They flew to the Moon, descended to within 14 km of the surface, and were under strict orders NOT to land — to this day, their lunar module drifts alone in solar orbit.
Apollo 10 launched on May 18, 1969, as the final full dress rehearsal before the Moon landing. The crew — Thomas Stafford, John Young, and Gene Cernan — tested every single system and procedure, with one exception: they were not allowed to touch down.
The lunar module was nicknamed Snoopy, and the command module Charlie Brown, after the Peanuts comic strip characters. Charles Schulz personally approved the use of his beloved characters for the historic mission.
Stafford and Cernan flew Snoopy to within 14.4 kilometers of the lunar surface — close enough to see individual boulders — before firing the engine to climb back up to rendezvous with Young in Charlie Brown. Some have since joked NASA was afraid the temptation to land might be too great.
During the critical ascent stage separation, the lunar module suddenly began spinning wildly. An abort guidance switch had been thrown to the wrong position, causing the spacecraft to gyrate out of control. Stafford wrestled it back within eight seconds, averting disaster.
On the return trip, the crew set a speed record for any crewed vehicle that still stands: 39,897 km/h (24,791 mph) relative to Earth's surface. No human has ever traveled faster.
The ascent stage of Snoopy was jettisoned into space after the mission and never recovered. It is believed to be orbiting the Sun somewhere in the inner solar system — making it one of the very few lunar modules to survive intact.