The Normandy travels through the Omega 4 Relay after acquiring a few upgrades. No ship has ever returned and for the crew it is likely a one way trip. To stop the Collectors, Shepard has made it clear the objective to survive comes second. There may not be a second attempt to stop the Collectors from taking more human colonies for unknown experiments being conducted.
Once through, there is no going back. If you're playing ME2, after completing the Suicide Mission you can restart and change a few things on the same mission.