Apollo 9 to land on the Moon; when the crews for Apollo 8 and 9 were swapped, their backup crews were also swapped. Since the rule of thumb was for backup crews to fly as prime crew three missions later, this put Neil Armstrong's crew (Borman's backup) in position for the first landing mission Apollo 11 instead of Pete Conrad's crew, who made the second landing on Apollo 12. Apollo 9 was the first space test of the complete Apollo spacecraft, including the third critical piece of Apollo hardware besides the command and service module and the Saturn V launch vehicle—the lunar
Journey of Apollo 15 to the Moon fuel on the LM, allowing it to land with more equipment and consumables on board. A major issue for all lunar orbiting spacecraft are mass concentrations or "mascons". These are regional areas where denser material is under the surface. Apollo 15 travelled over regions that had never been covered on previous flights, so it was unknown how its orbit would be affected by them. Mission Control predicted that by the time the crew awoke the next "morning", the lowest point of their orbit, perilune, would have dropped to about 8.6 nm (16.1 km) but this turned out to be an