Selenology: Snapping the Hidden Face

As every sky-gazer knows, only one face of the moon is visible. Terrestrial gravity has locked onto the moon’s near side, which always faces the earth. Some scientists have theorized that the hidden side bore many more craters and pockmarks than the visible face. That concept was first shaken by Lunar Orbiter 4, which mapped some 60% of the far side. Last week, Lunar Orbiter 5 knocked the notion completely into a cocked hat while completing the map.

What Lunar Orbiter 5’s strikingly clear wide-angle and telephoto pictures show, according to NASA scientists, is that the hidden-side scars do not differ markedly in number from those on the earth-oriented face. They only seem fresher and more numerous because the far side has not undergone the vast, more recent flooding of dark, possibly volcanic, material so evident on the near side. The disparity should prove a boon to scientists, since the result is that the moon’s early history is that much more legible on the hidden side.

By the time Lunar Orbiter 5 finishes snapping its 426 pictures this week, space experts should have even more to crow about. The last of the Orbiters is taking detailed pictures of lunar features such as the Aristarchus crater, a thermally anomalous “hot spot” that has long provoked scientific interest and speculation. Lunar Orbiter 5 has also sent back photographs indicating a likely future landing site near a permanently shaded area in a region adjacent to the Lunar North Pole. “In such shaded areas,” says Harold Masursky, of the U.S. Geological Survey at Menlo Park, Calif., “we can find out what the present escape heat is from the center of the moon, whether there is radioactive material, and whether it was hot in the past and is still cooling off.”