An instrument pod mounted on the hull of a Constitution-class starship to take readings inside ion storms, crewed by one officer and ejected if it endangers the ship.
The ion pod is central to the episode Court Martial. During a severe ion storm, Lieutenant Commander Benjamin Finney is assigned to the pod of the Enterprise to gather data. Standing orders require the captain to jettison it once the ship goes to red alert, after giving its occupant time to get out.
Finney is apparently still inside when the pod is released. The ship's computer log shows Kirk pressing the jettison button while the ship is only at yellow alert, and he is tried at Starbase 11 for culpable negligence.
Spock finds that the computer's records have been altered, and a search using amplified heartbeats shows that Finney is alive and hiding in engineering, having faked his death to ruin the captain he blamed for his stalled career.