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During Expedition 41 Reid Wiseman and Alexander Gerst replaced a failed pump on the outside of the ISS. I am guessing they took the broken pump back inside the ISS and returned it to earth in one of the cargo capsules.

Would it instead be possible to just throw the pump real hard in the opposite direction of the stations movement so it would travel slower and deorbit? I know the ISS travels roughly 8km/s relative to the earths surface, but one pump is probably pretty light and astronauts are pretty fit.

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    $\begingroup$ Throwing a thing as hard as you could will be about 10 or 20 m/s, that is nothing compared to 8000 m/s. $\endgroup$
    – Uwe
    Commented Jun 21, 2018 at 13:50
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    $\begingroup$ @Uwe yes, but the ISS needs constant boosts to prevent deorbiting, so those 10-20m/s will make a significant difference to the deorbit time. $\endgroup$
    – Jack
    Commented Jun 21, 2018 at 13:57
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    $\begingroup$ No, most likely they brought back with themselves inside, and then they will send it back with the next soyuz (not in the landing unit, but in the part for the waste, to burn on reentry). Producing space waste intentionally is a very serious thing. $\endgroup$
    – peterh
    Commented Jun 21, 2018 at 13:59
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    $\begingroup$ Of course it depends how quickly you want it to deorbit If you don't mind waiting a few months, you could just leave it outside the hatch. It will get left behind by station reboosts and (depending on its shape, size and mass) eventually deorbit. If you want to do it more quickly, you need to lower the perigee of the object enough that it experiences greater drag. $\endgroup$
    – Steve Linton
    Commented Jun 21, 2018 at 14:09
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    $\begingroup$ Possibly related: How long does trash jettisoned by hand from the ISS fall before burning up on reentry? $\endgroup$
    – Dan Pichelman
    Commented Jun 21, 2018 at 16:01

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@user3715778's answer is correct. Throwing will not make enough of a difference in the orbit to reenter Earth's atmosphere de-orbit promptly. Let's run the math using the vis-viva equation:

$$v^2 = GM \left(\frac{2}{r} - \frac{1}{a}\right)$$

           periapsis       apoapsis          semi-major       apoapsis
           altitude (km)   altitude (km)     axis (km)        velocity (m/s)
initial        400.           400.             6778.             7668 
 final          80.           400.             6618.             7575

difference                                                         93

world's record                                                     45

edit: Scott Manley gets about 90 m/s as well.

According to this Quora answer the fastest thrown ball was in Cricket with a speed of about 161 kph or about 45 m/s, which is about half of the speed necessary to promptly de-orbit. This would definitely shorten the time to reentry but it would be quite a challenge while wearing a space suit!

Order hundred meters per second is certainly achievable with a simple slingshot, if sufficiently large and the item sufficiently small. Just a random example: Monster Slingshot New World Record Shot in YouTube, from Joerg Sprave's Slingshot Channel. You might be able to do this in a standard space suit.

A simple sling conceivably might also do it, depending on several details...

However, At 400 km altitude everything could be considered as deorbiting! Drag limits the lifetime of many spacecraft there, and the ISS regularly boosts itself to even maintain it's altitude. The rate is quite variable and activity of the Sun can heat the upper atmosphere and increase decay rate a lot.

For example, Scott Manley says that a tool bag lost from the ISS took only 9 months to reenter the atmosphere for example.

See all the good answers to How long does trash jettisoned by hand from the ISS fall before burning up on reentry? for example. Basically, if you keep something on the ISS it will not de-orbit, at least until the ISS's end of life. However, if you toss it "overboard" it will find it's way to the Earth fairly quickly, and so this is done. In fact, it is done both intentionally and unintentionally, as is explored in the various answers to What kinds of things have been tossed out of the ISS?

GIF:

enter image description here

above: from the question Was this large pieces of “space junk” just released from the ISS in the “nadir and retrograde” direction?

edit: According to Scott Manley's video this is 1998-067NM 43203 ISS DEB

According to Space.com's 02-Feb-2018 article Cosmonauts Break Russian Spacewalk Record During Space Station Antenna Repair:

The cosmonauts spent the day replacing an electronics box for a high-gain communications antenna outside the Zvezda service module. Instead of holding on to the outdated piece of equipment, the cosmonauts tossed the original electronics box overboard, dooming it to burn up in Earth's atmosphere.

This answer shows a plot for a generic satellite, though each size and shape will be different. At 400 km the reentry time ranges from a few years to a few months depending on spacecraft shape and orientation (see this answer for example) and solar activity. More about drag in this asnwer and links within.

This answer links to a site called lizzard-tail.com where you can try a calculation yourself. It won't be accurate, again because spacecraft shape and solar activity can vary, but you can play with some general spacecraft and solar parameters there to get an idea. The website also links to this documentation

enter image description here

above: borrowed from this answer.

enter image description here

above: example of ISS altitude vs time, borrowed from this answer.

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    $\begingroup$ Baseball pitcher Aroldis Chapman holds a record for a fastest thrown pitch 105mph, still not enough delta-v to deorbit, but slightly closer. $\endgroup$
    – user3715778
    Commented Jun 22, 2018 at 13:25
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There is a great video by Scott Manley, which specifically addresses your question:

Could An Astronaut Throw Something From Orbit To Earth?

The short answer is no, humans can not provide enough delta-v by muscle power to deorbit an object by throwing it from ISS.

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    $\begingroup$ The long answer says that your short answer is the correct answer! $\endgroup$
    – uhoh
    Commented Jun 21, 2018 at 17:03
  • $\begingroup$ Well, the question was not from "from orbit", but "from the ISS" - which is still inside Earth's atmosphere. Astronomers consider the atmosphere of a planet belonging to that planet, so anything on/in/at the ISS is still "on Earth" (I know that there is the arbitrary chosen 100km limit where space begins and Earth ends...officially). Also, it's questionable whether the ISS orbits Earth or it is performing powered semi-ballistic flight (because it is actually de-orbiting already all the time and requires reboosts to stay up). Other than that, your answer is correct. $\endgroup$
    – Klaws
    Commented Jul 2, 2019 at 15:16
  • $\begingroup$ To de-orbit promptly, no... But throwing something retrograde from the ISS at just 20m/s will shorten that object's lifespan before reentry by 84%. (20m/s will shove its perigee down by 76 km!) This is still likely to be months not hours or even days. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Commented Jul 25, 2021 at 16:10
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0 m/s
At the altitude of the ISS, atmospheric drag—the effect of particles stealing your momentum—will decay most if not all orbits. The ISS has to make regular adjustments to it's orbital speed to keep it in the proper orbit. Without such adjustments, it would most certainly decay. Now, if you wanted the object thrown to decay immediately, it would have to be thrown at the orbital velocity of the ISS, which is about 7.7 km/s, in the opposite direction of travel. In doing so, the object would fall straight down towards the Earth, ceasing acceleration once it hit the terminal velocity of the atmosphere at its specific ∆altitude. Depending on the material, it would likely end up intact as well.

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