Free Astronomy Magazine July-August 2019

17 ASTRONAUTICS It was the prospect of a large crater on the flank of an Apennine massif, capable of producing a sample of the ancient crust, or a rock rolled from the summit and proving to be ejected from Serenitatis, that made the Apennine the main geological purpose of Apollo 15. Once this mission ended, and with the results of four Apollo lunar landings in hand, geologists were confident in their understanding of the 500 million years since the for- mation of Mare Imbrium, and they could not wait to send a new mis- sion to a mountain site. Previously, the flight dynamics team had been reluctant to search for a high site, as the approach line would have been rough and the available space would have been insufficient for a large landing ellipse. But after Apollo 15 flew over a mountain range to land on an adjacent plain, the ‘highlands’ did not seem so in- timidating. Propellant considera- tions limited the choice of the site to the central highlands.