I'm currently following US EVA-24 with Rick Mastracchio (EV1) and Mike Hopkins (EV2) tasked to replace a failed Pump Module (S1 PM), one of two external pump modules of the US section of the International Space Station (ISS). The schedule is also visible on this screen taken off live stream a few minutes before the EVA, scheduled to start on December 21, 2013 at 12:10 UTC (which it did):
Spacewalks overview for the US EVA-24 as broadcast on NASA TV (credit: NASA)
Currently, good half an hour into EVA 1, Mike and Rick are already 15 minutes ahead of schedule, and it seems they were even asked to perform a few additional checks (e.g. gloves check) that were not on the checklist as read to them by Koichi Wakata inside the station. So this naturally begs the question:
Why did NASA assess that they will require so much time (3 spacewalks each over 6 hours long) to replace a single Pump Module? Is this merely scheduling procedures with a good measure so astronauts don't have to hurry? Or do they always prepare flexed schedules like this for some other reason?
Update: A good hour into EVA, the two astronauts are roughly 25 minutes ahead of schedule. Here's another screen captured off the live stream, showing Pump Module design: