If you get a message back about "Excellent job. No new assignments at this time. Continue with your mission or return to base at your option." or something along those lines, that's it. You can try again in a couple of hours usually, when the envelope becomes "whole" again, and you are on the surface. Any assignment is based upon how much fuel you have left, how much ammo, how much "experience" you crew has, and how close (distance) you are from any other valid missions that you haven't already been assigned. If you're not close enough to something else, you won't get another. You can go find some "targets" and call in a ContactReport, and maybe get assigned that, but not always there either. Kind of a "roll of the dice", along with location and other factors...
