The Yrrillian battle cruiser is originally where the lifepod / lifecraft Aquila came from. The Yrrillians themselves were from a planet called Deneb, with the ships they made been 'built to last' - as shown by Aquila's ability to blast through solid objects without even a scratch. From what Aquila's computer tells Geoff and Tom, the battle cruiser that Aquila came from was apparently destroyed in the 'Fourth Yrrillian War' about 6000 years ago, with Aquila (a lifecraft/survival pod) been jettisoned from it before its apparent destruction and been driven to Earth by one of the crew.