After more than 11 hours of ramming thick sea ice to rescue the stranded Akademik Shokalskiy, Chinese ice-breaker Xue Long has retreated to open water, leaving the rescue operation to Australian ice-breaker Aurora Australis.
Aurora captain Murray Doyle said it was always possible the ice would be too thick for the Xue Long.
''We were always option B,'' he said. ''We were always going to be sent on until the end.''
Reports from the Chinese ice-breaker and the Akademik Shokalskiy say pack ice in the area is 10/10ths, which means the vessel is surrounded, and some ice floes are between three and four metres thick.
The Aurora could comfortably slice through ice up to 1.35 metres thick, and crash through thicker slabs by reversing and ramming.
But Mr Doyle was unsure how the ice-breaker would handle ice thicker than three metres. ''It's not what we're built for,'' he said.
The Aurora is due to arrive near the pack ice that surrounds the Shokalskiy about 10pm on Sunday. ''Then it's up to the rescue co-ordination centre in Canberra and what they want us to do,'' Mr Doyle said. ''They might want us to go in or just stand by and wait.''
But it would be a waste of time trying to ram ice that was too thick. ''It's like driving your car into a brick wall,'' he said.
At 6.33am (AEST) on Saturday, Mr Doyle received a situation report from the rescue co-ordination centre in Canberra saying the Xue Long had arrived within six nautical miles of the Shokalskiy when it started breaking through ice.
But it had managed to move only two nautical miles in the past half-day and made its way back to open water.
''The Xue Long advises that a more powerful ice-breaker would be required to assist the Akademik Shokalskiy,'' it read.
Mr Doyle said he would be wary of driving too far into the pack ice in order to avoid the Aurora also getting trapped.
During the past few days, the Shokalskiy was blasted by south-easterly winds, which applied more pressure to the pack ice around the ship. This would made the rescue operation harder still.
''But by the time we get there the weather could change,'' Mr Doyle said. A westerly wind would ease the pressure on the ice and help break it up, he said.
The Xue Long and French ice-breaker L'Astrolabe will remain in the area should the Shokalskiy's crew and passengers need to be evacuated.