International Space Station (ISS) is in trouble again. One of the two Thermal Control System (TCS) loops (Loop A) is malfunctioning due to a troublesome Flow Control Valve (FCV). This is nothing new for the ISS and such malfunctions have happened before (e.g. in 2010 during ISS Expedition 24).
The whole current situation is nicely explained (as nice as such complex systems can be, see for example this try of mine to describe US segment TCSthis try of mine to describe US segment TCS for perhaps half a page shorter version) in this Spaceflight101 article titled Space Station encounters Thermal Control System Failure (ISS Expedition 38).
Pump Module (PM) NH3 Quick Disconnects & Electrical Interfaces (Image credit: NASA)
ISS Updates on Twitter assures there is no immediate danger to the crew onboard, but answers a question on contingency procedures onboard the ISS should Loop B fail as well:
ISS Updates, 4:27 AM - 12 Dec 13: Power down US Systems to the bare minimum & move crew to Russian segment until repairs can be made. Otherwise - de-crew ISS.
So while there is no reason to worry about the safety and wellbeing of Expedition 38 crew, and I have no doubt they will manage to convince the misbehaving FCV to respect its operational limits, or replace it with another during an unplanned spacewalk if the ground controllers fail to tame it (pump module in question in on the external Temperature and Humidity Control, or THC loop), I do have two questions:
- How long can a single US segment TCS provide adequate thermal control and some of its less mission critical client equipment and/or whole ISS modules need to be shut down?
- Have any scientific experiments suffered permanent damage or serious delay during past such failures of the US segment TCS?
Additionally, is there any predetermined order in which equipment is to be shut down or unplugged from ISS's Temperature and Humidity Control, and how far in the list are ISS laboratories and their experiments?
Edit to add: For quick reference, here's a diagram of the ECLSS (Environmental Control and Life Support System):
On-Orbit ISS ECLS Hardware Distribution as of February 2010 (Source: NASA/ISEC)
And current readings of certain TCS sensors are available on Space Station Live displays: