Why does Earths Stargate have an iris?
The iris was installed in the first episode (“Children of the Gods”) as a protection against unwanted arrivals. Once it was determined that anyone on another planet could dial up Earth, and that many of the people who would want to do that were hostiles, it was decided that some form of protection was warranted. On a planet like Abydos, the answer was simply to bury the Stargate, but Earth wanted to explore — and so needed to find another way to control incoming traffic. Every member of an Earth SG team carries a G.D.O. (short for “garage door opener”), which allows the team member to transmit a numerical ID signal to Earth so that iris will be opened.
