That's the Agena Target Vehicle. Generally speaking the Agena was a rocket upper stage that was used for satellites and such, during Project Gemini it was launched on an Atlas rocket to practice rendezvous and docking with manned Gemini capsules.

Gemini 8 first docked with an Agena Target Vehicle, flown by a couple of rookie astronauts named David Scott and Neil Armstrong. Shortly after the docking, as in, a minute or two later, the mated pair of ships began to inexplicably roll to the right. Thinking it was a malfunction of the Agena, they shut it down, but the roll kept increasing. It was caused by a stuck control thruster on the Gemini capsule. The roll got so violent the astronauts vision started narrowing. Armstrong flew the ship back into stable flight by hand looking at the attitude indicator. They called an abort and landed safely in the Pacific ocean. They'd planned to land in the Atlantic, making this to this day the farthest a manned NASA mission has missed its intended landing target.

David Scott flew as Command Module Pilot of Apollo 9, the LEO test flight of a lunar module, becoming one of three men to perform the first docking of two manned spacecraft. And became the seventh human to set foot on the moon as commander of Apollo 15. 15 was the first mission to carry a lunar rover; which Scott set the extraterrestrial land speed record that still stands today: 12 miles per hour. It was Scott that collected the sample of lunar anorthosite dubbed the Genesis Rock. He spent over 22 days in space, He is currently 93 years old.

Neil Armstrong had a much shorter space career than Scott, spending a mere 8 days in space. Armstrong would fly only one other mission, commanding a little milk run called Apollo 11, during which he became the first human to set foot on a celestial body other than the Earth. If you're a big enough space nut you might have even seen the footage. Niel Armstrong passed away in 2012 at the age of 82.

It's odd to think, for longer than my entire life, only 24 men had seen the far side of the moon with their own eyes, including Armstrong and Scott. This morning that number increased to 27 men, 1 woman.