In 1871 he signed up to go as a surgeon and naturalist with the disastrous Hall expedition to the Artic on the "Polaris." When a the ship's leader, Captain Hall, became violently ill soon after having ingested a cup of coffee, Bessels treated him, but Halls's condition worsened. Soon Hall was displaying signs of insanity and refused to eat. Eventually he succumbed to the illness, dying on November 8, 1871.
Because there was a lot of friction between Bessels, other German crew members and Hall prior to his illness, when the expedition returned, a second mate on the expedition accused Bessels of murder, claiming that he had poisoned Captain Hall with morphine rather than administering him with quinine. Bessels was acquitted of the charges and Hall was found to have died of natural causes. To this day, however, there are many who are convinced Bessels and other plotters were responsible.
Bessels continued studying the Arctic up until his death of a stroke in 1888.