Free Astronomy Magazine May-June 2019

30 MAY-JUNE 2019 ASTRONAUTICS backup site on the meridian and the reserves in the western hemi- sphere. Mare Fecunditatis was too far to the east to provide a com- fortable margin for the final navi- gation update, leaving Apollo 11 assigned to Mare Tranquillitatis, where two useful sites were lo- cated. The trajectory of Apollo 8 (launched on December 21, 1968, when the Surveyor and Lunar Or- biter programs were almost a year past completion) was programmed to visualize the easternmost site, ALS-1, under ideal lighting. Apollo 10, on the other hand, performed a flyby of ALS-2 and reported it was generally appropriate, even though the furthest part of the landing el- lipse was approximate. A number of factors determined the launch windows for a lunar landing mission. These considera- tions included the lighting condi- tions at the time of launch, the launch pad azimuth, the geometry of translunar injection, the eleva- tion angle of the Sun at the lunar landing site, the number and loca- tion of lunar landing sites, and the lighting conditions during the re- turn to Earth. The time of a lunar landing was determined by the po- sition of the site and the acceptable range of elevation angles of the Sun, which spanned from 5 to 14 degrees in an east-west direction. Under these conditions, the visible shadows of the craters would have helped the crew to recognize the topographic features. The number of launch opportuni- ties from Earth for a given lunar month was equal to the number of candidate landing sites. The launch time was mainly determined by the allowed variation in launchpad az- imuth and by the position of the Moon at the arrival of the space- craft. The spacecraft had to be launched on an orbital plane con- taining the position of the Moon and its antipode at the arrival of the spacecraft. A launch pad az- imuth variation of 34 degrees al- lowed a launch period of four and a half hours. This period was called the “daily launch window”, the timeframe in which the launch di- rection was within the required range to intercept the Moon. Every day, two launch windows were open: one was available for translu- nar injection out of Earth’s orbit near the Pacific Ocean; the other was near the Atlantic Ocean. The opportunity of injection over the Pacific Ocean was privileged because it generally allowed a day- time launch. Apollo 11 demonstrated the lunar module’s ability to land on the Moon, but the fact that it was moved away from the established site was embarrassing. The flight technicians developed a simple method to correct the disturbances of the mascons and were certain that it would have worked to re- duce the size of the target ellipse. In addition, they decided to reduce the requirement from two backup sites to one. There were five main sites in the shortlist for the first landing. The ALS-1 and ALS-2 sites, located east of Mare Tranquillitatis, were sup- ported by ALS-3 in Sinus Medii, leaving ALS-4 and ALS-5 in Oceanus Procellarum as reserves in case of a prolonged launch delay. It would have been natural to send Apollo 12 to one of these sites, but the conservative constraints imposed during the first landing had led to the selection of open sites, and ge- ologists were eager to sample the ejecta from a large-enough crater. Indeed, even prior to the Apollo 11 flight, landing site selectors estab- lished a list of craters for this even- tuality. In principle, it had to be a simple matter of reviewing the sites rejected for the first landing A POLLO 12 − ALSEP deployment. [NASA, Project Apollo Archive]