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How much does it cost to return 1 kg from the ISS to the Earth? What are the parameters influencing this price?

What kinds of waste does the ISS dispose of? To mind come:

  • human waste*
  • household items like torn clothes or a broken toilet brush
  • poisonous chemicals**
  • food wraps
  • damaged tools, systems or even hull components.

* - why not compost those and use for plants?
** - resulting from what?

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    $\begingroup$ You would not believe how much junk mail the ISS gets… $\endgroup$
    – Jon Custer
    Jan 8 at 1:11
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    $\begingroup$ Human solid waste requires a relatively large amount of processing to be safely used as compost. Urine is relatively safe, solid waste is not. That's true on Earth too. $\endgroup$ Jan 8 at 4:04
  • $\begingroup$ Can you tell us what waste you think the ISS should dispose of, and how? Do you hope this will be jettisoned and left in orbit while that hopefully falls towards Earth and is burned up in the atmosphere? $\endgroup$ Jan 13 at 0:34

3 Answers 3

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What kinds of waste does the ISS dispose of? To mind come:

  • human waste

Excrement is not recycled. The ISS does not have a composting station. Urine is mostly recycled. However, the concentrated brine that results becomes trash that either is returned to the Earth's atmosphere to burn up or is vented (although might not be a good idea).

Water used for washing is similarly mostly recycled. The gray water from urine and washing becomes new wash water (and sometimes new drinking water). Water in the form of excess humidity in the air is recycled as drinking water.

  • household items like torn clothes or a broken toilet brush

Clothing typically doesn't get worn out on the ISS. It gets thrown out well before that. The ISS does not have a laundry. Astronauts toss their underwear after three or four uses. For cosmonauts, it's a bit longer.

  • poisonous chemicals (resulting from what?)

Some potentially hazardous materials are brought up to the ISS as, for example, experiments on crystal growth. These are supposed to be well isolated. Equipment and materials outgas. The environmental control and life support systems either collects those or vents the gases into space.

The most common poisonous chemical, carbon dioxide, is created by humans. There has been an experiment to use water and CO2 to create methane and oxygen, but it didn't work well. The CO2 scrubbed from the breathing atmosphere is mostly vented.

  • food wraps.

That's a sizable fraction of the trash sent to be incinerated by reentry into the Earth's atmosphere.

In addition to the above, there are

  • Used canisters / tanks such as those use to send water, oxygen, and nitrogen to the ISS
  • Equipment, etc., from experiments that have concluded or stuff that is being upgraded
  • Used office supplies such as pencils, pens, and paper
  • Printed instructions such as specific instructions for a spacewalk that has been successfully concluded or steps for an experiment that will not be repeated.

Given the huge added expense of sending stuff back to the Earth's surface and reused, it's oftentimes cheaper to treat it as trash.

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    $\begingroup$ I was aware EVAs use 'pen and paper' but not that it's extensively used inside the station. Kids nowadays are all like 'I have a laptop and a smartphone, why would I need paper!?'. Perhaps for the better(or not). $\endgroup$
    – Vorac
    Jan 8 at 2:59
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    $\begingroup$ it's oftentimes cheaper to treat it as trash I have a feeling that with the exception of things such as: experiments to be returned to Earth for further processing, broken equipment to be examined to determine failure modes/etc., personal effects (e.g., musical instruments, personal photographs, diaries, etc.) and perhaps some very expensive equipment to be refurbished (e.g., EVA suits) there is pretty much nothing where it pays to send it all the way back to Earth. $\endgroup$ Jan 8 at 4:02
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    $\begingroup$ @Vorac I suppose it's still easier to sling yourself around with office furniture in your pocket, than carrying a laptop in one arm. Although when it came to school (or university), I wish I could benefit from typing on a keyboard to transcribe what the teacher was saying. Wouldn't have to write in an unreadable way on a piece of paper. $\endgroup$
    – Clockwork
    Jan 9 at 14:53
  • $\begingroup$ Just asked a complementary question on the first point. $\endgroup$
    – Vorac
    Jan 9 at 15:53
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    $\begingroup$ @Vorac The use of paper increases drastically with age. A 70 year old unretired engineer would be completely lost without a nice-sized pad of paper and a working writing instrument. (I'm close to that age; I love my single-sided green engineering pad.) Except for stickies, a young engineer fresh out of college wouldn't know what to do with paper and a writing instrument (working or not). Old engineers' desks are full of paper. Young engineers' desks have a few stickies. Astronauts tend to be middle aged, so are probably halfway between the old farts and young farts with regard to paper usage. $\endgroup$ Jan 9 at 17:49
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David Hammen's answer gives a great overview of common ISS trash categories.

One category that is large by volume that was omitted is packing material. Supplies and equipment launched to the ISS are packed in protective material against the forces of launch. This material tends to be quite bulky (e.g. foam) and takes up a lot of volume in the ISS.

The linked presentation says that 30% of cargo volume is used for packing material.

Much of this packing material is not needed once on orbit [and] becomes a substantial component of the waste stream

enter image description here

Source: Alternative Packaging Study (emphasis mine)

See also Cargo ship inner organisation

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    $\begingroup$ I like the edible packaging idea! Didn't think about it until the slides mentioned it. $\endgroup$
    – user71659
    Jan 8 at 23:04
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In places where there is routine garbage pickup, there is usually restrictions against particularly large or toxic/dangerous items.

Those require special handling.

From the (currently unanswered) question How many kilograms of nickel particles will be dispersed in Earth's atmosphere by dumping old ISS batteries overboard?

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.


Further reading:

and I'll inject my own uninformed opinion from this answer to Do space industry companies want their defunct satellites back?

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  • $\begingroup$ Having the arm or some other mechanism simply eject the trash into space for subsequent orbital decay is yet another way of dealing with the trash. An experimental Nanoracks airlock module recently did just that. Whether that is sustainable (as claimed) or environmentally safe (as assumed) is perhaps debatable. $\endgroup$ Jan 9 at 17:27
  • $\begingroup$ The general assumption is that those batteries will burn up on reentry. That of course is to be confirmed. If they don't, that raises a new set of problems. Debris from uncontrolled reentry has rained down on the surface before. It appears your unanswered question is mostly concerned with the consequences of those batteries being completely incinerated by reentry as opposed to the consequences of those batteries surviving reentry intact, at least to some extent. $\endgroup$ Jan 9 at 17:40
  • $\begingroup$ I appreciate this aspect, but right now the answer is only a list of links. Would you mind adding the pertinent points from the links to your answers in a compact manner (i.e. - how many main batteryoes does it have? Do we need corrective actions?). The links can of course stay for reference. $\endgroup$
    – AnoE
    Jan 10 at 11:40
  • $\begingroup$ @AnoE except that it is not so much of "only a list of links" as it is several quotes from relatively authoritative sources that describes in some detail the actual nature of several tons of ISS trash. "... old nickel-hydrogen batteries..." "Weighing 2.9 tons... the heaviest single piece of garbage to be jettisoned from the International Space Station..." that "won’t remain as space junk indefinitely... as the whole lot will burn up when it enters Earth’s atmosphere in several years’ time." Other answers cover other aspects. $\endgroup$
    – uhoh
    Jan 10 at 14:00
  • $\begingroup$ @AnoE If there is something specific you'd like address in greater detail about the information in my answer, please let me know. But there is nothing wrong with including links to additional information beyond the answer itself. There's no rule, guideline or even general practice or obligation to summarize every single link that one includes. $\endgroup$
    – uhoh
    Jan 10 at 14:04

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