Ninety bathtub toys were hurled into a drainage hole on the Greenland ice in September - an experiment to see how melt waters find their way to the base of the ice sheet.
It was hoped the ducks would flow along subglacial channels and eventually pop out into the sea. They may still, but nothing has been seen of them so far.
"We haven't heard anything from them yet," said Nasa's Alberto Behar. "If somebody does find one, it will be a great breakthrough for us."

Dr Behar is a robotics expert with the agency at its Jet Propulsion Laboratory in Pasadena. He has been studying the tubular crevasses that appear on the surface of the Greenland ice known as moulins.

These "plug holes" can drain vast lakes of melt water that settle on the top of the ice during summer months. Scientists would like to know how and to what extent this water can help lubricate the base of the ice sheet, moving it faster towards the ocean.

If plastic fowl seem a very low-tech way for a man who has also worked on Mars rovers to investigate the problem, it should be stressed he also has a more sophisticated approach.

Dr Behar has been developing a Moulin Explorer, a probe that can travel through the chutes.

"We did not hear a signal back so it probably got stuck under the ice somewhere," said Dr Behar. "It was a bit of a long shot but we thought it was worth a try. We've got to go back and scratch our heads and think about what we do next."

These are early days. In the distant future, the descendents of the Moulin Explorer could be deployed by Nasa on icy moons in the outer Solar System that are thought to harbour subglacial oceans.