Or picture President Thomas Jefferson sending a party to scout passage to the Pacific Ocean in 1803, then saying, don’t touch a thing, especially not the ocean — because Lewis and Clark are scheduled to do that the following year.

It seems unfathomable, to go all that way, to take all of those risks and then pull back, not grabbing the brass ring and reaping the rewards. In a sense, though, those were the instructions, and that was the burden, borne by the relatively unheralded crew of Apollo 10 fifty years ago this month.

Spurred by President John F. Kennedy’s 1961 speech challenging the nation to “commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the Earth,” NASA went on an 8-year lunar sprint. This bold endeavor would employ close to a half million engineers, technicians, scientists and others both in government and industry. It also cost the lives of three heroic astronauts — Gus Grissom, Ed White, and Roger Chaffee — who perished in the 1967 Apollo 1 fire.

Successive Apollo flights had to become both safer and more daring at the same time to meet Kennedy’s deadline. Delays in the completion of the lunar lander, also known as the Lunar Excursion Module, meant that Apollo 8 would be the first crewed lunar mission to fly the command module only, from which 1968’s famed “Earthrise” photo was taken. It fell to the crew of Apollo 9 in March 1969, to fly the first test mission of the lander into space, spending 10 days in Earth orbit.

The stage was set, then for a full dress rehearsal by the next crew to the launchpad. Apollo 10’s officers had all earned astronaut wings during Project Gemini, NASA’s precursor to Apollo. Their mission aboard was simple: Practice and work out the kinks and set the stage for a successful landing on the moon (and safe return to Earth).