Canceled Apollo missions Schirra's crew would fly the C mission, first manned CSM (Block II CSM-101, retrofitted with the cabin safety improvements) as Apollo 7 in October 1968. McDivitt's crew and mission were kept as the first manned development LM flight (D mission); this was planned to be Apollo 8 in December 1968, now using a single Saturn V launch vehicle instead of two separate Saturn IB launches. The E mission was planned as an elliptical medium Earth orbit test of the operational LM with the CSM in a simulated lunar mission to an apogee of , to be commanded by Frank Borman
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