Slayton was the Director of Flight Crew Operations and effectively chose the crews for the flights. He did not intend to give astronauts two lunar landing commands but, according to historian Michael Cassutt, as late as the summer of 1969—when 10 landings were still scheduled—Slayton planned to give Lunar Module Pilots Fred Haise, Edgar Mitchell, and James Irwin the opportunity to walk again on the Moon as Commanders. During the early Apollo missions he used a rotation system of assigning a crew as backup and then, three missions later, as the prime crew; however, by the later Apollo flights, this system was used less frequently as astronauts left the program, Slayton wanted to give rookies a chance, and astronauts did not want to take backup positions that no longer could lead to prime-crew spots.