Complex sentence: The Norwegian spiral anomaly of 2009 was a phenomenon that appeared in the night sky over Norway on 9 December 2009, probably caused by the failed flight of a Russian RSM-56 Bulava missile.
Split into simpler sentences: The Norwegian spiral anomaly of 2009 was a phenomenon that appeared in the night sky over Norway on 9 December 2009. The most probable explanation is that it was caused by the failed flight of a Russian RSM-56 Bulava missile.