The practical obstacles were severe. Ice. The Arctic Ocean and its connecting channels freeze for most of the year, and even in summer the ice cover varies dramatically from year to year. A route that is open in one summer may be blocked in the next. The multi-year pack ice that fills Victoria Strait and the surrounding waters west of King William Island is particularly unforgiving — unlike seasonal ice, which forms and melts with the annual temperature cycle, multi-year ice is ice that has survived multiple summers and grown progressively thicker, denser, and more resistant to the warming that would eventually clear seasonal ice.
The problem with the specific route chosen by Franklin — south through Peel Sound and down Victoria Strait — is that it ran directly into some of the worst multi-year ice in the Canadian Arctic. Later expeditions, working from the evidence of where Franklin’s ships were found and from accumulated knowledge of ice conditions in the region, identified a more southerly route through Simpson Strait that would likely have been navigable in the ice conditions that prevailed during Franklin’s time. The irony of the expedition’s fate is that the Northwest Passage was, in a geographical sense, navigable during the period Franklin was attempting it — but that the specific route he chose ran into ice conditions that made it impassable, while an alternative route a relatively short distance to the south might have succeeded.
The Royal Navy’s Arctic exploration program in the first half of the nineteenth century had been, by the standards of the time, relatively successful. William Parry’s expeditions in the 1820s had mapped considerable stretches of the Arctic coast and demonstrated that ships could winter successfully in the high Arctic. John Ross and James Clark Ross had mapped significant additional territory. The Franklin expedition of 1845 was intended as the culminating effort — a final push through the remaining unmapped stretch that would complete the Northwest Passage from the Atlantic to the Pacific.