No rank without credits, no credits without safely bringing the data home. Captain's Log is an external application (see another thread on this page) that will, if you tell it what you've scanned, give you an approximate value. The credits you get will depend on first discoveries though - 50% more on some systems can skew the total quite a lot - and that goes to the first explorer to get the data back and sell it so an estimate from out in the black will be missing an unknown number of those.