Free Astronomy Magazine May-June 2019

29 MAY-JUNE 2019 ASTRONAUTICS lighted backup sites, in case the launch had been postponed by sev- eral days. The launch window for a particular site opened only once a month and it was thought that it was better to visit a secondary site a few days later than to wait a month for the optimal site to reap- pear. This required that the pri- mary candidate site was in the eastern hemisphere. The landing time was to be right after local dawn on the site, as the Sun had to be placed very low on the horizon to project a shadow sufficient to indicate the topography of the sur- face. Since the rotation of the Moon is synchronized with its or- bital period around the Earth, it ro- tates once a month and the Sun crosses the lunar sky at a speed of 12 degrees in 24 hours, requiring the backup sites to be spaced apart 12 degrees in longitude so that the lighting was correct for each day of delay in the launch. On the other hand, the main site should not have been too far to the east, as this would not have allowed enough time, after crossing the limb, to check the navigation before start- ing the powered descent. Second, the landing site had to be in a narrow band within 5 degrees of latitude from the lunar equator. A site at higher latitude would have involved a trajectory with greater propellant consumption, and the propellant economy was a priority for the first landing. In addition, not only should all considered sites be flat to minimize the need for maneuvering to avoid obstacles in the final phase of the descent, but the approaching ground needed to be level so as not to complicate the task of the landing radar. These safety constraints limited the first landing to one of the eastern maria on the equator, establishing the main landing site in Mare Tran- quillitatis or Mare Fecunditatis, the meridian, JPL sent Surveyor 5 to sample a sea in the eastern hemi- sphere. The probe landed in Mare Tranquillitatis, about 22 km away from the A3 site that had passed the Lunar Orbiter inspection and that had been shortlisted for the first Apollo landing. Surveyor 5 also landed in a crater, this time on a slope of 20 degrees. Instead of a shovel, it carried an instrument to study the chemical composition of the regolith. After taking a single reading, the probe activated its thrusters to “jump” a little lower down the slope to sample a second patch of regolith. The results indi- cated calcium, silicon, oxygen, alu- minum and magnesium. This im- plied the existence of basalt, but the high ratio of iron-to-titanium suggested that it was slightly differ- ent from its terrestrial counterpart. Surveyor 6 was instead sent to Sinus Medii (among Mare Insu- larum and Mare Vaporum) to re- place the science of its lost prede- cessors and landed without issue on November 10, 1967. The results of the chemical analysis indicated an iron-rich basalt. As we have seen, the goal of the Lunar Orbiter program was to pin- point possible landing sites for Apollo. Since there was not enough film to search for other sites, tech- nicians focused on those that seemed suitable based on tele- scopic studies. Possible sites for the first lunar landing were studied by the Apollo Site Selection Board for over two years. The thirty original candidate sites located on the near hemisphere, less than 45 degrees from the meridian and 5 degrees from the equator, were reduced to three by operational factors. First, the flight dynamics research team insisted that the site should be located east of the lunar merid- ian in order to allow more space to the west for one or two sufficiently