The class-A probe was a type of probe that was carried aboard Federation starships in the late 24th century. They were used in studying gas giant planets and the birth of stars.
In 2369, while trapped inside a holodeck program without realizing this, Captain Jean-Luc Picard ordered the launch of four class-A probes to study the collision of two gas giants in the Detrian system and the formation of a new star from the impact. The launch command was canceled by Moriarty, who took command of the computer command functions of the simulated USS Enterprise-D in the program. (TNG: "Ship in a Bottle")