The USCG heavy icebreaker entered service in 1976 while one of a class of two heavy icebreakers, her sister ship Polar Sea is no longer operational. Today the Polar Star is the United States only operational heavy icebreaker. She and her sister were built by Lockheed Shipbuilding. The icebreaker Healy is considered a medium icebreaker. Polar Star is the only ship in the United States' fleet large enough to break the heavy sea ice to access McMurdo, the U.S. research station in Antarctica.