Success over the challenge. There's a table there which says "Critical Failure, Failure, Success, Critical Success." It's the third one in that list. You can succeed at the challenge by moving at slow pace (at the cost of time and Supply, of course). If you're in a hurry, that's obviously not an option available to you.This may be my lack of knowledge about exploration challenges, but Slow pace says its counts as a success, but how I don't see anywhere describing how many success are needed.