# How precisely do we know mass of the ISS?

Everyone knows that in space, every gram counts. Knowing how much mass an orbital object has is necessary for precise motion calculations.

So how precisely do we know the mass of ISS? I'm sure we can estimate it's weight to a few dozen kilograms. But there is lots of stuff going in and out and stuff might even be leaking a little. How much effort is put into bookkeeping ISS's mass? To clarify, I'm talking about everything that is on or inside ISS at this moment, including people, cargo or connected spacecraft.

And what precision is needed to keep ISS running? If we weren't able to achieve that precision, what problems would there be in running ISS?

• I think the precision is below 1g. Jun 22, 2018 at 12:15
• @peterh If you have any source to that, can you add an answer? Jun 22, 2018 at 12:15
• I am pretty sure that based on the wording of the question, the OP is more interested in how accurately, the mass is known, rather than the number of decimals with which the sum is expressed.
– uhoh
Jun 22, 2018 at 14:51
• @Uwe Ok, but positioning the station (both in trajectory and in orientation by the giros) is a very costly thing, thus it likely requires very precise calculations, and these require very precise input data. And calculating the mass could be done probably easily by measuring the forces on the station and calculating their accelerations. For example, if there is an orbital correction maneuver, it must be known very precisely, how long should it be done and it requires to know the mass of the ISS precisely. Maybe a not very precise knowledge is also enough, and the propulsion at the end of the Jun 22, 2018 at 16:19
• On Earth all material going to the ISS may be weighted with good precision before launch. But the mass of all garbage removed from the ISS could not be measured precisely in zero gravity. There are also gases vented by ISS. Therefore the actual total mass of the ISS could be determined with low precision only.
– Uwe
Jun 25, 2018 at 14:03

I spoke to Mike Lammers, the Flight Director for the ISS and asked him about the mass uncertainty of the ISS; he mentioned that it is ±5000kg or about 1% of the total mass of 411,000kg.

Most of the uncertainty comes from waste going back to earth. Every cargo vehicle goes back down with tons of return cargo and trash. There’s no scale on ISS so as the crew fills cargo transfer bags with discarded foam, wet trash, unused food, old clothes, old equipment, etc. there are some assumed densities of the bags, and they aren’t be known to better than ±10%.

Knowing the mass distribution on the ISS is not that critical; the mass has the most significant effect on reboost. However, most of the reboosts use closed-loop guidance, so any discrepancies are corrected for by the guidance systems, and the mass is updated by looking at the reboost performance.

Mass distribution is not very important either as the space station does not have very stringent pointing requirements. The onboard systems are designed to be robust to error, mostly since the crew can move around tonnes of cargo in the space station and the robotic arm can move around, etc.

However, mass distribution is critical for vehicles entering the atmosphere, both manned and unmanned.

• Very nice and very surprising answer! Some slightly related information in answers to Does any site track the mass of the ISS? where the mass was reported in June to be 417,501.56 kg, eight digits of precision
– uhoh
Aug 27, 2018 at 3:02
• I have been thinking about "...and the mass is updated by looking at the reboost performance." This sounds a bit like saying the mass of the ISS is measured by looking at the delta-v for a given burn time or given quantity of fuel burned and assuming an engine Isp. Is that what is happening?
– uhoh
Aug 29, 2018 at 5:30
• That is my understanding, the ISS's position is known to better than 3,000ft I assume they know the trust they are producing with some accuracy, combined with the measured Delta-V you could derive the mass Aug 29, 2018 at 19:02
• @uhoh I understand what you are saying, all the information in the answer came directly from Mike Lammers including the bit about updating the mass from reboost Aug 30, 2018 at 1:33
• @MarkOmo okay so you are right, it could be the basis of a new question. If the mass of the ISS is really only known to 1%, then it is possible, that's not inconsistent. I mostly just wanted to check that that's what you meant. Thanks!
– uhoh
Aug 30, 2018 at 1:42