Horizon is the code name of a Strategic weapons system developed by the Tau'ri.
The system consists of a large, bullet-shaped delivery module containing ten missiles, six with Mark IX tactical warheads and four decoys. The device can be carried in the bomb bay of a 304 like the Apollo. Deployed from orbit, the module's three rocket engines propel it into the atmosphere; these rockets shut down and the missiles themselves are released about twelve seconds prior to impact, finding individual targets and detonating simultaneously. The total time from launch to detonation is about one minute.
So far, the system has been used once in combat conditions, by the Apollo in a pre-emptive strike against the Asurans in the Pegasus Galaxy, who were building Aurora-class battleships. The system performed without a hitch, destroying all of the primary and most of the secondary targets identified for the mission. The resulting six multi-gigaton Naquadria-enhanced explosions vaporized an entire continent on Asuras making the Horizon the most powerful weapon ever to be safely deployed by the Tau'ri. (SGA: "First Strike")