UNMANNED PROBES

In 1959, the Soviet Union continued a habit of achieving space firsts when they took the first images of the Moon’s far side. Though grainy and of low resolution, these were sufficient to show a dearth of mare landscape. They added to their tally when they softlanded 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 of the Moon 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. For a manned landing to be attempted, as Kennedy had directed in 1961, it was essential that the engineers designing the lunar module be aware of the nature of its surface. The best Earth-based images at the time showed 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 a simple probe; little more than a platfonn for slow scan television cameras that imaged the Moon as it fell at cosmic speed to its doom. Initi­ally, the series had little success because either the launch vehicle or the probe itself would fail before the target was reached.

NASA were on a steep learning curve about how unforgiving the practice of operating rockets and

spacecraft could be, but these The Ranger spacecraft. (NASA)
failures taught them about the need for extremely high reliability in the design and construction of spacecraft and their launch vehicles. Three space­craft in the series eventually met with success, beginning with Ranger 7’s impact on a patch of mare west of the centre of the Moon’s visible face. In view of our new knowledge of its surface, this previously unnamed area between the vast Oceanus Procellarum (Ocean of Storms) and Mare Nubium (Sea of Clouds) was renamed Mare Cognitum (Known Sea). Ranger 8 was targeted at another smooth area in the southern stretches of Mare Tranquilli – tatis, just east of the meridian, 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. This mission was also of interest to commercial TV networks, which broadcast the spectacular live images streaming back from the spacecraft so that the public could watch its 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, the presence of large rocks sitting on the soil strongly suggested that the surface would be able to support a LM.

Ranger’s plummet to the lunar surface could yield only limited coverage. Apollo’s planners wanted to survey large areas of the equatorial near side for possible landing sites and for landmarks to aid navigation. They also wanted high-resolution images of a number of sites to certify them for human visits. Meanwhile, scientists wanted imagery from across the entire Moon in order to improve their understanding of its complexities. The unimaginatively named ‘Lunar Orbiter’ series were able to achieve both of these tasks in a single 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 surface with two cameras, one of which could capture surface details as fine as a metre across. Its imaging system chemically processed photographic film on board and later scanned and transmitted it 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, almost the entire surface had been mapped, with much of the near-side equatorial zone imaged at high resolution.

Forty years later, a team of engineers acquired one of the old tape machines that had archived the recordings from the Lunar Orbiter transmissions and they nursed it back to life. Once they could replay the archive tapes and digitise their imagery, an almost impossible feat in the mid-1960s, they were able to coax pictures out of them that hugely exceeded the originals. These can be compared to images taken in recent years to detect changes in the lunar surface across the decades.

Running concurrently with Lunar Orbiter were the last of NASA’s pre-Apollo probes, the Surveyors. Their primary task was to prove that a 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, who sent the spacecraft 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 in support of a manned landing, a deeper understanding of its composition and history had to await the results of the Apollo missions. Neither the Rangers nor the Lunar Orbiters went further than imaging the Moon in optical wavelengths. No spectral data, not even simple colour was obtained and therefore the composition of the lunar soil could not be studied. The last three Surveyors did carry small experiments to study the composition of the soil. These showed that, based on three sites, the maria soils were basalt-like and were richer in iron and titanium, while the highland soil near Tycho was richer in aluminium and calcium. This hinted at the bigger picture that would not become clear until lunar material returned by astronauts was analysed in terrestrial laboratories.