Thirty years ago this month, Surveyor 7, America's last lunar softlanding probe, touched down upon the Moon. Its predecessors, Surveyors 1, 3, 5 and 6, all landed on relatively featureless plains near the lunar equator. Surveyor 7 set down in the southern uplands on the hilly outer flanks of the mighty impact crater Tycho, just 30 km from its northern rim. It was a spectacular setting, deliberately chosen to contrast with the locations of previous missions.
There were disappointments. Two Surveyors malfunctioned disastrously in the final moments of their flight - Surveyor 2 crashed southeast of Copernicus, and Surveyor 4 tumbled into the Sinus Medii, adding two more tiny impact craters to the Moon's pockmarked face. This was somewhat disconcerting, as the Apollo landings were only a couple of years away.
Surveyor 7 carried more scientific equipment than its forerunners, including a television camera with polarizing filters, a surface sampler, an alpha-scattering instrument to analyse surface chemistry, plus magnets on the footpads and surface scoop. "Dentists" mirrors provided views of areas beneath the spacecraft, and lunar dust deposited in various nooks and crannies around the spacecraft.Operations began shortly after landing on 10 January, and ended 80 hours after local sunset on 26 January. Activity on the second lunar day took place between 12 to 21 February. Surveyor 7 returned more than 21,000 TV pictures. A superb panoramic view of Tycho's hilly ramparts gives us a good idea of what astronauts might have seen. The foreground shows a pile of broken rocks which was probably caused by a small meteoritic impact. Of the most interesting images were those showing two Moon-ranging laser beams emanating from observatories on the Earth's night side.
To the naked eye the southern half of the Moon appears bright. Tycho can be seen as a brilliant spot just underneath the mouth of the "man-in-the-Moon". Full of impact craters, the Moon's southern uplands are spectacular to view through binoculars or telescopes. Some of the craters, like Sasserides (90 km across, just north of Tycho), are highly eroded and very ancient. Tycho, 85 km in diameter, is one of the youngest large lunar craters, formed by the impact of a small asteroid around 100 million years ago. Its floor lies nearly five kilometres beneath the clean-cut rim, and its bright central peaks rise to 1,600 metres above the floor. Tycho still looks remarkably fresh.Tycho's impressive system of rays radiates for hundreds of kilometres. In the days when the volcanic theory of lunar crater formation was taken seriously, the rays were explained as fractures in the Moon's crust, caused by irresistible internal forces, through which lighter coloured lava had welled. The rays actually consist of billions of tonnes of material excavated by the impact explosion, melted and recrystallised by the tremendous heat. Though they look thin and translucent, in places the rays are many tens of metres deep. Surveyor 7 landed in Tycho's dark collar - a 50 km wide zone that has escaped being overlain with ejecta.The best time to observe the superb topography of the Tycho / Surveyor 7 area this month will be on the evenings of 7 to 9 January (local lunar sunrise), and a fortnight later on the mornings of 20 and 21 January (local lunar sunset). Tycho's ray system becomes apparent as the Moon grows to full on 12 January.
