I put out a tweet earlier today mentioning that on a long, unsupported polar expedition for which there is little precedent and lots of unknowns, certain items are 'mission-critical'. Items of equipment or systems which are more important than the urge to save weight and move fast. Items without which, it's all over - for either the expedition, or us.
There were some good suggestions, ranging from hot chocolate to pants to M&Ms. A few expeditions over the last few years have failed when otherwise in good shape due to not having backups for these special pieces of kit. So, here are the items I'll be rating above all others and also the desperate need to lighten my sledges:
Communications - two independent systems to send word of injury or emergency. Two Iridium 9555 sat phones with 2x batteries (one always kept full), 2x GPS and a Yellowbrick tracker, as well as a McMurdo EPLB.
Floatation methods - in snow, not water. Walking in deep snow isn't an option, so we'll have a pair of skis with a repair kit for the bindings, plus a pair of snowshoes each.
Stoves - Without a stove, there is no heat and no water. We'll have three MSR stoves with 6 pumps (which can be unreliable in extreme low temps).
Extremities protection - frostbite will attack hands and feet first, plus mitts especially are at risk of loss in high winds. Multiple sets of gloves and mitts (from Montane) will be kept in drybags. They, plus socks (from Bridgedale) and liners, are the only items which would need to be immediately replaced in the case of falling in the water. Other clothing can be dried using body heat whilst hauling.
Lighting - on a winter/early spring expedition, you can't see without light! 2 lamps each plus more than enough AA battery power.
Theoretically, if all of these bases are covered and backups are kept in separate locations, an expedition can proceed, injuries notwithstanding, regardless of what happens as long as resilience and resourcefulness are maintained.