P4C-970 is the homeworld of the Aschen. SG-1 visited the planet in the year 2000, and formalized an alliance, becoming a part of the Aschen Confederation. Ten years later, SG-1 uncovered a sinister Aschen plot to eliminate the population of Earth. Using the stargate, SG-1 sent a message back in time to the SGC, in order to change their fate. The warning note said, "Under no circumstances go to P4C-970. (signed) Colonel Jack O'Neill." P4C-970 was locked out of the dialing computer, and the course of history was changed.
Cross Reference: Aschen, Mollem, P3A-194, Time Travel 2010
Episode Reference: 2010, 2001