Fuel cell
From Halopedia, the Halo wiki
The Fuel Cell is a grey circle located behind the left wing on the Ghost in Halo 2 and 3. It provides the engines of the Ghost with it's exploding fuel. Worth noting is that an explosion big enough to destroy the ghost can be trigged when hitting it with a single bullet from such weapons that are able to cause a Headshot.
File:Halo 3 ghost angle.jpg
The fuel cell located above the left wing.
