Alan Hubert and Dixie Dansercoer collect snow-depth data for ESA's CryoSat-2 mission. They're seasoned polar explorers so they are used to dealing with physically demanding and downright harsh conditions but what they encountered during their three and a half-month expedition from Russia to Greenland via the North Pole was dangerous even for them.

They didn't expect that the Lincoln Sea in the Arctic Ocean, the very ice they were on, would begin to disintegrate. Aside from the dangerous conditions they also had to circumnavigate the ice-free water leads.

Satellite video of the Lincoln Ocean collapse

"I noticed in satellite radar images that the Arctic Arc expedition was heading straight into very difficult and dangerous ice conditions, with large areas of thin ice and open water," says Leif Toudal Pedersen Danish National Space Centre. "Luckily I was able to follow the changes in ice conditions and ice drift thanks to the daily coverage provided by the Envisat ASAR sensor."

Alain Hubert and Dixie Dansercoer ‘stepped’ onto the sea ice off the coast of Siberia on 1 March 2007 and have since covered an amazing 1 600 km, each pulling a 130-kg sledge holding supplies and equipment.

Struggling over rough sea ice during the Arctic Arc expedition. Making headway over the sea ice can be very difficult. Sometimes compression zones can be up to five metres high and recently open stretches of water has caused problems for the expedition. Credits: Arctic Arc 2007 - International Polar Foundation

The Arctic Arc expedition was originally planned to continue on to southern Greenland, but bad ice conditions have made progress slow, so a decision as to whether the expedition will continue or not will be made once the two explorers reach the mainland.

Alain Hubert commented on 10 June, "We seem to have come through the boggy mass of the past few weeks in the Lincoln Sea, particularly where it narrowed. We can see now that the ice we are on is old ice again, and so more stable. What happened in the Lincoln Sea, I will never understand. But I think in any case that the worst of the problems are finally behind us and we can now look forward to the final part of our expedition with greater peace of mind."

The image illustrates the dangerous ice conditions that lay along the original Arctic Arc expedition route to the Greenland coast. Thin ice and open water conditions are visible as dark areas in the radar image and are prominent in the danger zone highlighted in red. The expedition was rerouted to the east through the 'safe' zone highlighted in green and characterised by thicker and more stable ice conditions. The yellow arrows in the image are ice drift velocities derived from a temporal series of Envisat ASAR radar images. Credits: ESA - Leif Toudal Pedersen, Danish National Space Centre, Danish Technical University