Fail Safe

"Fail Safe" is the seventeenth episode of the fifth season of Stargate SG-1.

Plot
A civilian is viewing through his telescope and sees what looks like a giant asteroid heading towards Earth. He is "convinced" by the U.S. military to keep it quiet.

At the SGC, SG-1 and Hammond discuss their options. They have only 11 days and 16 hours to stop the asteroid otherwise all life on Earth will be wiped out. The SGC gets no response from trying to contact the Tok'ra. They have been overextended and hard to reach since the assault on Revenna. The SGC contacts the Asgard through the Hall of Wisdom on K'Tau, but learn that the Protected Planet Treaty prevents the Asgard from taking any action to avert a natural disaster.

SG-1 and an engineering team go back to Revanna to fix the downed Tel'tak Jacob/Selmak and Daniel crashed. (Last Stand) The team plans to plant a naquadah enhanced nuclear weapon on the asteroid which would detonate with the force of 1,200 megatons.

They arrive in Earth's solar system and after their engines fail they are on a collision course with the asteroid. Both SG-1 and Stargate Command realize that the ship doesn't have enough space to decelerate to land. While the SGC sees them hit the asteroids surface, the scout ship actually enters a massive canyon, and it gives them the extra distance to brake enough to put the ship down, but their communications with Earth are severed. General Hammond believes them to be dead and an evacuation on Earth commences.

Having landed on the asteroid, Sam Carter completes detailed scans of the asteroid and discovers that it's made almost entirely from naquadah. Since naquadah doesn't naturally occur in the solar system, SG-1 conclude that the Goa'uld must have launched it to make it look like a natural disaster. If the asteroid is detonated, the resulting explosion would be big enough to burn Earth's atmosphere, and boil the oceans. Due to a cave-in that occurred while they were back at the ship, the control keypad isn't functioning to stop the bomb from detonating. Jack has to open the bomb and disarm it manually by clipping 'the red wire' but upon opening the casing, he finds the wires are all yellow. He is forced to cut them at random, and after cutting four of the five finds the correct one with only a few seconds left. After considering their options, Sam manages to open a hyperspace window large enough for the asteroid to pass through, long enough for it to miss Earth entirely. Knowing beforehand that their engines will burn up and lifesupport will fail a couple of hours after that, SG-1 succumbs to their fate.

Fortunately, not long after, a Tok'ra ship appears to give them a ride home.

Notable Quotes
Following the failed attempt to enlist the Asgard to save Earth.

Hammond: And then what happened?

Jackson: Let's just say Jack made a reference to Freyr's mother.

Hammond: We'll discuss Colonel O'neill's diplomatic shortcomings later...

O'Neill is disarming the bomb, with some difficulty.

O'Neill: You know, I'd like to take this opportunity to say that this is a very poorly designed bomb and I think we should say something to somebody when we get back.

Carter: I agree Sir.

Carter: Now find the wires leading from the timer to the detonator and cut the red one.

O'Neill: Carter. They are all yellow.

Carter: Say again.

O'Neill: There are five wires, all yellow.

O'Neill: (looking out the window of a Goa'uld cargo ship, approaching the Earth) Carter....I can see my house!