Complex sentence: Development rapidly increased in the 1970s, when the Norwegian authorities became actively engaged in Svalbard politics; their aim was that Longyearbyen should become a family community, like other towns in Norway.
Split into simpler sentences: Development rapidly increased in the 1970s, when the Norwegian authorities became actively engaged in Svalbard politics. Their aim was that Longyearbyen should become a family community, like any other town in Norway.