This Galaxy Ain't Big Enough...

 is a Fallout 3 quest and achievement in the Mothership Zeta add-on.

Kill the Alien Captain
Once you arrive on the Bridge, your next objective is to kill the Alien Captain. The captain himself is fairly weak, but he does carry a powerful sidearm, the Captain's Sidearm. There are also several other Aliens in the room, armed with Alien Atomizers. Once he is dead, a giant Alien head will speak to you, but as always, it is impossible to understand. Once he's done talking, another Alien ship passes by yours, and attacks the Mothership with its Death Ray.

Destroy the attacking Alien Ship
Your allies (if they have survived) will come to the bridge. Sally remarks that the control room looks exactly like the one from Captain Cosmos and that she knows exactly what to do. She orders everyone to take a specific position on the bridge. Paulson, however, is asked to "guard our backs"; he remains in the area with the teleporter.

Now you must destroy the attacking Alien Ship. You can command your ship's Death Ray, and determine how the power in the ship should be dispensed;
 * Shields: Most of the ship's energy is dispensed to the shield, giving you extra protection but weakening your Death Ray.
 * Intermediate: The ship's energy is dispensed equally amongst the shields and the Death Ray. This is the initial stance.
 * Death Ray: The ship's energy goes to the Death Ray, severely weakening your shields but giving your Death Ray much more power.

After shooting a couple of times Somah remarks that the ship's generators are offline. You must reactivate them via the four buttons located in the Bridge.

Once the enemy's shields have been halved, a group of hostile Aliens will attack, coming in via the teleporter.

Once the hostile ship has been destroyed and the fireworks have ended, speak with Sally to finish the quest. You may now use the Captain's Quarters as your own home, as for the rest of the ship, majority of the sections will be inaccessible.