Gemini X was one of the most ambitious missions of the Gemini program, accomplishing the first dual rendezvous in the history of spaceflight. Launched on July 18, 1966, from Cape Kennedy Launch Complex 19, the mission carried Command Pilot John Young and Pilot Michael Collins into orbit. Their mission plan called for docking with their own Agena X target vehicle, using its propulsion system to change orbits, and then rendezvousing with the derelict Agena VIII left in orbit from the Gemini VIII mission four months earlier.

After reaching orbit, Young and Collins rendezvoused and docked with Agena X on the fourth orbit. They then fired the Agena’s primary propulsion system — a 16,000-pound-thrust engine — to boost themselves to an apogee of 475 miles, the highest anyone had flown at that point. The burn was a pivotal moment: it proved that the Agena’s propulsion could be used to perform significant orbital changes while docked to a Gemini spacecraft, a concept essential for future space operations.

From the higher orbit, Collins performed a standup EVA, standing in the open hatch to photograph stellar ultraviolet emissions. The crew then used the Agena X engine again to lower their orbit and rendezvous with the Agena VIII target vehicle that had been orbiting since March 1966. Collins performed a second EVA — an umbilical spacewalk — in which he jetted over to the Agena VIII and retrieved a micrometeorite experiment package from its exterior. This was the first time an astronaut had transferred from one spacecraft to another in orbit.

However, the mission was not without difficulties. Collins found maneuvering in space exhausting, and the Gemini spacecraft used more fuel than planned during the rendezvous maneuvers, cutting short some planned activities. The mission splashed down in the Atlantic Ocean on July 21, 1966, and was recovered by the USS Guadalcanal. Gemini X had demonstrated that complex multi-vehicle operations were possible in space — a capability that would be essential for the Apollo lunar missions.