From The Stargate Omnipedia

Small planet on the outskirts of the Milky Way Galaxy. A science team from Stargate Command was dispatched to this planet to set up and transmit a subspace message to the Earth ship Daedalus, traveling in-between the Pegasus Galaxy and the Milky Way.


Critical Mass - Per Dr. Lee's suggestion, General Landry sends a team to P4M-399 to make contact with the Daedalus before a bomb explodes in Atlantis.