This is an Atlas Agena D rocket (officially designated the Atlas SLV-3). It is also sometimes referred to as an Atlas TLV (Target Launch Vehicle). It is an Atlas first stage mated to an Agena D second stage, and was used to launch the [unmanned] target vehicle for the later Gemini missions.

The Agena D was a modified version, and was fitted with a docking collar to allow the manned Gemini spacecraft to dock after rendezvous. This was called a Gemini Agena Target Vehicle (or GATV).

The first Atlas Agena D carrying a GATV was GATV 6, launched on October 25, 1965. It exploded minutes after launch. The next GATV launch was for Gemini VIII, and was successful. The GATV launched for Gemini IX failed to achieve orbit, but all others (Gemini X, XI, and XII) were successful.