Free Astronomy Magazine May-June 2019

34 MAY-JUNE 2019 ASTRONAUTICS considered by some as volcanic mouths along a geological fault. After making a precise landing with Apollo 12, the flight dynamics team felt confident enough to reduce the target ellipse and allow the next mission to aim for a smaller site on more rugged terrain. Since an inclined orbit is more wasteful than an equatorial one, the thresholds of the propellant had been increased to escape the confinement of the equatorial zone. In addition, the need for a backup site had been eliminated. From that moment, the delay in the launch of a maximum of three days would have been remedied by landing with a less favorable Sun angle. This relaxation of the con- straints, however, did not “open” the Moon, because the high lati- tude sites were still out of reach. Still, these relaxations offered an appreciated degree of flexibility. It was the narrowing of the target el- lipse and the rejection of the re- quirement that the landing site should be free of reliefs that al- lowed for the consideration of more interesting sites. Some geologists proposed to land inside a large crater like Hipparcus or Censorinus, but the preferences were for the territory to the north of Fra Mauro crater. In 1962, E. M. Cordonnier and R. J. Hackman pub- lished a stratigraphic study of Mare Imbrium. In extending this map, R. E. Eggleton decided to indicate the peripheral wavy ground as Im- brium ejecta and named it the Fra Mauro Formation. Although it was a single unit, it was divided into iso- lated areas on the periphery of the basin. At the time, the understanding of the first lunar history was based on how the Imbrium ejecta had spread over thousands of kilometers, carv- ing furrows. Dating Imbrium was the most important item on the A POLLO 14 − Alan Shepard on the lunar surface from the lunar module looking NW. [NASA, Project Apollo Archive]