Space

Deadly to all, except Cyborgs and Aliens, it houses many mysteries, and many asteroids. Any part of the station exposed to space, and not behind an airlock, solid wall or secured window will quickly lose breathable air.

Generally, unless you are in a proper space suit with functioning internals, you will freeze to death in the airless, bone chilling void, unless you happen to be hit by a stray meteor, or Space Dust.

Hazards

 * It is well known that space is not a safe place. Without a source of breathable air you will quickly pass out before you can accomplish anything, so set up internals. If you see someone passed out in a de-pressurized zone you should make it your priority to drag them somewhere safer and give them some Salbutamol to treat oxygen deprivation.
 * Space is cold. REALLY FUCKING COLD. Your jumpsuit is not enough to safeguard you from this hazard and you will eventually become critical if you spend too much time outside. While freezing, you move at a snail's pace and you might not be able to get to safety in time. It might be a good idea to announce your situation over the radio and have someone come get you if you're not sure you'll make it. Leporazine is the drug of choice for keeping you warm, but coffee or tea help somewhat. Morphine is also an excellent choice for aspiring astronauts as it helps you move in bulky space suits and reduces the slowing effects of the cold.
 * Low pressure is your first sign that the air is too thin to breathe. The low pressure of space will deal brute damage if your body is exposed to it.
 * Syndie scum and other sorts of traitors have a close relationship with space. They can often be found hovering around outside the station in stolen space suits. They will shoot you if they catch you out there. While the station engineers do their best to keep the horror that is The Great Outdoors separate from the rest of the station, traitors will seek to acquaint you with this region in a variety of ways. Don't let them.

Ghetto Space Suit
Located in many maintenance tunnels throughout the station and in other places, gas masks, fire suits and extinguishers can all be found. These items are ridiculously common. You are fairly protected from the cold in this equipment but you may want to bring some Leporazine as mentioned earlier. Firesuits no longer protect you from cold, but you can still use extinguishers. To propel yourself, just aim the fire extinguisher in the opposite direction you want to go in and voila! You are a true, blue, bona fide motherfucking rocket man. Extinguishers last surprisingly long when used this way and don't cut into your air supply like a jetpack does, but the downside is that it's much less maneuverable. Still, it should be good for simple work around the station. Guns and sprayers can also be used to maneuver in space.

Being Adrift In The Void
Alright, calm down. The situation may not be as hopeless as it seems. Too many newbies have died when they could have been saved but gave in to despair. If you have any of the aforementioned items your chances of survival are greatly increased. Cyborgs may have it a lot easier because they are innately immune to cold and vacuum. Standard and engineering models even have a built in fire extinguisher for propellant, but other models can't throw anything.

Your first priority is to activate internals if they weren't already. If you know where the station is and have a fire extinguisher, use it. If you don't, try throwing some items to propel yourself back to safety. If you were thrown out an airlock by a traitor, now's the time to let people know of your situation and where it happened. Generally they will have stripped you of all your protective gear so your time will be limited, but if they left you ANYTHING to throw you may at least have your body discovered. It might be a pain in the ass to retrieve it, but security will probably want you cloned to interview you. Maximizing your suit sensors increases this possibility dramatically. Even if you don't survive, you may be avenged by blowing their cover over the radio, but this doesn't always work. Remember you can cling to the hull of the station and walk around, but you should walk so you don't slip.

If you don't have any compunctions with metagaming, note that space in SS13 is random. You won't generally be in space forever -- if you die your body will probably hit something eventually. Paranoid traitors may try to give you some spin when they space you for this reason since you're more likely to hit an asteroid or outer station instead, or they'll send you along a vector where they know you won't be coming back.

Sometimes you just don't have any idea where the main station is. This tends to happen after blinking or walking into a portal set as a trap. If you have the equipment necessary to survive in space for an extended period of time, rejoice! Just set yourself on a diagonal vector and you'll encounter some form of civilization eventually. Besides the main station, there's at least one telecomms satelite with a fixable teleporter on it and a mining station you can call a shuttle from. If you're not seeing any stations and you're getting impatient, you may try changing direction. If and when you make it back, it'll make a fine story to tell everyone at the bar, assuming the whole place hasn't gone down the shitter while you were away.

Remember, space is more or less 'randomized' in that not everything is in a set direction from one another. Unless you specifically know where you're going, chances are where you end up can be almost anywhere. Just don't stray too far from the station and this should not be a problem.