In The Descent Orbit

The descent orbit of the final four missions was a particularly exciting affair as the spacecraft gently descended from its 110-kilometre high point over the far side to skim across the mountain tops on the near side with a clearance of barely 15 kilometres. The northerly paths taken by Apollos 15 and 17 over the near side were especially notable for the spectacular ride they offered the crews. Descending from their apolune, these spacecraft passed over Mare Crisium then Mare Serenitatis. On Apollo 15, the smooth basalt plain of Mare Serenitatis was already lit by the morning Sun and the mountains on its western shore rose like a wall coming towards them as they gently descended across the expanse of the mare, which was so large that its curvature was readily apparent. Capcom Karl Henize, who must have been imagining the approaching mountain range, jokingly enquired about their safety. "Fifteen, does it look like you are going to clear the mountain range ahead?" Irwin replied, "Karl, we've all got our eyes closed. We're pulling our feet up.'' "Open your eyes. That's like going to the Grand Canyon and not looking.'' This range also formed the eastern margin of the great Mare Imbrium. It was within an embayment seated among these peaks that Dave Scott and Jim Irwin would eventually land.

On Apollo 17, Jack Schmitt found his calling as a teller of stories of the Moon. There was no place in this geologist's mind for gushing wonderment at the stark beauty of Luna's ancient surface. No. As soon as the spacecraft had emerged from behind the Moon after LOI and he had completed his report on the SPS propellant utilisation, he started to bend the ear of Capcom Gordon Fullerton with a running commentary of the terrain below, breaking off at one point to remark, "One little minor problem, Gordy, is that we're breathing so hard that the windows are fogging up on the inside for a change.''

It was little wonder. The only trained scientist to reach the Moon was going to give a master class in observational geology, but coming over Mare Crisium he was just getting warmed up. "Oh, boy, there is Picard [Crater] - or Peirce, one of the two. Okay, Gordy, all those dark and light albedo changes around Picard and Peirce are not obvious at this particular angle yet. There's some hint of them.'' "Roger," confirmed Fullerton.

Schmitt stuttered on as the TV camera broadcast the view to Earth. "The rim - Is there one farther south of Peirce? Which - is it far - Is the one farthest - Picard, yes. Picard, I think, is the one I'm looking at. Yes, it is. Yes, and I can see Peirce now just behind the rendezvous radar. And, yes, way out there, you ought to start seeing them.''

Jack Schmitt had been trained by NASA to fly jets as part of being an astronaut. However, he simply did not think like a pilot, for pilots are trained to stay off the radio unless there is something operationally important to say, and this was the case for most crews. However, Schmitt's natural tendency, honed by years of scientific observation, was to describe. And this he did in spadefuls. Even during their first near-side pass, as they passed over the night-time side of the Moon, he was making use of the cool, dim Earthlight illuminating the landscape below. "I've got a visual

The Apennine Bench Formation at the southeast margin of Mare Imbrium.

on Eratosthenes and Copernicus. They are obviously different-age craters in this light. You can see the ray patterns in Copernicus moderately well. You can even tell that they do cross Eratosthenes. Stadius shows up as a very clear dark area to the southwest of Eratosthenes.''

Later in the flight, he had an opportunity to observe one of the Moon's most distinctive craters, Archimedes, located in the middle of Mare Imbrium. Archimedes is important to lunar geology because it is part of a series of lunar features that allowed geologists like Schmitt to apply the principle of superposition to construct a stratigraphic history of the region. The crater is flooded with the lavas that also filled the Imbrium Basin so it is older than the era of lava outpouring. To its south and southeast is a light patch called the Apennine Bench Formation, the Apennines being the mountain range that forms the eastern rim of the Imbrium Basin, but which he referred to simply as the Imbrium Bench, that predates the crater because we can see damage from Archimedes across its surface. Finally, the bench seems to be a sheet of a different kind of lava that formed soon after the creation of the Imbrium Basin itself. Schmitt told all this to Capcom Gordon Fullerton.

''This is one of the first opportunities that I've had to look closely at Archimedes, which is one of those craters that, in the early days of the lunar mapping programme, helped to establish some of the fundamental age relationships between the various units that were visible in the Earth-based photography." History lesson over, he began his description: "In this particular case, it related to the sequence of events that created Imbrium, cratered it, and then flooded it with mare. And Archimedes is a completely closed circle as a crater, and it is filled with mare. And it, in itself, is superimposed on one of the main benches of the Imbrium crater. Now, to have mare filling that crater and actually filling all the depressions of approximately the same level in the vicinity of a large mare region, it's one of the things that's suggested to many people that rather than single sources for mare lavas, you have a multitude of sources in a very fractured lunar crust. The ultimate source in depth, though, is still certainly a subject for controversy. Some of the ridge and valley structure of the Archimedes impact blanket is not covered by mare and extends to the southeast out onto the Imbrium Bench. That was also one of the pieces of evidence used in those early days of photogeologic mapping of the Moon. You'll have to excuse the reminiscing, Gordy.''

On and on he went, before and after his visit to the surface, providing lunar scientists with a journal of geological observations to stand for all time as the sights of the first scientist to visit the Moon.

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