Impelling

The problem of interstellar travel was so enthralling that it sometimes became an obsession even to a fairly well-developed Utopian world. This could only occur if in the constitution of that world there was something unwholesome, some secret and unfulfilled hunger impelling the beings. The race might then become travel-mad.

The extraordinary spectacle was a good many years ago, in school days, when theology is less impelling than curiosity ... […]