Free Astronomy Magazine May-June 2019

24 MAY-JUNE 2019 ASTRONAUTICS mediately of interest to Apollo se- lectors, were of scientific interest. In addition to its targets, Lunar Or- biter 2 had taken high-resolution images of the best sites of its prede- cessor. With all the candidate sites documented, the US Geological Survey produced terrain maps for Apollo designers. In addition to photographing the most promising sites from different angles to facilitate stereoscopic stud- ies for detailed topographic analy- sis, the next mission, the one of Lunar Orbiter 3, mapped approach routes. After achieving the project’s goal using the first three space- crafts, NASA left the remaining ve- hicles to scientists, who decided to fly them into near-polar orbits at higher altitudes for more general mapping. In just one year, Lunar Orbiters not only fulfilled their purpose of probing sites likely to be Apollo’s first landing locations, but they also returned the first clear views of the opposite hemisphere, signif- icantly advanced knowledge of the regional geology of the near hemi- sphere, and identified more fea- tures of sites that could have been visited during subsequent missions. The Lunar Orbiter program had been a remarkable success. Even after they had finished shooting, the spacecrafts gave a glimpse of the lunar interior. Although Lunar Orbiter 1 was deorbited before the arrival of its successor, it was noted that its orbit had been disturbed, which meant that the Moon’s grav- itational field was inhomogeneous. To investigate the matter further, the subsequent spacecrafts were not deorbited until their attitude- control propellant was almost ex- hausted. By exploiting vehicles in both equatorial and polar orbits, it was possible to map the gravita- tional field in sufficient detail to reveal that the seas flooded by lava A POLLO 12 − Astronaut walking on the Moon and col- lecting surface samples. [NASA, Project Apollo Archive]