Drama continued to unfold Friday in the frozen sea around Antarctica as a Chinese ship that aided the trapped Russian vessel, M.V. Akademik Shokalskiy, found itself also stuck in the heavy ice. (Related: "Antarctic Ship Rescue: 5 Lessons From the Trapped-Vessel Drama.")

The struggles of the Chinese ship, Xue Long, or Snow Dragon, which had provided the helicopter used in the airlift rescue of 52 passengers aboard the Shokalskiy, raise new questions about icebreaking vessels and the limits of their capabilities.

The Australian Maritime Safety Authority (AMSA), which coordinated the rescue, said the Chinese ship was in no immediate danger but appeared to be trapped. The Australian icebreaker that is carrying the rescued passengers from the Russian ship, the Aurora Australis, was standing by to offer possible assistance, while officials assess whether changing conditions would free the ship. (Related: "Best Pictures From Dramatic Antarctic Ship Rescue.")

National Geographic talked to one of the leading experts on icebreaker technology, retired U.S. Coast Guard Rear Admiral Jeffrey M. Garrett, who served as the first commanding officer of the icebreaking cutter, the USCGC Healy, when it was delivered in 1999. Garrett served on a National Academies of Science committee in 2005 that urged that the United States invest in upgrading its icebreaking capability. (See "Arctic Shipping Soars, Led by Russia and Lured by Energy.")

Garrett spoke by telephone from Seattle, where he is now a private maritime affairs consultant.

What is an icebreaker?

It's a complicated question. There's a whole spectrum of ice capability for ships. There are ships with some extra hull protection and some extra protection for propellers and rudders that can go through very light ice, and it goes all the way up to strong and powerful ships that can go through just about anything. And there's not a real good terminology. It's like saying something is a "truck." Well, that can mean anything from a pickup to a huge semi. People ask, "What's an icebreaker like?" There are all kinds of them, and you've got to dig a lot deeper to know what it's capable of doing.

What about the Shokalskiy and the rescue ships in the Antarctic?

The Shokalskiy is an ice-capable ship, built in the Soviet era, and its main function is now expedition and tourism. The three ships that made attempts to reach it are all probably on the high end of the ice-capable scale, but I wouldn't classify them as icebreakers. (Also see: "Who's on That Russian Ship Stuck on Antarctic Ice?")

What's the metric that people use to tell the difference between an ice-capable ship and an icebreaker?

One of the rules of thumb is how many feet of ice could it break at a given speed. The U.S. has two of the most powerful non-nuclear icebreakers in the world, the Polar Sea and the Polar Star. They can break over 6 feet [1.8 meters] of ice continuously at [a speed of] three knots. [One of Russia's largest nuclear-powered icebreakers, considered the strongest in the world, could break] probably upwards of ten feet [three meters]. [A ship that is merely "ice-capable"] would break 3 feet [0.9 meter] of ice or less at that speed.

What is making conditions so hard for these ships in the Antarctic?

The toughest kind of ice is sea ice that's under pressure from wind. Ice has a rough texture, and wind will push an ice field close together and actually pile the ice up. I think this is essentially what happened to the Russian ship. The blizzard or heavy winds put the ice under pressure and jammed it up. (See your Antarctica photos.)

Aren't there forecasts or warnings that could help ships avoid such situations?

There is a lot of weather information available. The Antarctic, of course, [is] not as well covered. So sometimes forecasts are over such a wide area, you don't know how forecasts apply to localized areas. There's a lot of trash talk on the Internet about how they didn't look at the weather. But it's often tough to predict—especially in polar conditions, which can change quickly. You can always "Monday-morning quarterback" these things. Should they have gone in there? Well, clearly not, you can see now. (See also: "Ship Stuck in Antarctica Raises Questions About Worth of Reenacting Expeditions.")

If Russia's nuclear icebreakers are so powerful, why wouldn't they be dispatched to help?

It's not like sending the AAA tow vehicle. It's a 20,000-mile [32,100-kilometer] journey. You're talking weeks and weeks. Also, the Russians are now in winter. Their icebreakers are all employed [in the Arctic]. We are in the Antarctic summer, so those ice conditions are probably going to get better. There's a very good chance the wind will die down or shift and they'll be able to get out.

Also, there's kind of an urban legend that Russian nuclear icebreakers can't go into warmer water because their cooling systems wouldn't be able to cool the nuclear reactors adequately. [Although we don't know if that's the case,] the Russians have never sent one of their nuclear [icebreakers] near the equator. Add to that the possible uproar about sending a nuclear ship into the pristine Antarctic, and it is not something the Russians are likely to do.

How different is an icebreaker from other vessels?

The question sometimes comes up of whether we can just retrofit existing ships—why don't we just put more steel in the hull and then it can go in ice?

But there's a significant difference in open-water vessels and ice-capable ships, and if you want an icebreaker it really must be built that way from the keel up.

You've got to protect things as mundane as the intakes where seawater comes in. [Intakes have to be screened] because ice can break into small pieces and enter the ship and clog the intakes.

You don't just take a normal ship's hull and make it into an icebreaker. The hull shape must be different. It has to have a bow that can crush ice, and the rest of the form has to be shaped in a way that it can go through ice efficiently. It's not just the size and thickness of the hull framing. It has to be very strong, but you also have to have all the ship systems strengthened so they can deal with cold weather: propellers, rudders.

What about power?

You have to have lots of horsepower and an ability to readily reverse and stop the direction of thrust quickly. The Polar-class ships have a very unique system, with two kinds of propulsion: a diesel-electric system for steaming over long distances, and gas turbines—like those on a commercial jet—that [provide extra power]. But the gas turbines burn a whole lot of fuel very fast. So the captain constantly has to make the decision on how much turbine power to use. [An icebreaker like the Polar Star would carry about 1.3 million gallons of diesel fuel aboard.]

How much does an icebreaker cost?

If you want something powerful, you are looking at almost $1 billion. You could probably design a more moderate ship that would meet most U.S. needs for something less, but you're talking about an expensive vessel.

Are we at the point where we need more ships with icebreaking capability, not just the U.S. but around the world?

The simplistic question you often get is, "With global warming, why do you need icebreakers?" But in the Arctic, you have more thinning and decrease in the ice pack, and that's making ice conditions more unpredictable. It's not like everything's getting easy. In some ways, global warming has increased the need for icebreaking and ice-capable vessels in both locations.

The Arctic is seeing a big increase in energy development and more shipping, and in the Antarctic you have more tourism. (Related interactive map: "The Changing Arctic.")

Now it appears that the Akademik Shokalskiy is not in immediate danger, but if a ship gets stuck and it gets dragged aground by the moving ice field, the hull could be punctured and this could have severe environmental consequences.