On its 12-day round-trip cruises from Bergen to Kirkenes, Norway, from October through March, Hurtigruten (888/387-2566, www.hurtigruten.com) guarantees that if the northern lights are not seen during the cruise, each passenger will qualify for a free 6- or 7-day cruise.
A northern lights "sighting" is based on crew logs and announcements, not on the length of time the lights were visible or whether or not each passenger, himself, saw the lights. Any passenger qualifying for a free cruise would be booked in an inside cabin on a sailing between Oct. 1, 2019, and March 31, 2020.