Young and Collins ride Agena's engine to 412 nautical miles; no human has flown higher
NASA still needed proof that astronauts could dock with one target, then hunt down a second, and work outside the capsule without running out of time or fuel. On July 18, 1966, John Young and Michael Collins launched from Cape Kennedy aboard Gemini 10, docking with an Agena target vehicle just hours later that same day and using its engine to reach 412.4 nautical miles, the highest altitude any human had flown. Collins made two spacewalks during the roughly three-day mission, retrieving a micrometeorite experiment from a leftover Agena still in orbit from Gemini 8's aborted rendezvous. The crew splashed down July 21, recovered by USS Guadalcanal just three miles off target.
Gemini 10 closed out NASA's rendezvous and EVA checklist before Apollo could risk a lunar module docking 240,000 miles from home. Collins's two spacewalks and the record altitude run gave Houston engineers the propellant and thruster data that shaped Apollo's own high-stakes dockings, turning a NASA program built on fixing Gemini 8's near-disaster into the final rehearsal for the moon.
