And if you’re wondering why ATV’s hatch is closed for the Russian EVAs, read our post from the 2012 ATV-3 mission.
Information contributed by Jean-Michel Bois at ATV-CC: To complete the explanation, the closure of the hatches during an EVA is done to minimise the tasks on board in case of any internal problem (e.g. leakage). As the number of crew members remaining inside the Station is reduced during an EVA, their priority in case of any issues would be to assist crew mates who are outside the Station. Actually, the closure of an ISS module can be performed quickly by the crew, but several things must be dismounted and removed (ventilation hoses, possible cables between the modules, etc.), so it is better to do that in advance, and not during an urgent, time-limited situation. Thus, during an EVA, several ISS modules are ‘pre’-prepared and isolated.
The ATV hatch closure is specific and takes more time because, as a visiting vehicle, it must be able to leave the Station at any time to free the docking port. To ensure a safe ATV undocking, the ATV navigation subsystems have to correctly pilot the vehicle, and in particular these systems need to know the precise ATV mass and centre of gravity. To calculate these, the levels of fuel, water, etc., in the ATV tanks can be checked on ground at any time by ATV-CC via telemetry, but the layout of the cargo bags and their content is information that can only come from the crew. So, before closing the ATV hatch, the crew performs, if necessary, a reorganisation of the bags and other items and carries out a last check of the ATV Pressurised Module; final close-out photos are taken and sent to ATV-CC. With this info, the ATV-CC in Toulouse is ready to conduct a safe undocking, if it should be required.