Two months before Apollo 11, astronauts aboard Apollo 10 undocked their lunar module and descended to within roughly 15.6 kilometers of the lunar surface, testing every procedure for landing short of actually touching down. The mission proved the hardware and techniques that would let Neil Armstrong and Buzz Aldrin land safely that July.
Apollo 10 was NASA's dress rehearsal in every sense. Commander Thomas Stafford and lunar module pilot Eugene Cernan separated the lunar module—nicknamed 'Snoopy'—from the command module 'Charlie Brown' and flew it down into a low orbit skimming the lunar surface, testing the descent and ascent engines, the rendezvous radar, and communications exactly as the actual landing crew would need to.
At the point of closest approach on May 22, 1969, Stafford and Cernan could see boulders and craters rushing beneath them at an altitude lower than most commercial airliners fly on Earth. NASA had deliberately left Snoopy without enough fuel to actually land, a safeguard against any temptation—officially and unofficially—to attempt an unauthorized landing.
The mission validated the entire mission profile: navigation, staging, and communication between lunar and command modules, all functioned as designed. Cernan later joked that the near-miss left him more nervous than any of his other spaceflights, precisely because they came so tantalizingly close without being allowed to finish the job. Two months later, Apollo 11 would complete what Apollo 10 had so carefully prepared.
Key people: Thomas Stafford, Eugene Cernan, John Young