Last week in response to this update on loading ATV-5 with trash we received another question through Twitter:
— Gloria Graizzaro (@gloriagraizzaro) January 30, 2015
We forwarded this to Kirsten Macdonell, now Senior Columbus and Payload Operations Engineer:
The answer is, although the mass of ATV, including its trash, must be known, nothing is weighed on orbit. Everything has already been weighed on the ground before launch. Therefore, when an item is trashed we already know its mass from before it was launched. For items like food waste and used clothing, there are dedicated waste loading bags in which to put this waste. The average mass of these bags, filled with either used food containers or clothing, has been calculated from measurements on the ground and these values are used as the mass of the trash bag on orbit. Even the foam used in packing hardware is weighed before launch and is tracked by part numbers so that we know its mass when it gets trashed.
We need to know the mass, centre of mass and moment of inertia of an ATV for the Guidance, Navigation and Control system software, which controls vehicle manoeuvres. The ATV Control Centre Attached Phase Operations Manager keeps track of this information (as well as the available volume for trash) throughout the ATV mission.