Original: Lightcurve data for Europa has been particularly tricky to interpret, so much so that for a long time its period of rotation was in dispute (ranging from 5 and a half hours to 11 hours), despite numerous observations.
Simplified: Lightcurve data for Europa has been very hard to interpret, so much so for a long time its period of rotation was in dispute (5 and a half, or 11 hours?)