P4X-351

P4X-351 is a planet in the Milky Way galaxy on which the Tau'ri outpost of Icarus Base resided. The planet was 21 light-years from Earth and had a uniquely powerful radioactive naquadria core, which is why Icarus Base was established on it.

History
In 2009, the George Hammond was sent to transport Doctor Nicholas Rush, Eli Wallace, Senator Alan Armstrong and Chloe Armstrong to Icarus Base on P4X-351 for an attempt to dial the Stargate's ninth chevron. The Stargate was not used to travel to the base because the planet contains a radioactive core made of Naquadria, which could possibly become unstable after the dialing of an incoming wormhole. This powerful core proved to be the only power source capable of dialing in the ninth chevron.

Attack
In 2009, a force of several Ha'tak motherships, Death Gliders, and troop transports attacked Icarus Base, prompting an evacuation. In the ensuing battle, Col. Telford engaged the attacking forces in an F-302. In desperation, Dr. Nicholas Rush, with the help of Eli Wallace, figured out how to dial the nine-chevron address. Using Earth's point of origin, they cracked the code, and Rush dialed the Stargate to the Ancient vessel Destiny instead of Earth. The base's personnel were able to evacuate. It is unclear how Dr. Rush was able to dial the Gate's nine symbol address, because, Earth's point of origin should not have been located on the gate. The Icarus gate had it's own point of origin, as seen when the team first attempted to dial the nine-symbol address.

When the base came under attack, the core of the planet destabilized, resulting in the complete destruction of the planet itself. The attacking forces (rumored to be the Lucian Alliance) failed to anticipate this outcome, resulting in the complete destruction of the attacking fleet. The George Hammond, which was defending the base from orbit, managed to locate all but approximately 80 of the Icarus base personnel and evacuate before the planet was destroyed. Colonel Samantha Carter reported that the remaining personnel who could not be accounted for were inside the base and could not be beamed out, however, readings indicated that the Stargate was active for six minutes before the planet's core went critical. It wasn't until Dr. Rush reported to the SGC from Destiny via long range communications, that General Jack O'Neill and others learned that these people were safe, albeit stranded, aboard the ship.

Behind the scenes
The Tau'ri designation of this planet was only referred to by name in a scene in the extended version of Air, Part 1, in which Dr. Rush brings it up in a conversation with General O'Neill.