Plot twist: The actual reason for the problems of the LMS Explorer wasn't some stupid asteroid field*. No: in reality Chief was too stingy** to repair the reactor/ whatever system with real replacement parts and instead used random parts (for example from the canteen) before they explored space. Therefore the shield broke down, when the asteroid hit the ship and finally they landed in another galaxy. That's also the reason, why there isn't a canteen as a regular building: they were used to "repair" the ship.
*although using a broken Game Boy or a printed brick as a navigation computer isn't a very smart idea in the first place
**even duct tape was too expensive for him