the lunar surface through a telescope at Washington’s Naval Observatory. Gilbert had heard a lecture about meteorite fragments that had been collected near a feature known as Coon Butte in northern Arizona. Mineralogist Albert Foote described these iron meteorites and noted their proximity to Coon Butte, but did not go so far as to connect the two in origin. Gilbert decided to study Coon Butte as a possible impact crater. By carefully measuring the shape of the crater, he calculated the likely size of an impacting iron meteorite. He postulated that the remnants of such an object must currently exist beneath the floor of the crater and used a magnetic dip needle (designed to show variations in Earth’s magnetic field) to search for what he believed should be an enormous buried iron body below the surface. But after intensive mapping failed to reveal the buried meteorite, Gilbert reluctantly (and wrongly) concluded that the Coon Butte crater must be a volcanic steam vent.5 Today, Coon Butte is known as Meteor Crater and is considered the world’s first documented meteorite impact site. How did Gilbert get its origin so wrong, especially since he had specifically tested the impact idea?
Gilbert did not understand that an impact at extremely high velocities (greater than 10 km/second) produces such enormous energies that the projectile essentially vaporizes as a point-source release of energy; left behind is a big hole with no buried iron body beneath the crater floor. An impact event is very similar to the detonation of a nuclear bomb. In fact, the formation of Meteor Crater fifty thousand years earlier by the impact of an iron meteorite must have looked very much like a nuclear explosion, complete with blinding flash and subsequent mushroom cloud. Documentation that this crater formed by impact opened the floodgates to the recognition and cataloging of dozens of impact craters on Earth (a process that continues to this day). Study of these features taught scientists to recognize the physical and chemical effects of high velocity impact, knowledge that would become critical in future interpretations of samples from the Moon and for a startling new interpretation of Earth’s history as well.
The Moon as Destination: The Space Race
The idea that we might someday travel to the Moon was often the subject of imaginative fiction, but such a journey could not be seriously contemplated until Konstantin Tsiolkovsky, Hermann Oberth, and Robert Goddard had developed the basic principles of rocketry and spaceflight.6 The technology of rockets made great strides under the impetus of war, as Germany developed the world’s first intercontinental ballistic missile (ICBM), the A4 (or “V-2” as Hitler dubbed it). In the years following World War II, intensive work toward the development of larger and better ICBMs as weapons of war led to the advent of Earth-orbiting satellites (Soviet Sputnik in 1957 and American Explorer 1 in 1958) and ushered in the Space Age. War and space were tightly coupled from the beginning, since the first use envisioned for space revolved around its possible value as a battleground.
Given this background, it was inevitable that the Moon would emerge as a key object in the exploration of space. Indeed, trips to the Moon began shortly after the beginning of the Space Age with the flight of Luna 2 in 1959. This Soviet robotic probe hit the Moon after a three-day journey, making it the first man-made object to reach another extraterrestrial body. Because of the Moon’s prominence in the sky and its proximity to Earth, it quickly became the focus of the first race into space between the United States and the Soviet Union. In May 1961, responding to a growing sense of geopolitical competition, President John F. Kennedy declared a national goal of a human lunar landing by the end of the decade. It was widely assumed that the USSR had accepted America’s challenge and that the “Race to the Moon” was on. A series of activities in Earth orbit conducted by both nations soon followed, filling that decade with new space accomplishments, which included extravehicular activities (spacewalks), the rendezvous and docking of two orbital spacecraft, long-duration flights (up to two weeks), flights to extremely high altitudes in the hundreds of kilometers, and the mastery of complex orbital changes. All of these techniques would be needed for a human mission to the Moon.
Meanwhile, the United States launched a series of robotic spacecraft to examine and scout the Moon. These missions probed its surface, landed softly on it, examined the soil, took high-resolution images of its surface features, and prepared the way for future human missions. The Ranger (impactors), Surveyor (soft landers), and Lunar Orbiter series gave us a first-order understanding of lunar surface features, processes and history.7 Scientists and engineers learned that the surface was dusty, yet strong enough to support the weight of a lander and astronauts. Craters covered every square millimeter of its surface, ranging in size from microscopic to enormous basins spanning thousands of kilometers. The landscape of the far side of the Moon turned out to be very different from its near side, with a near-absence of the dark, smooth maria that cover much of the Earth-facing hemisphere. Many unusual landforms of non-impact origin were found in the maria, strongly suggesting its origin as volcanic lava flows. Assuming that most craters were formed by impact, their density and distribution suggested that the Moon was an ancient world. Its surface told a story of having being exposed to space for many millions to billions of years.
The results of the Apollo missions, along with 380 kg (842 pounds) of rock and soil samples returned to Earth, largely confirmed and extended these inferences.8 We found that the Moon is made up of some of the same rock-forming minerals widely found on Earth and that it formed almost 4.6 billion years ago, about the same time as Earth. The samples suggested that the early Moon had been nearly completely molten, covered by an “ocean” of liquid rock. After this magma solidified at 4.3 billion years, a barrage of asteroids and comets bombarded the Moon’s surface for the next 400 million years, mixing-up the crust and creating a rough, heavily cratered surface. A final cataclysmic series of large impacts about 3.9 billion years ago formed the youngest basins, including the large, prominent Imbrium basin on the near side. The low areas of impact basins slowly filled with volcanic lava over the next 800 million years. For most of the last couple of billion years, the Moon has been largely inactive, with only the occasional large-body impact punctuating the slow and steady “rain” of micrometeorites that continue to grinds the surface into a fine powder.
This brief sketch of the history and evolution of the Moon describes a more complex planetary body than had been imagined before the Space Age. The Moon’s scarred, ancient surface records not only its own history, but also that of impacts in the Earth-Moon system as well. Because the Moon has no atmosphere or global magnetic field, the dust grains of the lunar surface also record the particle output of the Sun for the last three billion years. With the Moon as a “witness plate” to events in this part of the universe, this geologic time capsule remains virtually untouched, waiting to be recovered and read. Although we found that the Moon is depleted in volatile elements compared to Earth, we have only explored the lunar surface with people at six sites, all relatively close to the equator and on the near side. One cannot help but wonder what possible surprises await us at the regions near the poles or on the far side.
Most people are familiar with the political and pop-culture effects the Space Race had on the world, but they are not as well versed on the profound scientific impact of the Apollo missions. For the first time, we had collected samples from another world, taken from sites of known location and geological context. We took what we learned from these physical samples and coupled it with the global data gained from the robotic precursors. Added to this knowledge was information attained from regional areas through remote sensing. Combining
all of these data allowed us to reconstruct the story of the Moon with a high degree of fidelity. The most important discovery of the Apollo studies was recognition of the critical importance of the process of impact on the history and evolution of the solar system. From an elusive and questionable idea in the pre–Space Age era, the collision of solid objects became recognized as the dominant, fundamental process in planetary formation and evolution. Because we had learned to recognize the physical and chemical effects of hypervelocity impact through the study of the lunar samples, we soon recognized that large body impacts had occurred on Earth in the distant past. In particular, the extinction of the dinosaurs 65 million years ago was recognized to have happened simultaneously with the impact of an asteroid 10 kilometers in diameter. This observation, suggesting that impacts might cause mass extinctions of life, was soon extended to other extinction events evident in Earth’s fossil record.9 Some scientists now think that mass extinctions caused by impact may be one of the principal drivers of biological evolution. Thus, because we went to the Moon more than forty years ago, we now understand something very profound about the history of life on our home planet—an understanding that holds clues about our past and poses some sobering implications for our future.
The Moon as an Enabling Asset
For all of its impressive scientific and technical accomplishments, the Apollo program left many space advocates wanting. Because it was primarily driven by geopolitical conflict and designed to demonstrate our technical superiority, once Apollo had achieved its objective of “landing a man on the Moon and returning him safely to Earth,” as President Kennedy’s proclamation put it, there was no longer any reason to continue returning to the Moon or to go beyond into the solar system. Thus, the program held within itself the seeds of its own demise. The rates of expenditure acceptable during the Apollo program were simply not politically feasible for any follow-on space program.10 So the decision was made to make an attempt to lower the cost of spaceflight via a reusable space shuttle. While this effort did not succeed in lowering costs, the development of the shuttle led to some significant and unique capabilities. More importantly, it pointed the way toward an alternative architectural template for spaceflight, one in which small pieces, incrementally launched and then assembled in space and operated as a large system of systems, might multiply spaceflight capabilities carried out over a longer, more sustainable period of time. This template of operations reached its acme with the completion of the International Space Station (ISS).
As for missions to the Moon, there was only silence and isolation. Several attempts to fly an unmanned orbital mission to obtain additional global remote sensing data (which would permit better interpretation of the superb Apollo sample database) were unsuccessful. With the focus of the human program centered on the space shuttle and the subsequent building of a space station in low Earth orbit, little interest in additional lunar exploration was evident. Then, in the mid-1980s, a confluence of events occurred to focus attention once again on the Moon, an interest that continues to the present. First came the realization that after the building of the space station, an orbital transfer vehicle designed to reach high orbits, such as geosynchronous (~36,000 km or 22,000 miles high), was the obvious next step. A vehicle that can reach geosynchronous Earth orbit (GEO) can also reach the Moon. Thus, a series of studies focused on the possibility of lunar return, with an emphasis on longer, more permanent stays on the surface.
Figure 1.2. Orbital geometry of the Earth and Moon. The Earth-Moon system orbits the Sun within the plane of the ecliptic. The Moon’s orbital plane is inclined 5.1° from the ecliptic, and the Moon’s spin axis is tilted 6.7°. This results in a nearly perpendicular orientation of the Moon’s spin axis to the ecliptic (called obliquity) of 1.6°. This is in contrast the Earth’s obliquity of 23.4°.
The idea that we might want to remain on the Moon for longer periods of time inevitably led to the concept of obtaining some supplies locally, from the materials and energy found and available on the Moon. This concept, called in situ resource utilization (ISRU), is an essential skill for humans to master if we are to be significantly and permanently present in space and on other worlds.11 That realization led to a renewed interest in getting additional lunar data—most especially, data for the unique local environment found at the Moon’s polar regions. Because the spin axis of the Moon is nearly perpendicular to the ecliptic plane (figure 1.2), the Sun is always on the horizon at the poles. Some areas are in permanent darkness and hence, very cold. It was recognized that these “cold traps” might contain deposits of ice, along with other volatile substances deposited over geological time as water-bearing comets and asteroids collided with the Moon’s surface. Additionally, other areas near the poles might be bathed in permanent sunlight. This near-continuous energy source allows for the generation of electrical power during the long, two-week lunar night. At the time, we did not know the details of these hypothesized properties or even if they actually existed. However, over the past twenty years, a number of lunar robotic missions have revolutionized our knowledge of the Moon, and in particular the environment and deposits of the poles.
In 1994, the Department of Defense Clementine mission mapped the mineralogy and topography of the entire Moon from orbit. An improvised experiment on this flight used the spacecraft transmitter as a radio source to illuminate dark areas within craters near the poles. Analysis of radio echoes from the south pole suggested the presence of water ice in the crater Shackleton. This discovery was confirmed a few years later by the Lunar Prospector spacecraft, which found enhanced amounts of hydrogen at both poles. These discoveries stunned the lunar science community, since earlier results from the study of the Apollo samples had suggested that the Moon was bone-dry and always had been. Now, that concept—and our understanding of the Moon and its history—had to be reevaluated. Over the next few years, additional results from sample studies, remote sensing, and theoretical modeling culminated in the unequivocal detection of water vapor and ice during the impact of the LCROSS spacecraft, thus demonstrating beyond any doubt that significant deposits of water ice are present at both lunar poles. Conservative estimates of the amount of water ice run between several hundred million to more than a billion tons at each pole. Additionally, we have found that small areas near both poles are illuminated by the Sun for extended periods of time, some for more than nine-tenths of the year. All of this new lunar data has countries around the world planning ways to access the energy and resource bonanza at the poles of the Moon, available to those who arrive first.
Materials and energy are available on the Moon, two critical requirements for extended human presence. Water, in its decomposed form of hydrogen and oxygen, not only supports human life but is also the most powerful chemical rocket propellant known. Near-permanent solar energy is available proximate to the water-rich cold traps at the poles. The previously misleading image of the Moon as a barren, useless wilderness (as painted by Apollo results) has given way to a richer, more inviting, useful persona. The world now knows that the Moon is not simply another destination in space—but that it is an important enabling asset for spaceflight. Our current understanding of the Moon is vastly different from those early humans who first gazed up, grateful that they had the Moon to mark their calendars and chart the seasons. We now understand that the Moon is a world in its own right, an object located in our cosmic backyard whose resources we can access and use to travel throughout the solar system.
Our Future on Luna
Space engineer and visionary Kraftt Ehricke once said, “If God had intended man to be a space faring species, He would have given him a Moon.”12 This tongue-in-cheek statement is even more applicable today than when Ehricke first said it more than thirty years ago.
Why is the Moon a destination for humanity? Because it can be used to open up the frontier of space through the development of its material and energy resources. By harvesting the water ice and solar power available at the poles of the Moon, we
create the ability for long-term human presence on the Moon and in near-Earth space. Water can fuel a permanent, reusable space transportation system that can access not only the lunar surface but also every other point between Earth and Moon. This zone, called cislunar space, is where 95 percent of our satellite assets reside. The ability to reach these places with people and machines will allow us to build space systems of extraordinary power and capability. Moreover, such a system can also take us to the planets beyond Earth and its Moon.13
We can use the Moon to learn how to live and work effectively and productively on another world. This goal requires us to learn how to build protective shelters, safe from the thermal and radiation extremes of deep space. To provision ourselves, we must learn how to extract our supplies from local resources, including life support consumables, and learn how to build infrastructure using local resources for construction materials. Once established on the lunar surface, we will use these new capabilities to explore our nearest neighbor in space as well as to build a “transcontinental railroad” in cislunar space and establish a permanent beachhead off Earth. On the Moon, we will learn how to explore a planet using the optimum combination of people and robots, each doing the tasks at which they uniquely excel. Finally, we will reveal and decipher the record of planetary and solar system evolution recorded in the rocks of the Moon. Some mysteries uncovered by the Apollo explorations revolutionized Earth science. Additional exploration will reveal even more startling secrets and continue to revolutionize our understanding of the world and universe around us.
The Value of the Moon Page 2
