Apollo 10 was a May 1969 mission which acted as a practice run for the Apollo 11 mission. The mission was set up in the same way as Apollo 11 would be two months later, with a crew of two in a lunar module descending near to the surface of the moon while the main capsule remained in orbit. As such, it was the first fully manned Apollo mission to orbit the moon and the second manned orbit of the moon after Apollo 8. NASA refers to the mission as "an actual crewed landing, except [for] the landing."
The mission was manned by John Young, Thomas Stafford, and Gene Cernan, with the latter two commanding the lunar module while the former stayed in the command module. All three of them would return in later Apollo missions despite some issues with NASA management which highlighted the politics of the program.
NASA was worried that the astronauts would attempt to land on the moon, and, as such, they decided to short fuel the lunar module. The module had approximately ten percent less fuel than the Apollo 11 mission, just enough to complete the mission. Had they attempted to land, they would be left unable to return to the command module. The astronauts arrived within 8.4 kilometers of the moon and collected important information regarding the moon's gravitational field, allowing for calibration of the Apollo 11 landing system.
This mission was part of an ongoing collaboration between Charles Schultz and NASA. After trying to rebuild their public image in the wake of the Apollo 1 disaster, NASA chose Snoopy as their safety mascot. Apollo 10's lunar module was named Snoopy, and its command module Charlie Brown.