Iris

Fitting of its name, an iris covers Earth's Stargate during its operation at the S.G.C. Originally constructed of pure titanium, the iris is composed of more than 20 interlocking plates that hide away within the housing of the Stargate's inner track. The plates can swiftly eject, sliding into place at a single command from the base's computer system or manual override.

The purpose of the iris is to keep unwanted intruders, be it a person or object, from entering the S.G.C. Once deployed, the iris rests a few micrometers in front of the event horizon, preventing matter from reintegrating as it arrives through the wormhole. Objects are destroyed, and living beings are killed instantly upon "impacting" on the iris. Known substances capable of passing through the gate despite the iris include radio waves and accelerated particle beams (capable of moving at near-light velocities). The iris has only been breached by Tollan technology, capable of passing through any solid matter.

During an encounter with the gravity well of a black hole, the original iris was torn from its housing and lost through the Stargate. After the accident, a new trinium-titanium composite replacement was installed. It is capable of withstanding extreme temperatures.

When an incoming wormhole arrives at the S.G.C., the iris is immediately deployed, and can only been opened when a deactivation code signal (I.D.C., or "iris deactivation code") arrives via radio transmission from the G.D.O. of an SG team returning to Earth. (Some allies, including Bra'tac and the Tok'ra, have also been given I.D.C. transmitters.) Sometimes automatically, but usually manually, the iris is opened -- retracting to allow admittance to Earth. The base computer is equipped with a hand reader for opening the iris, so that the command may only be executed by authorized personnel.

KEY EPISODES

Children of the Gods - The first iris is installed and immediately put to work in the S.G.C. just after the first mission to Chulak.
The Enemy Within - Having only recently installed the iris, members of the newly commissioned S.G.C. have reservations as to how long the barrier will hold out against an onslaught of incoming Goa'uld attacks.
Bane - The iris is closed to prevent a swarm of large, flying insects from entering the S.G.C.
A Matter of Time - The original iris is destroyed during a deadly confrontation with a black hole, and later replaced with a stronger, trinium-enhanced iris.
Show and Tell - After a Re'tu uses the base computer to open the iris, a palm scanner is installed into the S.G.C. control room to prevent unauthorized personnel (or non-palmed aliens) from opening or closing the iris.
Pretense - The Tollan Narim and his cat, Shroedinger, penetrate the iris with their technology to deliver a message to the S.G.C.
Shades of Grey - The Tollan once again get past the iris, this time to retrieve their stolen technology.
The Other Side - Colonel O'Neill orders the iris closed, deliberately killing Alar, a genocidal alien leader who tries to follow them back to Earth.
Prophecy - Jonas Quinn foresees a future event in which Jaffa forces use a G.D.O. code to get past the iris and destroy the base. Fortunately, this scene does not come to pass.
Full Circle - A massive explosion that wipes out Abydos follows SG-1 through the wormhole back to Earth, but is barely cut off when the iris closes.
Lost City, Part 2 - Anubis attempts to send a powerful Goa'uld bomb through Earth's Stargate, only to have it stopped by the iris.