45-years ago on Sunday (20th July 1969), in fuzzy black and white television images across the globe, Apollo 11 astronauts Neil Armstrong and Buzz Aldrin were witnessed by millions of people becoming the first humans to set foot on a world beyond our own.
The significance of that momentous instant in human history is well documented. But what of the science? What were some of the important contributions to science made by the Apollo programme, and what have we learned about our nearest cosmic neighbour in the time since then?
The geological history of all worlds is governed by how they evolve over time. Relatively speaking, the Earth is a very active body, constantly reworking it’s surface through plate tectonics, volcanism, biology, and the weather. With the exception of infrequent meteorite impacts, the Moon has undergone no major geological re-working for most of the last 3 billion years and it has only the most tenuous of atmospheres. The Moon has therefore preserved in its rocks and surface formations a record of conditions in the early solar system which have largely been erased on Earth. Coupled with the fact that it is our nearest cosmic neighbour, and readily accessible with current technology, the Moon thus provides us with a planet sized preserve of the early solar system and, hence, sheds light on our own origins.
One of the principle objectives of the Apollo missions was to return samples of lunar material to Earth for chemical and isotopic analysis. In the years since Apollo, numerous analyses of these samples demonstrate that the composition of lunar rock is very similar to that of Earth. These results supported the now predominant view in lunar science that the Moon formed due to a giant impact between a Mars sized object and the Earth some 4.5 billion years ago, in the early solar system. The impact blasted a cloud of rocky debris into orbit about the Earth. Over time this cloud of debris coalesced under gravity to form the Moon. The “giant impact hypothesis” was first proposed in the 1940s, but was only settled upon as a result of the samples returned by Apollo.
Today the Moon is a largely silent world, with a total surface area equivalent to the continent of Africa, and indeed is sometimes referred to as the eighth continent. From the early 1950s until the late 1970s there was a flurry of explorative activity, including Apollo, and a whole series of American and, at the time, Soviet robotic orbiters and landers. The last of these was the Soviet robot Luna 24 in 1976. Then for the next 20 years there was something of a dearth of interest. Beginning in the mid 1990s and continuing today, however, is a resurgence of lunar exploration with multiple robotic missions, including the first from Europe, China, and India, as well as more from the USA. One of the most tantalizing discoveries of this more recent era is evidence for the existence of frozen water in impact craters close to the Lunar South Pole, which are never exposed to direct sunlight. This, of course, raises the future possibility of a more sustainable human presence on the Moon.
One further intriguing possibility is the future use of the Moon as a source of a form of Helium, called Helium-3. Helium-3 is a Helium atom with two protons, and a single neutron. Helium-3 is present in a continuous outflow of material from the Sun, called the Solar Wind, to which the lunar surface has been directly exposed for most of its geological history. Helium-3, rare in nature on Earth, is an important element in any future human economy powered by commercial nuclear fusion and has already attracted limited interest from a number of governments.
How humanity chooses to view the Moon, whether as a place for exploration, exploitation, or both, remains to be seen. In studying the development of aeronautical technology in the 20th century, the anecdote is sometimes offered that humanity went from the Wright brothers first powered flight to landing on the Moon in a single generation. The time between the final Apollo mission lifting off from the lunar surface for its return flight to Earth and now is just 20 years shy of the time between the Wright brothers and Armstrong. The Apollo astronauts were both the first and the last people to walk upon the Moon. Perhaps the 45th anniversary of humankinds first foray to another world should serve as a reminder that the Moon still awaits a more thorough exploration.
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