Halo: Combat Evolved question?

Why did Master Chief and the other marines land on Halo in the first place? I remember the first level your fighting on a ship but did the covenant blow it up, I remember that you get into some little escape pod and someone asks "what is that", and then the lady says "I dont know but were landing on it" I just dont remember why they left, the only thing I can think of is the ship would blow up. So what was the name of the ship and why did they leave it