A type 4 asteroid.
In planetary classification, a type 4 asteroid was a type stellar body deemed significant enough by Starfleet to be included in a routine geological survey, during the late-23rd century.
In 2270, the USS Enterprise conducted such a survey near the Romulan Neutral Zone. Just as they were preparing for approach on the final asteroid, they were ambushed by three Romulan battle cruisers that were hidden behind the asteroid. (TAS: "The Practical Joker")