Not a lot. But in space, even a small mistake can have huge consequences.
Both astronauts will be tethered to the space station, meaning they can't go drifting off into the void like Sandra Bullock. Both of them will be wearing 'Simplified Aid for EVA Rescue' suits - giant backpacks with small rocket thrusters that will give them the ability to "fly" back to the station should they become untethered.
"The worst possible example is if one of the crew members is completely incapacitated and so he has to be bought back inside", said ESA astronaut Luca Parmitano - who has previously conducted space walks.
Both astronauts have trained for this eventuality in swimming pools back on Earth. Fingers crossed they don't have to call on this training today.