# When jettisoning heavy objects from the ISS e.g. 2.9 tons of batteries+, how much angular impulse does the station get? Corrective actions necessary?

Discussion below this answer to Why does it take so long for ISS garbage to fall out of orbit? addresses the jettisoning of the ~2.9 ton pallet full of used batteries full of nickel and other things. It seems that the object was given something like a 10 to 40 cm/sec "nudge" on its way. This means it will start at a nearly circular 400 km primarily chosen to not hit the ISS again, and atmospheric drag will slowly bring it down over months or years until it burns up in the atmosphere an produces an unknown number of nickel particles in our atmosphere (see How many kilograms of nickel particles will be dispersed in Earth's atmosphere by dumping old ISS batteries overboard?) unless Gwynne Shotwell grabs it first and brings it (and the hypothetical sensitive item I've invented for fun) safely to Earth.

But I digress.

Question: When jettisoning heavy objects from the ISS like 2.9 tons of batteries (and other things), how much [angular impulse] does the station receive? How much angular momentum is imparted? Are corrective actions necessary to maintain attitude? Did the ISS' control moment gyroscopes (CMGs) speed up to a fevered pitch? Did attitude control thrusters start firing?

The imparted angular momentum (if any) will depend on the on the impulse and the lever arm the impulse forms with the center of mass (COM)of the ISS. If the vector is aligned, no angular momentum.

Calculating the angular impulse requires data on the impulse vector, its coordinates WRT to ISS COM, and the moment of inertia of ISS. The ISS information is available for historical confirmations (2008) but I am unable to source either the location of the COM in the ISS's current conformation or the geometry of the ejection.

The battery pack in question was pitched by the Canadarm during an EVA, so it's vector is unlikely to align with any of the airlocks or other reference structures.

The data needed would likely need to be reverse-engineered from the corrective actions.

• "Given that the ISS masses 4.2 10^5Kg and the ejection impulse is small, I’m sure any acquired momentum is well within the comfort zone for the attitude determination and control system" is strictly an unsupported guesstimate and so not sufficient for a Stack Exchange answer, and the ISS' moment of inertia and the geometry of the ejection are more relevant than the mass. For the first see What are the ISS moments of inertia around design axes?...
– uhoh
Commented Nov 2, 2021 at 6:23
• ...and for more information on the geometry and speed of the ejection, please check the post(s) I've linked to in the question, as well as This ISS trash deployment looks more like 2 feet than 2 inches per second, was it too fast or are these articles incorrect?
– uhoh
Commented Nov 2, 2021 at 6:23
• The reference you site was the most recent I could source, as well. Conformation changes since 2008 likely necessitate updated data to answer the posted question. I enjoyed the Air and Space link. Thanks. Commented Nov 2, 2021 at 15:09

Acording to the Facts and Figures by NASA, the ISS has a mass if 419725kg (which is oddly specific in my oppinion).

So by pushing a mass of about 2900kg to 10cm/s to 40cm/s retrograde, the station will accelerate in the opposite direction (prograde) with the amount of energy it took to push the batteries away. This will rise the orbit of the station slightly and at the same time lower the orbit of the batteries (due to the nature of orbital meachanics this will accelerate the batteries and make the station slower). But pushing 2.9t from a 419t mass will have neglible effect on the bigger mass, so all calcualtions in that regard are purely academical in nature since other effercts will have a bigger impact.

About corrective maneuvers: If the vector of force of pushing the batteries away was not trough the center of mass of the station, it will impart a rotational force. This will be corrected by the rate gyros on board the station (if they become saturated, than they have to be despun which will impart a rotation on the station which will than in turn be countered by attitude thrusters of the station). So such an action could go in both directions. It could force a correction by thruster sooner but it could also help delaying such an action.