Space Prison (SF Novel). Tom Godwin Godwin

A ship heading from Earth to Athena, a planet 500 light years away, is suddenly attacked by the Gerns, an alien empire in its expansion phase. People aboard are divided by the invaders into Acceptables and Rejects. The Acceptables would become slave labor for the Gerns on Athena, and the Rejects are forced ashore on the nearest 'Earth-like' planet, called Ragnarok. The Gerns say they will return for the Rejects, but the Rejects quickly realise that that isn't going to happen. Ragnarok has a gravity 1.5 times that of Earth, and is populated by deadly, aggressive creatures and it contains little in the way of usable metal ores. This, combined with a terrible deadly fever that kills in hours, more than decimates the population.