The asteroid passing Earth's Moon
The Naquadah asteroid was an asteroid created by the System Lord Anubis, and sent on a collision course with the planet Earth. The asteroid was approximately 137 kilometers from end to end and contained a high concentration of Naquadah. Knowing that the Tau'ri would use a Nuclear warhead to destroy the asteroid, Anubis intended the explosion to devastate the entire planet as the Naquadah would have greatly enhanced the power of the explosion.
SG-1 was first made aware of the asteroid in 2002 and later assumed that, since the asteroid popped up out of nowhere, a Goa'uld plot was in the works. The team gated offworld to the planet Revanna with a damaged Tel'tak that an SG team was in the process of repairing. They later went to the asteroid to plant a bomb. After discovering the contents of the asteroid, they deactivated the bomb and sent the asteroid though hyperspace using the Tel'tak's hyperdrive, so that it could pass and clear Earth, which it did. (SG1: "Fail Safe")
At least one civilian saw the asteroid, but was hushed by the United States Air Force. Later in 2005, Joe Spencer noticed the news article on the asteroid and thought it was the works of SG-1. (SG1: "Citizen Joe")