A small Starfleet sensor probe capable of warp nine, one of which is converted into a cramped one-person capsule to rush K'Ehleyr to the Enterprise-D.
A class-8 probe is an unmanned Starfleet probe about two metres long, able to travel at warp nine. It is designed to carry instruments and has no room or comfort for a passenger.
One is used as a courier vessel in The Emissary. Starfleet needs to get a special envoy to the Enterprise-D faster than any available ship could carry her, so the probe is fitted with basic life support and launched from Starbase 153. The Enterprise meets it and beams it aboard, and the crew are surprised to find a living occupant: the half-Klingon, half-human ambassador K'Ehleyr, who complains about the ride.
She has come to help deal with the T'Ong, a Klingon sleeper ship sent out decades earlier whose crew are about to wake believing that they are still at war with the Federation. Her arrival also brings her back into contact with Worf.