The missions before the lunar landing were designed to test out some of the most difficult manoeuvres of the Apollo Program. During Apollo 10, the Lunar Module experienced a malfunction that left the astronauts spinning out of control.
The Lunar Module, or LEM would be the crucial bridge to the moon. The LEM was the vehicle that would take two astronauts from the Command-Service Module to the lunar surface and back again. In 1969, the Apollo 9 crew would be the first to operate this spacecraft in orbit and following a successful mission, Apollo 10 launched just two months later. Building off of Apollo 9’s momentum, this new mission would encompass all aspects of a crewed lunar landing, except the landing part.
The crew consisted of veterans Thomas Stafford, John Young and Eugene Cernan. On the mission’s fourth day, Stafford and Cernan entered the lunar module, undocked it from the command module, and began their descent. The pair flew their LEM closer to the Moon’s surface than ever before, gaining insight into the lunar gravitational field and scouting a possible landing site for Apollo 11. However, just twelve kilometers from the moon’s surface, the LEM began to spin out of control. The spacecraft was nearing gimbal lock - which essentially meant that the guidance system could no longer trust its computer. According to a NASA engineer, the result is similar to when a car spins out on an icy road. Fortunately, the astronauts were able to quickly regain control, but if they hadn’t, the spacecraft would have lost its navigation system and could have slammed into the moon.
Despite that near disastrous moment, the mission was relatively smooth. Most importantly the Lunar Module did its job, meaning the United States was finally ready to put a man on the Moon.