Pause the game with the 'SPACEBAR' and press full stop. A requester will appear, now type "GZAIMASEN" and you should hear a sound letting you know it worked. Once the cheat is in effect you are invicible and if you call the requester and type "GIMMESHIPx" (where x is the mission A-Z). This only works before you enter the sector with the spacetruck.