The Kelvan Expeditionary Return Probes were a trio of probes built and launched in the 23rd century with the purpose of crossing the Galactic Void between the Milky Way and Andromeda galaxies. The probes were sent by the Federation to convey a message to the Kelvan Empire offering worlds suitable for Kelvan colonization.
The probes were large and bulky in shape, powered by conventional matter and antimatter reserves, the probes operated reverse engineered Kelvan warp drive designs and had an approximate mass and size to a 23rd century Constitution-class starship. Despite their advanced engines, the probes were not expected to reach the Andromeda Galaxy for 200 years.
In 2375, the USS Monitor was thrown out of the Milky Way galaxy during a transwarp test, it found itself 350,000 light years outside the galactic halo and directly between the two galaxies. The Monitor discovered the remains of one of the expeditionary probes which had been pulverised - no debris remained larger then the size of the average human fist. Captain John Lewinski was determined to use another probe to cannibalize the advanced warp technology to replace the failed Borg transwarp coil that was used in the ship. (TOS novel: Captain's Blood)
In 2378, the Monitor discovered a second probe, however, at the same time, it also discovered the Totality on approach to the galaxy. Abandoning the plans to use the second probe to return to the Galaxy, the Monitor used technology from the probe and it's own power systems to send a hyper-subspace signal to Starfleet Command, warning them of the incoming arrival of the Totality. The resulting message destroyed the second probe. (TOS novel: Captain's Glory)