You have to protect the Norad II, which has crashed in a little crater pretty much by itself. You can't just fly Raynor and two drop ships in (your "mission objective") because the crater is surrounded by enemy forces. You have to eat your way through them.

You're on a time limit because your Norad protectors can't survive for very long. First, repair anything that's hurt and surround your own base with bunkers. They do a lot of hit-and-run attacks. Next, build up a wraith/vulture combo force. Eat your way through the forces across the middle. Once you get into the middle of the screen, switch to all vultures and tanks and get all the missles. Wrap around into the "crater" and you're done! Don't even bother going further north than you have to, and make your transports follow that exact same "safe" path.

StarCraft Walkthough Index