How icebreakers work — and why they sometimes don't work

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By Alan Boyle, Science Editor

Icebreakers are indispensable for traffic through polar seas, but they're not invincible — as the current plight of a stranded Russian ship illustrates.

For the past week, 74 scientists, tourists, journalists and crew members have been stuck aboard the Akademik Shokalskiy in the midst of an expedition to celebrate the centennial of Australian geologist Douglas Mawson's polar odyssey. Thick ice and bad weather have foiled efforts to free the ship. The Shokalskiy's passengers have ample supplies — but at some point, rescuers had to decide whether to keep trying to break the ship loose or use a helicopter to take them to safety.

The Chinese icebreaker Snow Dragon (Xue Long) tried to approach the Shokalskiy over the weekend, but had to stay miles away because it couldn't crack through ice that ranges up to 10 feet (3 meters) thick. An Australian icebreaker, the Aurora Australis, tried to get through on Monday but was turned back as well.

On Tuesday, the Australian Maritime Safety Authority said all 52 passengers would be evacuated to the Snow Dragon by helicopter. The Shokalskiy's 22 crew members would remain with the vessel.

"This rescue will be a complex operation involving a number of steps and subject to factors such as weather," AMSA's Rescue Coordination Center said in a statement. "The helicopter is unable to fly in the current weather conditions, and will hold off on rescue until conditions improve."

'Bathtub with a big engine'
In this case, the ice was just too thick for the icebreakers — and AMSA said crew members on the Aurora Australis were worried that the ship "would be at risk of becoming beset by ice itself if it continued to make further rescue attempts."

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Icebreakers don't make their way through the thickest icepacks by cutting through it like a knife. Instead, their rounded keels slide over the ice and crash down into the water below. Greg Mortimer, co-leader of the Australasian Antarctic Expedition, told The Guardian that the basic design is "like a bathtub with a big engine inside it."

On Monday, the Aurora Australis tried getting to the Shokalskiy, but had to back off to a position 20 miles away due to poor visibility and 35 mph winds.

The Aurora Australis is designed to break through level ice that's up to 4 feet (1.23 meters) deep, at a speed of 3 mph. It can handle thicker layers if it backs up and rams the ice. But the fused-together ice floes that surrounded the Shokalskiy were just "too thick to penetrate," Captain Murray Doyle told the Sydney Morning Herald.

That left a helicopter evacuation to the Snow Dragon as the fallback option.

America's icebreakers
AMSA's Rescue Coordination Center is in charge of the rescue effort, but the fact that a Chinese ship is playing such a key role points to the international cooperation that's often required for Antarctic operations. The U.S. Coast Guard currently has two Seattle-based polar icebreakers in service: the medium-class Healy, which recently wrapped up an Arctic expedition; and the heavyweight Polar Star, which is en route to Antarctica to clear a channel to the U.S. base in McMurdo Sound.

The Polar Star is just returning to service after a years-long, $90 million overhaul. It's designed to get through a 6-foot-thick (2-meter-thick) layer of ice when it's sailing at 3.5 mph — and when it's in its backing-up-and-ramming mode, it's capable of handling ice that's up to 21 feet (6.5 meters) thick.

"Sometimes you have to get a running start," the Polar Star's captain, George Pellissiere, told NBC News from the ship.

The expedition leader on the Shokalskiy, Chris Turney, told TODAY that the Polar Star could "definitely get us out" — but that wasn't in the cards. Pellissiere said his ship was between Honolulu and Sydney, and would require a week or so to sail to Antarctic if needed.

"As of right now Australia is coordinating the rescue operations," Coast Guard spokeswoman Allyson Conroy said in a statement. "The nearest U.S. Coast Guard asset is more than 3,000 nautical miles away. We are communicating with Australia and standing by if our assistance is indeed needed."

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Alan Boyle is's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.