When you attempt to dock at the Illusive Man's station, Hackett will talk with you over vidcomm, mentioning that this attack will expose the Crucible to Reaper eyes and must be considered the first phase of the assault on the Reapers themselves. He will give you the chance to back out, which you should take seriously: this is the game warning you that you are about to hit the point of no return. In other words: got anything else you want to do first? Do it before hitting Cronos Station.