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Weighing 2.9 tons... this heap of old batteries is now the heaviest single piece of garbage to be jettisoned from the International Space Station.

begins Gizmodo's ISS Ditches 2.9-Ton Pallet of Batteries, Creating Its Most Massive Piece of Space Trash

Digital Trends' What was inside the space station pallet jettisoned into space on Thursday says:

On Thursday, March 11, mission controllers in Houston commanded the space station’s Canadarm2 robotic arm to jettison an external pallet containing old nickel-hydrogen batteries into Earth orbit.

The nickel-hydrogen batteries were once used for the ISS’s power system but have since been replaced with newer lithium-ion batteries featuring improved power capacity, smaller size, and lighter mass.

Fortunately, the pallet and the batteries inside it won’t remain as space junk indefinitely (there’s enough of that already orbiting our planet), as the whole lot will burn up when it enters Earth’s atmosphere in several years’ time.

I suppose that it's fortunate for other spacecraft in LEO that they won't stay in orbit for even longer than "several years' time" but without taking any measures to increase drag like attaching some Terminator Tape or equivalent it's still a non-zero risk.

But for those who breathe all that nickel doesn't just go away, it becomes a long term resident of Earth's atmosphere.

Scientific American's Some airborne particles pose more dangers than others; New evidence suggests that breathing nickel and other metals can lead to lung and heart damage, and even death is from 2009, and the science of the effects of PM2.5 and smaller particles on human heath is rapidly evolving.

Questions:

  1. Considering all of the Nickel Hydrogen that were ever on the ISS that have now all been replaced, what fraction were incinerated in the atmosphere we breathe, and how many total kilograms of nickel does this represent?
  2. Were there any estimates as to what fraction would end up as nickel-containing PM2.5 particles? (the size at which particles tend to remain in our lungs and can sometimes move into the bloodstream and lodge in different organs)

Related:

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    $\begingroup$ Each day, roughly 43.3 (metric) tons of meteoric matter enters the Earth's atmosphere. That material is ~1.72% nickel. So we get ~740 kg of nickel per day from natural sources. (FWIW, there's a fairly wide range of values for the total meteoroid flux on the Net, ranging from 10⁷ to 10⁹ kg/year, but the value I used seems to be the one most frequently used in recent work). $\endgroup$
    – PM 2Ring
    Mar 13, 2021 at 12:13
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    $\begingroup$ Note that the batteries on ISS were nickel-hydrogen, not nickel-metal-hydride. They contained gaseous hydrogen. $\endgroup$
    – Tristan
    Mar 14, 2021 at 6:04
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    $\begingroup$ Wait. So let me get this. We're getting Nickelback? No, somebody stop the music :D $\endgroup$
    – user39728
    Mar 15, 2021 at 2:59
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    $\begingroup$ I actually worked with Nickel in the lab in grad school. The pure metal oxidizes super fast in the atmosphere at high temperatures (e.g., if it's burning like a meteorite). We'll get lots of Nickel oxide, but probably no metallic nickel is my guess. $\endgroup$
    – user39728
    Mar 15, 2021 at 3:02
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    $\begingroup$ It actually depends on the oxide film that forms. If it were aluminum, the film would block the diffusion of oxygen through the metal, so the oxidation would stop and the rest of the metal would be protected and stay pure. If it were iron, the oxide would form porous scales through which oxygen would diffuse to keep eating away at the iron inside until eventually only iron oxide was left. I think Nickel is more like iron than aluminum, but I don't remember. $\endgroup$
    – user39728
    Mar 15, 2021 at 3:44

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