In 1909 Nansen combined with Bjørn Helland-Hansen to publish an academic paper, "The Norwegian Sea: its Physical Oceanography", based on the "Michael Sars" voyage of 1900. Nansen had by now retired from polar exploration, the decisive step being his release of "Fram" to fellow Norwegian Roald Amundsen, who was planning a North Pole expedition. When Amundsen made his controversial change of plan and set out for the South Pole, Nansen stood by him.