The crew of the ISS had a rather exciting weekend; a valve failed on coolant loop A, which circulates ammonia through the US segment of the station to draw excess heat off of the electronics and carry it to the massive radiators on the station’s truss. When the valve failed, NASA had to switch them over to the redundant loop B, which also meant powering down some experiments to reduce load, and shifted to troubleshooting the valve. They’ve now decided to go ahead and mount a series of spacewalks to replace the entire pump unit containing the valve. Fortunately, there are three complete spares on the station’s exterior, delivered before the end of the Shuttle program, and while the water leak that nearly drowned Luca Parmitano has yet to be resolved, they’re confident that if they just use different suits than he’d used, they’ll be fine. This will require a minimum of two spacewalks, and possibly three, and NASA is debating the exact timing. In particular, they generally try to avoid Christmas Day, so that mission controllers can operate on a reduced staff that day and the astronauts can get some downtime, but no decision has been made yet on the exact timing.
One thing is sure: this will mean the first operational Cygnus flight will have to wait a bit longer. It’s already been delayed due to other issues in the crowded ISS flight manifest, but NASA considers the spacewalks to fix the coolant system more important. Cygnus will have to wait until January at the earliest.