Lunas face

When casually viewed from Earth, the Moon exhibits a mottling of dark grey patches against a lighter grey landscape. We now know that the light grey areas are rough and heavily cratered, forming an extremely ancient highland terrain that goes back to the earliest era of the solar system, beyond 4 billion years ago. The dark patches, called mare (pronounced 'maa-ray', plural maria) are great plains of basalt that solidified from immense effusions of lava that flowed early in the Moon's history. In many cases, these outpourings of molten rock filled large circular basins that had been excavated some time previously by cataclysmic impacts. Peppered across its face are bright sprays of material that emanate from some of the craters. These are rays of ejecta - shocked and pulverised rock thrown out by more recent high-speed collisions by somewhat smaller bodies. Given immense time, these rays will fade and darken to match their surroundings.

Down the centuries and all across the planet, peoples have continued to portray the Moon as a deity. The Greeks associated it with their goddess, Selene, while the Romans worshipped Luna. Despite this deification of the Moon, one Greek philosopher, Hip-parchus, was able to determine its distance and size by clever interpretation of naked-eye observations. It would be nearly 1,500 years before Europeans learned to stand on the shoulders of Hipparchus by applying scientific principles to the gaining of knowledge.

Four hundred years ago, Galileo Galilei acquired an early version of the telescope and turned it towards the Moon and its markings. His written descriptions and drawings reveal that he saw it not as a perfect celestial body that merely reflected the imperfect landscape of Earth, as some of his contemporaries believed, but as a world in its own right, with plains, highland areas and ranges of mountains. Although he, like others, called the dark areas 'seas', his perception was sufficiently developed to suggest that they were just as likely to be dry plains.

As the telescope and its use increased in sophistication, a series of maps were drawn by ever more capable selenographers, notably by Giovanni Riccioli who instituted the scheme of nomenclature that is used today and has gradually evolved

Galileo's 1610 drawings of the Moon, the first depiction of the rugged lunar landscape.

to name most of the large features that can be seen from Earth. The finest maps of the pre-photographic age were drawn by two German cartographers, Wilhelm Beer and Johann Madler. Later photography naturally became the staple medium of lunar research and good atlases were produced, showing the near side with oblique lighting that displayed lunar topography well.

The question that most intrigued lunar scientists concerned the origin of craters -circular landforms that appeared ubiquitous on the Moon and whose sizes ranged from many hundreds of kilometres down to the limits of detection. Craters could be found occasionally on Earth, although their size seemed to be limited to a few kilometres at most, and all were associated with volcanoes. Many tried to bend the volcano hypothesis to explain the origin of lunar craters but it was a geologist, Grove Karl Gilbert, who postulated accurately that the major process forming the lunar landscape was impact, sometimes on an utterly cataclysmic scale. Volcanism did occur and was responsible for laying down the vast mare plains; however, there are no volcanic peaks on the Moon that would rival a Mount Fuji, Mount Kilimanjaro, Mount Vesuvius or Mount Etna. Instead lunar volcanism produced low mounds with small craters at their summits.

Gilbert's work was not fully acknowledged by the lunar science community for decades, but this difficulty in accepting the new occurs in science far more often than many people realise. Too many scientists were wedded to their imaginings of massive volcanic events to grasp how time and a rocky rain from space could form such consistent structures. They reasoned that if craters came from falling rocks, they should arrive from many angles and produce elongated craters. Lunar craters were notable by their circularity.

Then, at the turn of the 1960s, Eugene Shoemaker carried out an elegant study of the great crater Copernicus which had been exquisitely photographed during the testing of the 2.54-metre telescope at Mount Wilson to provide the best imagery of the Moon available before the age of spacecraft. His investigation finally drove home the importance of impact as the prime sculptor of the Moon's face. The study was coupled with findings from ballistic trials that demonstrated how extremely violent explosions that resulted from cosmic impacts would produce circular craters for all but the most steeply angled impacts. Related studies identified the manner in which impacts shock rock, thereby providing a tool by which sites of terrestrial impacts could be identified. One significant product of this knowledge was the dawning realisation that impact is still reworking not only the surface of the Moon, but also the surface of Earth.

As the space age developed and Cold War politics aimed America to the Moon, lunar scientists found themselves with an undreamt-of opportunity to extend their discipline which, up to that point, had accomplished about as much as could be achieved with blurry Earth-bound photographs. NASA was aware that justifying its existence only on Kennedy's political whim was bureaucratically dangerous, so it turned to science as a valid, long-term reason for flying to the Moon. Although the primary driver for Apollo was international prestige and technical supremacy, science would give the crew of the first mission something useful to do once the United States' flag had been planted, and then go on to become the rationale for the missions that followed.

UNMANNED PROBES

The Ranger spacecraft.

The Soviet Union continued a habit of achieving space firsts when they took the first images of the Moon's far side in 1959, which showed a dearth of mare landscape.

They added to their tally by soft-landing a probe in early 1966 and returned the first picture from the surface. However, it was the American unmanned missions that gained prominence in acquiring high-grade knowledge prior to the Apollo programme. In the process, NASA learned how to design reliable spacecraft for the lunar environment and how to operate them from a distance.

If a manned landing was to be attempted, as Kennedy had directed in 1961, it was vitally important that the engineers designing the lunar module were aware of the kind of surface to expect. The best Earth-based images at the time could show features no smaller than about a kilometre across, and were hardly suitable for finding rocks and slopes that could topple a lander.

NASA initiated the Ranger project to take their first close look at the lunar surface. In its final form, Ranger was

I hroe spaeoeraf^^ in the series ^^^ eventually met with success when Ranger 9's target, the crater Alphonsus. One Ranger 7 impacted on a patch of frame from the descent imagery.

mare west of the centre of the Moon's visible face. This was previously an unnamed area between the massive Oceanus Procellarum (Ocean of Storms) and Mare Nubium (Sea of Clouds), but the scientific community renamed it as Mare Cognitum (the Known Sea), in view of our new knowledge of its surface. Ranger 8 was targeted at another smooth area in the southern stretches of Mare Tranquillitatis that planners believed might offer a good site for a future manned landing.

The final probe in the series, Ranger 9, was given over to the scientists who programmed it to dive into Alphonsus, a large, distinctive crater near the centre of the Moon's disk. They were particularly interested in a number of unusual dark patches within the crater, which appeared to be the result of volcanism, but it was also of interest to commercial TV networks who broadcast the spectacular live images streaming down from the spacecraft, allowing the public to watch the suicidal dive in real time. The final frames from these probes showed surface features as small as half a metre across and, to the relief of the lunar module designers, showed large rocks sitting on the soil. If the surface could support rocks, it would surely support a LM.

As Ranger's dive to the lunar surface could yield only limited coverage, Apollo's planners wanted to make a close inspection of the equatorial near side for possible landing sites and to aid spaceflight navigation by the sighting of landmarks. Meanwhile scientists wanted to gather imagery from across the entire

Oblique view over the crater, Copernicus, from Lunar Orbiter 2.
The Lunar Orbiter spacecraft.

Moon in order to improve their understanding of its complexities. The unimaginatively named 'Lunar Orbiter' series fulfilled both of these roles over a very successful year of operations. This was a much more sophisticated probe. It went into a controlled elliptical orbit around the Moon that had its perilune over the near-side equatorial zone. It photographed the lunar surface with two cameras, one of which could capture surface details as fine as a metre across. Its imaging system used film-based photographic technology that was chemically processed on board and later scanned and transmitted to Earth. All five probes in the series were successful, although the first suffered operational problems that limited its usefulness. By the time Lunar Orbiter 5 was intentionally crashed to clear the way for Apollo, nearly the entire lunar surface had been mapped, with much of the nearside equatorial zone imaged at high resolution.

Running concurrently with Lunar Orbiter were the last of NASA's pre-Apollo probes, the Surveyors. Their prime mission was to prove that a Moon landing could be made using a leg technology similar to that being planned for the LM. Seven missions were launched, of which five were successful. Most were sent to characterise the surface near prospective Apollo landing sites along the equator. The final mission was given over to scientists and sent well south to land in the highlands on the ejecta blanket of Tycho, one of the Moon's most prominent craters.

Though these missions gave NASA a solid overview of the Moon's topography, surface strength and texture - at least in support of a manned landing - a deeper understanding of the Moon's composition and history had to await the results of the Apollo manned missions. Neither the Ranger nor the Lunar Orbiter probes went further than imaging the Moon in optical wavelengths, and no data was obtained that allowed the composition of the lunar soil to be studied. The last three Surveyor craft carried small experiments to study the composition of the soil, showing that, based on three sites, the maria soils were basaltlike and richer in iron and titanium, while the highland soil near Tycho was richer in aluminium and calcium. These results hinted at the bigger picture that would be deduced in the light of the torrent of data that would flow from Apollo.

APOLLO REACHES THE MOON

By the time the Apollo missions arrived at the Moon, scientists knew that the Moon was a rock-strewn, battered world where very little happened. By day, sunlight

The Surveyor 3 spacecraft as photographed by the Apollo 12 astronauts.

blasted its surface, unfiltered by any kind of atmosphere, until it was hotter than any landscape on Earth. By night, whatever heat the surface held was quickly radiated out into space, chilling the landscape colder than the depths of the Antarctic. They had surmised and confirmed that the surface was basically a rubble layer called the regolith that had built up over aeons by the incessant pounding of incoming hypervelocity meteoroids, from sub-microscopic dust to mountain-sized rocks and comets. They were sure that volcanism on a large scale had created the maria but didn't know whether it had also occurred in the highland regions. They had a few theories, all largely unsupported by hard data, to account for the Moon's existence and why Earth deserved such a large satellite in comparison to its size.

"Apollo 8, Houston. What does the ole Moon look like from 60 miles?" Capcom Gerry Carr could not suppress his desire to ask the obvious question when the crew of Apollo 8 came around from behind the Moon on their first pass in orbit. CMP Jim Lovell had dreamed of this day from childhood, and took the opportunity to reply. It seems unsurprising now, but what he saw was very similar to the view anyone can see through a telescope, only from a much closer perspective. "Okay, Houston. The Moon is essentially grey, no colour; looks like plaster-of-paris or sort of a greyish beach sand.''

Bill Anders later spoke of his impressions of the Moon's far side. "The back-side looks like a sand pile my kids have been playing in for a long time. It's all beat up, no definition. Just a lot of bumps and holes.'' He was not telling the scientists anything they didn't know from the pictures sent by Lunar Orbiter.

Apollo 8 added little to our understanding of the Moon, as would be expected of a short, pioneering reconnaissance mission. Its role during the 20 hours it spent in lunar orbit was to give its crew and the mission control team some experience of operating a manned spacecraft in the lunar environment. While they were there, they could also inspect two possible landing sites on the southern plains of Mare Tranquillitatis. Planners were keen for the crew to study them visually from orbit and to inform future crews of what to expect, given that they had arranged for these sites to have the same early morning illumination that the landing missions would expect. The Apollo 10 crew likewise concentrated on operational matters as they rehearsed the steps that would lead to a landing. Both flights had Hasselblad cameras and took many 6 x 6-centimetre photographs of selected swathes of the lunar surface, and while they repeated much of the work already achieved by Lunar Orbiter, they had the great advantage of having the film brought home for processing. It was not until Apollo 11 that a crewman was finally left alone to look at the lunar landscape while his colleagues explored the surface.

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