User blog comment:Legionwrex/How did Saren reach the Citadel?/@comment-1440333-20130507094740/@comment-1440333-20130507191448

I imagine he passed through space with no air in the same way Shep did in ME3 - there is no evidence that while in Mass Relay Transit that the travelling body is actually exposed to space and other astrological phemonena (as suggested above it is in fact more likely they are not), ergo the corridor between the Mass Relays must be contained, similar to a wormhole or teleporter, and that therefore Saren and the Mako are in a self-contained pocket travelling with them, in this case that would be a pocket of Ilos' atmosphere.