This Galaxy Ain't Big Enough...

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

Reunite with the others
You exit the Space Walk in a airlock on the Observation Deck. Activate the switch to repressurize the airlock; you can now remove your spacesuit. In the next room is a teleporter back to the Engineering Core; activating this allows Sally and any surviving abductees to join you. Sally then crawls through a duct to power down the energy gate leading to the next part of the level.

Disable the Death Ray and make your way to the Bridge
After the door is open, head down the hallway into the room with the glass floor. Enjoy the view of Earth. After a short dialog, Sally will go to open another door. Immediately afterwards, a hologram of an Alien will appear over the glass floor, and you will learn about the Death Ray. Once Sally returns, proceed through the newly opened door. You will find a deactivated teleporter and be tasked with finding the other end and turning it on.

Upon entering the Weapons Lab, you will face Aliens, turrets, and robots. After defeating them, turn left and go down the hallway on the far side. In a room to the right, you'll find a Drone Pod with a Guardian Drone. Once you reach the T-intersection, turn right. Follow the hallway to the second floor of the Weapons Lab. Go down the stairs and through the door in the middle. Once in another room, turn left and go upstairs. Go thorough the circular door and follow the room to the left. Go into the hallway to the northeast and up the stairs into the Experimentation Lab.

Once in the Lab, veer right into another room and go down the hallway before you. Follow the hallway until you reach a large room with a force field covering a door on the other side. Activate and destroy the power generator in the middle of the room to disable the force field. Head down the hallway and enter the Biological Research Lab.

Head up the stairs on the opposite side of the room. Turn right and go across the bridge. You will find a shielded door and switch to the right. Go through the door and turn right. Walk straight across the platform to the teleporters. Note: once you use the teleporters to the Death Ray Hub, you cannot go back until after the quest is over. If you're going for Alien Archivist, make sure you have all the previous recordings. There should only be one more to get after this point if you've been doing it correctly.

Fight your way through the Death Ray Hub to Death Ray Control, where you'll face a room full of hostile aliens. Once they are defeated, press the button with the cylinder graphic above it four times, causing the four generators to rise. Repeat the same pattern to overload each one. Note that once the first generator explodes, another wave of several aliens will attack you. If you'd like, you can redirect the death ray from this room as well.

Once all four generators have been destroyed, the door across the way opens to the Living Quarters. You'll encounter portable shields here for the first time, which can be destroyed by several hits. Fight past turrets, aliens and the occasional Abomination to reach your final destination: the teleporter to the Bridge.

Kill the Alien Captain
Note: Once you enter the Bridge, you are committed to finishing This Galaxy Ain't Big Enough... and will not be able to re-enter most areas of the ship. Be sure you've finished exploring Mothership Zeta to your satisfaction before entering the Bridge.

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 weapon, the Captain's Sidearm. There are also several other Aliens in the room, armed with Alien Atomizers. Note that your allies (if any) may kill the Alien Captain without you. Once he is dead, a large holographic 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 Shields, and determine how the power in the ship should be divided between them:
 * Full Shields: Most of the ship's energy is routed to the defensive Shields, giving you extra protection but weakening your Death Ray. This is the leftmost button.
 * Intermediate: The ship's energy is divided equally between the Shields and the Death Ray. This is the initial condition.
 * Low Shields: The ship's energy goes mostly to the Death Ray, severely weakening your Shields but giving your Death Ray much more power. This is the rightmost button.
 * Fire Death Ray: This is the fourth button, to the right of the three Shield buttons.

After you've fired the Death Ray a few times, Somah remarks that the ship's generators are offline. You must reactivate them via the four buttons located on the east and west walls of 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 still-accessible parts of the ship (mostly the Bridge, Engineering Core, and Steamworks) as your own home.

After you leave the mothership, Somah and Paulson are removed from the game and you can never see them again.

Bugs

 * When trying to access the teleport (Xbox 360) to the death ray (that was the only one active out of the three, one of which is the bridge) it does nothing and when pressed a buzz noise as if it is inaccessible.
 * When entering the observation area, Sally sometimes will go to the large window in the middle of the room and stay there. Unable to talk with her, she'll stay there and won't open the door for you, making it impossible to continue.
 * Sometimes, after you enjoyed the view of Earth, Sally should go to open another door. But instead of opening the door, she'll wait you at the the teleporter behind the "key locked" door.(unconfirmed, PC)
 * Sometimes, when you are near the teleporter to get to the Death Ray Control Room/Death Ray Hub (and you haven't yet destroyed the Death Ray Generators), the game will report that you've completed all objectives and instruct you to go the bridge - which you can't unless you destroy the generators...
 * After returning to Earth, the player's Pip-Boy and sometimes even weapons may glow orange, similar to the visual effect when the player is on fire. This orange glow is exaggerated in Megaton for some reason.

Videos
Videos on the Discussion page.