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After reading about the old ammonia leak here, I began wondering if and how that lost ammonia is replaced. Some ideas I can think of are:

  • Launch some aboard the next supply run?
  • Synthesize some on-board? There's science-y stuff up there to make chemicals with, right?

It might also be something else entirely.

How is lost ammonia replaced on board the ISS?

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  • $\begingroup$ If you would like to read about how that leak scenario played out, this paper is very readable. ntrs.nasa.gov/api/citations/20190029027/downloads/… They worked very hard to not need to install one of the spare ATAs described in my answer. $\endgroup$
    – Organic Marble
    Commented Aug 15 at 23:31
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    $\begingroup$ Note that the synthesis of ammonia is an extremely energy hungry process which requires high pressures and temperatures. Sending a reactor for this to space would certainly not be mass-efficient. $\endgroup$
    – TooTea
    Commented Aug 16 at 8:51

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The Ammonia Tank Assembly (ATA) is an Orbital Replacement Unit (ORU).

Here is a schematic showing the ATA in the ammonia coolant loop. The ATA makes up leakage and acts as an accumulator (yes there is another accumulator in the pump package - that is not big enough for constant use, but allows the loop to run while the ATA is replaced). A nitrogen tank assembly (which is also an ORU) supplies regulated nitrogen to pressurize the two ammonia tanks in the ATA through bellows in the tanks.

The schematic is from https://www.nasa.gov/sites/default/files/atoms/files/iss-operating_an_outpost-tagged.pdf

During the shuttle days ATAs that were full of ammonia were carried up (for example, on STS-131) and empty ones taken down.

There are two spare ATAs filled with ammonia stored externally on the ISS. I do not know whether or not ATAs can be carried up in a Dragon trunk, or even if any more ATAs exist, but hopefully two is enough for the remaining ISS lifetime. This visualization shows the location of the two spare ATAs (highlighted yellow).

The ATA is the rectangular box at the back of the payload bay in the photo below.

The ATAs were swapped out using the robot arms and EVA crewpersons.

Last two images are from images.nasa.gov

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    $\begingroup$ because the question asks if it's synthesized, I think it's worth noting that the nitrogen in the ATA is used to inflate bellows to pressurize the ammonia tanks, not chemically reacted into ammonia. A couple figures prior (Chapter 11 Figure 3) illustrates this more explicitly. $\endgroup$
    – Erin Anne
    Commented Aug 15 at 22:37
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    $\begingroup$ I remember when they did the ammonia swap out EVAs, prior to ingress into the airlock the astronauts would do a “bake out”, lounging in the Sun for ten minutes or so, in order to allow any ammonia that had leaked onto their suits to evaporate. This was to avoid bringing any ammonia into the interior since concentrated ammonia is very toxic. They also inspected each other’s suits for ammonia, which I think was detectable by color. I seem to remember one time some ammonia was detected on a suit requiring some extra bake out time but I'm not sure. $\endgroup$
    – Steve Pemberton
    Commented Aug 16 at 13:16
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    $\begingroup$ @StevePemberton you might be remembering some of the leak repair EVAs. space.stackexchange.com/a/64966/6944 IIRC swapping out the ATAs was pretty drama free at least on STS-131 $\endgroup$
    – Organic Marble
    Commented Aug 16 at 13:38
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    $\begingroup$ That must have been what it was, thanks. And it seems they started the bake out “timer” while still at the worksite after verifying no visible ammonia. If I am correctly remembering lounging at the airlock it might have been an occasion when there was some remaining time in the bake out period when they arrived at the airlock. $\endgroup$
    – Steve Pemberton
    Commented Aug 16 at 13:59
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    $\begingroup$ Are you sure an ATA wouldn't fit in a Dragon? Looking at the NASA docs they're like 2x1.2x1.4m and weigh about half a ton with about an additional quarter ton of ammonia... That's not actually very big compared to most of the SpaceX listed cargo volume and mass capacities... Unless the 2 meters is a problem, but I wouldn't think so... $\endgroup$
    – Perkins
    Commented Aug 16 at 16:50

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