Surface investigations

As the mechanical properties of the lunar surface would influence the design of the LM the Apollo planners said, in October 1962, that the development of JPL’s ‘soft landing’ spacecraft should receive a higher priority than the orbital spacecraft. However, the development of the Centaur stage was so protracted that the surface investigations could not start until 1966. The planners for these Surveyor missions were faced with the same dilemma as their Apollo counterparts: where should they send their first mission? Although safety issues obliged them to select one of the dark plains, this was consistent with characterising the surface in the equatorial zone in which the Apollo targets were located. When Surveyor 1 was launched on 30 May 1966, the ‘old hands’ at JPL might well have wondered whether they were in for a rerun of the teething troubles that had plagued Ranger, but on 2 June the spacecraft landed safely near Flamsteed, in a crater that appeared to have been breached by the Ocean of Storms. As with Ranger, the single instrument was a television camera. Its first picture showed the spacecraft’s foot pad resting on the surface, which was barely indented. It then proceeded to take a multitude of individual frames from which a panoramic mosaic was later produced. There was a profusion of small craters and rocks, but the area was generally flat. Although the site seemed to be consistent with a flow of low viscosity lava, this was disputed. The camera continued to send panoramas to document the appearance of the surface under different illumination, and then the solar-powered spacecraft went into hibernation for the long lunar ‘night’ – and, to everyone’s surprise, not only did it awaken with the return of the Sun, it did so each ‘morning’ for the rest of the year. Having succeeded at the first attempt, the engineers were disappointed when Surveyor 2 tumbled during a course correction on its way to the Moon, and was lost. On 20 April 1967 Surveyor 3 set down in a 660-foot-diameter crater in the

Ocean of Storms, bouncing several times prior to coming to a halt. The inner wall was pocked by smaller craters, one of which had excavated large blocks of rock. This vehicle had an arm with which to determine the mechanical properties of the loose surficial material, dig trenches to reveal the subsurface, and roll rocks to determine the extent to which their state of erosion was selective. In contrast to its hardy predecessor, Surveyor 3 survived only one lunar night. Next, contact was lost with Surveyor 4 several minutes before it was scheduled to land. Having sampled two dark plains in the western hemisphere and failed twice to reach a site on the meridian, JPL dispatched Surveyor 5 to sample the Sea of Tranquility. It landed on 11 September 1967, just 14 nautical miles from the 2-P6 site on the short­list for the first Apollo landing. Instead of an arm, it had an instrument to investigate the chemical composition of the surficial material. After taking one reading, the spacecraft ‘pulsed’ its thrusters in order to ‘hop’ several feet, to sample a second patch. The results indicated calcium, silicon, oxygen, aluminium and magnesium, which implied basalt, but the high ratios of iron and titanium meant that the lunar basalt was subtly different from its terrestrial counterpart. Surveyor 6 was sent to the Meridian Bay to fill in for its lost forerunners, and landed without incident on 10 November 1967. The results of its chemical analysis indicated an iron-rich basalt. Since the dark plains across the Apollo landing zone had proved to be remarkably similar, NASA released the final spacecraft to the scientists, who decided to send it to Tycho, a bright ‘ray’ crater in the southern highlands, where it landed on 10 January 1968. By cutting margins, JPL enabled it to employ both the robotic arm and the chemical analyser – which proved fortunate because the analyser became stuck, and if it were not for the arm nudging it free the scientific study would have been undermined. In addition, rather than make the spacecraft hop so as to sample different patches of surface, the arm was used to place the instrument on a patch of excavated soil in order to check that this was the same as the material on the surface, and later to place it on top of a rock. Some researchers interpreted the elemental abundance data to mean that the lunar highlands were an alumina-rich basalt, but Eugene M. Shoemaker, head of the Astrogeology Branch of the USGS, argued that the dominant rock in the Tycho ejecta was anorthositic gabbro, which had interesting implications.2