The Agena Target Vehicle (/əˈdʒiːnə/; ATV), also known as Gemini-Agena Target Vehicle (GATV), was an uncrewed spacecraft used by NASA during its Gemini program to develop and practice orbital space rendezvous and docking techniques, and to perform large orbital changes, in preparation for the Apollo program lunar missions.

The combined spacecraft was a 26-foot (7.92 m)-long cylinder with a diameter of 5 feet (1.52 m), placed into low Earth orbit with the Atlas-Agena launch vehicle.

[3] They would fly the combined spacecraft in a stabilized mode and perform a number of experiments: After detaching from their respective Gemini capsules, Agena target vehicles continued to execute post-mission operations under ground control.

The vehicle was eventually placed in a 220 nmi (410 km; 250 mi) circular decay orbit, facilitating its observation during the Gemini 10 mission.

While this would not be a problem for the Agena D with its single restart, the multi-restart GATV would eventually lose all of its oxidizer before the stage's operating life (which would last weeks instead of hours) could be completed.

[6] A few questions were raised about the compatibility of the ATDA with the Atlas booster, since it had much lower mass than the GATV, potentially throwing off the launch vehicle's aerodynamics and calibrated settings.

Hidden behind clouds, the Atlas's B-2 engine gimbaled hard to right starting at T+120 seconds and remained fixed in that position, flipping the launch vehicle 216° around and sending it back towards Cape Kennedy.

The Agena's engine did not activate since the proper altitude and velocity had not been attained, preventing the guidance system from sending the start command.

The loss of the lock on the ground prevented normal engine cutoff signals from being transmitted to the Atlas; BECO was generated by the staging backup accelerometer, SECO at T+273 seconds due to LOX depletion, and VECO and Agena staging from a backup command generated by the missile programmer.

However, the true cause of failure surfaced when the Air Force released film taken by tracking cameras at Melbourne Beach, Florida, which showed the Atlas pitching over and heading downward.

However, the shroud that protected the docking adapter during launch failed to separate, due to lanyards being incorrectly secured with adhesive tape.

Gemini 9A was launched on June 3, and when in orbit, the crew observed that the shroud of the ATDA had partially opened and was described by Stafford as "looking like an angry alligator".

Gemini 8's docking with the Agena featured in the short science fiction drama DARKSIDE produced by nrgpix as an entry to the London Sci Fi film festival 2020.