Reroute ADS power 2/3
Get back to the main atrium, saving the game on your way there. After you leave the elevator a group of bipedal necromorphs will attack you. After dealing with them go around the elevator and enter the second one, going to the Level 01.
There are a few containers and a Save station here. After getting through the door next to the station you will step into vacuum. Don't worry, the distance here is not that long, and you can visit a nearby Store. There are also some lockers nearby.
The mining administration room contains a package of necromorphs, which you shouldn't have problem dealing with. There's a Store to the left, but what should really interest you is the MA Computer room (to the right from the entrance) which contains a Text log, along with some other items. And within the storage area you can find a locker with a Power Conduit. Get back to the main room and go further inside. Avoid the unstable parts of the floor and eliminate the appearing necromorphs. At the very end of the room there's a terminal which is our main interest - after using it get back to the elevator.
Reroute ADS Power 3/3
Level three contains a few lockers, Force gun schematics, and a Save station. Enter the room, kill the enemies and activate the terminal. The door on the left hand side will lead you to your next objective.
Destroy the Asteroids
Go up the elevator and head for the large round gate leading into space. Duck behind the first cover, let the meteors pass by and recharge the oxygen, then make a run for the next cover.
Hide behind a second cover and let the wave of meteors pass you by. Keeping in mind that we're in zero-g you should aim for the station on your right, refill your oxygen and hide behind it.
Now just continue jumping from cover to cover until you reach the last one. Refill your oxygen, jump towards the large gate and enter.
Behind the door awaits a Save station, a crate, and a huge cannon. Approach the console, press E and start shooting at them asteroids. You'll need to last a bit over a minute. There's a hull integrity information screen to your right and you cannot allow it to reach 0. Also, you need to be sure not to overheat the cannon, so don't keep the LMB and RMB buttons pressed for too long.
Go to the tram station
All that needs to be done now is to get back to the tram. The spacewalk part will be a lot easier now, as there are no more asteroids to dodge, but some necromorphs will appear instead. You shouldn't have problems reaching the atrium (take the elevator to level 02). On your way there you will encounter some bizarre-looking heads with legs. When you reach the atrium head to the tram station and into the tram.