(CNN) -- After nearly a week trapped by deep Antarctic ice, the 74 passengers aboard an expedition vessel could soon be freed.
But based on the troubles of the last few days, no one's sure exactly when that might happen.
An Australian icebreaker ship Aurora Australis is headed toward the Russian-flagged vessel and is expected to arrive around midnight (8 a.m. ET) Sunday, the Australian Maritime Safety Authority said.
It follows the Chinese icebreaker Xue Long, or Snow Dragon, which was just six nautical miles away from the trapped vessel when it couldn't get any closer due to unusually thick ice.
The Snow Dragon went back to open water but remains in the area to provide support, said Andrea Hayward-Maher, spokeswoman for the AMSA. The Chinese ship has a helicopter on board that could assist in evacuation.
The plan is to see if the Aurora Australis can break through the ice further than the Chinese ship could.
"If not, the aerial transfer of passengers on board maybe a possibility," Hayward-Maher said.
A French icebreaker was also en route to assist, but AMSA called off that vessel's mission Saturday after it became clear that ship wouldn't get farther than the Chinese boat. Also, the French and Chinese ships can break ice only about 1 meter thick, where as the Australian ship can pierce through floe about 1.35 meters thick.
"Some reports say that the ice is at least 2 meters deep. It's a possibility that the Aurora Australis might not be able to break through that type of ice," Hayward-Maher said.
The rescue icebreakers were battling the planet's coldest environment while trying to reach the Akademik Shokalskiy ship, whose 74 researchers, crew and tourists remained in good condition despite being at a frozen standstill since Monday.
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The ship is carrying scientists and passengers led by expedition leader Chris Tunrey, an Australian professor of climate change.
"The vessel is fine, it's safe and everyone on board is very well," Turney said. "Morale is really high."
The ship got stuck in the ice 15 days after setting out on the second leg of its research trip.
According to Turney, a professor at the University of New South Wales, the ship was surrounded by ice up to nearly 10 feet (3 meters) thick. It was about 100 nautical miles east of the French base Dumont D'Urville, about 1,500 nautical miles south of Hobart, Tasmania.
On Christmas morning, the ship sent a satellite distress signal after conditions failed to clear.
The crew had a "great Christmas" despite their situation, Turney told CNN. He said crew members have used the delay to get more work done.
"We've just kept the team busy," he said.
The expedition is trying to update scientific measurements taken by an Australian expedition led by Douglas Mawson that set out in 1911.
The expedition to gauge the effects of climate change on the region began November 27. The second and current leg of the trip started December 8 and was scheduled to conclude with a return to New Zealand on January 4.
Turney said the ship should still be back in New Zealand on time.